DONOR-ACCEPTOR METHODS FOR BAND GAP REDUCTION IN

DONOR-ACCEPTOR METHODS FOR BAND GAP REDUCTION IN CONJUGATED POLYMERS: THE ROLE OF ELECTRON RICH DONOR HETEROCYCLES

By CHRISTOPHER A. THOMAS

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2002

Copyright  2002 by Christopher A. Thomas All rights reserved.

ACKNOWLEDGMENTS I thank my parents, Nancy and Larry, for their continuous support and attempts to understand and encourage me during what has been simultaneously the most enjoyable and most stressful part of my life. They made this document possible by encouraging and participating in experiences that ensured I would have the backgound and interest in trying to figure out how the world works. The decision about whom to work for in graduate school is one of the events I have agonized the most about in my life. As promised, the graduate advisor-student relationship is a complicated entity consisting of advisor, boss, counselor and friend that can change its active role without warning. Despite this, I am positive that there is no other person that I would rather have worked with during this process. I especially appreciate being given an unusual amount of decision making and direction setting power in the projects I was involved with and the freedom to explore aspects of science that interested me even when they did not overlap cleanly with Professor Reynolds’ research interests or funding. For their major contributions to my enjoyment and interest in chemistry, I thank Joe Carolan for his contagious enthusiasm, and Joel Galanda, who immensely affected the three years of my life he was involved with my chemistry and physical science education. I especially appreciate his role in enabling my employment at the Rockwell International Science Center, one of the experiences I consider most valuable in my education. I thank my mentor at Rockwell, Les Warren, who, from high school through undergraduate and graduate school, has influenced my decisions on how to approach life, my education and iii

career more than any other person I have met. To Tim Parker, thanks go for largely directing my research experiences as an undergraduate and showing me how to do synthesis even after being hit by my snowboard. Several coworkers have had an important role both in this document and graduate school. Thanks go to Jacek Brzezinski and Kyukwan Zong for making the fluorenones and XDOPs respectively, allowing me to spend my time doing things I found to be more interesting. To Roberta Hickman, thanks go for friendship, helping me, for making lab-life more fun, and for your contributions to the data which eventually became Figure 5-5. Thanks go to C. J. Du Bois, the reigning Reynolds’ group lab clown, for the Yuengling-Tücher inspired conversations at the Copper Monkey and always finding the reference that I knew existed but could not find. I also appreciate the rest of the members of the Reynolds group, especially Carl Gaupp and Irina Schwendenman, for being helpful when I needed it and generally fun to be around. I also appreciate the help provided by Lori Clark, Donna Balcolm, and Professor James Deyrup for assitance in navigating through the UF bureaucracy, Lorraine Williams for making life on the polymer floor easier and Khalil Abboud and Peter Steel who solved the crystal structures included in this document. Finally, my time here would not have been nearly as fun without the friends that provided the distractions necessary to have made this work possible to get through when it felt like work. Thanks go to Dominic Rice, who throws the best parties I have ever been to and was always up for hanging out when I needed to blow off steam; Kevin Boone for the football weekend fun and the good friends I have met through him; the KA’s for fun times tubing and at the lake house; Andrew Cottone, the Mobsters and the Bandits for the softball and baseball experiences; and Megan Odroniec for a fun couple of years.

iv

TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................................................................................ iii ABSTRACT.......................................................................................................................vii CHAPTERS 1

BAND GAP: THE DEFINING QUANTITY FOR CONTROLLING THE PROPERTIES OF CONJUGATED POLYMERS .......................................................1 Brief History of Conjugated Polymers ...................................................................1 Origin of Bands and Conductivity in π-Conjugated Organic Materials .................4 Why Band Energies Are Important ......................................................................12 Conductivity Mechanisms in Organic Materials ..................................................15 Ionic Conductivity and Redox Conductivity ..................................................17 Polarons, Bipolarons and p-Dimers ................................................................20 Doping Induced Optical Transitions .....................................................................24 Conjugated Polymer Synthesis and Device Incorporation ...................................26

2

ELECTROCHEMICAL AND COUPLED IN SITU EXPERIMENTAL METHODS IN CONJUGATED POLYMER RESEARCH ................................................................36 Introduction ...........................................................................................................36 Monomer Crystallization ......................................................................................37 Spectroelectrochemistry ........................................................................................38 Colorimetry ...........................................................................................................43 Cells and Reference Electrodes for Electrochemistry ..........................................47 General Electrochemistry and Cyclic Voltammetry (CV) ....................................52 Differential Pulse Voltammetry (DPV) ................................................................61 In Situ Conductivity ..............................................................................................62 Four-Point Conductivity on Free-Standing Films ................................................69 Electrochemical Quartz Crystal Microbalance (EQCM) ......................................72

3

DONOR MEDIATED BAND GAP REDUCTION IN A HOMOLOGOUS SERIES OF CONJUGATED POLYMERS ..............................................................................76 Introduction and Literature Overview ..................................................................76 Synthesis and Monomer Properties ......................................................................81 v

Monomer Structural Features and Crystallography ..............................................85 Computational Explanations of Monomer Properties ..........................................90 Polymer Optical Properties ...................................................................................93 Polymer Electrochemistry ...................................................................................106 Conclusions and Perspective ..............................................................................115 Experimental Section ..........................................................................................122 4

THE ROLE OF INTERGAP REDOX STATES IN CONJUGATED POLYMER REDUCTION PROCESSES: THIENYL AND EDOT CONTAINING FLUORENONES ......................................................................................................129 Introduction .........................................................................................................129 Monomer Synthesis and Properties ....................................................................133 PBEDOT-DCF and PBTh-DCF Synthesis, Electrochemistry and Spectroelectrochemistry ......................................................................................136 Monomer Electrochemistry ................................................................................148 Conclusions .........................................................................................................151 Experimental Section ..........................................................................................153

5

POLY(ALKYLENEDIOXYPYRROLES): AQUEOUS COMPATIBLE CONDUCTING POLYMERS WITH LOW FORMAL REDOX POTENTIALS ..160 Introduction .........................................................................................................160 Monomer Synthesis and Polymer Electrosynthesis ............................................165 Polymer Electrochemistry ...................................................................................169 Polymer Spectral Characteristics ........................................................................174 Electrochemical Quartz Crystal Microbalance Studies on Ion Transfer ............180 Conclusions .........................................................................................................184 Experimental Section ..........................................................................................184

APPENDIX A CRYSTALLOGRAPHIC INFORMATION FOR CYANOVINYLENE MONOMERS ...........................................................................................................188 B CRYSTALLOGRAPHIC INFORMATION FOR FLUORENONE MONOMERS ...........................................................................................................210 REFERENCES ...............................................................................................................217 BIOGRAPHICAL SKETCH ..........................................................................................226

vi

Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy DONOR-ACCEPTOR METHODS FOR BAND GAP REDUCTION IN CONJUGATED POLYMERS: THE ROLE OF ELECTRON RICH DONOR HETEROCYCLES By Christopher A. Thomas May 2002 Chair: Professor John R. Reynolds Major Department: Chemistry Keywords: conducting polymers, conjugated polymers, donor-acceptor, thiophene, EDOT, EDT, electropolymerization, pyrrole, 3,4-alkylenedioxypyrrole, EDOP, ProDOP Three families of conjugated polymers based on cyanovinylenes, fluorenones and 3,4-alkylenedioxypyrroles (XDOPs) were investigated to determine the role of the donor heterocycle in donor-acceptor (D-A) polymers (cyanovinylenes and fluorenones) and aqueous compatible conjugated polymers (PXDOPs) which are stable in the presence of strong biological reductants. When donors such as thiophene (Th), 3,4-ethylenedioxythiophene (EDOT, EDT) or 3,4-ethylenedioxypyrrole (EDOP) were used as donors and electropolymerized in a D-A-D configuration where the acceptor was cyanovinylene, spectroelectrochemistry confirmed band gaps ranging from 1.6 eV to 1.1 eV. Cyclic voltammetry confirmed p-type doping typical of conjugated polymers containing these heterocycles, and in situ conductivity indicated that a conductivity increase of at least 5 orders of magnitude occurred upon p-type doping. Spectroelectrochemistry and colorimetry experivii

ments establish a color change from a deep blue (neutral form) to a transmissive blue (ptype doped) form when switched in acetonitrile on indium tin oxide. The cyanovinylene family of polymers exhibit an uncharacteristically sharp reductive cyclic voltammetric response, but do not change colors upon reduction. In situ conductivity indicates that the change in conductivity on reduction is small, and the symmetrical shape is evidence it is predominantly due to redox conductivity and not electronic conductivity as is the case for p-type doping. These results combine to suggest that the strongest donor necessary for band gap reduction in these systems is EDOT as increasing the donor character by using EDOP did not reduce the band gap further. They also suggest that acceptor units be present in the polymer backbone in a 1:1 ratio (to donors) and that the acceptor unit not have strong acceptors pendant to the main conjugation path for improved n-type conductivity. PEDOP and PXDOPs in general are uniquely suited for use as aqueous compatible conjugated polymers in applications where poly(pyrrole) (PPy) is now used. This interest is largely due to the PXDOPs having the lowest reduction potentials of any conjugated polymer reported to date, lending them improved stability in the p-type doped conducting state. This stability was tested with the biological reductants dithiothreitol and glutathione which were found not to reduce the PXDOPs but did reduce PPy. Colorimetry established color changes for the PXDOPs from red/orange (neutral) to transparent blue/gray (oxidized) with band gaps between 1.9 to 2.2 eV by spectroelectrochemistry.

viii

CHAPTER 1 BAND GAP: THE DEFINING QUANTITY FOR CONTROLLING THE PROPERTIES OF CONJUGATED POLYMERS 1.1 Brief History of Conjugated Polymers Research interest in conjugated and electroactive polymers has been invigorated with the discovery that these materials can be incorporated into flexible, ultimately low cost displays for a variety of applications. The field of conjugated polymers1 (CPs) originated near the end of the 1970’s with the discovery that poly(acetylene) (PA, 1) can be made highly conductive by doping.2 Subsequently, poly(aniline) (PANI, 2), poly(pyrrole) (PPy, 3), poly(thiophene) (PTh, 4) and more recently poly(3,4-ethylenedioxythiophene) (PEDOT, 5) (Figure 1-1) have been extensively studied and the field itself is the subject of many reviews.3 Originally it was thought that the principal application for CPs would be to replace relatively dense metals such as copper and aluminum in weight sensitive applications such as air and space travel. While heavily doped poly(acetylene) (PA) can be made more highly conducting than copper on a conductivity per weight basis, its instability

1. While the terms conjugated polymer and conducting polymer are often used interchangeably throughout the literature, “conjugated polymer” is preferred here since the materials made are often not highly conductive, especially when undoped, and many properties described herein make use of the non-conductive state. 2. Chiang, C. K.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G. Phys. Rev. Lett. 1977, 39, 1098. 3. (a) Roncali, J. Chem. Rev. 1992, 92, 711-738. (b) Chan, H. S. O.; Ng, S. C. Prog. Polym. Sci. 1998, 23, 1167-1231. (c) Roncali, J. J. Mater. Chem. 1999, 9, 1875-1893. 1

2 under ambient conditions which include water and oxygen precluded the practical use of PA practically. Search for alternative CPs with superior stabilities led to the discoveries of PANI, PPy, PTh, and PEDOT which, although blessed with improved stability in the doped, conducting form, are still much less conductive than PA and have thus reduced research efforts into CPs as light-weight metal replacement conductors. Far from marking the death of the field, the successors to PA have found use in a wide range of applications including thin film transistors (TFTs),4 flexible light emitting displays (OLEDs),5 chemical sensors for a wide array of analytes,6 and nerve cell guidance channels and biological substrates.7 Largely in recognition of the important role that these materials are expected to play in the near term technology infrastructure of the internet, portable electronic devices and computers, the Nobel Prize in Chemistry was awarded in 2000 to the discoverers of PA, Drs. Heeger, MacDiarmid and Shirakawa. Poly(acetylene) (PA), PANI, PPy, and PTh represent the first generation of CPs, and as such, are not the ideal materials for most applications for reasons that are detailed throughout this chapter. PEDOT has been the most successful of the 2nd generation CPs 4. Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498-500. 5. (a) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bredas, J. L.; Logdlund, M.; Salaneck, W. R. Nature 1999, 397, 121-128. (b) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. Engl. 1998, 37, 402-428. 6. (a) Albert, K. J.; Lewis, N. S.; Schauer, C. L.; Sotzing, G. A.; Stitzel, S. E.; Vaid, T. P.; Walt, D. R. Chem. Rev. 2000, 100(7), 2595-2626. (b) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100(7), 2537-2574. (c) Ellis, D. L.; Zakin, M. R.; Bernstein, L. W.; Rubner, M. F. Anal. Chem. 1996, 68(5), 816-822. 7. C. E. Scmidt, V. R. Shastri, J. P. Vacanti, R. Langer, R. Proc. Natl. Acad. Sci. USA 1997, 94, 8948.

3

x

H N

H N

S

S x

x

x

x O

PA, 1

PANI, 2

PPy, 3

PTh, 4

O

PEDOT, 5

Figure 1-1. Common conjugated polymers because of its reduced band gap compared to PTh and increased environmental stability as a doped conductor. As such, it has been commercialized by Agfa and Bayer and found applications in transparent static protection of electronics during shipping, improved soldering processes for circuit boards, photographic film production and as a component in organic light emitting devices (OLEDs). Because of PEDOT’s unique properties, research interest and related publications have increased exponentially over the last several years from the first one in 1991 to 65 in 1999.8 Simple heterocycles have largely been exhausted as possible candidates for 3rd generation materials because a combination of lower band gap, aqueous compatibility and the ability to be n-type doped without necessitating heroic efforts to exclude air and water are needed for these applications. Because of these requirements, it seems likely that next generation materials will come from more complicated monomers built up from smaller functional segments to incorporate the electronic properties needed while maintaining the ability to be effectively polymerized. Designing optimized materials requires a fundamental understanding of how structural modification is related to the ultimate material properties. Recent research efforts by several groups indicate that the quantities

8. Groenendaal, L. B.; Jonas, F., Freitag, D.; Pielartzik, H.; Reynolds, J. R. Adv. Mater. 2000, 12, 7, 481-494.

4 that are most significant to the control of CP properties are the band gap and position of the edges of the conduction band and valence band. This chapter establishes the molecular origin of band gap, how it manifests itself in current materials, what needs to be done to optimize some materials, and the impact that band gap has on materials in terms of conductivity. 1.2 Origin of Bands and Conductivity in π-Conjugated Organic Materials While PA is essentially a defunct material in terms of current research interest, its structural simplicity provides a convenient entry into the band gap discussion. PA is composed of a chain of sp2 hybridized carbon atoms linked by alternating single and double bonds, (CH)x. The theoretical work was done long before the actual synthesis and PA was assumed to have a structure like that shown in Figure 1-2 A where resonance causes the double bonds and single bonds to have the same bond length on average and there is extensive delocalization along a chain. The communication along the main chain of the polymer is due to the overlapping p orbitals forming π bonds and this material was expected to be highly conductive as a result. The anticipated orbital diagram is shown in Figure 1-2 A, and when the electrons are added in, there should be no energy difference between the full VB arising from the HOMO of acetylene and the unfilled CB from the LUMO. This explains the expected high conductivity which requires a partially filled band. Physicists have an alternative model to chemists for describing the band structure of materials that adds insight into the behavior of these materials at the expense of being slightly harder to visualize. In this model the energy of the orbitals, and thus bands, are viewed in reciprocal k space used for crystallography. For hypothetical PA, the VB runs up in energy and the CB runs down to meet at a point indicating no band gap. Chemists tend to be uncomfortable

5 A. HYPOTHETICAL POLY(ACETYLENE) BAND STRUCTURE

k(E)

=

DOS(E)

E k

DOS

B. POLY(ACETYLENE) AFTER PEIERLS DISTORTION

=

EF

E

k

DOS

C. DEVELOPMENT OF THE BAND STRUCTURE OF POLY(THIOPHENE) B3P86-30%/cep-31g*

S

3.0

x

0.0 Energy /eV

CB -3.0 Eg -6.0 VB -9.0 -12.0 -15.0 0

1

2

3

4 5 6 Number of Rings (x)

∞

~DOS

Figure 1-2. Development of band structure from monomer to polymer. (A) Poly(acetylene) is composed of a series of p orbitals which were initially expected to have the band structure shown on the right. (B) PA actually undergoes a Peierls distortion causing a band gap to open with the approximate density of states shown. Adapted from Hoffman, R. Angew. Chem. Int. Ed. Eng. 1987 26, 846-878. (C) The PTh band structure is based on orbitals from monomer through oligomer overlapping as shown. Adapted from Salzner, U.; Lagowski, J. B.; Pickup, P. G.; Poirier, R. A. Synth. Met. 1998, 96, 177-189. visualizing k space and as a result, density of states (DOS) is utilized frequently as a model that incorporates the important information from k space while preserving the intelligibility

6 of real space for non-physicists. The derivation and methods of approximating the DOS of a material given the band structure are detailed by Hoffman.9 When PA was synthesized, it was found to be an insulator with a conductivity of ca. 10-13 S cm-1 for the cis-PA form.10 Isomerization occurs incompletely by heating and efficiently upon n-type doping and charge compensation with AsF5.11 trans-PA effectively dimerizes by a Peierls distortion,12 opening up a band gap (1.48 eV by photoconductivity for trans, ~2 eV for cis) at the Fermi level and dramatically reducing the conductivity because electrons have to be thermally excited across the band gap to delocalize in the partially filled CB. The band structure and approximate DOS results of this process are represented in Figure 1-2 B. Because of this, PA must be doped to either partially fill the CB by adding electrons (n-type doping) or partially vacate the VB by oxidation (p-type doping). This doping process, while possible and quite effective at improving the conductivity to a maximum of 2(10)4 S cm-1, does not result in an air stable material and caused chemists to look elsewhere for other polymers that could be doped more easily. In the early 1980’s it was discovered that electron rich poly(heterocycles) could be chemically or electrochemically oxidized to from CPs that have relatively high conductivities. Pyrrole and thiophene are two examples of these heterocycles. The 9. Hoffman, R. Angew. Chem. Int. Ed. Eng. 1987, 26, 846-878. 10. trans-PA has a neutral conductivity of 10-5 - 10-6 S cm-1. 11. Chien, J. C. W. Polyacetylene Chemistry, Physics and Material Science; Academic Press, Harcourt Brace Jovanovich: Orlando, 1984; Chapter 5. 12. The bond length alternation has been experimentally determined to be ~0.08 A. (a) Fincher, C. R.; Chen, C. E.; Heeger, A. J.; Macdiarmid, A. G.; Hastings, J. B. Phys. Rev. Lett. 1982, 48, 100. (b) Yannoni, C. S.; Clarke, T. C. Phys. Rev. Lett. 1983, 51, 1191.

7 evolution of the band structure of PTh is shown in Figure 1-2 C for monomer through hexamer and eventually polymer. Figure 1-3 shows the same formation of bands from overlapping molecular orbitals for poly(pyrrole) PPy as determined by Density Functional Theory (DFT) calculations. These calculations are becoming widely used in CP research due to their accounting of electron correlation, accuracy in band gap prediction and relatively light computational requirements.13

DEVELOPMENT OF THE BAND STRUCTURE OF POLY(PYRROLE) H N

B3P86-30%/cep-31g*

3.0

x

Energy /eV

0.0

CB

-3.0

Eg

-6.0

VB

-9.0 -12.0 -15.0 0

1

2

3

4 5 6 Number of Rings (x)

∞

Figure 1-3. Evolution of the band structure of PPy from monomer through hexamer extrapolated to polymer as determined by density functional theory. Adapted from Salzner, U.; Lagowski, J. B.; Pickup, P. G.; Poirier, R. A. Synth. Met. 1998, 96, 177-189.

There are five classes of conductivity that can be identified and are largely differentiated by their conductivity magnitude and temperature dependence. Of these, four can be easily explained by simple band diagrams (Figure 1-4) while the fifth, 13. Salzner, U.; Pickup P. G.; Poirier, R. A.; Lagowski., J. B. J. Phys. Chem. A, 1998, 102 (15), 2572 -2578.

8 superconductivity, has a more complicated mechanism. Metals are the most familiar conducting materials to most and are characterized by a zero band gap originating from a partially filled band or, alternatively, a contacting VB and CB. Semi-metals are relatively uncommon and examples are mostly limited to alkali and alkaline earth elements. The conductivity mechanism in semi-metals involves overlapping orbitals creating a partially filled band which enables electrical conductivity. With the exceptions of PA, PPy, PEDOT (which when doped with specific dopants are metallic)14 and poly(sulfurnitride) (an inorganic semi-metal), the vast majority of CPs exhibit semiconductor or insulator-like conductivity. These operate under an identical mechanism where population of the CB is thermally activated and the conductivity magnitude is determined by the temperature and the size of the band gap. The difference between a semiconductor and an insulator is rather arbitrarily determined at a band gap of 3 eV in most cases. The temperature dependence of conductivity is the primary indicator of conduction mechanism for newly discovered materials. Figure 1-4 B shows the temperature dependence for a prototypical material for each class of conductivity. Since metals and semi-metals have no band gap, the thermal component to metallic conductivity serves to decrease conductivity with increasing temperature by scattering electrons through increased phonon (or lattice vibration) intensity. This is observed for Cu and (SN)x. Conductivity is described by the equation σ = neµ where σ is the conductivity, n is the number of charge carriers, e is the sign of the charge carrier and µ is the charge carrier 14. (PPy) Lee, K.; Miller, E. K.; Aleshin, A. N.; Menon, R.; Heeger, A. J.; Kim, J. H.; Yoon, C. O.; Lee, H. Adv. Mater. 1998, 10(6), 456-459. (PEDOT) Aleshin, A. N.; Kiebooms, R.; Heeger, A. J.; Synth. Met. 1999, 101, 369-370.

9 A. CLASSES OF CONDUCTIVITY NOTE: Superconductivity does not fit neatly into this model CB CB CB

Eg > 3

CB Eg = 0

Eg < 3

Eg = 0

VB

VB

VB

VB

METAL

SEMIMETAL

SEMICONDUCTOR

INSULATOR

B. CONDUCTIVITY vs. TEMPERATURE FOR VARIOUS MECHANISMS 108 Copper (METAL)

A

106 (SN)x (SEMIMETAL)

Conductivity /S cm-1

104 (TTF)(TCNQ) (TYPE III LINEAR CHAIN CONDUCTOR)

102

1

Mobility of Charge Carriers -α µ=T

B

Conductivity σ = neµ

10-2

PPy-DBSA (SEMICONDUCTOR)

10-4 Concentration of Charge Carriers n = e-Eg/2kT

10-6

Temperature

10-8

0

50

100

150 200 Temperature /K

250

300

350

Figure 1-4. Conductivity temperature dependence. (A) Simple band diagrams showing the difference between metals, semi-metals, semiconductors and insulators. (B) Conductivity vs. temperature profiles for the four types of conductivities indicates metals and semimetals have a negative correlation of conductivity vs. temperature where semiconductors and insulators (not shown) have a positive correlation. This results from the competing relationship of charge carrier mobility and concentration (B, inset) for semiconductors. For metals, only the mobility is important since there is no band gap. Adapted from Reference Epstein, A. J.; Miller, J. S. Sci. Am. 1979, 241, 4, 52-61.

10 mobility. All three quantities that determine conductivity have different temperature dependences (Figure 1-4 B inset) and it is the competition between these quantities that determines the overall relationship between σ and T. The number of charge carriers, n, follows an Arrhenius activation and depends on an exponential relationship of band gap and temperature. The charge carrier mobility is related to T and decreases with increasing temperature because of scattering. For PPy doped with dodecyl benzene sulfonic acid (PPyDBSA), the conductivity increases with temperature due to increasing thermal population of the CB (increase in n).15 An alternative visualization to the mechanism of conductivity differences between metals and semiconductors is shown in Figure 1-5. In metals, the mobile electron can be viewed as being scattered by a lattice vibration (phonon) that grows with increasing temperature. At low temperatures, the electron has enough energy to surmount the phonon barrier height where at high temperatures, the electron is scattered. In contrast, semiconductors use a hopping mechanism where the electron is initially trapped in a lattice defect. Increasing temperatures causes the lattice defect to deform and the electron can move under the influence of a potential field. Minimizing the band gap is an important goal for maximizing the neutral conductivity of CPs. Most CPs synthesized to date have band gaps greater than 2 eV characterizing them as mid- to high band gap polymers. Polymers with band gaps lower than 1.5 eV are considered relatively low band gap materials and few confirmed examples are available of polymers with band gaps below 0.8 eV.16 PITN is a notable example of one

15. Aleshin, A. N.; Lee, K.; Lee, J. Y.; Kim, D. Y.; Kim, C. Y. Synth. Met. 1999, 99, 2733.

11 of the earliest low band gap polymers, made so because of the competition between aromaticity and quinoid geometry in the polymer.17 For reference, silicon has a band gap of 1.1 eV. A. BAND-TYPE CONDUCTION (METALS)

LATTICE VIBRATION

DELOCALIZED ELECTRON

INCREASED TEMP

ENERGY

SCATTERED ELECTRON

B. CONDUCTION BY HOPPING (SEMICONDUCTORS) ELECTRON LOCALIZED BY DEFECTS OR DISORDER

LATTICE VIBRATION

DISTANCE IN ELECTRIC FIELD

Figure 1-5. Conductivity mechanism (A) A delocalized electron is scattered by a lattice vibration in metals which increases at increasing temperatures. (B) In semiconductors, conduction by hopping is assisted by lattice vibrations and increases with increasing temperature. Adapted from Epstein, A. J.; Miller, J. S. Sci. Am. 1979, 241, 4, 52-61.

16. Some poorly defined materials with extremely low band gaps have been reported but further confirmations are necessary: (a) Huang, H; Pickup, P. G. Chem. Mater., 1998, 10(8), 2212 -2216. (b) Huang, H; Pickup, P. G. Chem. of Mater.; 1999; 11(6); 15411545. 17. Wudl, F. J. Org. Chem 1984, 49, 3382.

12 1.3 Why Band Energies Are Important While it is practically the band gap that determines the conductivity and color of the neutral polymer, the positions (energies) of the band edges determine how easily doped (ptype or n-type) a polymer is and how stable it is in the doped states compared to the neutral forms. Figure 1-6 describes the four ways in which most CPs can be classified. These are either high or low band gap with either a high or low VB or CB. The most common scenario by far is the polymer with a band gap of 2 eV or greater and a relatively high VB. Examples of polymers with these characteristics are PPy, PTh, PEDOT and poly(3,4alkylenedioxypyrroles) (PXDOPs), an extreme example of a high VB whose properties are described in Chapter 5. These polymers are all prepared by oxidative polymerization routes and are easily p-type doped18 by virtue of their high lying VB. Because of their high band gap, the CB is generally out of reach for stable electrochemical n-type doping and PPy cannot be n-type doped. PTh does exhibit n-type doping at very negative potentials and PEDOT is more difficult to n-type dope than PTh. Another situation is the high gap/low VB polymer such as PPV and its analogues. PPV is not easily doped electrochemically,19 but its bands are positioned so that charge carriers of either sign can be injected making this class of polymer useful in light emitting devices where the emission energy color is tuned by modifying the band gap. Low gap polymers are a relatively recent topic of research interest and these types of polymers have many interesting applications. Regardless of how the band energies line 18. p-type doping is the creation of charge carriers by partial oxidation of a polymer or semiconductor creating a partially vacated VB. Conversely, n-type doping is the creation of charge carriers by partial reduction creating a partially filled CB. 19. Tanaka, S.; Reynolds, J. R. J. Macro. Sci., Pure Appl. Chem. 1995, A32, 1049-1060

13 up, the low band gap often causes these polymers to be transparent in the doped state (p- or n-type) (see section 1-5 for a discussion of how band gap translates into colors). This makes them useful in a variety of devices such as electrodes and windows where this transparency can be exploited. As with the high gap polymers, the color of low gap polymers is dictated by the magnitude of the band gap. Since the distance between the edge of the VB and CB is smaller by definition in low gap systems, the CB is generally more accessible to n-type doping even in low gap polymers with relatively high VBs. This class of materials is perhaps the most intriguing, offering the transparency of low gap systems with the ability to be both n-type and p-type doped in the same polymer. While it is obvious that the VB needs to be relatively low in energy for facile n-type doping, the question arises, how low does it need to be for the n-type, conductive state to be stable in devices? The answer to this question is somewhat complicated and lies in the available redox couples that can interfere with the n-type state described and is shown in Figure 1-7. For a p-type polymer, the reaction of interest is the O2 to H2O redox couple where oxygen could oxidize the neutral form of a polymer to the p-type doped state. This redox couple is very near the Eº’ (formal redox potential, see Chapter 2) for several conducting polymers and p-type doped polymers with an Eº’ more anodic of the O2 to H2O redox couple are reduced by H2O and are stable in the neutral form. Polymers with a high enough VB to have an Eº’ cathodic of the O2 to H2O couple are stable in the p-type, oxidized state. For n-type conducting polymers the reaction of interest is the H2O to H2 couple. A polymer with an Eº’ cathodic of this couple (every authentic n-dopable polymer to date) will be oxidized by H2O to the neutral form of the polymer. This couple appears at ca. -0.65 V vs. SCE. With typical overpotentials it is necessary to have a polymer with an

14 CB -2.0

-1.5 CB CB Potenial /V vs SCE

-1.0

CB

-0.5

0

VB

VB

Eg > 1.5 eV

Eg < 1.5 eV

+0.5 VB

+1.0

VB High Eg Polymers

Low VB (HOMO) Easily p-type doped Easily n-type X doped Example PPV

Low Eg Polymers

High VB (HOMO)

High CB (LUMO)

Low CB (LUMO)

X

X

X

X

X

Cyanovinylenes (Chapter 3)

PCDM few others

Poly(pyrrole) Poly(XDOPs) (Chapter 5) many others

Figure 1-6. Possibilities for positioning of band edges in both high and low band gap polymers. The common case is a high band gap polymer with a high VB. For many applications, a low band gap polymer with a low CB is desirable and few examples are available. Eº’ more anodic of -0.5 V vs. SCE for the n-type doping process for the polymer to be stable in the presence of water.20 This is a far more significant problem than the need to have an

15 Eº’ for p-type doping cathodic of ca. +0.5 V vs. SCE to be stable in the oxidized state since even moderately electron rich polymers are well cathodic of this value. To have a polymer that is stable to water in its n-type form and stable to oxygen and water in its p-type form, the polymer needs to have a band gap of less than 1 eV and be very easy to reduce compared to current materials. Identifying the structural factors that could enable this are the impetus for much of the research in this work. 1.4 Conductivity Mechanisms in Organic Materials While the ability to dope a material at potentials that are compatible with ambient environmental conditions is important for electrochromic devices and sensors, the conductivity magnitude is an important property for microwave camouflage devices for military applications. CPs offer a unique property in that their conductivity can be tuned over 8 or more orders of magnitude in the same material. Several different conductivity mechanisms are available for electrically conducting CPs depending on their structure and hybrid materials can have multiple mechanisms. Figure 1-8 shows the typical conductivity ranges for materials commonly considered metals, semiconductors and insulators although the factor that determines this classification is really the temperature dependence of conductivity rather than the conductivity magnitude. The three most common conducting polymers shown in Figure 1-8 span the range of conductivities from metal to insulator depending on doping level. This property is unique to CPs. The mechanism for neutral polymer conductivity has been discussed in section 1-2. Neutral polymers tend to be semiconductors or, less frequently, insulators since the band gap is generally < 3 eV yet is

20. de Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met. 1997, 53-59.

16 RED FORM -2.0

OX FORM

PTh-

PTh°

H 2O

H2

A

C

-1.5

Potenial /V vs SCE

-1.0

-0.5

0

+0.5

{ PEDOT° PPy°

PEDOT+ PPy+

PANI° 2H2O

PANI+ O2

PPV°

PPV+

}

B +1.0

A

B C

p-type dopable polymers in the neutral state with E1/2(° to +) in this range are oxidized by O2 (p-doped form stable under ambient conditions). n-type dopable polymers in the reduced state with E1/2(° to -) in this range are oxidized by O2 (neutral form stable under ambient conditions). p-type dopable polymers in the neutral state with E1/2(° to +) in this range are stable to O2 (neutral form stable under ambient conditions). n-type dopable polymers in the reduced state with E1/2(° to -) in this range are oxidized by H2O (neutral form stable under ambient conditions).

Figure 1-7. Stability of CPs in their oxidized or reduced forms depends on where the formal oxidation potential (Eº’) lies (a direct consequence of the VB (p-type) or CB (ntype). A p-type polymer with a low Eº’ is stable in its neutral state. Adapted from de Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met. 1997, 5359.

17 too high for efficient population of the CB by kT. Doping by introducing charge carriers (positive or negative) has a drastic effect on conductivity as noted in Figure 1-8 and the details of this are important for designing new materials. SILVER COPPER IRON BISMUTH

METALS

POLYACETYLENE σmax > 2 X 104 S cm-1

106

POLYTHIOPHENE σmax = 2000 S cm-1

104 102

InSb

(SN)x TTF•TCNQ NMP•TCNQ KCP

DOPED

1 GERMANIUM

SEMICONDUCTORS

10-2 10-4

SILICON

TRANS (CH)x

10-6 SILICON BROMIDE GLASS

INSULATORS

DNA

10-8 10-10 10-12

DIAMOND

10-14

SULFUR

10-16

QUARTZ

CIS (CH)x

MOST MOLECULAR CRYSTALS

UNDOPED

S

POLYPYRROLE σmax = 500 S cm-1

x

H N x

10-18 Ω-1 cm-1

Figure 1-8. The conductivity range available to CPs spans those common for metals through insulators. Adapted from the Handbook of Conducting Polymers.

1.4.1 Ionic Conductivity and Redox Conductivity A wide variety of conductivity mechanisms are seen in a variety of polymeric materials. Non-conjugated organic polymers can display conductivity in certain situations. The simplest form of conductivity in organic polymers is ionic conductivity (Figure 1-9 A). Nafion,21 a polymer used in fuel cells, and poly(ethyleneglycol) (PEG),22 used in batteries, are the canonical materials that exhibit ionic conductivity. The mechanism for this involves

18 ions (Li+, H+) moving in the presence of an electric field. In Nafion, phase segregation of the hydrophobic perfluorinated sections from the hydrophilic sulfonic acid groups forms cavities ca. 4.5 nm across which have highly concentrated acid groups. Proton conduction occurs through these channels. In PEG, an intercalated carbon electrode injects Li+ ions into the polymer which are then allowed to move by segmental motion of the PEG chains. In some materials anions are the dominant mobile species and the transference number is the ratio of the mobility of the cation to that of the anion. Another conductivity mechanism that is observed in organic materials is redox or self-exchange conductivity studied in detail by Royce Murray’s group (Figure 1-9 B).23 One class of material that exhibits this type of conductivity includes transition metal bis-αdiimine structures functionalized with PEG chains around them. This results in a matrix of easily oxidizable or reducible transition metal centers which are fixed in location between two electrodes. A maximum in conductivity is attained when there is a mixed valent condition, often with nearly the same concentration of neutral and charged sites. Since the redox centers cannot move throughout the film, electron or hole hopping between adjacent sites of different charge state allows conductivity to occur.

21. (a) Hsu, W. Y.; Gierke, T. D. J. Membr. Sci. 1983, 13, 307. (b) Srinivasan, S.; Manko, D. J.; Koch, H.; Enayetullah, M. A.; Appleby, J. J. Power Sources 1990, 29, 367. (c) Heitner-Wirguin, C. J. Membr. Sci. 1996, 120, 1. 22. Gray, F. M. Solid Polymer Electrolytes-Fundamentals and Technological Applications; VCH: Weinheim, Germany, 1991. 23. (a) Williams, M. E.; Masui, H.; Long, J. W.; Malik, J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 1997. (b) Long, J. W.; Kim, I. K.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 11510. (c) Williams, M. E.; Crooker, J. C.; Pyati, R.; Lyons, L. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 10249.

19 A. IONIC CONDUCTIVITY (Nafion 117 & PEG) Nafion 117™ [ ( CF2CF2)m CF2CF2 ]n O CF2 m = 5 - 13.5 CFCF3 k n = ca. 1000 O k = 1, 2, 3, ... CF2CF2SO3

[ ]

Nafion Tertiary Structure

CONDUCTIVITY MECHANISM IN PEG Li° Li+

~ ~~ ClO - ~ O 4 O O O O Li+

OO

O

O O

O

O

~~

O

CsLi+

O

O

O O

~~ O

Li+ O

~~

~~

~~

O

O

O O

~~

~~ O

~~

~~

A-2

~~

A-1

O

O O ClO4O

O O

CsLi+

Li°

Li°

B. REDOX (SELF-EXCHANGE) CONDUCTIVITY (Example: Polyether Hybrids) O

(O (

x O

(

3X

O

O )x O N

=

N

Co(III)

+ M = Fe, Co, Ni x = 3, ~7, ~11, ~15 B-1

e-

or Co(II)

B-2

e-

B-3

ee-

Gate electrode maintains 3:17 Co3+ : Co2+

Electrodes are typically Pt IME or bands

Figure 1-9. (A) Ionic conductivity is the predominant mechanism of charge transport in Nafion fuel cells and polymer electrolytes for batteries. (B) In redox conductivity, fixed redox centers pass charge by electron hopping between adjacent sites of different oxidation state.

20 1.4.2 Polarons, Bipolarons and π-Dimers In doped PA, a conductivity mechanism is evident that is different than ionic or redox conductivity (Figure 1-10 A). PA has a degenerate ground state, that is, there are two structures composed of exchanging carbon-carbon double bonds that have the exact same total energy. PA chains with an odd number of carbon atoms have an unpaired electron (a neutral soliton). Upon oxidation or reduction, a radical cation or anion is generated which moves along a polymer chain by the mechanism shown in Figure 1-10 A.24 This charge has the properties of a solitary wave, thus the soliton terminology from physics, and does not dissipate as it traverses the chain. Solitons have a width because the spin density (for a neutral soliton) or the charge density (cationic or anionic solitons) is not localized on one carbon but is rather spread out over several. Passing from one side of a soliton to another, the double bonds become gradually shorter, the single bonds gradually longer until they are equal in the center of the soliton.25 This double bond pattern is reversed on the other side of the soliton. All CPs other than poly(acetylene) have non-degenerate ground states. In poly(heterocycles) there is an aromatic state and a quinoidal state of higher energy (except for PITN). Oxidation of a poly(heterocycle) creates a radical cation called a polaron where the delocalization is over ca. 4 to 5 rings. This radical cation exhibits an EPR signal since there is an unpaired spin and moves under the effect of the applied potential along the polymer main chain. Further oxidation at higher doping levels in poly(heterocycles) can 24. Su, W. P.; Schrieffer, J. R.; Heeger, A. J. Phys. Rev. Lett. 1979, 42, 1698. 25. (a) Rebbi, C. Sci. Am. 1979, 240, 92. (b) Boudreaux, D. S.; Chance, R. R.; Bredas, J. L.; Silbey, R. Phys Rev. B: Condens. Matter. 1983, 28, 6927. (c) Thomann, H.; Dalton, L. R.; Tomkiewicz, Y.; Shiren, N. S.; Clarke, T. C. Phys. Rev. Lett. 1983, 50, 533.

21 A. SOLITON CHARGE CARRIER IN POLY(ACETYLENE)

[OX]

A-1 A-2 PA is degenerate

A-3 B. POLARONS AND BIPOLARONS AS CHARGE CARRIERS B-1 S

S S

S S

S

S

S

S

x

Neutral Polymer 1 e- OX

B-4

ClO4-

B-2 S

S S

S

S

S

x

poly(heterocycles) are non-degenerate

A Single Polaron 1 e- OX ClO4-

B-3 S

ClO4-

S S

S

S

S

S

S

S S

S

S

S

S

x

A Bipolaron

C. π-DIMERS AS AN ALTERNATIVE TO BIPOLARONS

A-

*

A-

O

x O

MEH-PPV

Figure 1-10. Charge carriers in conjugated polymers. (A) Solitons are the primary method of charge transport in doped PA. (B) Polarons and bipolarons are suggested as the charge carriers for doped poly(heterocycles) with non-degenerate ground states although π-dimers (C) are proposed as an alternative to polarons, especially in oligomeric materials. create either a second polaron or a dication, called a bipolaron. A bipolaron is an EPR silent dication delocalized over 4 to 5 rings. Considerable debate about the nature of charge carriers in CPs has ensued but we know certain facts. First, at low doping levels, an EPR active signal is observed consistent with polarons. Second, as doping level increases, the

22 EPR signal vanishes consistent with a spin paired conducting state. Third, oxidized PTh shows a two peaked absorption structure in the UV-Vis that is partially consistent with bipolarons as the charge carriers. The theory of electronic transitions for polarons and bipolarons was originated in the Su-Shrieffer-Heeger (SSH) model which predicted bipolarons as the stable charge carriers.26 This model was enhanced by a continuum electron-phonon coupled model proposed by Fesser, Bishop and Campbell (FBC).27 Early calculations have shown that the bipolaron is more stable than two polarons28 by 0.45 eV (the difference between the bipolaron binding energy (0.69 eV) and the polaron binding energies (2 X 0.12 eV)).29 More recent calculations have shown that up to an oligomer with ten monomer units, a bipolaron is the stable charged species. Beyond a decamer, charge repulsion between the two cations sharing the same lattice distortion suggests that two polarons are the more favorable charged species.30 The conundrum is that if this two polaron state is indeed favored, it is difficult to explain the EPR data suggesting that there are no spins present in highly doped polymers. A proposal that provides a partial solution to this problem is the π-dimer (Figure 1-10 C) as the stable charge carrier in highly doped conducting polymers. This would allow the predicted two polaron state to exist but solves the EPR problem by pairing spins on two cofacially aligned chains. This has been demonstrated for oligomers of PPV31 and poly(thiophenes).32 The optical properties of 26. Su, W. P.; Schrieffer, J. R.; Heeger, A. J. Phys. Rev. Lett. 1979, 42, 1698. 27. Fesser, K.; Bishop, A. R.; Campbell, D. K. Phys. Rev. B. 1983, 27, 4804. 28. Heeger, A. J.; Kivelson, S.; Schrieffer J. R.; Su, W.-P. Rev. Mod. Phys. 1988, 60, 781. 29. Bredas, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309-315. 30. Sakamoto, A.; Furukawa, Y.; Tasumi, M. J. Phys. Chem. B 1997, 101, 1726-1732.

23 bipolarons are different than those of π-dimers as described in the next section and enable distinguishing between the two. In most p-type doped CPs, the anions move in and out of the film on doping and dedoping and can be exchanged in electrolyte spontaneously, but are immobile in the film when dry once a constant doping level has been reached. Typically, the charge carriers must hop from one anion to the nearest anion along the chain as they move. Thus, there is a pinning potential for separating the polaron cation from its anion. At low doping levels, when the pinning potential is larger than the potential across the polymer, low conductivity results. As the doping level increases, charge carriers have to migrate much shorter distances through the film to find the next anion and the pinning potential is thus smaller.33 Effective conjugation length or coherence length is independent of molecular weight and is the number of repeat units that an electron or charge carrier can effectively access. There is no firm consensus in the literature on what the effective conjugation length of most conjugated materials is. Single-molecule spectroscopy on PPV indicates that this material can be treated as a polymer that has a coherence length of only about 10-15 rings despite molecular weights (Mw ~ 100,000) which would allow much greater effective conjugation lengths.34 In other materials this may be much shorter and even approach the 31. Sakamoto, A.; Furukawa, Y.; Tasumi, M. J. Phys. Chem. B 1997, 101, 1726-1732. 32. Hill, M. G.; Penneau, J.-F.; Zinger, B.; Mann, K. R.; Miller, L. L. Chem. Mater. 1992, 4, 1106-1113. 33. Reynolds, J. R.; Schlenoff, J. B.; Chien, J. C. W. J. Electrochem. Soc. 1985, 11311135. 34. (a) Vanden Bout, D. A.; Yip, W.-T.; Hu, D.; Fu, D.-K.; Swager, T. M.; Barbara, P. F. Science 1997, 277, 1074-1077. (b) Hi, D.; Yu, J.; Barbara, P. F. J. Am. Chem. Soc. 1999, 121, 6936.

24 length where the bipolaron is the most stable charged species in highly doped materials. In electropolymerized materials where molecular weight information is difficult to obtain, there may only be oligomers in the sample. Regardless, it is clear that optical and electrical properties saturate very early in the formation of polymer from monomer, much earlier than the physical properties for most polymers. 1.5 Doping Induced Optical Transitions Poly(acetylene). Section 1.4 showed that PA is unique among CPs because it has a degenerate ground state. Neutral PA has one allowed optical transition, that from VB to CB as shown in Figure 1-11. Three options are then possible for creating charge carriers on a PA chain which appear as a localized state at mid-gap. In trans-PA with an odd number of electrons, a neutral soliton forms from the odd p orbital and single π electron. Oxidation creates an empty state and reduction creates a doubly occupied state at mid-gap, all of which are spectroscopically observed. Poly(pyrrole) and poly(thiophene). The nondegenerate ground state due to the energy difference between the aromatic form and quinoidal form of poly(heterocycles) necessitates different charge carriers and thus optical responses. The quinoid structure has a larger affinity for charges as suggested by calculations which show a lower ionization potential and larger electron affinity than the aromatic structure.35 PPy is perhaps the most well-studied of the poly(heterocycle) systems and provides a view that is consistent with other similar conjugated polymers such as PTh. Theoretical and optical studies place the band gap of PPy at 2.7 to 2.8 eV with a λmax of 3.2 eV. Single electron oxidation creates a

35. Bredas, J. L.; Themans, B.; Fripiat, J. G.; Andre, J. M.; Chance, R. R. Phys. Rev. B: Condens. Matter. 1984, 29, 6761.

25 chain with a polaron on it and two intragap states that are roughly 0.5 eV inside the original band edges (Figure 1-11 B). According to Bredas and Street (Acc. Chem. Res. 1985), when a second electron is taken out of the chain, a bipolaron is formed, as is consistent with their calculations. More recently, Furukawa revisited the spectrochemical data for neutral and oxidized forms of PTh, PPV poly(2,5-thienylenevinylene) and polyaniline.36 The original papers on these polymers had identified the stable charge carriers as bipolarons in the heavily doped state based on the two bands observed in the UV-Vis-Near-IR of the p-type doped form. The FBC model predicts that the ratio of absorbance intensities Eb1/Eb2 should be 14. In all cases the observed ratios are much smaller, on the order of 1.2. This was termed the “intensity anomaly” and several theoretical approaches were pursued in attempt to explain it while still maintaining the bipolaron assignment. Furukawa reassigned the bands to correspond to polarons based on contemporary oligomer studies and reevaluated the peak intensities. Similar transitions for polarons (solid arrows in the polaron diagrams of Figure 1-11 B) compared to bipolarons are allowed. Specifically, the VB to lowest polaron band is allowed for polarons as it is for bipolarons, but the lowest polaron to highest polaron band transition is allowed for polarons and is not for bipolarons. Additionally, the VB to highest bipolaron band (Eb2) is polarized in the direction perpendicular to the chain axis making it extremely weak and explaining the intensity anomaly. Consequently, reassignment of all the polymer’s charge carriers in the heavily doped state to arrays of polarons proves more consistent with theory than the assignments as bipolarons. The lack of EPR signal is explained by the polarons forming lattices or arrays (π-dimers) that attenuate spin density. Heavily doped PTh shows metallic like behavior such as a Pauli spin 36. Furukawa, Y. J. Phys. Chem. 1996, 100, 39, 15644-15653.

26 succeptibility, linear temperature dependence of the thermoelectric power and a negative correlation of conductivity with temperature. This has traditionally been explained by the lowest bipolaron band growing in size until it meets the VB allowing a partially filled band necessary for the metallic conductivity mechanism. Furukawa’s explanation is that the polaron lattice, built up of an array of singly occupied states, would form essentially a half filled band, enabling a metallic conductivity mechanism as well. Oligomers, and how their properties relate to polymers, continue to be an active research area and it appears that the idea of π-dimers, polarons and arrays or polarons as the dominant charge carriers in conjugated polymers is gaining acceptance. Due to scarcity of easily n-type doped materials, the optical properties of these polymers are only recently beginning to be understood. It is postulated that the charge carriers are the same regardless of the sign and this seems to be true, at least for p-sexiphenyl, for which polarons are the stable n-type charged species.37 1.6 Conjugated Polymer Synthesis and Device Incorporation Electrochromic devices. The previous background has been provided to introduce the concepts that dictate the properties of devices fabricated from conjugated polymers, namely band gap, position of the band edges and conductivity. This section details how these design components are unified to make electrochromic and light emitting devices. Electrochromics, driven by consumer interests in displays and military interests for absorbers, have also found use in commercial applications such as electrochromic mirrors in cars and eventually in dial-tint windows for residential and business uses. With the

37. Koch, N.; Rajagopal, A.; Ghijsen, J.; Johnson, R. L.; Leising, G.; Pireaux, J.-J. J. Phys Chem. B 2000, 104, 1434-1438.

27 A. POSSIBLE OPTICAL TRANSITIONS FOR PA

Neutral Poly(Acetylene)

Neutral Soliton

Positive Soliton

Negative Soliton

B. POSSIBLE OPTICAL TRANSITIONS FOR A NON-DEGENERATE POLY(HETEROCYCLE)

Eb2

Eg Eb1

Neutral Polymer

Polaron States

Bipolaron States

Bipolaron Bands

Metallic-Like States at High Doping Level

Figure 1-11. Optical transitions for doped conducting polymers. (A) neutral PA, p-doped and n-doped PA all have a mid-gap optical transition. (Adapted from Bredas, J. L.; Street, G. B. Acc. Chem. Res. 1985, 18, 309-315) (B) The non degeneracy of poly(heterocycles) causes the charge carriers to be polarons or bipolarons at high doping levels. (Adapted from Fesser, K.; Bishop, A. R.; Campbell, D. K. Phys. Rev. B. 1983, 27, 4804) exception of a Gentex product based on an organic electrochromic, consumer electrochromics are based almost solely on inorganic tungsten trioxide which, while robust, suffers from slow switching times and is not easily extendable to other colors. Organic electrochromics, especially those based on conjugated polymers, offer faster switching times38 and are much more easily modified to provide different colors based on research in 38. Welsh D. M.; Kumar A.; Meijer E. W.; Reynolds J. R. Adv. Mater. 1999, 11(16): 1379-1382.

28 band gap modification. Figure 1-12 shows a typical device used for testing organic electrochromic materials. In this device an indium tin oxide (ITO) electrode on either glass or a flexible substrate like Mylar or poly(ethylene) is to electropolymerize a conjugated polymer. Two different complimentary CPs are used on opposite electrodes with a transparent gel electrolyte between them. Anodically and cathodically coloring polymers. The color of these devices is based on the band gap of the polymer as shown in Figure 1-12. Typically a high band gap and a low band gap polymer are used and are assembled in the device where one is in its neutral form while the other is in its doped form. Low band gap polymers tend to have the π to π* transition for the neutral form in the visible region and are thus cathodically coloring, meaning that the neutral state is the colored state. High band gap polymers tend to have the π to π* transition shifted to higher energies into the UV and are thus near colorless in this state. When oxidized however, an absorption in the visible region becomes apparent making high band gap polymers anodically coloring. Low band gap polymers tend to have the absorbances that appear upon doping pushed to low energies in the NIR and are thus colorless or nearly so in the doped state. Thus, this complementarity works quite well for devices that include these materials since when the neutral polymer is oxidized at one electrode, the oxidized polymer is reduced at the other electrode and the effect is a pair of polymers that are either both nearly colorless or both deeply colored at the same time. Single polymer devices can also be used when a specific color transition is required rather than the clear to opaque transition described above. Currently, no materials are available for the n-type to neutral polymer transition because materials with low enough CBs and high n-type doped conductivities that are stable to repeated switching do not exist.

29 A. ELECTROCHROMIC DEVICE SCHEMATIC ITO on Substrate Electroactive Polymer 1 Gel Electrolyte Electroactive Polymer 2 1.50

B. Colored State

Absorbance

1.25

Visible Light

1.00

Region B-1

0.75

Region B-3 Oxidatively Doped High Band Gap Polymer

Region B-2

0.50 0.25

Neutral Low Band Gap Polymer

0 0 1.50

1

3

4

C. Bleached State

1.25 Absorbance

2 Energy /eV

Visible Light

Neutral High Band Gap Polymer

1.00 0.75 0.50 0.25

Oxidatively Doped Low Band Gap Polymer

0 0

1

2 Energy /eV

3

4

Figure 1-12. Electrochromic Device Schematic. (A) typical makeup of a two polymer electrochromic device operating in either the opaque to transparent or conducting to insulating modes. (B) In the opaque to transparent mode, the colored state is derived from a doped high gap polymer and neutral low gap polymer combining to both contribute absorbance in the visible region. (C) The bleached state is formed by the compliment to the colored state - a neutral high gap polymer with transitions in the NIR and a doped low gap polymer with transitions in the UV. Adapted from Sapp, S. A.; Sotzing, G. A.; Reynolds J. R. Chem Mater. 1998, 10(8), 2101-2108.

30 Electrochemical polymerization. Several methods are available for the preparation of CPs. For preparation directly on an electrode, oxidative electrochemical polymerization is certainly the simplest alternative. The mechanism proceeds with oxidation of a heterocyclic monomer such as Py, Th or EDOT at an electrode. The highest HOMO density is located on the heteroatom (S or N) but the highest spin density in the lowest energy form is found at the 2 position (See Figure 1-13). Initially in the polymerization there is only monomer and oxidized monomer radical cations available. In one possible mechanism, two radical cations dimerize to form the dication bis(heterocycle). The lower energy of the aromatic form drives the rearomatization of the dimer to form the neutral species. The extended conjugation in the neutral dimer causes it to have a lower oxidation potential than the monomer and it is preferentially oxidized. The radical cation dimer is likely present at lower concentrations than the radical cation monomer and either meets with this monomer to form the trimer or dimerizes to form the tetramer (less likely). Rearomatization proceeds as before to afford the neutral species. As chain length increases, solubility decreases and the oligomers precipitate out on the electrode. In order to continue depositing on the electrode, the neutral oligomers formed must be oxidized to the conducting form so that electrons can be transferred through the forming film to continue deposition. The films formed are thus insoluble and infusible. Figure 1-13 B shows the possible ways of coupling radical cations of simple poly(heterocycles). Since the spin and charge density are not entirely on the 2 or 5 positions, coupling at the 3 or 4 positions (the β positions) is possible. These αβ and ββ coupling products can form in electrochemical polymerizations and are viewed as detrimental since they block conjugation. In EDOT, the 3 and 4 positions are blocked with

31 A. ELECTROCHEMICAL POLYMERIZATION MECHANISM (X = S, NH, O) - eEpa

X

X H

2

X

X

X H

X

+ 2H+

X

dimer X

-e

-

X

X

X

5

H

X X

4

X

H

X

X X X

X

H H

3 X 1

2

Numbering Key X

+ 2H+

trimer

B. ALTERNATIVE COUPLING POSSIBLITIES IN HETEROCYCLE POLYMERIZATION β X

αα' coupling α

X αα' coupling

X

X

X

X

X

X β

X

X

α

αβ' coupling

X X

X

X

X

Figure 1-13. Oxidative polymerization mechanism for a generalized poly(heterocycle). (A) Monomer is oxidized to the radical cation which then couples and rearomatizes releasing two protons. The dimer formed has a lower oxidation potential than monomer and either dimerizes or couples to another monomer to form trimer. (B) In simple poly(heterocycles), the 3 and 4 positions are unprotected making conjugation blocking αβ and ββ couplings possible. a cyclic ether making polymerization proceed exclusively through the 2 and 5 positions and simplifying structural assumptions. Electrochemical polymerization is often maligned, especially by traditional polymer chemists, who feel that the inability to characterize these films by even the most standard polymer characterization techniques to determine molecular weight and structure negates the usefulness of these polymers. While some of these arguments are valid,

32 electropolymerized polymers will remain quite useful for several applications for the following reasons: First, despite the difficulty in obtaining molecular weight by standard polymer solution methods such as gel permeation chromatography (GPC), vapor pressure osmometry (VPO) and light scattering, radio labelled end group analysis of tritiated derivatives has estimated the degree of polymerization to be about 20 in PPy and can be used when this information is necessary to gather for characterization.39 Grazing emission XRF has also been developed to estimate molecular weight in other solid state polymer films.40 While this degree of polymerization is low by polymer standards, it is certainly past the point where the electrical properties saturate (vide supra). Second, when it is polymer film on a conducting surface that is required, there is no easier way to prepare this than by electropolymerization. There are no catalysts to prepare or complicated workups and purifications to perform on solution polymerized materials. It is also necessary to address electrode confinement issues and the redissolution of solution polymerized soluble materials from the electrodes after they are cast or spin-coated onto an electrode. Third, electropolymerized polymers are much more electroactive than even solution cast polymers with nearly identical repeat units. This is due to the memory of the film for the dopant ion used during electrosynthesis and the morphology adjustment that must be made on doping and undoping. Finally, it is generally easier to synthesize monomers for electropolymerized materials than for their soluble counterparts. Typically all that must be done is to attach electron rich heterocycles known to polymerize around a core. Since it is

39. Nazzal, Street, G. B. Chem. Comm. 1984. 40. Blockhuys, F.; Claes, M.; Van Grieken, R.; Geise, H. J. Anal. Chem. 2000, 72, 33663368.

33 the radical that is coupling, it is the purity of the radical, not the purity of the monomer that determines molecular weight. This core can tolerate a variety of functionalities making it far more facile to build up libraries of compounds to test theories on band gap modification or transport properties. Electrochemically prepared materials are unsuitable in applications such as conductive fibers in which case traditional polymer processing techniques are necessary. Additionally, when more traditional properties such as glass transition and lack of crystallinity are more important than electroactivity, solution polymerized materials with much higher molecular weights are the obvious choice. Electropolymerized materials are also justly criticized for the difficulty that ensues when trying to convert their synthesis to continuous manufacturing processes and the yields from electropolymerization are quite low since the only monomer that is reactive is the fraction that diffuses into the electrochemical double layer. Light emitting devices are quickly becoming the most important application in conjugated polymeric materials. As of mid 2001, these materials are emerging from the labs and finding their way into devices such as car stereo and cell phone color displays where wide viewing angle, low cost and intrinsic lighting are more important than resolution. While lifetimes are not yet comparable to LCDs or more traditional CRTs, they are improving rapidly. PPV and specifically, MEH-PPV is now the most widely used material in OLED research. It is synthesized (Figure 1-14) by either radical or anionic chain growth addition type polymerization of a symmetrically functionalized PPV with either a sulfonium, sulfone or xanthate salt. This chain growth method allows high molecular weights (100,000s) of a polymer precursor that can be cast, spin coated or spun into fibers. Subsequent thermal elimination forms the active PPV light emitting material. Additionally,

34 Heck and other organometallic polymerizations are also used to produce functionalized PPV derivatives. L

Nu:

-HL Monomer

L

Nu

L

[X]

L

L = Cl, Br, sulfonium sulfone, or xanthate

[X] [X]

-HL

L

L

x

x

Precursor Polymer

PPV

eCB

VB EF

EF

CB

VB

h+ Metal

Organic HTL

Organic ETL

Metal

Figure 1-14. Polymerization of PPVs. (A) Chain growth radical or anionic addition polymerization is used to make high molecular weight PPV precursor polymer which is then thermally eliminated to make PPV or a derivative. (B) PPVs are widely used in light emitting devices, the simplest of which have the setup shown above. From the left, holes (h+, hollow dots) are injected from a low work function electrode such as Al or Ca and migrate through a hole transporting layer where the recombine radiatively with electrons (solid dots) injected from the transparent electrode (ITO or PEDOT).

An energy diagram is included in Figure 1-14 showing the simplest of the light emitting devices. In this setup, a hole injected from a low work function metal such as Ca or Al meets an electron injected from a transparent electrode (EDOT or ITO) inside the active layer. A device engineering contribution to this field is the recognition that the hole moves at different speeds than the electron and hole transporting layers are now included

35 into devices to ensure productive charge carrier recombination inside the active area of the material. To create the next generation of materials, the factors that influence band gap, the edges of the semi-metals VB and CB, and the conductivity must be understood. Currently, there are no materials that have high p-type and n-type conductivity in the same material, few materials that have a band gap under 1.5 eV and relatively few CPs that are suitable for biological applications. The subsequent chapters address the structural components of these issues and provide a foundation for the design of materials that meet the requirements for next generation materials.

CHAPTER 2 ELECTROCHEMICAL AND COUPLED IN SITU EXPERIMENTAL METHODS IN CONJUGATED POLYMER RESEARCH 2.1 Introduction Chapter 1 described the importance of band gap and the properties that arise from it, namely, doping (both p- and n-type), color and conductivity changes. The experimental methods used to quantitatively characterize these properties has largely been the realm of electrochemists who are interested in describing and optimizing charge storage capacity, dc electronic conduction and ion conduction in CP films and composites. The doping of CPs affects the conductivity, electronic and vibrational spectra, spin concentration and physical structure. Most of these properties are best studied by in situ methods to avoid the effects of exposing the polymer to ambient conditions while in a metastable state. Coupled techniques to study the in situ changes in electronic spectra (Section 2.3), conductivity (Section 2.8) and mass transport (Section 2.10) are described in this chapter. Despite several recent reviews related to CP synthesis,1 there is no review available which addresses the techniques common to CP characterization with the intended audience being the synthetic chemist who must characterize a newly prepared material. Complete characterization of an electrochemically prepared CP includes determining the potential where polymer deposition occurs, the polymer’s band gap, formal potentials for

1. (a) Roncali, J. Chem. Rev. 1992, 92, 711-738. (b) Chan, H. S. O.; Ng, S. C. Prog. Polym. Sci. 1998, 23, 1167-1231. (c) Roncali, J. J. Mater. Chem. 1999, 9, 1875-1893. 36

37 p-type doping and n-type doping, doping level, conductivity onset and magnitude, mass changes upon doping, color transitions and changes in optical absorbances. These values and methods are referred to repeatedly in the subsequent chapters and the intent of this chapter is to provide the necessary references and background material to understand and perform these experiments. Where possible, examples using PEDOT data are shown. 2.2 Monomer Crystallization Monomer crystal structures provide information on the bond length alternation, relative orientation of rings and degree of planarity, most of these quantities having some potential to be extrapolated to the polymer. Additionally, monomer crystal structures provide a starting point and a method of validation for computational probes of polymer electronic and optical properties. Crystallization of organic compounds remains somewhat of an art and several methods were used to obtain single crystals for x-ray diffraction in this work. Ideally monomers were left in capped NMR tubes to slowly evaporate over the course of several weeks although narrow bore glass tubing is a more economical way to achieve the same effect. In some cases this worked with NMR solvent, CDCl3 but in most cases several solvents were tried. The high surface area and low volume of an NMR tube makes this environment more suitable for crystallization than larger vessels with different aspect ratios. Typical solvents used for the planar aromatic compounds described in this document are chloroform, toluene, benzene, acetonitrile, ethanol, and dichloroethane. In all cases, the monomer solution was nearly saturated and was filtered before addition to the NMR tube to avoid particulates that may cause nonproductive nucleation. In cases where several vessels were used with the above solvents and no quality crystals developed, vapor diffusion was used. In this method, a small open vial containing

38 a nearly saturated solution of monomer in a good solvent was placed in a larger vial containing a non-solvent for the monomer with a high vapor pressure (usually pentane). This larger vial was sealed so the contents of the two smaller vials could reach equilibrium over the course of several days. During this process the volume of the non solvent decreases and the volume of the good solvent, monomer and diffused poor solvent increases. It is important to leave enough free volume in the smaller vial so that the poor solvent can diffuse in without overflowing the monomer solution. It is also important that the two solvents be miscible and to dissolve the monomer in a solvent with a sufficiently high vapor pressure so that the process of solvent mixing equilibrium occurs mostly in the smaller vial. Solvent diffusion through an inverted U-tube was also used successfully to obtain crystals of monomers. In this method, a 4-5 mm tube was bent in a U-shape and filled with a good solvent for the monomer with some means of sealing this U-tube to a vial (usually a vial cap with a hole drilled in it and the U-tube epoxied in place). While inverted, one vial is filled with saturated solution of monomer in good solvent and the other is filled with a poor solvent that is miscible with the monomer solution. The U-tube is mated with these two vials and the setup is allowed to reach equilibrium in the dark in a place where it is isolated from shock and most vibrations. Even though the color throughout the setup becomes homogenous quite quickly, it takes several days for crystals to appear in the top of the U-tube. Other methods of monomer crystallization are diffusion cooling and slow monomer sublimation for volatile monomer solids. 2.3 Spectroelectrochemistry Spectroelectrochemistry is an important and routine method used in the characterization of CPs. It provides the band gap (Eg) of a polymer, information about the

39 intraband states that appear on doping, and can be used to establish the formal redox potential through spectroelectrochemical titration. Spectroelectrochemistry is best performed in a UV-Vis-NIR instrument (a Cary 5e was used throughout this work). 1 X 1 X 4.5 cm cells were used which were transparent from 2600 nm although smaller ranges are typically used. Figure 2-1 shows the cell and electrode arrangement used throughout this work for potential control in a cuvette vs. a Ag wire pseudo reference electrode. In this method, a Teflon cap is drilled with holes to accept (1) an ITO glass slide through a slit shown in the front, (2) a tube to accept the Pt counter electrode which has a rectangular hole cut in it slightly larger than the beam from the spectrophotometer, (3) a hole to accept the Ag wire pseudo reference electrode which passes through a tube that keeps it from shorting with either the ITO/polymer or the Pt counter electrode and (4) a hole to accept a needle for degassing the solvent prior to collection of spectra subsequently maintaining an inert gas blanket over the cell.

Figure 2-1. Spectroelectrochemical cell design showing scaffolding used to keep the electrodes from moving within the cuvette. This cell utilizes a Pt counter electrode with a rectangular hole cut in it, a Ag wire pseudo reference electrode, a hole for degassing and a slit for the insertion of ITO/glass.

40 In a typical spectroelectrochemical experiment, solvent (ACN, [0.1 M] TBAP and blank ITO are placed in both the sample cuvette and the blank cuvette. Both cuvettes are degassed with argon for 5 minutes and are placed in the spectrophotometer for background collection. The background is collected over the range 1600 nm to 250 nm. While the spectrometer, sample and cuvettes can be scanned over a much larger range, at wavelengths longer than 1600 nm noise due to water is visible in the spectra and is not easily removed. Additionally, data is collected every 1 nm but reported vs. eV where 1 eV = 1240 nm making the near IR region highly compressed (many points cover a small range in eV). Thus, 1600 nm = 0.775 eV and 2600 nm = 0.477 eV, so collecting data in this noisy range amounts to only ca. 0.3 eV of extra data. After background collection, the polymer on ITO/ glass is placed in the sample cuvette and leads are attached. For most of the spectra in this work, the solution was degassed only prior to background collection. In all of the polymers studied, this was sufficient to limit degradation in the neutral state. Active bubbling of argon through the sample solution tends to cause the film to flake off throughout the experiment and does not appear to attenuate the noise at wavelengths longer than 1600 nm. The polymer is then switched over several potentials while monitoring the absorbance to determine the potentials where the polymer is completely oxidized and completely charge compensated (neutral). With these potentials established, the polymer film is broken in with a potential square wave between the oxidized and neutral potentials. p-Type dopable polymers typically break in over roughly 20 double potential steps and this process is monitored by following the current and charge during application of the potential square wave. The majority of polymers in this work are stable in their oxidized state and undergo spontaneous reaction with oxygen in the neutral state to be reoxidized (see Figure 1-7),

41 sometimes with degradation of the polymer. In order to collect good data, the oxidized state was probed first ensuring that the film would not be held in the usually unstable neutral state for a long period of time before the spectra for the p-type doped state was recorded. The potential was then poised where the polymer is completely oxidized and held under potential control while the energy was scanned from 1600 nm (0.775 eV) to 250 nm (4.96 eV). For most polymers, the potential was then stepped in 100 mV increments until the range of polymer states between oxidized and neutral had been examined. Example data for PEDOT (collected every 50 mV) is shown in Figure 2-2. λmax π to π*

S x O

O Eb1

+0.87 V Neutral Polymer

0.8

Absorbance

Eb2

Eg

1.0

1.2

Absorbance at λmax

1.2

1.0

~ E°'

0.8 0.6 0.4 0.2

Bipolaron States

-1.0

-0.5 0.0 0.5 1.0 Potential /V vs SCE

Eb1 0.6

0.4

0.2

-0.93 V

0.0

Eg ~ 1.6 - 1.7 eV 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Energy /eV

Figure 2-2. Spectroelectrochemistry for PEDOT. Film was deposited from PC [0.1 M] TBAP, [~300 mM] EDOT and switched in ACN [0.1 M] TBAP. Inset shows Absorbance vs. potential. Band gap is determined by extrapolating the onset of the π to π* absorbance to the background absorbance. The Eb1 transition is allowed and is visible at intermediate doping levels.

42 From the data shown in Figure 2-2, it is possible to determine the band gap by extrapolating the onset of the π to π* absorbance to the background yielding a value of 1.6 to 1.7 eV for PEDOT. In order for these results to be valid, it must be clear that the polymer is completely neutral, that is, there are no shoulders merged with the π to π* transition that may be taken as the band gap. Given the above limitation, this method of estimating band gap is the most reliable of those mentioned in this chapter because only one onset has to be estimated. The positions of intragap states are also able to be determined from spectroelectrochemistry. At intermediate doping levels, there is a peak that appears that is consistent with Eb1. Since the corresponding absorbance for Eb2 is not allowed, it is not observed in the spectra but is equidistant from the CB edge as Eb1 is from the VB edge. From the absorbance vs. potential data at λmax, it is possible to determine the polymer reduction potential, E°’, where there is 50% neutral polymer and 50% oxidized polymer as shown in the inset of Figure 2-2.2 The assumptions made are that there is 100% neutral polymer at λmax and 100% oxidized polymer when the absorbance stabilizes at low absorbance values at λmax. From the Nernst equation, vide infra, it is then possible to determine E°’ from the point where there is 50% oxidized and neutral polymer. This method can be useful when electrochemical methods are inconclusive due to highly capacitive films. While the cell shown in Figure 2-1 proved adequate, several modifications are suggested to ease collection of spectra. First, the cell cap should be made of a polymer such as poly(etheretherketone) PEEK3 that is nearly as solvent resistant as Teflon but is more 2. Heineman, W. R. Denki Kagaku 1982, 50(2), 142-148.

43 physically robust, can hold screws without stripping, and can be wet by epoxy. It should also have a longer section that mates with the cuvette so the tendency to rock is reduced. This cell cap should also have a wider opening for the ITO so that it is easier to avoid having the polymer touch the cell cap when inserting the polymer/ITO slide. Given the 1 cm2 opening at the top of the cuvette, it would be difficult to fit in even the small SCE electrodes used herein but a flared glass double potential junction could be used in the center of the cap to allow incorporation of the greater stability of the SCE electrode. This also alleviates the problem of having to make sure that the reference electrode contacts neither the film/ ITO or the counter electrode, the former case almost always results in destruction of the film. The Pt counter electrode with the rectangular hole cut into it to allow the passage of light is overkill for fast switching polymers containing EDOT although slower materials will require a counter electrode with matched area. Films switch reliably and homogeneously with Pt wires (straight or coiled) run down the side of the cuvette despite a lower surface area. This counter electrode assembly as shown has the tendency to twist in the cuvette sometimes coming in contact with the reference wire or working electrode. Improvements to this cell are also useful in colorimetry experiments. 2.4 Colorimetry Since markets that are emerging for CPs concentrate on displays, whether electrochromic, OLED, or OLEC, the color of light absorbed or emitted from polymers is quite important to quantify. Colorimetry, in its simplest form, is spectroscopy that corrects 3. PEEK (VictrexTM 450G, 381G or 450GL30) is a high temperature thermoplastic with excellent resistance to a wide range of solvents except strong acids and long term exposure in chlorinated solvent. It is finding wide use in medical applications as well as in HPLC fittings. Other options are Delrin, a polyacetal used in bearings spacers and gears and Duroid, a PTFE ceramic material.

44 for the nonlinearity of the human eye.4 Light entering the eye is imaged on the retina by two classes of photoreceptors called rods and cones. Rods are highly sensitive and are capable of detection down to the single photon level making them suited to detecting small amounts of light. Cones are less sensitive than rods and are the active detection mechanism in well-lit rooms and daylight situations. There is only one type of rod and three types of cones. Therefore, low light vision is in grey scale (using the single rod type), and well-lit situations appear in color due to the differing spectral sensitivities of the cones. These cones are dubbed L, M, and S because of their sensitivity to long, medium and short wavelength light respectively. The basis for colorimetry is the measured sensitivities of the three cones which overlap across the visible region and have a gaussian form. Signals that result in the same cone signals are perceived as the same color to the brain. Since there are many more L and M cones compared to S cones, coupled with the fact that short wavelength light is scattered more in the optics of the eye, the eye is less sensitive to colors in the blue and purple region of the visible spectra. While colorimetry can be performed on standard spectrophotometers with linear detectors and then deconvoluted in software, to date, colorimetry is best performed with a dedicated instrument using the same cell shown in Figure 2-1. Representing color using the three cone values is not convenient and several systems have evolved to specify colors in more useful ways. One of the most popular methods of determining whether two colors match is the color scale of the CIE 1931 standard observer based on data from seven to ten color-normal individuals in the 1920s in England. From these spectral sensitivities

4. Berns, R. S. Billmeyer and Saltzman’s Principles of Color Technology, Third Edition , Wiley & Sons: New York, 2000.

45 tristimulus values are available. Color is then described by multiplying the relative spectral power of a standard light source, the reflectance factor of an object and the tristimulus values of the standard observer. The tristimulus values of the object are then calculated. These values are represented by chromaticity coordinates (x and y) and the luminance (Y) in a chromaticity diagram. The standard chromaticity diagram is a horseshoe shaped spectrum locus such as that shown in Figure 2-3. The colors are shown in this figure as a rough estimate of the actual colors represented. Inks used to print these diagrams cannot fully encompass the exact color since there is no space for black, gray or brown. Because of this, and since the luminance value (the z coordinate not shown) is absent, subsequent figures referring to color measurements will show only the horseshoe and not the approximate colors it represents. The experimental setup for this experiment is nearly identical to spectroelectrochemistry. The polymer film is poised at a potential and held under active potential control while the colorimeter (in this work a Minolta CS 1000) is used to obtain the Luminance (Y), and CIE x and y coordinates as a complete description of color. Besides the Y, x and y data from the colorimeter for the polymer film, the values for an illuminating light source are also required to translate the above values into L*a*b values that can be input into image processing programs to conveniently represent real colors. The light source in these experiments is nominally a 5000 K source, but is measured through ITO/ glass and solution before or after each experiment to correct for drift or warm/cool spots in the source. From the color values, it is possible to establish the dominant wavelength (λd), and the color purity of the polymer at a specific potential, usually at the end of the doping range. The dominant wavelength is the wavelength of color (on the spectrum locus) whose

46

y 0.9

S x O

O

0.8

0.7 Purity (+0.87 V) = 0.0588 Purity (-0.93 V) = 0.475

0.6

0.5 Purity(+0.87 V)

0.4

0.3

0.2 Purity(-0.93 V)

0.1

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x

Figure 2-3. CIE Colorimetry data for PEDOT-TBAP in ACN relative to the 1931 standard observer. Film was identical to that used for spectroelectrochemistry (Figure 2-2). Color purity is designated by the ratio of the distance from the light source to a point and the distance from the light source to the pure color on the edge of the locus. chromaticity is on the straight line between the sample and the illuminant source. The color purity is the distance between the illuminant and a point divided by the distance between the illuminant and spectrum locus and indicates how close a sample point is to a pure

47 wavelength. Colorimetry data for PEDOT switched in ACN/TBAP is shown in Figure 2-3 and the indicated values for color purity are shown as well. 2.5 Cells and Reference Electrodes for Electrochemistry Electrochemical methods have proven the most useful means of characterizing CPs throughout recent years. With the exception of the combination in situ methods (spectroelectrochemistry, colorimetry and EQCM), most electrochemical experiments can be performed in the relatively simple cell shown in Figure 2-4. This cell consists of a glass vial and a Teflon cap milled at the University of Florida with several holes drilled into it. Typically, a Pt flag electrode fabricated from a piece of Pt foil and wire spot welded together is used as an auxiliary electrode. This is placed through the Teflon cap which is then assembled with the body and taped together with electrical tape so torque applied from the leads does not move the cap. A Pt disc (source: BAS, often referred to as a button) working electrode is inserted through a hole in the cap and is secured a slight distance from the bottom of the cell by either friction or an o-ring placed on the outside of the cell. The hole for the working electrode has a small groove cut into it so that it can accept ITO/glass slides in the standard dimensions produced by Delta Technologies. This groove is cut so the ITO face will be perpendicular to the auxiliary electrode. The reference electrode is inserted through a similar hole and held in place by the same methods and there is a small hole for a piece of either Ag or Pt wire placed close to the larger reference hole so either of these pseudo reference electrodes could be used instead. Solution is added through one of the electrode holes and the cell is filled to the point where all of the electrodes are bathed in solution. Typically 10 mL of [10 mM] monomer solution is prepared and 2-4 mL is used at a time. Higher monomer concentrations (up to 0.1 M) are used for extensive film

48 deposition on ITO electrodes but for deposition on Pt, 10 mM is sufficient for several experiments. The leads for each of the above mentioned electrodes are connected to a potentiostat by either alligator clips or preferably micro clips since the current magnitude tends to be small. Electrochemistry throughout this work was performed on one of several Perkin-Elmer (formerly EG&G) Princeton Applied Research (PAR) potentiostats models 263A, 273 or 273A controlled via GPIB (National Instruments) bus by a computer running Windows 98 or 2000 and Scribner Associated CorrWare 2 electrochemical software. Additionally, a Pine bipotentiostat (model ABP-AFCBP1) and a programmable constant current source was used for certain experiments (noted). Data is evaluated by plotting in either CorrView (Scribner) or Microcal’s Origin. Several different types of reference electrodes were used depending on the circumstances; all with the objective of measuring the interfacial potential at the working electrode. The ideal reference electrode for most aqueous experiments is the saturated calomel electrode (SCE). The redox process governing the potential of this electrode is Hg2Cl2 + 2 e- ' 2 Hg + 2 Cl-. This electrode is available as a kit (BAS RE-2) and is assembled by adding mercury prior to use. It consists of an H-cell where one arm contains a pool of mercury with calomel paste (Hg2Cl2) followed by a saturated KCl solution on top of this paste. A small amount of glass wool is placed on top of the calomel paste to keep it from moving if jarred and the electrode is filled with saturated KCl solution so that two halves of the H-cell are in contact with the same solution. A Vycor tip separates the KCl solution from the sample medium. This tip is kept immersed in saturated KCl(aq) when not in use and a U-tube was fabricated at the University of Florida glass shop to hold two SCE electrodes for comparison. These electrodes (four total were used) were compared two at a

49

2 cm

Saturated KCl Solution KCl Crystals Calomel Paste

6.5 cm

7.5 cm

Hg

6 mm 6 mm

6 mm

Figure 2-4. Schematic of electrochemical cell suitable for most electrochemical techniques. Working and reference electrodes are obtained from BAS. Cell, cap and auxiliary materials are fabricated on-site. Both SCE electrodes (shown in cell) and Ag/Ag+ electrodes (top right) are shown for comparison. time by connecting a multimeter to the Pt wire protruding from the electrode body and measuring the potential difference between the two references. In no cases was this ever more than 10 mV and was typically near 1 mV indicating excellent stability with regard to potential drift. While prolonged use in aqueous solutions poses no problems for SCE electrodes, organic solvent use can damage these electrodes after some time. Specifically, KCl is poorly soluble as is KClO4 in ACN. This leads to plugging of the frit by precipitate

50 which eventually increases both the impedance and junction potential of the reference. While frequent calibration was used here to ensure that the potentials measured were valid, prolonged use suggests a slightly modified SCE electrode. NaClO4 and tetraethyl ammonium chloride TEACl are much more soluble in ACN than the Li-salt and while reference electrodes containing these salts are not available commercially, they can be easily prepared. All reference electrodes here were calibrated by the use of a 5 mM standard solution of ferrocene (Fc) in 0.2 M LiClO4 - ACN. A cyclic voltammetry experiment was conducted to measure the E1/2 of the Fc/Fc+ redox couple. Typically, scanning over the potentials 500 mV to +600 mV at 100 mV sec-1 included the peak associated with this transformation. Averaging the peak potentials yielded the E1/2 and this was noted for comparison to other reference electrodes and to account for drift. This was consistently at +344 mV vs. SCE, a value consistent with current literature values of 356 mV.5 In experiments requiring the absence of air and water, SCE is unsatisfactory due to two factors. First, water leaks through the Vycor frit contaminating the solution. This can be alleviated by the use of a salt bridge or a double junction with a second Vycor frit separating the reference from the sample solution. The middle solution contains organic electrolyte. This works in principal, but this adds the uncertainty of another junction potential. The cell design used throughout does not adequately seal out air and these types of experiments are best performed in an Ar dry box. SCE electrodes, double junction or not are incompatible with the dry box atmosphere. In these types of situations two options are

5. Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications Wiley: New York. 2001; p 54.

51 available. First, a Ag/Ag+ electrode can be constructed from a silver wire immersed in a solution of 1-10 mM AgNO3 in 0.1 M TBAP in ACN separated from the solution by a Vycor frit. This electrode works reasonably well on the benchtop but the silver ions leak through the frit, are reduced by DMF and are photochemically unstable necessitating near daily preparation of the reference and reference solution. Additionally, the Vycor frits available from BAS seem to require some water in their preparation in order to have a low enough junction potential to be used as a reference. When prepared in rigorously dry conditions, they simply do not work. Thus in the dry box, a Ag wire pseudo reference is used and its potential is checked repeatedly since it depends on the sample solution composition. This potential is often -30 mV compared to SCE but can be as much as -300 mV vs. SCE when checked by the standard Fc solution above. The values of several available reference electrodes and their conversions to SCE are shown in Figure 2-5. The value for Ag° is shown as a range since this potential drifts. SCE appears to be the most popular electrode used throughout the literature and, thus, all potentials here are reported vs. SCE. Rigorous electrochemistry demands that when Ag wire pseudo references are used, Fc is added at the end of the experiment and the potential reported vs. Fc. However, since the standard Fc solution containing ACN and perchlorate is nearly identical in composition to most of the solutions used here to characterize polymers, the potential difference should be negligible thus removing the need to use Fc as an internal standard for most experiments. The accuracy of electrochemical potentials reported in this work is affected by several factors. The absence of any temperature control adds an uncertainty of 5-10 mV to the potential values measured. Drift in the reference electrode adds another 10 mV of

52

-0.5 E[Vacuum] = EAg/Ag+ + 4.66 V

Ag/Ag+

-0.4 ESCE = EAg/Ag+ + 0.2578 V

-0.3

Ag (Pseudo) SCE

-0.2

ESCE = EAg/AgCl - 0.0442 V

Ag/AgCl

ENHE = ESCE + 0.2412 V ESCE = E[Vacuum] - 4.4 V

-0.1 EAg/AgCl = ENHE - 0.197 V

NHE

0 E[Vacuum] = ESCE + 4.4 V

+ 4.16

[Vacuum]

Figure 2-5. Reference electrode conversion values. Note: Ag wire as a pseudo reference is typically converted to SCE by subtracting 30 mV but can vary by several hundred mV and needs to be calibrated with each use. uncertainty and for those experiments conducted with Ag wire pseudo references, the uncertainty is probably as much as 50 mV given the dependence of potential on solution composition. Considering the comparatively larger effects of small changes in polymerization conditions, these values are acceptable. 2.6 General Electrochemistry and Cyclic Voltammetry (CV) Several electrochemical methods are used to characterize CPs. In general these can be divided into controlled potential (potentiostatic) methods and controlled current (galvanostatic) methods. The Nernst equation governs the applied potential E, the formal redox potential E°’ and the concentrations of analyte at the electrode and has implications for all of the electrochemical methods discussed here. The polymers in this work are electrochemically prepared, a process that can be done by potentiostatic, galvanostatic or CV methods. In a CV experiment, the potential is swept at a constant rate while the current

53 is measured.6 The data for the standard Fc solution described above is shown in Figure 26 and the shape of the trace indicates several things about the kinetics of electron transfer to the electrode. The Fc/Fc+ couple is known as a reversible couple, a term that has very specific meanings to electrochemists that go beyond the simple similarity to other reversible systems. Reversible redox processes are those that have concentrations of oxidized and reduced species that are predicted by the Nernst equation. Additionally, the peak to peak separation (∆Ep) at 25 °C is 58 mV although processes that have ∆Ep of 6065 mV are usually considered reversible. The ratio of the peak currents, ip,a and ip,c are 1 and the peak current scales linearly with the square root of the scan rate. The shape is a reversible CV and is characterized by a generally increasing slope with a peak superimposed on it. Upon potential reversal, a similar shape is observed with the opposite current sign and slightly shifted potential at the peak. The peak is due to the diffusion of analyte through the solution and the limitations this places on effective mass transport to the electrode. The peak current is proportional to the rate of electrolysis at the electrode and higher rates of potential change correspond to higher electrolysis rates while the concentrations of electroactive species are adjusted to comply with the Nernst equation.

RT C O E = E 0′ + ------- ln ------nF C R

Equation 1

6. More specifically, since digital instruments are used, the potential is stepped by very small amounts dictated by the resolution of the DAC in the potentiostat. This process is essentially analogous to an analog sweep. Bott, A. W. Current Separations, 1997, 16(1), 23-26.

54 Ferrocene electrochemistry is quite different than that of a CP because ferrocene is dissolved in solution and diffuses to the electrode where it is oxidized while an electropolymerized CP is usually electrode adhered. Figure 2-6 shows the waveforms for the CV experiment and data obtained for the Fc standard solution under these conditions. Charging currents are responsible for most of the limitations of the CV experiment. These limitations are that there is a detection limit of ca. [10 µM], the charging current is directly proportional to the scan rate but the peak current is proportional to the square root of the scan rate, effectively limiting both the sensitivity and the scan rate. Figure 2-6 B shows the waveform for the common step method DPV that addresses some of these limitations and is discussed later. Since diffusion to the electrode is responsible for the offset in peak potential in CV traces, electroactive species that are electrode confined have nearly identical peak potentials on the forward and reverse scans. The redox processes for films of CPs in solution or on electrodes are rarely reversible because the extensive changes in polymer structure that occur on oxidative or reductive doping preclude compliance with the kinetic limitations of the Nernst equation.7 Figure 2-7 shows data from the electrochemical synthesis of PEDOT along with the CV for the polymer on a Pt electrode. Figure 2-7 A shows the first scan of a cyclic voltammetric deposition at a scan rate of 50 mV sec-1. No electrochemical response is evident at potentials cathodic of +1 V vs. SCE. At ca. 1.2 V vs. SCE the current increases rapidly peaking at ca. 1.4 V. These values indicate the onset of polymerization (Eonset) and peak polymerization potential for the monomer (Ep,m) and are collected on the first scan

7. Monolayers and very thin layers of polymer can appear much more reversible that the systems treated here.

55

Potential

+0.385 V

· · ·

·

CV Conditions: 100 mV/s Scan Rate 1 Point/mV

Fe

20 mA

E1/2(CV) = +0.335 - +0.344

Time

General Conditions: 5 mM Ferrocene 0.2 M LiClO4 Acetonitrile Pt electrode A = 0.02 cm2

Potential

Step Size + Amplitude Step Size

+0.285 V +0.302 V

+0.354 V

I2

Step Time

· ·I

5 mA i = I2 - I1

1

Time E1/2(DPV) = +0.344

DPV Conditions: Step Time = 0.0167 sec Step Size = 2 mV Seg #1 Amplitude = 0 mV, 4 Points Seg #2 Amplitude = 100 mV, 1 Point +0.333 V -0.5

0.0

0.5

1.0

Potential /V vs SCE

Figure 2-6. Comparison between CV and DPV methods and results for ferrocene. and used as a metric to compare different monomers. The return scan after the peak in the first scan crosses the anodic wave in a feature exclusive to CP polymerization and metal deposition called a nucleation loop. This is due to the quantity of polymer put down due to the first scan increasing the electrode area. Figure 2-7 B shows the remaining 5 scans of

56 PEDOT polymerization. The peak at 1.4 V remains and the peak current increases in value. The other feature that is evident is the increase in current over the potential region between -1 V and ca. 0.4 V due to doping and undoping of the polymer. CPs can be electrochemically deposited by potentiostatic deposition above Eonset or by galvanostatic deposition. Figure 2-7 A-1 shows the galvanostatic deposition of PEDOT at a current of 10 µA (500 µA cm-2). In this experiment, the potential is measured at constant current and initially peaks then slowly decreases because PEDOT is highly conducting and can deposit more easily on previously grown PEDOT than on bare Pt. The potential at the end of this polymerization is labeled Elim and under these conditions, the value of Elim for all the polymers discussed herein is important for other experiments. Potentiostatic deposition is also an effective means of depositing a polymer film and is used mainly when a specific amount of charge must be passed while the potential is maintained below a certain value. This is the primary method used to deposit films on ITO. Upon completion of the electrochemical deposition, the polymer coated electrode is rinsed in the solvent used for deposition and is placed in a solution of solvent and electrolyte without monomer. Figure 2-7 C shows the CV obtained for the PEDOT polymerization described above. This spectrum is similar in shape to that obtained for ferrocene except that the peak separation is wider and the peak definition is not as sharp. The peak values are typically reported for conjugated polymers and the average is taken as the E1/2. Frequently the redox process from the p-type doped state to the neutral state is poorly defined making determination of an accurate E1/2 difficult. For an electrode adhered film, the peak current has a linear dependence on scan rate which is routinely verified over the scan rates 10 mV sec-1 to 300 mV sec-1. Polymers often have a linear scan rate

57 14 12

A. First Scan from CV Deposition

8 6

x

1.20

Potential /V vs SCE

10

4

Ep, m

S

1.25 1.15

O

1.10

A-1 Galvanostatic Deposition 10 µA

1.05 1.00 0.95

O

Elim

0.90 0.85 0.80

2

0

60

120 180 Time

240

300

0

nucleation loop

Current Density /mA cm-2

-2

Eonset

12 10

B. Second Through Sixth Scans from CV Deposition

8 Polymer Doping Current

6 4 2 0 -2

4

Ep, an, polymer

C. PEDOT CV

3 2 1 0 -1 -2 -3

Ep, cath, polymer

-4 -5

-1.0

-0.5

0.0

0.5

1.0

1.5

Potential /V vs SCE

Figure 2-7. Some electrochemical data for PEDOT. (A) Deposition by CV (first scan only) at 50 mV sec-1 and by galvanostatic methods (A-1). (B) Second through sixth scans for PEDOT CV growth indicating increasing quantity of polymer on the electrode. (C) CV of PEDOT on A = 0.02 cm2 Pt from the above CV deposition in monomer free [0.1 M] TBAP ion ACN at 50 mV sec-1.

58 dependence between 10 and 100 mV sec-1 but only the fastest switching polymers remain in the diffusion controlled regime above 100 mV sec-1. This points out one of the difficulties of the CV experiment which is that it contains both kinetic (scan rate) and thermodynamic data (E1/2) in the same experiment. This is often difficult to separate and other experiments are employed to separate these energetic factors. CV deposition for a number of polymers allows the comparison of how changing the monomer structure and electronic properties affects Ep,m and Eonset. Figure 2-8 shows the CV deposition for EDOT as above and shows the peak polymerization potentials for several other monomers. From this several observations are made. First, when extending the monomer conjugation by going from Th to BiTh, the peak polymerization potential is decreased. Second, appending electron rich units onto the monomer unit further decreases Ep,m and the combination of increased monomer electron richness (EDOT) and extended conjugation (TerEDOT) results in a cathodic shift of Ep,m by nearly 2 V. Third, for a given monomer conjugation length of one ring, changing the heteroatom from S to N reduces Ep,m by 1.2 V. Utilizing the above observations, the organic chemist can avoid the poly(thiophene) paradox where the oxidation of monomer to form polymer occurs at potential where the polymer is unstable. It is this decreased Ep,m for EDOT resulting in more facile electrochemical synthesis that has been the reason being much of the current research activity surrounding this molecule. Galvanostatic, potentiostatic and coulometric methods are also utilized extensively to study polymer electrochemistry. Of particular importance are coulometric methods where the charge needed to p-type dope and neutralize a polymer is necessary to compare to the changes in optical absorbance during this transition to determine coloration

59

5 mA cm-2

S O

O

S

H N H N

S S O

O

O

O

O

O

O

S CH3

O S

S

S S

S O

-1.5

-1.0

O

O

-0.5

0.0

0.5

1.0

O

1.5

2.0

2.5

Potential /V vs SCE

Figure 2-8. Peak polymerization potentials for common monomers. Potentials where overoxidation are likely to occur are shaded in red. Extended conjugation decreases peak oxidation potential as does electron rich character of the parent heterocycle. efficiency. Figure 2-9 shows results from a potentiostatic deposition of EDOT at four potentials between 1.2 V and 1.5 V vs. SCE followed by immediate charge neutralization to -1 V. The experiment is started from open circuit potential which accounts for the variations in the first 30 seconds of Figure 2-9 B. This experiment is useful for determining the doping level of PEDOT by assuming a 100% deposition efficiency and using the amount of charge passed on neutralization to determine how many sites are occupied.8 Most polymers have a doping level of between 0.25 and 0.3 when highly doped which indicates a cation every three to four rings.

60 1.5

A. Potential Profile PEDOT Deposition

Potential /V vs SCE

1.0

S

1.5 V 1.4 V 1.3 V 1.2 V

x O

O

0.5

0.0

-0.5

Open Circuit (No Deposition) Neutral PEDOT

-1.0

0.05

B. Potential Square Wave for PEDOT Qgrowth

Charge /Coulombs

0.04

1.5 V

Qdedoping

1.4 V

0.03 1.3 V

0.02

0.01

0.00

1.2 V

0

30

60

90

120

150

Time /Seconds

Figure 2-9. Determination of doping level by chronocoulometric deposition. (A) Potential profile for determining charge passed during deposition and charge passed during dedoping. (B) Data for EDOT deposition and de-doping.

8. Randriamahazaka H.; Noel V.; Chevrot C.; J. Electroanal. Chem. 1999, 472(2), 103111.

61 2.7 Differential Pulse Voltammetry (DPV) Differential pulse voltammetry (DPV) has several advantages over CV while allowing the determination of the same basic values for polymer films. First, it is several times faster than CV over the same potential range. Second, it more easily allows determination of E1/2 values for a polymer since for a reversible system, the peak potential is the same on the forward and reverse scans and corresponds to E1/2. Figure 2-6 (above) shows both the CV discussed in the previous section and the DPV for the ferrocene standard solution. Qualitative evaluation of reversibility involves comparing the peak heights and peak to peak separation. As mentioned previously, conjugated polymers do not undergo reversible doping processes and so the peaks for the anodic and cathodic scans are not symmetrical and do not occur at the same potential but the DPV results are more symmetrical because of the more effective rejection of charging current. Third, DPV factors out charging current by sampling the current twice at each potential. The current is sampled first at time τ’ immediately before the pulse and then again at τ immediately after the pulse. The current reported is the differential current δi = i(τ) - i(τ’) at each base potential. The pulse height (step size + step amplitude) is maintained at 100 mV at each potential in the experiment and the timing is chosen to minimize noise and maximize experiment speed. The shape of the DPV relative to the CV of Fc can be explained by considering the concentrations of species in the double layer during oxidation of Fc to Fc+. At a potential cathodic of the formal redox potential of Fc (-0.2 V) early in the experiment no faradaic current (current associated with Fc oxidation) occurs before or after the 100 mV pulse because the pulse height is too small to cause oxidation. The differential current is thus near zero. At anodic potentials near the end of the experiment, Fc is oxidized at the

62 maximum rate and the step size is still too small to change this rate making the differential current again essentially zero. Therefore, only near the formal redox potential is any differential current observed because near this potential, Fc is oxidized at a rate that is submaximal and the step size is large enough to perturb this equilibrium and result in faradaic current. The discrimination against charging currents is due to the step size and from the pre-electrolysis (the three points collected and discarded before the step) establishing apparent bulk concentrations which are then observed by the pulse. 2.8 In Situ Conductivity Conductivity is perhaps the most important physical property of conjugated polymers and surprisingly, few papers contain this data. The worst effect of this is the misleading conclusions drawn from the current debate on n-type dopable and low band gap materials where the sole method of characterization and proof of n-type doping is a cyclic voltammogram. While the equilibrium in situ conductivity experiment described below is non-routine in that it involves two potentiostats and requires somewhat lengthy data workup, it is certainly within the capabilities of most groups. The setup described here allows the determination of conductivity onset, comparison of the magnitudes of p-type and n-type doped conductivity magnitudes, and the conductivity profile at high doping levels. Methods for determining absolute conductivity are not definitively described in this section although proposed methods for doing so are mentioned. The first mention of conjugated polymer deposition on interdigitated microelectrodes (IMEs or IDEs) from Wrighton describes the fabrication of moleculebased transistors and is the basis for the conductivity method used throughout this work with few changes.9 Subsequent Wrighton publications10 refined the technique which is

63

Pins 1 & 3: 10 µm line and gap widths 25 Pt digits on each bus

1 2 3 4

Chip Pins 2 & 4: 5 µm line and gap widths 50 Pt digits on each bus

Figure 2-10. IME photos and schematics. Photos courtesy of IMT, University of Neuchâtel summarized as follows. A monomer is electropolymerized galvanostatically (10 µA, 0.5 mA cm-2) for 250 sec (2.5 mC) on a 0.02 cm2 Pt button as an initial guess for the ensuing polymerization on an IME (Figure 2-10 A). The limiting potential at 250 seconds (Elim) is then used as the initial potential for deposition on an IME.11 Pictures of the IMEs are shown

9. Kittleson, G. P.; Wight, H. S.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 73897396. 10. (a) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441-1447. [polyaniline] (b) Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 5133-5140. [poly(3-methylthiophene)] (c) Ofer, D.; Crooks, R. M.; Wrighton, M. S. J. Am. Chem. Soc. 1990, 112, 7869-7879. [PTh, PPy, PANI]. 11. IMEs were obtained from Dr. Giovanni C. Fiaccabrino, Institute of Microtechnology, (IMT), University of Neuchâtel, Rue Jaquet-Droz 1, 2007 Neuchâtel. email: [email protected]. www-samlab.unine.ch. The devices used herein contain 2 IMEs on one chip, the first with 5 µm Pt line and gap widths, the second with 10 µm Pt line and gap widths. Each IME is 1 µm in length and for most monomers and cleaning methods, superior results are obtained with PECVD nitride passivation on the surface of the Pt so that only the edges are exposed.

64 in Figure 2-10. Two potentiostats are setup as shown in Figure 2-11 B where one potentiostat acts as a gate potentiostat and the other as a drain potentiostat measuring the interelectrode current.12 The gate potentiostat can be inexpensive as it is used mainly to supply a constant potential for polymerization and throughout this work a Pine bipotentiostat model AFCBP1 was used. When configured as shown, Elim is applied from the gate potentiostat followed immediately by application of a 10 mV drain voltage across the IME. The current is monitored for ca. 300 seconds at which time, if there is no drain current flowing, the experiment is terminated. The gate potential is then raised by 50 mV and the above experiment is repeated until a drain current starts to flow within 300 seconds indicating that the CP has bridged the electrode gap. Successful polymerization results for PEDOT-TBAP is shown in Figure 2-11 C. Polymer deposition is allowed to continue until the drain current reaches 65 µA. This value was arrived at empirically and assumes that the polymer is deposited in near its most conducting state. When the drain current is kept below these values, it ensures in most cases that the polymer is more resistive than the leads used to perform the experiment. In cases where the polymer in its most conducting state is more conductive than the leads, the conductivity trace plateaus at a resistance equivalent to ca. 160 Ω. In these cases a thinner polymer film must be redeposited. The viability of the polymer film on the IME can be tested in several ways. In early attempts to perform this experiment, the individual IME leads were cycled and compared to the CV when the leads were shorted. If the peak currents are nearly identical, then there is electronic communication throughout the polymer and both IME leads. In every attempt at verifying this, the cyclics were indeed identical and this diagnostic was later dropped 12. Schiavon, G.; Sitran, S. Zotti, G. Synth. Met. 1989, 32, 209-217.

65 A. DETERMINING DEPOSITION POTENTIAL

Gate Potential /V vs SCE

1.3

20 µA (1 mA/cm2)

1.2

Elim

1.1 10 µA

1.0

S x

0.9 O

0.8 0.7

Galvanostatic Deposition 0.02 cm2 Pt electrode

0.6 0.5

0

50

100 150 200 Time /Seconds

B. EXPERIMENTAL SETUP

250

C. POLYMER DEPOSITION ON IME 50

EGATE removed

Drain Current /µA

AUX (A) REF (R) CELL

V GATE

W

O

A V R DRAIN

W

40

EGATE = Elim EGATE = 1.15 V vs SCE

30

VDRAIN = 10 mV

20 Rlead =

10

V (= 10 mV) imax

R < Rlead ≈ 160 Ω

0

imax ≤ 62 µA 0

20

40 60 80 Time /Seconds

100

D. IN SITU CONDUCTIVITY 1 ∆iDRAIN = ∆VDRAIN RDRAIN

1S 1st Scan 2nd Scan

Drain Current

Conductivity

∝ σ

50 µA

1 mV/sec 4 Points/mV Drain Current 0.975

100 µA -1.0

-0.5 0.0 0.5 Gate Potential /V vs SCE

1.0

Figure 2-11. Procedure for In Situ Conductivity of PEDOT.

1.000 1.025 Gate Potential /V vs SCE

66 since appearance of a drain current during deposition of a well behaved CP is indicative of electronic communication across the previously insulating gap. Typically the polymer is broken in with the IME leads shorted in normal potentiostatic operation by repeated potential square waves encompassing the potentials over which conductivity data is to be collected. Typically 20 square waves are collected to ensure that peak current is reproducible. The gate-drain two potentiostat setup is then reconnected and polymer conductivity data is collected (Figure 2-2 D). A gate potential is applied so that the polymer is in its neutral, non-conducting state. When the current stabilizes and drops to near negligible values (< 1 µA although sometimes this value is not reached), a CV is run on the drain potentiostat between 25 mV around 0 V (0 V is effectively the gate potential value) at 1 mV sec-1 with 4 points collected every second. For n-type dopable polymers, where this data is to be collected, the polymer is scanned cathodically first. The gate potential is stepped in 50 mV increments and the drain CV is repeated after equilibration. The resulting data are several lines centered around each gate potential where the slope is the pseudoconductivity and the reciprocal of the slope is the resistance. This data is worked up in a spreadsheet and the pseudo-conductivity value is reported vs. potential. This data is called pseudo-conductivity (units: 1/Ω or S) because it does not carry the same meaning as volume conductivity (or just “conductivity”, units: S cm-1). Conversion to volume conductivity from pseudo-conductivity requires knowledge of the film thickness, a troubling issue that, even with the sacrifice of an electrode for SEM analysis, tends not to yield useful data. The problem is even more difficult with highly conducting polymers such as PEDOT where the polymer does not make a uniform film across the electrode, but where fibrils are sufficiently conductive to reach the designated 65

67 µA. Polymer conductivity data in the literature is typically normalized to poly(3methylthiophene) (P3MT) by assigning this polymer a conductivity of 60 S cm-1.13 This is anything but a fair assumption to make as film thickness, electrode geometry, dopant ion and several other variables affect the conductivity of polymer films. Cell constants have been determined by impedance methods with conductivity standards for IMEs and other electrodes and used to normalize conductivity but this method has not found widespread use among the chemists that are making and characterizing new materials probably due to its complexity and impedance requirements.14 It should be possible to normalize conductivity for the moderately conducting polymers proposed in this study by measuring the charge passed by the gate potentiostat during polymerization, assuming a 100% polymerization yield, polymer density of 1.5 g cm-3, 2.3 electrons per doped polymer repeat unit and uniform coverage of the electrode between the gaps (with known area A in cm2). However, polymers prepared throughout this work were polymerized using instrumentation lacking the capability (a coulometer) to measure charge passed and this was not attempted. Pickup has used a polymer film attached to a micrometer with thin film of evaporated gold to measure in situ conductivity.15 This method seems more difficult than the IME method. Several other groups use a banded electrode prepared from mylar Pt sandwiched between two sheets of Pt foil.16 13. Tourillon, G.; Garnier, F. J. Phys. Chem. 1983, 87, 2289. 14. (a) Sheppard, Jr., N. F.; Tucker, R. F.; Wu, C. Anal. Chem. 1993, 65(9), 1199-1202. (b) Paeschke, M.; Wollenberger, U.; Kohler, C.; Lisec, T.; Schnakenberg, U.; Hintsche, R. Anal. Chim. Acta. 1995, 305, 126-136. 15. Ochmanska, J.; Pickup, P. G. J. Electroanal. Chem. 1991, 297, 211-224. 16. Schiavon, G.; Sitran, S. Zotti, G. Synth. Met. 1989, 32, 209-217.

68 Cleaning of IMEs. Polymer coated IMEs were never subjected to physical cleaning (i.e. scraping or wiping) but were instead cleaned chemically. A solution of SC1 (5:1:1 H2O : NH4OH : H2O2) was prepared by adding the ammonia, then hydrogen peroxide to the water in a beaker. This cleaning solution was then heated to 50 °C under magnetic stirring and the tip of the IME was immersed in the cleaning solution with effort being made to keep the epoxied gold wires out of the SC1 as this is deemed a failure point for the Swiss electrodes used herein. The electrodes were kept in this stirred solution for 1 minute and then rinsed profusely with DI water (water pressure from the tap was used to attempt to loosen this film. For highly adherent films, this process was repeated but never more than a second time (total 2 minutes in SC1). The IMEs were shorted together and made the working electrode in a 3 electrode electrochemical cell and placed in a solution of 1 M H2SO4. The IMEs were then cycled at 20 mV/sec from + 1.15 V vs. SCE to -0.3 V vs. SCE; current passing at the negative extreme but not peaking. This was repeated 5 times consecutively and the scans compared. In cases where the scans were not equal, an additional 5 scans were performed until a consistent CV was observed. On particularly stubborn to remove films, the electrodes were cycled over -1 to -2 V vs. SCE in 1 M H2SO4 to evolve H2 from the electrodes followed by the oxidative cleaning/conditioning recipe shown above. This always resulted in electrodes that showed minimal current passed (10-8 A) when a potentiostat was connected across the IMEs and a 10 mV bias was applied. Occasionally, one side of the IME would be unusable (rendering the entire IME unusable as well) presumably from breakage of the attachment from the IME chip to the epoxy header. This situation is detectable during the cleaning phase by proceeding with the cleaning as described above with all the IME leads shorted and treated in unison until a

69 consistent response is observed. The IME leads are then decoupled and cycled individually. Overlay of the two corresponding IME CVs indicates that both leads are active. 2.9 Four-Point Conductivity on Free-Standing Films The electrochemical in situ conductivity method described in the previous section is useful for investigating the changes in conductivity upon doping and undoping and the conductivity turn-on potential in solution. Characterization of non-solvated polymer samples of constant doping level requires the use of different electrodes and techniques. Free standing films of CPs are prepared by either solution casting, spin coating or electrodepositing a material on a substrate, followed by rinsing, drying and removal of the CP from the substrate. For the electrochemically prepared materials described herein, a thick polymer film is prepared electrochemically on ITO/glass, stainless steel, titanium or glassy carbon. The nature and morphology of this electrode exerts considerable influence on the properties of the resulting film, especially with regard to the surface reflectivity of the film. Free standing films of the highest conductivity are generally prepared by slow galvanostatic deposition at reduced temperatures on polished electrodes in a low vapor pressure solvent such as propylene carbonate (PC) or γ-butyrolactone (GBL) that can act as a plasticizer. At a sufficient film thickness, the electrode film coated electrode can be removed from the solvent and the film dried and removed by peeling from the electrode surface. The film can be cut with a razor or scissors to afford pieces that are of appropriate size for conductivity measurement. Factoring out the effects of lead and contact resistance precludes the use of two-probe methods. Three types of volume conductivity measurements are typically performed on free-standing polymer films depending on the

70 available instrumentation and film geometry. These are all variations of the four-point probe method and are described in the following sections. Laboratories equipped for solid-state electronics research typically have a fourpoint probe device (such as the Signatone S-301-417 used in our laboratories) available with a predefined electrode geometry such as that shown in Figure 2-12. In this device, four equally spaced points on a line make electrical contact with a polymer film on an insulating surface. Between the outer pair of these points, a constant current (I) is applied while the potential difference arising (∆V) from this current is measured at the two inner contacts. For typical conjugated polymers in this work, currents ranging from 10 µA to 1 mA were injected through the films which produce potential differences between 10 µV and 10 V. For any given film, ca. six currents within the above range were selected and the volume conductivities (ρ, S/cm) were found according to Equation 2 and averaged to give a single value for a polymer film. In this equation, derived by Valdes,18 t is the film thickness in cm. Since the contacts are easily repositioned throughout the film, this process was repeated over several areas to gauge the homogeneity of the CP. This conductivity value is independent of electrode contact area as long as the distance between the points, d, is much greater than the film thickness, t.19

∆V ρ = 2πt ------- = 2πtR I

Equation 2

17. The particular probe is labeled as an 85 g weight, T.C. probe with a d spacing of 40 mils (40 mils X 0.0254 mm mils-1 = 1.016 mm) 18. (a) Blythe, A. R. Polymer Testing 1984, 4, 195-200. (b) Valdes, L. B. Proc. Inst. Radio. Engs. 1954, 42, 420.

71 +

1

2

I

-

V

I

w or b d°

d

d°

l a

Figure 2-12. Four-point resistivity measurement on a free standing film. t, not shown, is the polymer film thickness. This method is applicable to the use of four-point probe devices (Signatone, Equation 2) as well as wires Ag -pasted across the film (Equation 3). Adapted from Smits, F. M. Bell. Sys. Tech. J. 1958, 37, 711-718. Often, electrochemically prepared polymer films are brittle and not sufficiently robust to handle the application of the probe points from the previous device to make electrical contact. In these situations, and at times when the conductivity over a larger area of film is to be measured as a function of time, or in laboratories lacking a four-point probe device, leads may be attached directly to the film. This measurement is best performed on a film that is much longer than it is wide and can be taped to a glass slide with double sided tape. Four small wires (Ni or Cu was used throughout this work) are attached along the film so that there is a much greater distance between the contacts measuring ∆V than there is between the contacts injecting I (this can be visualized in Figure 2-12 where the outside d values are small and the middle d value is comparably large. Current values similar to those

19. a t/d ratio of 0.1 is sufficient but correction factors need to be applied for ratios larger than 0.4. Since our probe has d = 1.016, a thickness of 102 µm is sufficient but care must be exercised for films 400 µm or thicker.

72 above are applied and the measured potential difference and Ohm’s law (V = IR) are used to calculate the resistance R. The volume conductivity is derived from Equation 3 where l is the length (in cm) between the potential measuring leads and w is the width of the film (cm).

l ρ = ---------Rtw

Equation 3

Most CP systems can be treated by one of the two methods above. However, in devices or polymers where contacts must be irregularly attached, the equations and methodology for measuring conductivity in these systems have been described by van der Pauw.20 2.10 Electrochemical Quartz Crystal Microbalance (EQCM) During the deposition as well as oxidation and reduction of CP films, mass changes occur at the electrode. The coupling of electrochemical control to a quartz crystal microbalance is termed the electrochemical quartz crystal microbalance (EQCM) and is useful in elucidating some of the changes that occur in electroactive polymers.21 The EQCM is simple in concept and is based on the piezoelectric behavior (physical deformation in a potential) of a quartz crystal. In the experiment, an alternating field is applied between two Pt electrodes evaporated on opposite faces of a quartz crystal driving the oscillation parallel to the crystal faces. The Saurbrey equation (Equation 4) describes

20. (a) van der Pauw, L. J. Philips. Res. Repts. 1958, 13, 1-9. (b) van der Pauw, L. J. Philips, Res. Repts. 1961, 16, 187-195. (c) Montgomery, H. C. J. Appl. Phys. 1971, 42(7) 2971-2975. 21. (a) Deakin, M. R.; Buttry, D. A. Anal. Chem. 1989, 61, 1147A. (b) Buttry, D. A. Electroanal. Chem. Bard, A. J., Ed.; Dekker: New York, 1991; Vol. 17 p. 1. (c) Buttry, D. A.; Ward, M. D. Chem. Rev. 1992, 92, 1355. (d) Ward, M. D. Phys. Electrochem. (Rubinstein, I. ed.) Dekker, New York, 1995, p. 293.

73 the relationship between mass (m) and resonant frequency (∆f) of a quartz crystal where f0 is the fundamental frequency (ca. 8.9 MHz for experiments in this document but it varies), ρ is the density of quartz (2.648 g cm-2), µ is the shear modulus of quartz (2.947(10)11 g cm-1 sec-2) and n is the harmonic number of the oscillation (typically 1). The constants are

– 2f 20 mn - = – Cf m ∆f = -----------------( ρµ ) 1 ⁄ 2

Equation 4

usually lumped together to provide a single sensitivity factor (Cf, units: Hz cm2 µg-1) that should be checked for each crystal and sample solution used since the Saurbrey equation strictly applies only in the vacuum or gas phase. Because viscoelastic issues affect the resonant frequency of quartz, the rigidity of any polymer film must be checked by crystal impedance or conductance analysis at every oxidation state of the polymer studied to ensure the validity of the results and interpretation using the Sauerbrey equation. The most important use of the EQCM is as a probe of the often complex ion transport mechanisms in CPs which depend on the rate of electrosynthesis and this morphology, solvent viscosity and polarity, and size and charge of the counter ion. In general for a p-type CP, it is expected that the mass should increase as the polymer is oxidized and decrease as the polymer is re-neutralized, a condition called anion dominance since the anion and its associated solvent ions enter the film. This can be reversed in polymer composites where a dopant anion such as poly(styrenesulfonate) can be cosynthesized with the film and thus immobilized in it. Such films tend to be cation dominant at low potentials as cations from the solution enter the film to charge compensate the sulfonate anions which are available from the recently neutralized polymer.

74 In this work, EQCM results were obtained using a Seiko EG&G (now PerkinElmer) QCA 917 connected by BNC cable to the auxiliary in of a Perkin-Elmer PAR 273A potentiostat. The QCA outputs a signal ranging from 20 kHz V-1 or 2 kHz V-1. The 20 kHz V-1 range is used during potentiostatic or more typically galvanostatic polymerization to monitor the higher mass of polymer deposited compared to the smaller mass changes which occur on doping and dedoping (2 kHz V-1 range). Scribner Associates CorrWare 2 was used to control the 273A and monitor the aux channel which has a range of ± 10 V. For a typical deposition, the frequency at open circuit is followed for at least 10 minutes until it stabilizes followed by setting the zero frequency of the QCA to ca. + 5 V as measured on the aux in. This allows a 15 V X 20 kHz V-1 = 300 kHz range to be measured during deposition. It is common practice to express EQCM results in units of ng cm2 Hz-1 from Suerbrey’s original work. However, if the crystal is evenly covered (as inspected visually), the sensitivity can be expressed as Hz ng-1 and in this work is corrected for the theoretical sensitivity of the crystal22 used 1.09 ng Hz-1 and expressed as ng.23 Polymer rigidity was determined on a Hewlett Packard LF Impedance Analyzer model 4192A having a range of 5 Hz to 13 MHz. This instrument is not computer controlled and either a strip chart recorder is used or the full width at half maximum

22. The sensitivity of the crystal is determined in 0.5 M AgNO3 in 0.5 M HNO3 to deposit Ag at a current density of 0.25 mA cm-2. The calibration factor is calculated from the slope of the plot of mass of metal (converted from charge assuming 100% deposition yield) vs. frequency. It has been reported as 1.03 ng Hz-1 on the 2 kHz scale and 1.11 ng Hz-1 on the 20 kHz scale compared to the theoretical value of 1.09 ng Hz-1. These results are from Cameron, D. A. Ph. D. Dissertation, University of Florida. 23. Bruckstein, S.; Shay, M. Electrochim. Acta. 1985, 20, 1295. (B) Skoog, D. A.; West, D. M. Fundamentals in Analytical Chemistry; 4th ed.; Saunders: Philadelphia, 1982, p. 586.

75 (FWHM) for the QC resonance is deduced manually. The procedure is as follows: While still in the EQCM cell, the resonant frequency of the system is measured. The cell is then transferred to the impedance analyzer and scanned from 100 kHz below this resonant frequency to 100 kHz above it. The maximum is noted as is the FWHM. If the FWHM does not change appreciably on doping and is comparable to a bare crystal in solvent and dry, the film is assumed to rigid and the Sauerbrey equation is valid.

CHAPTER 3 DONOR MEDIATED BAND GAP REDUCTION IN A HOMOLOGOUS SERIES OF CONJUGATED POLYMERS 3.1 Introduction and Literature Overview The evolution of the field of conjugated electroactive polymers1 (CEPs) as replacements for inorganic materials in electronics devices has been fueled by the flexibility in tailoring specific electronic properties by rational chemical synthesis. To this end, organic light emitting and electrochromic devices are making rapid progress toward commercialization.2 The unique properties of CEPs are largely based on the ability to tailor the energy levels of the valence band (VB) and conduction band (CB) both relative to each other (band gap control) or in an absolute sense (modifying the ease of oxidation or reduction). Tailoring the band gap (Eg) of CEPs allows variation in emission wavelength, absorptive colors in electrochromic devices and conductivity in the neutral state. Changes in band energy allow optimization of interfacial energy level alignment between the polymer and electrode contacts in organic light emitting devices (OLEDs).3 Low band gap

1. Conjugated electroactive polymers are differentiated from conjugated polymers in that their propensity to undergo electrochemical doping and undoping reactions can be leveraged. 2. (a) Groenendaal, L. B.; Jonas F.; Freitag D.; Pielartzik H.; Reynolds, J. R. Adv. Mater. 2000 12(7), 481. (b) D. M. de Leeuw Physics World 1999 31. (c) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Brédas, J. L.; Lögdlund, M.; Salaneck, W. R. Nature 1999 397, 121. 3. Ishii, H.; Sugitama, K.; Ito, E.; Seki, K. Adv. Mater. 1999, 11, No. 8, 605-625. 76

77 CEPs are particularly interesting because of the possibility of high intrinsic conductivity in the neutral state, transparency in the p-doped conducting state, and their tendency to achieve reduced band gaps through conduction band energy lowering making stable n-type doping possible. Several factors affect the band gap in conducting polymers and the field in general has been well reviewed.4 These factors are detailed in Figure 3-1 and are: The stability of the quinoid form (reduction of bond length alternation reduces band gap) of a poly(heterocycle), planarity of the repeat unit, interchain effects, resonance contributions5 and donor-acceptor effects.6 While ensuring planarity of the repeat unit is difficult to achieve without compromising solubility,7 and interchain effects have been only sparsely described,8 most of the research effort has been devoted to increasing the stability of the quinoid character in the neutral polymer and capitalizing on donor-acceptor effects. Despite some recent success for poly(acetylene), poly(thiophene) and poly(pyrrole),9 there exists no accurate and general computational tools to probe the relationship between

4. Roncali, J. Chem. Rev. 1997, 97, 173. 5. (a) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382. (b) Kobayashi M.; Colaneri N.; Boysel M.; Wudl F.; Heeger A.J. J. Chem. Phys. 1985 82(12) 5717-5723. (c) Roncali, J. Chem. Rev. 1992, 4, 711-738. 6. Zhang, Q. T.; Tour, J. M. J. Am Chem. Soc. 1998 120, 5355-5362. 7. Yao, Y.; Lamba, J. J. S.; Tour, J. M. J. Am Chem. Soc. 1998, 120, 2805-2810. 8. (a) Smith, R. C.; Fischer, W. M.; Gin, D. L. J. Am Chem. Soc. 1997, 119, 4092-4093. (b) Hu, D.; Yu, J.; Barbara, P. F. J. Am. Chem. Soc. 1999, 121, 6936-6937. 9. (a) Brédas, J.-L.; Cornil, J.; Beljonne, D.; Dos Santos, D. A.; Shuai, Z. Acc Chem. Res., 1999, 32, 267-276. (b) Salzner, U.; Pickup, P. G.; Porer, R. A.; Lagowski, J. B. J. Phys. Chem. A 1998, 102, 2572-2578.

78 structure and electronic properties for materials that derive their properties from extended range inter- and intramolecular interactions. We have embarked upon a program to understand how substitution affects band gap in perfectly alternating Donor-Acceptor (DA) copolymers. The quintessential low band gap conducting polymer is poly(isothianapthene), a thiophene ring fused to a benzene ring across the 3 and 4 positions of the thiophene ring. Polymerization occurs through the 2 and 5 positions of the thiophene ring. This arrangement induces a competition for aromaticity in the monomer repeat since it is impossible for both thiophene and benzene to simultaneously be aromatic. Benzene itself has an energy of aromatization (Eres) of 1.56 eV while thiophene is 1.26 eV.10 These energetics predict that benzene will remain aromatic and force the thiophene to adopt a pseudo-diradical electronic state at the 2 and 5 positions. When polymerized, this amounts to the bond connecting the thiophenes existing in the quinoid state, which lowers the band gap of the system by decreasing bond length alternation. While this proves to be an effective method of lowering polymer band gap (PITN has a band gap of 1.1 eV, about 1 eV lower than PTh), This structural topology proved limited in scope. Studies over the last decade have been geared almost exclusively to manipulating band gap through either planarization of the repeat unit by fusing heterocycles together, or by creation of polymers that have alternating donor and acceptor moieties or some combination of these two approaches. The donor-acceptor route has by far the most utility in terms of diversity in synthetic possibility while avoiding solubility problems that plague planar organic molecules. The route described in this chapter and the next chapter is more 10. March, J. Advanced Organic Chemistry; Wiley: New York, 1985.

79 accurately described as a donor-acceptor-donor route since the need to electropolymerize these monomers enforces the condition that easily oxidized and electropolymerizable heterocycles are at the perimeter of the molecule. The donor-acceptor approach can be loosely classified into two distinct families; those monomers for which resonance structures can be drawn that include the acceptor group and main chain polymer conjugation together, and those that which cannot and use the acceptor to inductively modify the backbone as shown in figure 3-1. C12H25

C12H25

N

S

N

x

N N

N

PITN

N

N

N

C6H13 C6H13

n

m

Planar Poly(Pyridines)

PLANARITY

RESONANCE EFFECTS

S

S 2

S O O

2

x

PT S,S-Dioxides

BOND ND LENGTH ALT TERNATION

INTERCHAIN EFFECTS

C8H17

x

S

PA

S x

S C8H17

DONOR-ACCEPTOR EFFECTS CN

C8H17

P3OT

S

CN

CN

O

O

S O O

S

S

x

O

PBEDOT-CNV D-A Included in Main Chain Resonance Chapter 3 (This Chapter)

PBEDOT-DCF D-A Separate from Main Chain Resonance Chapter 4

Figure 3-1. Overview of methods for the modification of band gap. Cutout indicates methods discussed in this chapter and the following chapter.

The logic behind the donor-acceptor approach (hereafter D-A) is that the high HOMO of the donor and the low level of the LUMO are incorporated into the resulting

80 monomer and thus polymer electronic structure.11 Figure 3-2 illustrates this concept with data for PEDOT,12 poly(cyanoacetylene) (PCA)13 and anticipated data for PBEDOT-CNV (vide infra). In this example, the band gaps are established through the onset of the π to π* transition in the spectroelectrochemical series and the energies are estimated from the potentials for the electrochemical doping/undoping redox couples. Specifically, the HOMO for PEDOT was obtained by cyclic voltammetry, the band gap by spectroelectrochemistry, and the position of the LUMO was deduced by subtraction of the band gap from the HOMO position.14 PCA poses a similar set of problems as there is no observable oxidation but there is a reduction wave to which the band gap (UV-Vis of the neutral form) was added to arrive at the low lying HOMO energy. PBEDOT-CNV spectra is described in detail below and does not require similar compromises. In this contribution, the synthesis of a matrix of structurally homologous conducting polymers differing systematically in the density of electron rich chromophore in direct conjugation with the polymer backbone is described. In addition, the effect of increasing the electron density along the polymer main chain is detailed. This approach is an effective method of decreasing the band gap along a polymer chain and lends insight into 11. (a) van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, (Review not Released Yet). (b) E. W. Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Synth. Met. 1993, 55-57, 299. 12. PEDOT data was obtained as described in the experimental section detailing spectroelectrochemistry. 13. Gorman, C. B.; West, R. C.; Palovich, T. U.; Serron, S. Macromolecules 1999, 32(13), 4157-4165. 14. Only recently has n-type doping of PEDOT been demonstrated. Even under rigorously controlled atmospheric conditions its stability is unremarkable. Ahonen, H. J.; Lukkari, J.; Kankare, J. Macromolecules 2000, 33(18), 6787-6793.

81 -2.5

CB

Energy /V vs Ag/Ag

+

-2.0 -1.5

Eg

E1

-1.0 -0.5

Eg

E1

E2

E2

VB

Eg

0 +0.5 +1.0

x CN

S x O

S O

S O

O O

CN x

O

Figure 3-2. The Donor-Acceptor approach, Alternating donor and acceptor moieties results in a polymer that has the combined optical properties of the parent donor or acceptor monomers. techniques for modifying the electronic properties of conjugated materials in general. In this work, the generally held assumption that acceptor strength must continually increase in order to decrease the band gap of a conjugated polymer is examined and found lacking. Moreover, our approach to creating the acceptor unit through the high yielding Knoevenagel condensation has general applicability to the synthesis of a variety of high molecular weight, conjugated step growth condensation polymers which are soluble and processible for use in OLEDs.15 3.2 Synthesis and Monomer Properties Overall synthetic yields for D-A monomers are often hampered by the need to cross couple two aromatic heterocycles with the concomitant loss in yield for the preparation of 15. (a) Greenham, N. C., Moriatti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628-630. (b) Hanack, M.; Segura, J. L.; Spreitzer, H. Adv. Mater. 1996, 8, 663-666.

82 organo-tin, -ZnCl, or -boronate reagents and during the coupling itself. The cyanovinylene acceptor core described here lessens some of the complications of combining pre-made DA units and eliminates the need for expensive and toxic tin reagents completely while placing the lower yielding synthetic steps early in the monomer synthesis. In this work, the premonomers were prepared by using the reaction schemes outlined in Figure 3-3. Aromatic carboxaldehydes of thiophene (commercially available) EDOT and EDOP were synthesized by the Vilsmeier formylation of the parent heterocycle. The structural diversity of this family is due to its ability to react three unique aromatic carboxaldehydes with two unique aromatic acetonitriles to prepare six monomers. The inclusion of the recently rediscovered 3,4-ethylenedioxypyrrole (EDOP) unit and its ability to react by Vilsmeier chemistry to form EDOP carboxaldehyde increases the structural variety of monomers available.16 An aromatic acetonitrile acts as the second premonomer component and the two used in this work were prepared by a catalytic route shown in Figure 3-3 (EDOT-ACN) or purchased (Th-ACN). The Knoevenagel condensation of an aromatic aldehyde with an aromatic acetonitrile is used as the last step in monomer synthesis with simultaneous preparation of the cyanovinylene core.17 The monomers available with this route are listed in figure 3-4 and are described by the matrix which combines the two types of premonomer to make six monomers. The Knoevenagel reaction

16. (a) Merz, A.; Schropp, R.; Dötterl, E. Synthesis 1995, 795-800. (b) Savage, D. J.; Schell, B. A.; Brady, B. K. Imaging Element Containing Poly(3,4-Ethylene Dioxypyrrole)/Styrene Sulfonate. U. S. Patent 5,665,498, September 9, 1997. (c) Schottland, P.; Zong, K.; Gaupp, C. L.; Thompson, B. C.; Thomas, C. A.; Giurgiu, I.; Hickman, R.; Abboud, K. A.; Reynolds, J. R. Macromolecules 2000, 33(19), 7051-7061. 17. Ho, H. A.; Brisset, H.; Elandaloussi, E. H.; Frére, P.; Roncali, J. Adv. Mater. 1996, 8, 990.

83 is carried out in ethanol or t-butanol with a slight excess of potassium t-butoxide and routinely allows for unoptimized yields of 80 to 95 percent. Vilsmeier Formylation S

O

O S

a O

O

H O

Catalytic Alkylation S

O

S

b O

CN

O

O

Vilsmeier Formylation and N-benzyl Deprotection Bz N

O

Bz N

c O

O

H N

O

d

H O

O

Knoevenagel Condensation O S

H

S

O H O

CN

S

e CN S

Figure 3-3. Reactions used to synthesize cyanovinylene monomers. a. POCl3, 1,2dichloroethane. b. i. BuLi, THF, -78 C, 1 h ii. ZnCl2 iii. Ni(acac)2, PPh2Cy, BrCH2CN, THF. c. POCl3, 1,2-dichloroethane. d. cat. H2. e. KOtBu, EtOH, reflux, 1 h.

84

Aldehyde Component

H S

S

O H

Acetonitrile Component

CN

O H

S

CN

O

O

O

O

H

EDOP-CHO

EDOT-CHO

Th-CHO

CN

N

O

H

S

CN

N

O S

S

Th-ACN

O

S

O

S

Th-CNV-EDOT

BTh-CNV

O

Th-CNV-EDOP H

CN

CN

S

S

S

CN

S

CN O

S

O

O O

O

EDOT-ACN

O

O

EDOT-CNV-Th

O

O

BEDOT-CNV

N

S O O

O

EDOT-CNV-EDOP

Figure 3-4. Synthesis of Cyanovinylenes. Matrix describing synthetic access to the cyanovinylene family of monomers. Premonomers are condensed with KOtBu in EtOH (reflux). Electron richness for the three unique heterocycle cores in this study follow the trend EDOP > EDOT > Th as evidenced by monomer oxidation potential. Evidence that EDOP is considerably more electron rich than EDOT is noticeable as solutions of EDOP readily polymerize on standing to form deep blue/purple solutions while EDOT undergoes some initial chemistry but passivates and does not completely polymerize to dark solutions. Monomer UV-Vis spectra arranged in order of increasing λmax are shown in Figure 3-5. These follow the expected trend where the less electron rich monomers have blue-shifted λmax values compared to the more electron rich monomers. There are some other notable features of the UV-Vis monomer spectra. In those monomers where there is an EDOT that is directly adjacent to the cyano group (EDOTCNV-Th, BEDOT-CNV and EDOT-CNV-EDOP), a further transition at high energy

85 occurs for these monomers and appears above the 190 nm UV-cutoff for acetonitrile. Additionally, the monomers that have thiophene in the position adjacent to the cyano group, the complement of the previous selection, BTh-CNV, Th-CNV-EDOT and ThCNV-EDOP, all have higher molar absorbances at λmax than monomers with EDOT immediately adjacent to the cyano group. 3.3 Monomer Structural Features and Crystallography The x-ray crystal structure of BTh-CNV was determined at -100(2) °C. It crystallizes in the orthorhombic space group Pbca. The structure determination confirms the cis relationship of the thiophene not adjacent to the cyano group (ring B) and the cyano group. The sulfur atoms are cisoid to the carbon-carbon double bond. While there is considerable disorder in the crystal structure, it is due to the entire molecule being flipped end for end at certain sites. In contrast to EDOT-CNV-Th, which also contains a Th in the ring B position, there is no disorder about the thiophene-vinyl linkage with the thiophene adopting the configuration of the major contributor in EDOT-CNV-Th. The molecule is close to planar with the angle between ring A and the cyano group being 5(1)° and the angle between ring B and the cyano group being 3(1)°. The angle made between the planes of the two heterocycle rings is 5(1)°. The x-ray crystal structure of EDOT-CNV-Th was determined at -120 °C. It crystallizes in the orthorhombic space group Pna21, with a single molecule in the asymmetric unit, and has the thiophene ring disordered over two unequally occupied (79%: 21%) orientations. The structure determination confirms the cis relationship of the cyano and thiophene substituents. The whole molecule is close to planar. Specifically, the plane of the double bond is inclined to the plane of the EDOT thiophene ring at an angle of only

86 Molar Absorptivity 30,000 S

λmax 364 nm onset 418 nm εmax 30500

CN

S

BTh-CNV

O

λmax 378 nm onset 430 nm εmax 21600

O

S CN

S

EDOT-CNV-Th

S

λmax 383 nm onset 442 nm εmax 28300

λmax 394 nm onset 449 nm εmax 22300

O O CN

S

Th-CNV-EDOT

O

O O S

O CN

S

BEDOT-CNV

S

λmax 394 nm onset 454 nm εmax 29100

λmax 401 nm onset 458 nm εmax 27600

O O CN

N H

Th-CNV-EDOP

O

O O S

O CN

N H

EDOT-CNV-EDOP

200

800 500 700 600 Wavelength /nm Figure 3-5. Monomer molar absorbance (M-1cm-1) for UV-Vis region in acetonitrile. 300

400

3.4(3)° and to the plane of the other thiophene ring at an angle of 4.3(3) °, while the planes of the EDOT and thiophene rings make an angle of 7.2(3)°. In contrast to the isomeric compound Th-CNV-EDOT18, the conformation of the EDOT substituent is such that the

87 sulfur is cisoid with respect to the geminal nitrile substituent, while the major contributor to the disorder has the sulfur of the thiophene ring cisoid to the vicinal nitrile group, as observed in Th-CNV-EDOT, but with the minor contributor having a transoid orientation of these groups. The x-ray crystal structure of Th-CNV-EDOT was determined at -125 °C. It crystallizes in the monoclinic space group P21/n with a single molecule in the asymmetric unit. The structure determination confirms the cis relationship of the cyano and EDOT substituents. The conformations of the two thiophene rings are such that their sulfur atoms are cisoid to the carbon-carbon double bond. Most significantly, the whole molecule is close to planar. Specifically, the plane of the double bond is inclined to the plane of the EDOT thiophene ring at an angle of 6.6(3)° and to the plane of the other thiophene ring at an angle of 8.7(3)°, while the planes of the two thiophene rings make an angle of only 2.8(3)°. The cyano substituent points slightly out of the plane of the double bond. The bond lengths and angles are similar to those in other structurally related compounds and there are no unusually short intermolecular interactions. The x-ray crystal structure of BEDOT-CNV was unable to be obtained due to lack of suitable quality single crystals. All methods described in Chapter 2 were attempted for the purpose of obtaining an x-ray crystal structure using acetonitrile, ethanol, chloroform, methylene chloride, benzene and various other chlorinated solvents in different crystallization environments.

18. G. A. Sotzing, C. A. Thomas, J. R. Reynolds and P. J. Steel. Macromolecules, 1998, 31, 3750-3752.

88

BTh-CNV

EDOT-CNV-Th

Crystals Growing

Th-CNV-EDOT

BEDOT-CNV

Th-CNV-EDOP

EDOT-CNV-EDOP

Figure 3-6. Perspective and top views of all the monomers in the cyanovinylene family.

89 The x-ray crystal structure for Th-CNV-EDOP was partially established. However, due to instability of the monomer, the crystal decomposed before complete data collection was possible. The partially resolved structure is shown in the figure but angles were unable to be obtained. Table 3-1. Incline angles for the planes comprised of the thiophene, EDOT, EDOP and cyanovinylene units in the monomer crystal structures. Ring Aa-CNVb

Ring Bc-CNV

Ring A-Ring B

BTh-CNV

5.(1)

3.(1)

5.(1)

EDOT-CNV-Th

3.4(3)

4.3(3)

7.2(3)

Th-CNV-EDOT

8.7(3)

6.6(3)

2.8(3)

4.3(1)

21.9(1)

24.5(1)

Name

BEDOT-CNV Th-CNV-EDOP EDOT-CNV-EDOP

a. Ring-A is defined by the four carbon atoms that make up the heterocycle nearest the cyano unit (left-most heterocycle in the structures in Figure 3-5). b. CNV is defined by the three carbon atoms in the ethylene and cyano group. c. Ring-B is defined by the four carbon atoms in the heterocycle β to the cyano group (right most heterocycle in structures throughout this chapter).

The x-ray crystal structure for EDOT-CNV-EDOP was determined at -100(2) °C. It crystallizes in the triclinic space group P-1 with X molecules in the asymmetric unit. The conformation of the EDOT is identical to the EDOT conformation in EDOT-CNV-Th and the conformation of EDOP is identical to that in Th-CNV-EDOP. Specifically, the sulfur of EDOT is transoid to the carbon-carbon double bond while the nitrogen of EDOP is cisoid to the carbon-carbon double bond. The molecule deviates considerably from planarity compared to the other monomers in this family. Ring A (EDOT) is nearly planar to the cyano group with an angle of 4.3(1)°. The angle between ring B (EDOP) and the cyano

90 group is 21.9(1)°, the largest deviation from planarity for any of the monomers in this family. Consequently, the angle formed between the planes of EDOT and EDOP is 24.5(1)°. 3.4 Computational Explanations of Monomer Properties The interesting behavior elicited by the EDOT-CNV-Th monomer bears further scrutiny. It is the only monomer in this family which crystallizes in a form where the thiophene unit is found to be in two different orientations relative to the cyano group. These orientations are related by single bond rotation about the ethylene-thiophene bond and this type of rotation is not observed for BTh-CNV, the other monomer containing a thiophene in the Ring B position. The crystal structure of BTh-CNV, however, is not without its own disorder arising from what effectively amounts to translation of the cyano group relative to other molecules in the crystal rather than the rotation of individual bonds. Calculations were undertaken on structures19 of EDOT-CNV-Th and Th-CNV-EDOT in order to determine whether the cause of the disordered crystal structure is due simply to a lower barrier of rotation for the thiophene when it is β to the cyano unit in EDOT-CNV-Th compared to the case when it is α as in Th-CNV-EDOT. The crystal structure coordinates for both Th-CNV-EDOT and EDOT-CNV-Th were modified to vary the dihedral angle between the thiophene ring and the cyanovinylene unit. Semiempirical single point calculations at the PM3 level were performed on the unrelaxed monomer geometries and the total energy, HOMO and LUMO energies were extracted. PM3 is a semiempirical force field that accurately reproduces the

19. x-ray coordinates were modified to vary the dihedral angle between the Th ring and the remaining section of the molecule.

91

A

Total Energy /kcal/mol

X-ray Structure

0.5 kcal/mol X-ray Structure 8.2

HOMO-LUMO Difference /eV

B 8.0

7.8

7.6

Th-CNV-EDOT 7.4

EDOT-CNV-Th 7.2 0

30

60

90

120

150

180

Thiophene Twist Angle /degrees

Figure 3-7. Single Point PM3 calculations on un-relaxed x-ray data for Th-CNV-EDOT (blue) and EDOT-CNV-Th (red) with specified twist angle about the thiophene ring. Part A: Total molecular energy as a function of twist angle. Part B: HOMO-LUMO difference as a function of twist angle. bond lengths and angles observed in the crystal structures for this family of molecules and was therefore deemed to accurately model the energetics of these systems. Additionally,

92 these calculations were performed at the semiempirical level to minimize computational times and because of the recognition that lower levels of theory are acceptable when the trends are more important than quantitative values. Figure 3-7 shows these results plotted as a function of thiophene twist angle for these two monomers. From the results in Figure 3.7 A, several things are apparent. First, the lowest energy conformation appears to be at a twist angle that is remote from the two structures as determined by the crystallographic data for EDOT-CNV-Th and the single rotational structure found for Th-CNV-EDOT in the x-ray results. This minimum is found between 135 to 145 degrees and is presumably absent from the crystal structure due to crystal packing forces not accounted for in the calculations. Second, For Th-CNV-EDOT, the total energy. vs. twist angle data correctly predict the result observed in the x-ray data to be the most stable by roughly 0.7 kcal/mol, a number that is larger than kT at room temperature (ca. 0.5 kcal/mol), where both monomers were crystallized. For EDOT-CNV-Th, the data is slightly more ambiguous. It predicts the minor conformer observed in the x-ray results to be the most stable, albeit by a value that is quite a bit smaller than kT. This implies that relatively free rotation about the cyanovinylene-heterocycle single bond is allowed while the molecule is crystallizing and becomes locked into the nearest planar conformation upon crystallization. The inability to clearly predict the most stable rotational contributor to the EDOT-CNV-Th crystal data is likely due to lack of correction for relaxation of other bond distances and angles upon rotation. Additionally, from the relationship between the thiophene torsional angle and the HOMO-LUMO difference (Figure 3.7 B), it is apparent that the energy difference approximates the expected cosine function predicted for rotation about these bonds.

93 Furthermore, it appears that below a twist angle of 10 degrees on either side of perfectly planar, there seems to be minimal effect on the HOMO-LUMO difference for either material. This result has implications in the comparison of data derived from these monomers which, with the exception of EDOT-CNV-EDOP, are all planar within this range. Further discussion will assume that the differences between these monomers and their resulting polymers (except EDOT-CNV-EDOP and its polymer) are not due to the slight variation in ring twist angles. 3.5 Polymer Optical Properties Monomers in this family are easily polymerized by galvanostatic, potentiostatic or scanning voltammetric methods to yield electroactive films on Pt or ITO. Polymerization proceeds with the same general mechanism outlined in Chapter 1 with minor changes. Since the monomers in this study are all unsymmetrical about the polymerization sites, the resulting polymers formed are likely prepared with a mixture of head to tail (H-T), head to head (H-H) or tail to tail (T-T) topologies where the head of the polymer is defined somewhat arbitrarily as the heterocycle nearest the cyano group so as to ensure consistency among the other visual aids in this work as well as the monomer naming convention. Figure 3-8 shows the possible coupling geometries for these monomers. Determining the actual amounts of H-T, H-H or T-T couplings in electropolymerized polymers such as these has not been demonstrated experimentally, however some hypotheses exist. It is anticipated that one electron oxidation of a monomer in this family removes an electron from the HOMO which is located on the heteroatom (S or N) of the most electron rich heterocycle. This cation radical then rearranges to place the highest spin density predominantly at the 5 position (the site immediately adjacent to the heteroatom but not connected to the

94 remainder of the molecule) of the heterocycle which then undergoes dimerization as described for symmetrical monomers such as thiophene, pyrrole, EDOT and EDOP. This dimerization occurs presumably with another radical cation where the spin is similarly located primarily on the 5 position of the most electron rich heterocyclic moiety since this is the radical species that is anticipated to occur in the highest concentration early in the electropolymerization. Assuming that the radical can be located on the most electron rich heterocycle with some certainty, this produces a H-H or T-T dimer depending on whether the electron rich heterocycle is nearest the cyano group or not. CN

CN

T

S

S

H O

-e-

S

O

O

S

x

O

Regioregular (H-T) Polymer H-T CN S

O

CN S O

O

CN

S

S

S O

O

S

x

O

Polymer Containing H-H, T-T and H-T Couplings BiEDOT-(TMS)2 O

O

CN S

CN S

S

S

S

S

x

CN O

O

O

T-T

O

H-H

Figure 3-8. Regiochemistry of cyanovinylene polymerization showing head to tail (H-T), head to head (H-H) and tail to tail (T-T) couplings and planarity of T-T couplings from the crystal structure of polymer subunit BiEDOT-(TMS)2. From here multiple options exist. First, extended conjugation monomers have lower oxidation potentials than their conjugation-limited predecessors and would possibly oxidize next, encounter additional molecules of the initially described radical cation and

95 form the first H-T coupling in the ensuing trimer. The electrochemical deposition of ThCNV-EDOP is slightly different from the other monomers in that preparation at low current densities leads to nodular films rather than the smooth deposits observed for the other monomers under identical conditions. Due to the poor slow-growth film forming properties of Th-CNV-EDOP, the monomer with the greatest difference in electron rich character between the two heterocycles, we surmised that there was a potential floor, below which, the T-T dimer (thiophenes on the perimeter of the dimer) would not be oxidized and only this dimer would be formed. Semiempirical calculations on the monomer, H-H, T-T and HT dimers were used to test this hypothesis. Using orbital energies from the PM3 calculations on all likely dimers (εHOMO), Koopmans’ theorem20 and a correction factor from vacuum to SCE which includes a solid state polarization parameter,21 the ionization potentials (IP) were estimated for these dimers and the monomer. Koopmans’ theorem Koopmans theorem:

IP calc = – ε HOMO

simply states that the ionization potential can be derived from the HOMO energy while a solid state polarization correction is necessary to first convert the potential reference from the vacuum level to SCE, and second to take into account the nonspecific solid interactions that differentiate the actual dimer from the vacuum calculations. The actual values (as compared to the Koopmans’ estimates values) are off for the monomer by nearly 1 V but if the trends are to be believed, the calculations indicate that every possible dimer formed has a lower oxidation potential than the monomer. The most surprising result of these calculations is that for all the dimers of Th-CNV-EDOP, the H-H dimer (with EDOPs on 20. Koopmans Physica, 1933, 1, 104. 21. Brédas, J. L.; Heeger, A. J. Macromolecules 1990, 23, 1150-1156.

96 the periphery) has the highest oxidation potential. This could be the result of facile oxidation of the electron rich (EDOP)2 unit at the core of the T-T dimer (Figure 3-9) rather than the terminal heterocycle as proposed in the mechanism outlined above. This may result in spin localization on the interior of the monomer distant to where it needs to be to productively from polymer. These results suggest that the poor slow deposition film quality is due to poorly matched oligomer solubility with solvent conditions or poor coupling of the T-T dimer from spin localization rather than the existence of a potential floor which limits oligomer growth to dimer. Since the other monomers are all more closely matched in terms of flanking heterocycle electron rich character, the idea that all dimers have lower oxidation potentials than the monomers appears to be a generally valid assumption for this monomer family. Ionization Potential (Calculated) /V vs SCE

2.2

Th-CNV-EDOP MONOMER

O

H CN

N

H N

O

S

S

O

S

N H

CN O

2.0

O

CN

H-H DIMER

O

1.8 T-T DIMER O

1.6

H-T DIMER S O

O

O

CN

1.4

S

O

CN N H

S

O H N

CN N H

S CN

O

O

N H

Figure 3-9. Calculated ionization potentials for various dimers of Th-CNV-EDOP.

Figure 3-10 shows data for the polymerization of Th-CNV-EDOT that is representative of all the monomers in this family. The scanning voltammetric deposition curve (main part) displays the expected qualities for the deposition of a conducting polymer

97 film with continuously increasing surface area from scan 1 to scan 3 as evidenced by the increase in peak current on subsequent scans. Notable features of this polymerization include the nucleation loop observed during the first scan which is the crossover of the current trace on the cathodic scan immediately following the peak labeled ip,a and is indicative of an increase in the electrode surface area due to electroactive polymer deposition. While this deposition method is commonly used for the preparation of polymer films for electrochemistry on Pt buttons, it is not well suited to the electrosynthesis of films on ITO with reproducible qualities or for the preparation of films on IMEs. The inset of this figure shows the galvanostatic growth of Th-CNV-EDOT on a 0.02 cm2 Pt button (same as above) at 20 µA (1 mA/cm2) for 250 sec. Despite using slow-growth conditions as described above to arrive at a 25 mC/cm2 film, the limiting potential at the end of this experiment (Elim) is a good starting point for establishing the ideal potential to grow films for in situ conductivity experiments on IME electrodes of entirely different geometries. The optical properties of conducting polymers have been treated in a number of research papers and reviews.22 Films for spectroelectrochemistry on ITO were grown from ACN/TBAP solutions at a potential that is adjusted empirically to deposit a film at ca. 1 mC s-1 cm-2 (25 mC/cm2 total). An initial guess of the correct potential is generated based on Elim from the galvanostatic experiment on Pt. If the film on ITO takes longer than 25 seconds to reach 25 mC/cm2, the film is removed (see below) and the potential is increased by 20 mV until the entire deposition takes near 25 seconds. The trial films are removed with a lint free wipe soaked in acetone, resulting in multiple depositions on the same ITO before 22. (a) Patil, A. O.; Heeger, A. J.; Wudl; F. Chem. Rev. 1988, 88, 183-200. (b) Furukawa, Y. J. Phys. Chem. 1996, 100, 15644-15653. (c) Cornil, J.; Beljonne, D.; Brédas, J. L. J. Chem. Phys. 1995, 103(2) 834-841.

98 3rd Scan CN

S O

S O

ip,a

Potential /V vs SCE

1.4

2nd Scan 1st Scan

1.2 1.0

Elim

0.8 0.6 0.4

0

50

200 150 100 Time /seconds

250

0.5 mA

nucleation loop 0.5

1.0 Potential /V vs SCE

Figure 3-10. Polymer deposition by repeated potential scanning of Th-CNV-EDOT. Inset: galvanostatic growth at 0.05 mA on a 0.02 cm2 Pt button electrode for 250 sec. obtaining a spectra quality film. It has been established that the removal of first growth polymer films leaves small nodules which act as nucleation points for subsequent polymer depositions causing them to occur in a more homogenous fashion. Figure 3-11 shows the spectroelectrochemical series for monomers that are related by complete substitution of thiophene by EDOT. PBTh-CNV (top) has a band gap23 of 1.5 eV (as measured in our labs), defined as the onset of the absorbance for the π to π* transition of the neutral form. Contention about this polymer exists in the literature as its

23. Please see the spectroelectrochemistry section in Chapter 2 for definition and significance of this and related terms such as switching depth, carrier tail and isosbestic point.

99 band gap was initially reported to be 0.8 eV,24 the event which initially drew our attention to this family. The subsequent resynthesis and characterization for this work confirms that the polymer in the original study (Ho, Brisset, Elandaloussi, et. al.) was not completely reduced prior to estimation of the band gap resulting in a shoulder at lower energy being taken for the band gap. As this polymer was the first material synthesized in this family (and does not reflect the improvements evident here), it reflects some of the liabilities of this system, namely, PBTh-CNV has the highest oxidation potential for all of the monomers in this family and is the most difficult material to reduce completely. The spectrum for the neutral polymer (Figure 3-11 (top), curve h) represents a polymer that has been electrochemically reduced in the presence of ca. 0.25 mL hydrazine. Despite these rather forcing reducing conditions, PBTh-CNV still shows evidence of charge carriers present in the form of the bipolaron peak E1 at 1.2 eV. In contrast to the behavior of PBThCNV, PBEDOT-CNV demonstrates much cleaner spectral properties (Figure 3-11 bottom). It is easily reduced electrochemically and upon oxidation, a highly absorbing tail forms at low energy indicative of charge carrier formation at the expense of the π to π* peak in the neutral form. A well defined isosbestic point is also present which is taken as evidence that the polymer undergoes a state to state transition between p-type doped and neutral. The band gap for PBEDOT-CNV is ca. 1.1 eV, 0.4 eV lower than PBTh-CNV, resulting in a reduction in the energy of λmax from 512 nm to 695 nm. PBEDOT-CNV also undergoes a much more significant optical switching depth in the visible and NIR spectral regions compared to BTh-CNV.

24. Ho, H. A.; Brisset, H.; Elandaloussi, E. H.; Frére, P.; Roncali, J. Adv. Mater. 1996, 8, 990.

100 Figure 3-12 shows the spectroelectrochemistry results for the two monomers containing one cyanovinylene, one EDOT and one thiophene representing the effect of switching the position of the EDOT and thiophene units about the cyanovinylene group. This exchange leads to a decrease in band gap of ca. 0.2 eV from PEDOT-CNV-Th to PThCNV-EDOT. Similar to the polymers PBTh-CNV and PBEDOT-CNV above, the polymer with an EDOT group not directly adjacent to the cyano group shows improved spectral properties in the sense that PTh-CNV-EDOT undergoes greater optical switching depth, and displays a more well defined band in the NIR that is attributed to E1. In Figure 3-12 top representing PTh-CNV-EDOT, the ∆A at λmax is 0.6 absorbance units where in the bottom spectra for PEDOT-CNV-Th, the ∆A is only 0.1 absorbance units. Because of the above properties, and the well defined electrochemistry, facile monomer synthesis and good stability, PTh-CNV-EDOT represents one of the most useful polymers in this family. Figure 3-13 details the effects of further increasing the electron rich character of the heterocycle flanking the cyanovinylene group. The band gaps of PTh-CNV-EDOP and PEDOT-CNV-EDOP saturate at 1.1 eV and are lower than the non-pyrrole containing polymers discussed previously with the exception of PBEDOT-CNV. PEDOT-CNVEDOP has the cleanest isosbestic point of all the polymers described here and is the most easily oxidized. The spectroelectrochemical series for PTh-CNV-EDOP shows a broad π to π* transition and very little switching depth through the visible region, possibly due to film morphology. Suggestions for the non-standard film forming characteristics of PThCNV-EDOP have been mentioned previously and additional comment is due about the monomer stability of the pyrrole containing members of the family. Th-CNV-EDOP is the monomer with the greatest differential in electron rich character of the heterocycles around

101 cyanovinylene. While solutions of the Th-CNV-EDOP monomer in ACN/TBAP tend to be moderately stable, there is slight discoloration after standing capped for several days but the solution does not become completely dark as is the case for EDOT-CNV-EDOP monomer after standing for only hours. This observation lends further credence to the theory regarding stability of the T-T coupled dimer in Figure 3-9. A summary of the spectral features of these systems in presented in Table 3-2. From this table, which is in the same order as the peak absorbance values for the monomers, it is evident that the band gap decreases upon increasing electron richness. The λmax also increases with the exception of PTh-CNV-EDOP and the bipolaron transition E1 remains relatively constant. Although the band gaps for this family of polymers changes over a range of 0.4 to 0.5 eV, they are quite similar in terms of the qualitative color transitions that they undergo. Films on ITO change from a moderate, absorbing blue (undoped state) to a transparent light blue (p-type conducting). The transparency of the light blue undoped state appears greater to the eye for PTh-CNV-EDOT and PBEDOT-CNV than for the other polymers. The color changes during the p-type to neutral transition are illustrated quantitatively in Figure 3-14 for PTh-CNV-EDOT. Figure 3-14 part A shows the luminance as a function of potential and indicates that to the eye, the polymer undergoes a fifty percent change in luminance over its doping range. Figure 3-14 part B shows a section of the xy color space representing hue and saturation and the trace that PTh-CNV-EDOT undergoes as it is doped and undoped. The area surrounding the 1.53 and 1.38 V designations is near the white point, which, combined with the high luminance at this potential, causes the film to be highly transmissive.

102

1100 nm

1.0

x

λmax 512 nm

A

CN

S

S

PBTh-CNV Eg ~ 1.5 eV

Absorbance

0.8

0.6 a 0.4

0.2 h

Visible

0.0

B

1100 nm a

1.0

λmax 695 nm

x CN

S O

S O O

PBEDOT-CNV Eg ~ 1.1 eV

0.8

Absorbance

O

0.6

0.4

0.2

m

0.0 0

1

2

3

4

Energy /eV

Figure 3-11. Spectroelectrochemical series of PBTh-CNV and PBEDOT-CNV representing the effect of complete substitution of EDOT for thiophene on polymer optical properties. BTh-CNV (top): a = 1.53 V vs SCE to h = -0.47 V vs SCE in 0.5 V increments. PBEDOT-CNV (bottom): a = 0.98 V vs SCE to m = -0.77 V vs SCE in 0.5 V increments.

103 1.6 lmax 598 nm

A

x CN

S O

1.4 O

S

962 nm

Absorbance

1.2

PTh-CNV-EDOT Eg ~ 1.2 eV

a

1.0

0.8

0.6

0.4

0.2 Visible

j 0.0

x

lmax 531 nm

B

CN S

0.4 O

1020 nm Absorbance

S

O

PEDOT-CNV-Th Eg ~ 1.4 eV

0.3

0.2

a 0.1

0.0

k 0

1

2

3

4

Energy /eV

Figure 3-12. Spectroelectrochemical series of PTh-CNV-EDOT and PEDOT-CNV-Th (top): a = +1.33 V vs SCE to j = +0.43 V vs SCE in 0.5 V increments and PEDOT-CNV-Th (bottom): a = +1.03 V vs SCE to k = +0.03 V vs SCE in 0.5 V increments.

104 1.4

λmax 642 nm

~1100 nm

A

x

H N

CN

O

1.2

O

S

PTh-CNV-EDOP Eg ~ 1.1 eV

1.0 Absorbance

a 0.8

0.6

0.4

0.2 Visible

r 2.2 1400 nm

B

2.0

λmax 756 nm

N O

S O

1.8 O

a

1.6

O

PEDOT-CNV-EDOP Eg ~ 1.1 eV

1.4 Absorbance

x

H CN

1.2 1.0 0.8 0.6 0.4 0.2

s

0.0 0

1

2 Energy /eV

3

4

Figure 3-13. Spectroelectrochemical series for PTh-CNV-EDOP and PEDOT-CNV-EDOP (top): a = +X V vs SCE to r = -X V vs SCE in 0.5 V increments and PTh-CNV-EDOP (bottom): a = +1.23 V vs SCE to s = -0.77 V vs SCE in 0.5 V increments.

105 Table 3-2. Polymer optical properties obtained from spectroelectrochemical series.

Name

λmaxa /nm (neutral form.)

Optical Band Gapb (Eg)

E1 /nm

PBTh-CNV

512

1.5

1100

PEDOT-CNV-Th

531

1.4

1020

PTh-CNV-EDOT

598

1.2

962

PBEDOT-CNV

695

1.1

1100

PTh-CNV-EDOP

642

1.1

~1100

PEDOT-CNV-EDOP

756

1.1

1396

a. λmax, E1 obtained directly from spectral peaks. b. Eg obtained by extrapolating the absorption onset to zero absorbance.

While the next section (Section 3.6) discusses the electrochemical details of the ptype and n-typing doping processes, only the p-type spectroelectrochemistry is reported in this section. Spectroelectrochemical series were attempted in the region comprising the neutral to n-type doping region. These results were unsuccessful in that no observed change in spectral signature was observed before polymer degradation was evident. This is presumably due to one of two possibilities. First, sealing of the cell used to perform p-type to neutral spectroelectrochemistry is likely inadequate for the more rigorous demands imposed by reduced polymers. Even so, changes in optical characteristics in the dry box were not evident to the eye for these polymers even though the corresponding electrochemical processes are easily documented and are quite stable. A full explanation for this observation await the electrochemical results described next but this suggests the possibility that even though charges are created on the polymer chain with the concomitant

106 ion transport associated with doping, charge carriers are not produced and thus no color change occurs. 3.6 Polymer Electrochemistry In summary of the previous section, incorporation of increasingly electron rich moieties around the cyanovinylene acceptor results in band gap reduction which eventually saturates at 1.1 eV. What is not clear is the specific mechanism of band gap reduction. Is this due to increasing the HOMO energy, reducing the LUMO energy or some combination of the two (See Figure 3-2)? Electrochemical deposition on Pt button electrodes and IMEs followed by characterization with cyclic voltammetry (CV), differential pulse voltammetry (DPV) and In Situ Conductivity (σ) are used to explain these phenomena in greater detail. The voltammetric results for all six polymers are shown in Figure 3-15. These results indicate that there is a broad p-type doping process which occurs at potentials in the range of 0 V to + 0.8 V vs. SCE and is generally described by capacitive charging overlaying the faradaic polymer doping process. This broad oxidative process is coupled to an even broader charge neutralization process. Scanning cathodically of this oxidation/ neutralization, a plateau is reached where very little current passage occurs. Even further scanning results in a new set of peaks attributed to n-type doping of the polymer at potentials between -1.0 V to -1.7 V vs. SCE. Charge neutralization from the reduced form then occurs upon reversal of the scan direction. Voltammetric studies were performed by poising the polymer at a potential in the plateau region corresponding to the neutral form and scanning anodically to access the p-type electrochemistry. Scan direction reversal then neutralizes the polymer and accesses the n-type region. This process was repeated until the correct potential limits were determined to observe the polymer electrochemistry for stable

107

A

B x CN

x CN

S

S

S

S O

50 µA

O

50 µA

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 PBTh-CNV Potential /V vs SCE

1.0

C

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 PEDOT-CNV-Th Potential /V vs SCE x

D

CN

S O

S O O

O

x CN

S O

40 µA

O

S

50 µA

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 PTh-CNV-EDOT Potential /V vs SCE

E

x

H CN

F

N O

S

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 PBEDOT-CNV Potential /V vs SCE

O O

O

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 PTh-CNV-EDOP Potential /V vs SCE

N

S

O

40 µA

x

H CN

O

40 µA -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 PEDOT-CNV-EDOP Potential /V vs SCE

Figure 3-14. Polymer cyclic voltammetry on Pt button electrodes A = 0.02 cm2. Polymer deposition was by galvanostatic growth (Figure 3-10) to create a 25 mC/cm2 film. All electrochemistry is in a H2O and O2 free dry box.

108 processes. This has the added utility of breaking in the polymer morphology to support efficient ion transport. p-Type breakin for these polymers ranges from 10 to 20 scans at which point the polymer CV does not undergo any marked changes in line shape of peak current magnitude. n-Type doping occurs over 20-50 scans; this longer time frame likely corresponding to the larger dopant ion used for reduction (assuming oxidation results in anion insertion and reduction leads to cation insertion). Exceptions to this are found in the two polymers that contain a thiophene in the ring B position, namely PBTh-CNV and PEDOT-CNV-Th. The oxidation peak for these polymers is quite sharp and occurs near 0.5 V vs. SCE. PTh-CNV-EDOP (Figure 3-15 part E) is the exception to the above procedure where the polymer oxidative and reductive voltammetric processes are separated to illustrate the instability of the cathodic process in PTh-CNV-EDOP. On the first cathodic scan for this polymer, a small peak is observed at -1.4 V attributed to reduction of the polymer. On the subsequent return scan, this peak disappears owing to the instability of the reduced polymer. This type of heightened sensitivity in the reduced form and decreased current responses in the reductive regions of the polymers which contain EDOP (PThCNV-EDOP and PEDOT-CNV-EDOP, E and F respectively) is typical of most pyrrole containing polymers. Despite the relatively weak acidity of the pyrrollic proton, it is likely succeptible to nucleophilic attack from other reduced pyrroles. There is no analogous process in thiophene or EDOT containing polymers. Stability to redox switching is excellent in these polymers as determined by the magnitude of the n-type peak current after several scans. For PTh-CNV-EDOT, which was studied for extended periodof time, the peak currents decrease ca. 10 percent over then first

109 20 scans and then decrease only ca. 5 percent over the next 200 scans. Additionally, the shape of the reductive processes in PTh-CNV-EDOT and PBEDOT-CNV are the most symmetrical of all the polymers in this study which indicates that there is little preference for ion transport in the doping or dedoping processes. In addition to the two main redox processes available in this polymer family, there are several smaller peaks that occur inside the main oxidation process and the main reductive process. These peaks are more pronounced for samples of polymer that have been deposited under slow, more controlled conditions and as such, are presumed to be related to order in the polymer secondary structure. Figure 3-16 shows the cyclic voltammogram for PTh-CNV-EDOT deposited under these controlled conditions at a current of 10 µA (500 µA/cm2). This polymer displays two sets of two peaks, dubbed prepeaks in the literature,25,26 inside of the main voltammetric processes. Some question about the exact nature of these prepeaks appears in the literature.25 What is apparent from this work is that first, prepeaks only appear after a reductive potential excursion. Repeated scanning over the p-type region does not result in the observance of prepeaks. Second, these prepeaks near the main anodic and cathodic voltammetric processes are correlated to each other. That is, the outer two peaks, labeled bn and bp in Figure 3-16 and the inner two peaks, labeled an and ap are coupled and do not exist unless a potential more cathodic or more anodic of the peak is accessed during cycling. Figure 3-16 part C illustrates the coupled nature of the inner set of prepeaks. Part B partially illustrates the coupled nature of the outer set of prepeaks but appeal to the callouts must be made for definitive proof. The callout on the left (red) shows three sequential cyclic voltammograms over the region of bn, an and ap. On 25. Zotti, G.; Schiavon, G.; Zecchin, S. Synth. Met. 1996, 72, 275-281.

110 x CN

S

-0.94 V

O O

S

bp ap

bn

20 µA

A

an

+0.98 V

-1.14 V increasing scan #

increasing scan #

B (x2)

C (x2)

5 µA

5 µA -0.8

-2.0

-0.4 0.4 0.0 Potential /V vs SCE

-1.5

-1.0

-0.4

-0.5

0.0

0.5

1.0

0.8 0.4 0.0 Potential /V vs SCE

1.5

2.0

Potential /V vs SCE

Figure 3-15. Explanation for prepeaks observed in PTh-CNV-EDOT films. A, B, and C illustrate the effect of narrowing the potential window scanned over while the callouts, red and blue represent the effect of scanning over a moving window of 3 prepeaks. the first scan bn is visible but is absent from the second two scans indicating that it is necessary to access bp in order for bn to appear on subsequent scans. The complement to these results is shown in the blue callout of Figure 3-16. In this series of voltammograms, the potentials where an, ap, and bp appear are accessed and while bp appears on the first scan, it does not appear on the second or subsequent scans when bn is not accessed. The physical intuition for prepeaks is that they are due to trapped charge in the polymer.26 When PTh-CNV-EDOT is oxidized (ca. + 0.98 V), then charge compensated to the neutral form, there remain sites that are still oxidized where the dopant is either trapped in polymer

111 due to some dense morphology or there is a surface or near electrode interaction that prohibits this section of polymer from being charge neutralized. As the potential is scanned more negative, these sites an and bn are neutralized. The polymer n-type doping neutral to anionic transition is observed on scanning more cathodic of the prepeaks. On the return anionic-to-neutral transition, trapped anionic charge states are not neutralized until ap and bp are scanned. The prepeaks can be quite large compared to the actual polymer doping process as seen in Figure 3-15 D for PBEDOT-CNV. This implies that for this polymer where large prepeaks are observed and other polymers deposited with ordered tendencies, there is a considerable volume of polymer that is electrochemically inaccessible to charge compensation compared to the subset of polymer that is electrochemically active. The DPV and in situ conductivity results for PBEDOT-CNV are shown in Figure 3-17. Increases in the conductivity occur for both oxidative (p-type) and reductive (n-type) doping. The p-type conductivity onset for PBEDOT-CNV is ca. 0 V vs. SCE and occurs slightly later than the extrapolated onset in the CV although the latter must be extrapolated due to the large prepeak obscuring the p-type onset. The p-type conductivity exhibits the typical S-shape expected for conductivity results.27 The conductivity is not volume normalized but is rather reported as the reciprocal of the resistance since the methods of reporting volume conductivities on this type of electrode are of unsatisfactory precision without accurate film thickness measurements. The onset for n-type conductivity occurs 26. (a) Zotti, G.; Schiavon, G.; Zecchin, S. Synth. Met. 1996, 72, 275-281. (b) Crooks, R. M.; Chyan, O. M.; Wrighton, M. S. Chem. Mater. 1989 1, 2. (c) Borjas, R.; Buttry, D. A. Chem. Mater. 1991 3, 872. 27. (a) H. S.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 7389. (b) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441. (c) Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 5133.

112 around -1.2 V vs SCE giving this polymer a conductivity derived band gap of 1.2 eV. This compares to the optical band gap of 1.1 eV and the DPV-derived band gap of 0.9 - 1.0 eV. The optical band gap requires the accurate estimation of only one onset and should for this reason be more accurate. The DPV results for PBEDOT-CNV mirrors the CV results in that the p-type doping processes appear quite broad in the CV and do not peak in the DPV. The causes of this will be discussed below when the question of charge transport mechanisms is addressed. Similarly, the n-type doping electrochemical processes match in terms of peak potential for both the CV and DPV results. This reaffirms the intuitive answer that polymer doping, both reductive and oxidative, is not a reversible process since there should be a peak to peak separation of 0 for both the CV and DPV results for an electrode adhered film. This is due simply to the fact that the transport of ions and resulting film swelling that occurs upon polymer doping and undoping is slower than the electrochemical time scale used to probe it. Scan rate dependence experiments for PTh-CNV-EDOT shown in Figure 3-18 exhibit the typical linear dependence on scan rate expected for an electrode adhered film over the scan rates where doping is not diffusion limited. This occurs over 100 to 500 mV sec-1 for PTh-CNV-EDOT and deviates from perfect linearity only due to polymer breakin over the first 30 scans, an observation that is consistent with reports that n-type doping breaking takes more scans than p-type doping breakin at approx. 40-50 scans and 10-20 scans respectively.28 This scan rate linearity occurs over a much broader range of scan rates than the typical homo polymers of pyrrole, thiophene and EDOP that make up these polymers excluding EDOT which demonstrates exceptional linear scan rate dependence. 28. (a) Zotti, G.; Schiavon, G.; Zecchin, S. Synth. Met. 1996, 72, 275-281.

113 7

151 Ω

x CN

S O

S

6 Pseudo Conductivity /mS

O O

5

O

4 3 2 X 1000

1 0

66 MΩ

20 -1.3 V

15

Current /µA

10 5 0 -5

-10 -15 -1.46 V

-20 -2.0

-1.5

Electrochemical Eg ~ 0.9 eV

-1.0

-0.5

0.0

0.5

1.0

Potential (V vs SCE)

Figure 3-16. PBEDOT-CNV in situ conductivity and DPV results in 0.1 M TBAP/ACN (drybox). n-type conductivity is multiplied by 1000. Figure 3-19 shows the CV on an IME, the DPV on a Pt button and the In Situ Conductivity on an IME for PTh-CNV-EDOT. The CV results on the IME are similar to the CV results in Figure 3-15 obtained on a Pt button which indicates our ability to obtain

114 -300 x

j

CN

S O

-200

O

S

a 0 200

Current /uA

Current /mA

-100

100

100 0

-100 -200

200

-300

300

-1.4

-1.2

-1.0

-0.8

-0.6

0

-0.4

100

200 300 400 Scan Rate /mV/sec

-0.2

0.0

0.2

500

0.4

Potential /V vs SCE

Figure 3-17. PTh-CNV-EDOT reductive electrochemistry and scan rate dependence. Cyclic voltammetry for the n-type doping of PTh-CNV-EDOT at scan rates a = 50 mV/sec to j = 500 mV/sec in 50 mV increments. Inset: peak current vs. scan rate for cathodic and anodic peaks. useful cyclic data on the IME which is useful for direct comparison to the conductivity data. The positions of the peaks are labeled for comparison to the DPV and conductivity results. The DPV results are similar to those obtained for BEDOT-CNV where a broad p-doping wave is observed that does not peak like the comparatively sharper n-type DPV process. The DPV n-type to neutral transition (Figure 3-19 part B top left) wave peaks at nearly the exact potential as the corresponding CV process. In contrast, the neutral to n-type wave in the DPV experiment peaks at -0.98 V compared to the much more cathodic -1.26 V in the CV. For the in situ conductivity plot (Figure 3-19 part C) a comparatively thick film was

115 deposited in order to observe the n-type conductivity more accurately. This results in a bell shaped conductivity trace that is nearly symmetrical about the peak at -1.03 V vs. SCE. Vertical lines drawn from the peaks in the CV trace indicate that the peak in the n-type conductivity and indeed, all of the observable conductivity in this range occur between the peaks in the CV spectrum. From the plot of n-type conductivity it is not clear whether this particular conductivity profile is due to an intrinsic property of the polymer or is due to irreversible degradation of the polymer as the potential is scanned more cathodic of the peak conductivity. The experiment was duplicated after 72 hours of storage in an Ar dry box to determine the extent of change in the conductivity plot. The result is plotted in Figure 3-20 and indicates that very little degradation is evident in the polymer over this time scale and even less is to be attributed to the initial conductivity experiment itself. Experiments to probe the countercation tolerance were also performed. A film of PTh-CNV-EDOT was deposited from a solution of TBAP. Assuming anion dominant ion transport for p-type doping, this resulted in a film containing perchlorate. CV data was then collected over 10 scans with TBAP, TEAP, and finally LiClO4. The polymer stability in TBAP has been discussed. Peaks associated with n-type doping when TEAP or LiClO4 were involved decreased rapidly and were not observed again even upon return to TBAP. 3.7 Conclusions and Perspective The above results indicate the validity of concentrating on D-A effects as a method of band gap reduction in conjugated polymers. This study establishes that for a fixed acceptor, cyanovinylene, increasing the electron density around the acceptor reduces the resulting polymer band gap. Several more conclusions can be drawn from relationships

116 0.15

A. CV

x CN

- 0.88 V

O

0.10 Current /mA

S

S

0.77 V

0.05

O

1.09 V

0.00 -0.05

0.96 V

-0.10

300

- 1.25 V B. DPV

- 0.86 V

Differential Current /µA

200 100 0 -100 -200 -300 0.007

- 0.98 V C. In Situ Conductivity

0.006

Conductivity /S

0.005

- 1.03 V

0.004 0.003 0.002 0.001 0.000 -1.5

-1.0

-0.5

0.5 0.0 Potential /V vs SCE

1.0

1.5

Figure 3-18. PTh-CNV-EDOT electrochemistry in 0.1 M TBAP/ACN. A. CV at 100 mV/ sec on 10 µm gap and line width masked Pt IME. B. DPV on 0.02 cm2 Pt button. C. In Situ conductance on 10 µm gap and line with masked Pt IME.

117 5.0 A.

B.

x CN

S O

4.0 O

Conductivity /mS

S

3.0

2.0

1.0

0.0 -1.50

-1.25

-1.00 -0.75 Potential /V vs SCE

-0.50

-1.25

-1.00 -0.75 Potential /V vs SCE

-0.50

Figure 3-19. PTh-CNV-EDOT conductance after growth (a)and 72 h (b). between the electrochemical onsets for doping vs. degree of electron rich character around cyanovinylene data arranged in order of increasing electron density. Figure 3-21 shows this type of plot where the primary energy of the band onsets are derived from CV and are checked against in situ conductivity results when available. Despite the inclusion of thermodynamic and kinetic data in the CV results possibly obscuring the desired thermodynamic information, in this particular case the CV correlates with the spectroscopic results more closely. Higher priority is placed on conductivity results than CV onsets and absolute determination of band gap is left to spectroelectrochemical results. When spectroelectrochemical results contradicted CV results, the baricenter of the band gap was determined by CV and the band gap magnitude was superimposed upon this to get the valence band (VB) and conduction band (CB) onsets. In all cases, CV slightly overestimates the band gap when the onsets are not obscured by prepeaks. DPV results slightly underestimate the band gap and conductivity results come the closest to corroborating spectroelectrochemical results while providing added information about

118 band edges in energy space. DPV results are quite sensitive to any sort of ion transport process resulting from redox chemistry29 and in general the quantities change over two orders of magnitude. Conductance results often change over 5 to 7 (or greater) orders of magnitude and measure the contribution from redox chemistry to conductivity only. This allows a partial explanation of why the DPV and conductivity results do not match exactly. While the DPV trace is beginning to rise, there is very little contribution to the conductivity that is visible on a linear scale and thus the conductivity appears to lag the DPV. From the plot in Figure 3-21 it is apparent that the reduction in band gap is entirely the result of the VB being raised in energy while the CB remains fixed. This follows from the premise of this chapter (Figure 3-2), that the VB energy is set by the identity of the donor while the CB energy is set by the identity of the acceptor. Given the scope of donor and acceptor moieties in this work, there appears to be a lower limit to the band gap that can be achieved by simply increasing the electron rich character about an acceptor. Ultimately, a band gap of 1.1 eV was achieved whether EDOT was used alone or a combination of thiophene, EDOT and the much more electron rich EDOP. An argument can be made that the ultimate example of electron rich heterocycle around the cyanovinylene core (EDOT-CNV-EDOP) is not truly the lowest band gap that can be demonstrated by this system due to the considerable deviation from planarity (ca. 25 degrees) in this particular monomer. If one is simply trying to obtain the lowest band gap material, attention to the details that cause EDOT-CNV-EDOP and future monomers based on EDOP to diverge from planarity is necessary. However, given the synthetic

29. DPV is able to observe electrochemical changes that are not productive for increasing conductivity.

119 complexity of EDOP synthesis and the incorporation of EDOP into other systems due to the difficulty in lithiating EDOP and preparing protected analogues of EDOP, it is worth asking whether much is gained from this effort. Indeed, a fully substituted EDOT system appears to be the best compromise in terms of maximum donor strength and ease of synthesis worth investing into a conducting polymer system. EDOT is far more stable as a monomer than EDOP, is easily lithiated and stannylated for use in Stille coupling reactions, and is easily reducible when incorporated into monomers with an acceptor. The issue of reduction bears further comment. The main impetus of low band gap CEP research has been to make available materials that n-type dope in an accessible potential region. Taking into account the reduction potentials for oxygen and water while allowing for common overpotentials places the need to have a polymer n-type dope at a currently inaccessible -0.5 V vs SCE to be stable in air.30 It appears that a reduced polymer containing EDOP would not be stable in devices regardless of this presently out of reach potential target. Failing to absolve the all-thiophene and all-EDOT containing polymers from liability is the dismal n-type conductivities observed in these systems. This situation arises presumably from the necessity to use TBA+ as a dopant cation rather than the smaller and more n-type compatible TEA+ or Li+. Use of either of these alternative cations leads to rapid polymer degradation under cycling and little utility for devices containing the above polymers and TEA+ or Li+. The use of TBA+ electrolytes creates a scenario where the conductivity rises and falls to nearly zero within the confines of the doping processes

30. de Leeuw, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E. F. Synth. Met. 1997, 53-59.

+1.0

+0.5

0

-0.5

-1.0

-1.5

-2.0

S

x

O

S

x

1.4 eV

PEDOT-CNV-Th

O

S

CN

S O

x O

O

O

S

1.1 eV

VALENCE BAND

1.2 eV

O

x O

PBEDOT-CNV

S

CN

CONDUCTION BAND

PTh-CNV-EDOT

S

CN

Figure 3-20. HOMO-LUMO band edge progression in cyanovinylene monomers

1.6 eV

PBTh-CNV

S

CN

H N O

x O

1.1 eV

PTh-CNV-EDOP

S

CN

O

H N

O

x O

1.1 eV

PEDOT-CNV-EDOP

O

S

CN

120

121 determined by CV. This strongly implies that the total conductivity, which is the sum of the electrical conductivity and the redox conductivity, is dominated by the redox conductivity,31 that is, the conductivity derived from the gate potential forcing the polymer to be partially doped. In a polymer system suitable for n-type devices, the polymer architecture must be engineered to tolerate smaller, more mobile dopant ions so that the electrical conductivity dwarfs the conductivity derived from mixed-valent states along the polymer backbone. A fundamental requirement of conductivity matching also appears necessary where the n-type and p-type conductivities are within an order of magnitude of each other. In these systems, the n-type conductivity is generally four to five orders of magnitude less than the p-type conductivity. Aside from the above mentioned liabilities, the series of polymers discussed herein demonstrates some important concepts that should affect the strategies used to discover low band gap and n-type dopable polymers. Specifically, while EDOT has been identified as the ideal donor in terms of ease of synthesis and adequate donor strength, increased acceptor strength over the systems in this study appears necessary to decrease the band gap below 1.1 eV. In these systems, there are always twice the number of donor molecules as there are acceptors. Thus the donor should exert much more of an influence than the acceptor, an observation reinforced by Figure 3-2. For next generation low band gap systems, increased acceptor strength and number will be necessary while maintaining planarity. Additionally, the use of symmetrical monomers drastically decreases system complexity.

31. See Chapter 2

122 3.8 Experimental Section General methods. NMR spectra (1H and 13C) were obtained on a QE-300 spectrometer. Chemical shifts are reported in ppm relative to residual protio solvent (CHCl3 7.24 ppm (1H), 77.0 ppm (13C); DMSO 2.49 ppm (1H) (HOD). FAB-HRMS were obtained at the Mass Spectrometry Core Laboratory at the University of Florida. Elemental Analysis (CHNS) were obtained from Robertson Microlit Laboratories. UV-Vis-NIR spectra were recorded on a Cary 5E spectrophotometer in acetonitrile (ACN). IR spectra were recorded on a Biorad FTS-40A spectrophotometer on NaCl plates. Ethanol (Aaper, 200 proof) was used as received. ACN was distilled from CaH2 prior to use and stored over 3A molecular sieves in a dry box for electrochemical experiments in the reductive regime. DMF (anhydrous) was used as received. BTh-CNV,32 EDOP33 and EDOP-CHO34 were prepared as previously reported. Synthetic manipulations were performed under an argon atmosphere in flame or oven dried glassware using standard Schlenk techniques. Electrochemistry was performed in an argon filled dry box with either a PAR 273, 273A or 263A potentiostat/galvanostat controlled with Scribner Associates Corrware software. Platinum working electrodes were used in three electrode cells with Pt flags as counter electrodes and either Ag wire (dry box), SCE or Ag/Ag+ (bench top). Potentials are reported vs SCE and are calibrated vs. internal ferrocene standard. DPV results were

32. Roncali BTh-CNV 33. Merz, A.; Schropp, R.; Dötterl, E. Synthesis 1995, 795. 34. Synthetic procedures for the ethylenedioxypyrrole monomers and premonomers were developed by Kyukwan Zong and are denoted by a KZ after the compound name.

123 obtained using the waveform described in Chapter 2 using a step size of 2 mV, step time of 0.0167 sec and an amplitude of 100 mV. In situ conductivity data was obtained using the Wrighton method35 on 10 µm gap and line width (1 mm length, 25 pairs of bands) or 5 µm gap and line width (1 mm length, 50 pairs of bands) passivated Pt IME electrodes supplied by the Institute of Microtechnology, Neuchatel, Switzerland. The passivation layer was silicon nitride over Pt prepared by plasma enhanced chemical vapor deposited (PECVD) on Corning #7740 borosilicate glass. Polymer films were deposited potentiostatically on the above IMEs with a 10 mV drain potential applied between the IMEs to detect when the film bridged the gap. Deposition was continued until the drain current reached approximately 50 µA to ensure the polymer resistance in the conducting state was more than the lead resistance. Equilibrium data were collected every 50 mV (gate potential) by a +25 mV to -25 mV (drain potential) sweep at 1 mV/sec (4 points/sec). The resulting trace was fitted by linear regression for each gate potential and the conductivity at Vgate is reported as the slope of this line. Conductivities are reported directly, uncorrected for polymer film thickness. The experimental apparatus and data treatment are described in detail in Chapter 2. X-ray crystallography. Intensity data were collected with a Siemens SMART CCD area detector using monochromatized Mo Ka (λ = 0.71073Å) radiation. Crystal dimensions and atom coordinates, bond lengths and angles are available in Appendix A. Structures were solved by direct methods using SHELXS9036, and refined on F2 by full-

35. Schiavon, G.; Sitran, S. Zotti, G. Synth. Met. 1989, 32(2), 209. Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 7389. Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441. Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 5133.

124 matrix least-squares procedures using SHELXL9637. All non-hydrogen atoms were refined with anisotropic displacement coefficients. 2-(3,4-ethylenedioxythienyl)carboxaldehyde (EDOT-CHO). To a three necked RBF fitted with a reflux condenser, pressure equalizing addition funnel and a nitrogen purge was added 10 mL of 1,2-dichloroethane, 1.0 g (7.04 mmol) of EDOT, and 0.52 g (7.12 mmol) POCl3. The flask was cooled via an ice bath for 20 minutes before POCl3 addition over the course of 15 minutes. The solution was then heated to 90 °C for two hours, cooled, poured into ice, and the aqueous layer was washed with ether (2 X 50 mL). Light yellow crystals separated after 24 hours, yield 1.1 g (92%). mp 135-136 °C. 1H NMR (CDCl3) 3.93 (m, 2H), 4.63 (m, 2H), 6.55 (s, 1H), 10.21 (s, 1H). Elemental analysis calculated for C7H6SO3: C, 49.41; H, 3.53; S, 18.10. Found: C, 49.16; H, 3.49; S, 18.92. FAB-HRMS expected 170.0038, found: 170.0014. 2-(3,4-ethylenedioxythienyl)acetonitrile (EDOT-ACN). BuLi (14.1 mL, 35 mmol, 1 eq) was added to EDOT (5g, 1 eq) and 200 mL THF at -78 °C in a single necked flask fitted with a septum. ZnCl2 (4.8 g, 1 eq) was added and the reaction was stirred at room temperature for an hour. This solution was transferred to a pressure equalizing addition funnel and added slowly to a 500 mL 3-necked flask fitted with a reflux condenser and an Ar inlet containing Ni(acac)2 (0.90 g, 10 mol%), BrCH2CN (4.2 g, 1 eq), and cyclohexyldiphenylphosphine (0.94 g, 10 mol%) in 100 mL THF. After addition was complete, the flask was heated to 60 °C for 1 h. The flask was then cooled and cold 1 N HCl (100 mL) was added with 50 mL ether. The organic phase was washed with water, 36. Sheldrick, G. M., Acta Crystallogr., Sect. A, 1990, 46, 467. 37. Sheldrick, G. M., SHELXL-96, University of Gottingen, 1996.

125 dried with MgSO4 and the solvent was removed in vacuo. The oily residue was chromatographed in the dark on silica gel (20 X 5 cm) with 15% EtOAc in hexane. The product turns darks when standing neat in a freezer in the dark and was not submitted for elemental analysis. Yield, 33%. mp 52 (some clearing) - 72 (flow) °C. FAB/HRMS calculated for C8H7NO2S: 181.02, found: 181.0167. N-benzyl-1-cyano-2-(2-(3,4-ethylenedioxypyrrolyl))-1-(2-thienyl)vinylene. (KZ) To a solution of EDOP-CHO (0.60 g, 2.47 mmol) and 2-thiopheneacetonitrile (0.34 g, 2.72 mmol) in tert-butanol was added potassium t-butoxide (0.61 g, 5.43 mmol) at room temperature. The reaction mixture was stirred for 3 h at 50 °C. After cooled to room temperature, t-butanol was removed in vacuo and the residue was diluted with dichloromethane, washed with water and dried over MgSO4. The residue was purified by chromatography on silica gel using the eluent (hexane/ethyl acetate = 3: 1) to give the product as a yellow solid and the product was subjected to debenzylation in next step. 1H NMR (300 MHz, CDCl3) (7.18 (m, 3H), 7.19 (m, 2H), 7.13 (m, 2H), 6.99 (m, 1H), 6.96 (s, 1H), 6.44 (s, 1H), 4.97 (s, 2H), 4.35 (m, 2H), 4.27 (m, 2H). General condensation procedure for the preparation of the cyanovinylene monomers BTh-CNV, EDOT-CNV-Th, Th-CNV-EDOT, BEDOT-CNV, Th-CNVEDOP and EDOT-CNV-EDOP. 1-cyano-2-(2-(3,4-ethylenedioxythienyl))-1-(2thienyl)vinylene (Th-CNV-EDOT). To a three necked round bottom flask equipped with a reflux condenser and argon inlet was added 50 mL absolute ethanol, 0.73 g (5.9 mmol) of 2-thiopheneacetonitrile, 1.0 g (5.9 mmol) of 2-(3,4ethylenedioxythienyl)carboxaldehyde and 1.0 g (8.8 mmol) of potassium t-butoxide. The

126 solution was heated to 70 °C and allowed to stir for three hours, afterwhich, the solution was poured into water. 1-cyano-1,2-bis(2-thienylene)vinylene (BTh-CNV). UV-Vis (ACN) λmax 364 nm (30500 M-1 cm-2). FT-IR 3100, 2213 (CN), 1584, 1430, 1326, 1243, 1049, 850, 828, 713, 700, 578, 507 cm-1. Elemental analysis calc. for C11H7NS2: C, 60.80; H, 3.25; N, 6.45; S, 29.51. Found: C, 60.62; H, 3.11; N, 6.45; S, 29.22. 1-cyano-1-(2-(3,4-ethylenedioxythienyl))-2-(2-thienyl)vinylene (EDOT-CNVTh). Prepared via same method as Th-CNV-EDOT. Yield: 62%. mp 117-118 °C. 1H NMR (CDCl3):? 7.81 (s, 1H), 7.62 (d, 1H), 7.50 (d, 1H), 7.11 (dd, 1H), 6.39 (s, 1H), 4.35 (m, 4H). 13C

NMR:? 142.10, 139.58, 138.07, 132.53, 130.97, 129.05, 127.66, 99.68, 64.97, 64.87,

64.27. UV-Vis (ACN) λmax 378 nm (21600 M-1 cm-2). FT-IR 3107, 2211 (CN), 1574, 1488, 1436, 1419, 1362, 1324, 1208, 1170, 1019, 920, 897, 856, 709, 617, 594, 463 cm-1. FAB/HRMS calc'd for C13H9NO2S2: 275.01, found: 275.0073. Elemental anal. calc. for C13H9NO2S2: C, 56.71; H, 3.29; N, 5.09; S, 23.29. Found: C, 56.67; H, 3.29; N, 5.01; S, 22.84. Th-CNV-EDOT. The orange solid was collected via filtration and recrystallized from ethanol/H2O (90:10) to give 1.5 g (93%) of Th-CNV-EDOT. (mp 167-168 °C) 1H NMR (DMSO-d6) 4.25 (m, 2H), 4.36 (m, 2H), 6.99 (s, 1H), 7.10 (t, 1H), 7.25 (d, 1H), 7.50 (s, 1H), 7.76 (d, 1H); 13C NMR with APT (DMSO-d6) 64.31 (down), 65.34 (down, 97.87 (down), 105.64 (up), 111.89 (down), 117.15 (down), 125.71 (up), 127.81 (up), 128.53 (up), 128.36 (down), 141.46 (down), 144.98 (down). UV-Vis (ACN) λmax 383 nm (28300 M-1 cm-2). FT-IR 3093, 2203 (CN), 1583, 1558, 1519, 1477, 1442, 1329, 1269, 1188, 1064,

127 1010, 959, 910, 844, 825, 744, 697, 625, 600, 526, 478 cm-1. FAB-HRMS calculated for C13H9NS2O2: 275.0075, found 275.0067. 1-cyano-1,2-bis(2-(3,4-ethylenedioxythienyl))vinylene (BEDOT-CNV). 0.497 g (1) (2.9 mmol, 1 eq), 0.531 g (2) (1 eq) and 0.358 g KOtBu (1.1 eq) were combined in a 50 mL flask. 25 mL ethanol was added and the solution was refluxed for 2 h. At this time the reaction was cooled, 10 mL water was added and the product was filtered. Analytical samples for of BEDOT-CNV were obtained by soxhlet extraction into ethanol to afford the product in 87% yield. The other monomers are more easily purified by chromatography on a silica gel plug with CH2Cl2. mp 203-204 °C. 1H NMR (CDCl3): d 7.79 (s, 1H), 6.90 (s, 1H), 6.65 (s, 1H), 4.25 (pair of t, 8H). 1

13C NMR: UV-Vis (ACN) λ

max 394

nm (22300 M-

cm-2). FT-IR 3101, 2877, 2207 (CN), 1580, 1558, 1486, 1452, 1433, 1368, 1336, 1273,

1181, 1070, 1009, 959, 905, 730, 681, 631, 610, 462 cm-1. FAB/HRMS calculated for C15H11NO4S2: 333.01, found: 333.0157. Elemental anal. calc. for XXX: C, 54.04; H, 3.33; N, 4.20; S, 19.24. Found: C, 53.47; H, 3.15; N, 4.10; S, 18.54. 1-cyano-2-(2-(3,4-ethylenedioxypyrrolyl))-1-(2-thienyl)vinylene (Th-CNVEDOP). (KZ) The solution of the compound (?) (0.9 g, 2.6 mmol) in THF was very slowly added to the solution of sodium (0.15 g. 6.5 mmol) in NH3 (30 mL) at -78 °C. The reaction mixture was stirred for 3 h and 1.0 M NH4Cl aqueous solution (20 mL) was carefully added. The stopper on the reaction vessel was removed and allowed to ambient temperature. After evaporation of NH3, the aqueous phase was extracted with dichloromethane and dried over MgSO4. The residue was purified by chromatography on silica gel using the eluent (hexane/ethyl acetate = 3: 1) to give the product as a yellow crystal (0.45 g, 50%): mp 164-165 °C; 1H NMR (300 MHz, CDCl3) (8.60 (br, 1H), 7.30

128 (s, 1H), 7.16 (m, 2H), 7.02 (dd, J = 4.9, 3.8 Hz, 1H), 6.55 (d, J = 3.3 Hz, 1H), 4.30 (m, 2H), 4.22 (m, 2H); UV-Vis (ACN) λmax 394 nm (29100 M-1 cm-2). FT-IR (CDCl3) 3460, 2989, 2930, 2202 (CN), 1575, 1538, 1343 cm-1; HRMS (FAB) (M+) calcd for C13H10N2O2S 258.0463, found 258.0457; Anal. Calculated for C13H10N2O2S: C, 60.45; H, 3.90; N, 10.85. Found: C, 60.35; H, 3.89; N, 10.91. 1-cyano-2-(2-(3,4-ethylenedioxypyrrolyl))-1-(2-(3,4ethylenedioxythienyl))vinylene (EDOT-CNV-EDOP). (KZ) This compound was prepared in analogy to Th-CNV-EDOP except the BOC group was advantageously removed during the condensation. Yellow crystals were obtained after chromatography (1.20 g, 75%); mp 148 °C (decomp); 1H NMR (300 MHz, CDCl3) (8.60 (br, 1H), 7.34 (s, 1H), 6.49 (d, J = 3.8 Hz, 1H), 6.25 (s, 1H), 4.40-4.15 (m, 8H); UV-Vis (ACN) λmax 401 nm (27600 M-1 cm-2). FT-IR (CDCl3) 3459, 2989, 2934, 2859, 2202 (CN), 1569, 1531, 1459, 1344 cm-1; HRMS (FAB) (MH+) calcd for C15H13N2O4S 317.0596, found 317.0601; Elemental analysis calculated for C15H12N2O4S: C, 56.95; H, 3.82, N, 8.86. Found: C, 56.44; H, 3.89; N, 8.61.

CHAPTER 4 THE ROLE OF INTERGAP REDOX STATES IN CONJUGATED POLYMER REDUCTION PROCESSES: THIENYL AND EDOT CONTAINING FLUORENONES 4.1 Introduction Fluorenes, and their polymeric counterparts and derivatives have become an integral aspect of organic polymer LED and LEC construction1 due to their high photoluminescence and electroluminescence quantum yields and thermal stability.2 Poly(fluorenes) have been used as electron transport materials in light emitting devices and fluorene derivatives have been useful as the active layer in blue emitting LEDs due their high band gap (ca. 3 eV) which arises from simultaneously exhibiting a high LUMO and a low HOMO. Structurally, fluorenes and their derivatives offer facile functionalization about the 2 and 7 positions which can be used as sites for conjugation extension3 (Figure 4-1 BPyrA-FC6) and fixture of an electroactive (Figure 4-1 BiPyrA-FFc) or inductive group (Figure 4-1 A, B) effectively makes the fluorene a tunable planarized biphenyl.4 Figure 4-1 shows fluorenone derivatives with nitro and cyano substituents at the 9 positions

1. A review of polymers for OLED applications: Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem. Intl. Ed. 1998, 37, 402. 2. (a) Pei, Q.; Yang, Y. J. Am. Chem. Soc. 1996, 118, 7416. (b) Yoshida, M.; Fujii, A.; Ohmori, Y. Yoshino, K. Appl. Phys. Lett. 1996, 69, 734. (c) Grell, M.; Long, X.; Bradeley, D. D. C.; Inbasekara, M. Woo, E. P. Adv. Mater. 1997, 9, 798. 3. (a) Justin Thomas, K. R.; Lin, J. T.; Lin, Y.-Y.; Tsai, C.; Sun, S.-S. Organometallics (ASAP Received January 18, 2001). (b) Justin Thomas, K. R.; Lin, J. T.; Lin, H.-M.; Chang, C. P. Chuen, C.-H. Organometallics 2001, 20, 557-563. 129

130

O O

O

H17C8 C8H17

S

S

O

S

S

6x

BBTPE-FC8 R

R

S

S

NC

CN H17C8 C8H17

NO2

O2N

CN

NC

O

NO2

A

NO2

NO2

B

C

NO2

x

H O

N N H21C10 C10H21

N

N

Fe

N

H13C6 C6H13 S

2PA NLO

BPyrA-FC6

BiPyrA-FFc

Figure 4-1. A structural survey of existing aromatic dicyanomethylidene and related fluorene derivative molecules and polymers. having multiple reversible electrochemical half waves (E1/2) that are accessible in air.4 Fluorene derivatives are also able to be difunctionalized at the 9 position with either solubilizing groups or inductive groups capable of tuning the biphenyl electronic character but not participating in effective conjugation through the 2,7 positions. The sum of these functionalization sites makes fluorenes able to be highly electron accepting with suitably electronegative side groups making them desirable as acceptor moieties in donor-acceptor molecules, polymers and complexes. Both thiophene and EDOT fluorene monomers have been chemically polymerized to form soluble materials with either di-ethyl, -hexyl and -

4. (a) Perepichka, I. F.; Popov, A. F.; Orekhova, T. V.; Bryce, M. R.; Andrievskii, A. M.; Batsanov, A. S.; Howard, J. A. K.; Sokolov, N. I. J. Org. Chem. 2000, 3053-3063. (b) Mysyk, D. D.; Perepichka, I. F.; Perepichka, D. F.; Bryce, M. R.; Popov, A. F.; Goldenberg, L. M.; Moore, A. J. J. Org. Chem. 1999, 64, 19 6937-6950. (c) Perepichka, I. F.; Kuz’mina, L. G.; Perepichka, D. F.; Bryce, M. R.; Goldenberg, L. M.; Popov, A. F.; Howard, J. A. K. J. Org. Chem. 1998, 63, 6484-6493.

131 octyl side chains at the 9 position (Figure 4-1 BBTPE-FC8 and Figure 4-2 EDOT-FC8 and PBEDOT-FC8).5 The inclusion of fluorene containing materials in the increasingly important area of two-photon absorbing NLO materials (Figure 4-1 2PA NLO) underscores this chromophores utility in light harvesting and generating applications.6 O

O

O

O

H17C8 C8H17

H17C8 C8H17

S

S

S

x O

EDOT-FC8 NC

O

PBEDOT-FC8 NC

CN

CN

S S

S

x

S

S

z O

PCDM

yx O

PCDM-co-EDOT

Figure 4-2. EDOT-Fluorene model and polymer along with PCDM and PCDM-co-EDOT

In light of the ease and variety of functionalization possibilities and our experience modifying chemically rich multifunctional molecular fragments with terminal EDOT moieties to aid electropolymerization, it seemed natural to explore the use of 2,7fluorenone EDOT and thiophene derivatives as electropolymerizable monomers for our multicolor electrochromic, n-type dopable and band gap control goals. Continuing with the theme of electropolymerizable cyano-containing groups started in Chapter 3 and absent the

5. (a) Larmat, F.; Reynolds, J. R.; Reinhardt, B. A.; Brott, L. L.; Clarson, S. J. 1997, 36273636. (b) Donat-Bouillud, A.; Lévesque, I.; Tao, Y.; D’Iorio, Beaupré, S.; Blondin, P.; Ranger, M.; Bouchard, J.; Leclerc, M. Chem. Mater. 2000, 12, 1931-1936. (c) Belletête, M. Morin, J.-F.; Beaupré, S.; Ranger, M.; Leclerc, M.; Durocher, G. Macromolecules, 2001, 34, 2288-2297. 6. (a) Belfield, K. D.; Hagan, D. J.; Van Stryland, E. W.; Schafer, K. J.; Negres, R. A. Org. Lett. 1999, 1, 10, 1575-1578. (b) Belfield, K. D.; Scafer, K. J.; Mourad, W.; Reinhardt, B. A. J. Org. Chem. 2000, 65, 15, 4475-4481. and references therein.

132 constraint to make soluble polymers to test the viability of these ideas, the natural acceptor for the 9 position of fluorenone was the planar, highly electron withdrawing dicyanomethylene group. This group has been used successfully in both the PCDM family of polymers7 (Eg ~ 0.8 eV, Figure 4-2) which has been reported to copolymerize with EDOT to afford co-polymers with a band gap near 0.1 eV (Figure 4-2 PCDM-co-EDOT).8 The CDM core itself is reminiscent of an electron rich fluorenone because the conjugation path has the same resonance relationship to the tuning dicyanomethylene group. Alternate choices include the ketone precursor (Figure 4-3 molecules 7 and 8), or an ethyl or propyl ketal derivative of this which has found success in PCDM applications.9 While the fluorenone derivatives proposed here are not expected to be superior (in the sense of low band gap) to the previously reported CDM containing polymers due to the more extreme frontier orbital positions of poly(fluorenone) materials (HOMO in excess of 1 V, LUMO in excess of -2 V) compared to PCDM (HOMO ca. 0.3 V, LUMO ca. -0.5 V), band mixing is expected to bring the valence and conduction bands (polymer HOMO and LUMO) within an electrochemically accessible range for device inclusion. Despite the observation that sulfur containing heterocycles tend to quench photo- and electroluminescence yields,10

7. (a) Lambert, T. L.; Ferraris, J. P.; Chem Commun. 1991, 752-754 also, 1268-1270. (b) Neef, C. J.; Brotherston, I. D.; Ferraris, J. P. Chem. Mater. 1999, 11, 1957-1958. (c) Huang, H.; Pickup, P. G. Chem. Mater. 1999, 11, 1541-1545. (d) Salzner, U.; Kiziltepe, T. J. Org. Chem. 1999, 64, 764-769. 8. Huang, H.; Pickup, P. G. Chem. Mater. 1998 10, 2212-2216. 9. Kitamura, C.; Tanaka, S.; Yamashita, Y. J. Chem. Soc., Chem. Commun. 1994, 1585. 10. (a) Belletête, M.; Beaupré, S.; Beuchard, J.; Blondin, P.; Leclerc, M.; Durocher, G. J. Phys. Chem. B 2000, 104, 9118-9125. (b) Biczók, L.; Bérces, T.; Inoue, H. J. Phys. Chem. A 1999 3837-3842.

133 these monomers are expected to play a didactic role in our attempts to optimize new polymer’s redox states and controllable energy levels.

4.2 Monomer Synthesis and Properties R

R1

R

R

R1

R1

a, c

a, b S 1, 2

(Me)3Sn

Si(Me)3

S

3, 80 % 4, 90 %

Si(Me)3 S 5, 84 % 6, 86 %

1,3,5,7,9: R = R1 = H (Thiophene) 2,4,6,8,10: R, R1 = -OCH2CH2O- (EDOT) R1 (Me)3Si

R S

NC

CN

R

d

R1

R1

R

R

R1

O S

9 = BTh-DCF (89 %) 10 = BEDOT-DCF (92 %)

Si(Me)3

e

(Me)3Si

S

S

Si(Me)3

7, 86 % 8, 78 %

Figure 4-3. Synthesis of fluorenone monomers. a. n-BuLi, THF/hexanes, -78 0C, 2 h. b. Me3SiCl, -78 ºC 1h to r.t, 2 h. c. Me3SnCl, THF, -78 ºC, 1 h to r.t., 2 h. d. 2,7Dibromofluoren-9-one, DMF or PhMe. Pd(Cl2(PPh3)2 (cat.), 100 ºC, 36 h or 4 h. e. CH2(CN)2, PyH, r.t., 24 h. The synthesis of BTh-DCF (9) and BEDOT-DCF (10) monomers is depicted in Figure 4-3. In this synthetic sequence, thiophene and EDOT are lithiated at the 2-position with n-butyllithium. This anion is trapped with chlorotrimethylsilane to afford the TMS heterocycles 3 and 4. These intermediates need not be isolated and the 5-position can be subsequently lithiated in one pot and trapped with chlorotrimethylstannane to afford the compounds 5 (thiophene) and 6 (EDOT) which are suitable for Stille coupling. Vacuum distillation affords the products which are colorless crystalline solids. The trimethylsilyl group was introduced to counteract the efficient solid state packing and thus insolubility of the target molecules in this work and is known to be both compatible with electropolymerization and have beneficial effects for film deposition on ITO glass.11 The

11. Sotzing, G. A., Reynolds, J. R. Adv. Mater. 1997, 9, 795-798.

134 key step in this synthesis is the bis(triphenylphosphine)palladium(II)dichloride catalyzed Stille coupling reaction joining two equivalents of either 5 or 6 with 2,7dibromofluorenone. The products of these reactions are orange to red and are subsequently reacted with malononitrile under Doebner modified conditions of the Knoevenagel condensation to afford the target compounds 9 (88%) and 10 (82% last step). With the presence of the solubilizing TMS groups, these compounds are soluble in nonpolar solvents such as chloroform, methylene chloride and toluene. The monomer colors are purple (BThDCF, 9)12 and light blue (BEDOT-DCF, 10)13 in dilute solution and have high molar extinction coefficients (ca. 700,000 M-1 cm-1 range) making them deeply colored when concentrated.1H and 13C NMR spectra of compounds 3-10 are consistent with the proposed structures. In these spectra, satellites due to coupling between hydrogen and carbon atoms and 29Si, 117Sn and 119Sn isotopes are visible.14 The values of these coupling constants were especially helpful in the assignment of the signals to the hydrogen and carbon atoms in compounds 5 and 6. Monomer UV-Vis characterization was performed in methylene chloride and the relevant spectral information is reported in Figure 4-4. The differences in monomer color

12. CIE color coordinates for the monomers in methylene chloride solution. BTh-DCF [132 mM]: Luminance = 377, x = 0.257, y = 0.250. 13. BEDOT-DCF [78 mM]: Luminance = 580, x = 0.257, y = 0.329. 14. (a) Harris, R. K.; Kennedy, J. D.; McFarlane, W. Group IV-Silicon, Germanium, Tin and Lead. In NMR and the Periodic Table; Harris, R. K. and Mann, B. E. Ed.; Academic Press: London, New York, San Francisco, 1978; pp. 310-330, 342-366 and references within. (b) Harrison, P. G. Investigating Tin Compounds Using Spectroscopy. In Chemistry of Tin; Harrison, P. G. Ed.; Blackie & Son Ltd.: Glasgow, London, 1989; pp. 71-81 and references within. (c) Davies, A. G. Organotin Chemistry; VCH: Weinheim, New York, Basel, Cambridge, Tokyo, 1997; pp. 18-24 and references within.

135 are due to a roughly 10 nm shift in absorption onset when increasing the electron rich character of the perimeter heterocycle. Additionally, the spectra for BEDOT-DCF displays three discernible peaks. Despite the high molar absorptivity of ca. 750,000 mole-1 cm-1 of the main peaks, a broad band is observed for each monomer near 600 nm

800000 NC (Me)3Si

CN

S

S

Si(Me)3

700000 BTh-DCF λmax = 335 nm, 755,000 onset = 400 nm

Molar Absorptivity

600000 500000

O (Me)3Si

O

NC

CN

S

O

O

S

Si(Me)3

400000 BEDOT-DCF λmax = 339 nm, 736,000 367 nm, 721,000 386 nm, 597,000 onset = 410 nm

300000 200000 100000 0 300

400

500

600

700

800

Wavelength /nm

Figure 4-4. Monomer UV-Vis spectra in methylene chloride for BEDOT-DCF and BThDCF.

X-ray quality crystals of BEDOT-DCF were obtained by vapor diffusion of pentane into a small vial of monomer dissolved in benzene. After several days large blocks of dark diffracting BEDOT-DCF were observed. The x-ray crystal structure indicates that the molecule is close to planar with a molecule of benzene incorporated into the unit cell. BEDOT-DCF is slightly bowed and the conformation of the EDOT rings relative to the fluorenone are in the anticipated orientation. Coordinates are available in Appendix B.

136 BTh-DCF does not crystallize under these conditions or similar conditions. Much like BEDOT-CNV in the previous chapter, all the methods in Chapter 2 were attempted using similar extensive solvents and solvent combinations.

O TMS

O

NC

CN

O

O S

S

TMS

BEDOT-DCF

Figure 4-5. X-ray crystal structure for BEDOT-DCF crystallized by vapor diffusion from benzene/pentane.

4.3 PBEDOT-DCF and PBTh-DCF Synthesis, Electrochemistry and Spectroelectrochemistry The poor solubility of these monomers in ACN suggested that initial attempts to electrochemically deposit films of PBTh-DCF and PBEDOT-DCF be attempted in methylene chloride and binary combinations of methylene chloride and ACN. Attempts at deposition by cyclic voltammetric, galvanostatic or potentiostatic methods in ACN were unsuccessful owing to the high potential necessary to oxidize the monomer (>1.5 to 2 V vs. SCE in all cases) rendering any polymer formed at this potential electrochemically inactive.

137 A survey of available solvents suggested that toluene be combined with ACN to form a mixture that could simultaneously solvate the monomer and electrolyte while not being prone to oxidation under the electrochemical conditions necessary to effect electropolymerization. This was indeed the case for BTh-DCF and BEDOT-DCF which can be deposited optimally onto a Pt button (A = 0.02 cm2) from a solution of 4:6 toluene:ACN [0.1 M TBAP] (BTh-DCF) or 6:4 toluene:ACN [0.1 M TBAP] (BEDOT-DCF).15 Given the scarce use of toluene in electrochemical systems and near absence from conjugated polymer systems, no explanation for the utility of toluene and failure of methylene chloride is immediately obvious. Deposition proceeds by cyclic voltammetry, galvanostatic or potentiostatic growth with galvanostatic growth used to deposit all of the films on Pt button electrodes and potentiostatic growth used to form films on both ITO and Pt/IME electrodes. Galvanostatic electrochemical deposition of PBEDOT-DCF on a Pt button at 10 µA (0.5 mA cm-2) (Figure 4-6 B) is typical of conjugated polymer depositions at constant current. The potential profile observed is that of a potential peak to 1.1 V vs. SCE within the first 10 seconds followed by a rapid drop to < 1 V and a final settling near 1 V after 250 seconds. This correlates to a 2.5 mC film (0.125 mC cm-2) which typically results in a polymer that is both thick enough to have currents in the several tens of microamps (tens of mA cm-2 on 0.02 cm2 Pt buttons) during doping/undoping and thin enough to enable facile ion transport. The limiting potential (Elim) at the end of this polymerization is used throughout this work as a metric for both comparing deposition parameters and

15. BTh-DCF can also be deposited from the 6:4 toluene:ACN solution but is not as electroactive as that derived from the 4:6 ratio.

138 determining starting points for further electrochemical experiments such as IME deposition for in situ conductance. After electrochemical deposition, the polymer film is washed with toluene then ACN (~ 2 mL) and placed in a solution of ACN 0.1 M TBAP. Figure 4-6 A shows the CV obtained from the galvanostatic polymerization of BEDOT-DCF described above. The polymer undergoes the typical broad oxidation waves (approximate E1/2 = 0.65 V vs. SCE, current density multiplied by 5 for comparison to reductive scan rate dependence data) observed for nearly every electroactive polymer. The CV displayed in Figure 4-6 A is created by first finding the peaks in the CV indicating complete doping, then applying a 20 wave double potential step between the p-type doped and neutral potentials in order to break in the polymer. The p-type doped polymer adopts an open circuit potential (OCP, Voc) of ca. +0.25 to +0.35 V indicating the neutral form is the preferred form. Scanning cathodically of +0.1 V vs. SCE, at least two reductions are observed, accessing the second of which rapidly degrades peak current during CV (a measure of polymer electroactivity) and polymer life. The first reduction (E1/2 = -0.6 V vs. SCE) can be repeatedly accessed electrochemically without significant effects on polymer switching lifetime. Scan rate dependence (Figure 4-6 C) of this reductive electroactive process indicates that the peak currents scale linearly (10 mV sec-1 to 200 mV sec-1) with the scan rate, a result consistent with an electrode adhered film with diffusion parameters suitable for ion flux at these speeds (i.e. the redox process is not diffusion limited over this regime). Electrochemical results for PBTh-DCF are nearly identical with a slightly anodic p-type doping E1/2 (~0.7 vs. SCE) and identical shape in the reductive region. DPV results (Figure 4-6 D) are also typical of most conjugated polymers and appear similar in form to the cyanovinylenes discussed in Chapter 3 displaying a plateau near the

139 8 A 1.2

E /V vs SCE

200 mV/sec 150 mV/sec 100 mV/sec

6 4

1.0 0.8

B

0.6

J /mA cm-2

0.4 0.2

2 10 mV/sec

0

50 100 150 200 250 Time /Seconds

X5

0

8

Jpeak /mA cm-2

6

-2 -4

4 2

C

0

20 mV sec-1

-2 -4

TBAP

0

20 40 60 80 100 120 mV sec-1

D O

O

NC

CN

O

S

O

S

x

5

JDiff /mA cm-2

PBEDOT-DCF

0

-5 LiClO4 TBAP -10 -1.0

-0.5

0.0

0.5

1.0

Potential /V vs SCE Figure 4-6. Electrochemistry (CV) for PBEDOT-DCF (A) on a Pt button (A = 0.02 cm-2) when deposited by galvanostatic deposition as in (B). (C) Scan rate dependence for reduction and (D) DPV for PBEDOT-CNV in TBAP (line) and LiClO4 (dash). OCP which slopes sharply but does not peak on the anodic side of the OCP and slopes sharply and peaks on the cathodic side of the OCP. DPV traces are shown in Figure 4-6 D for PBEDOT-DCF in ACN with either 0.1 M TBAP (solid line) or 0.1 M LiClO4 (dashed

140 line). When changing electrolytes, the polymer film was rinsed with the second electrolyte and switched 20 times between the potentials outside of the CV limits to ensure complete transformation to the new dopant ions. The reductive side of the TBAP-switched film has a peak at -0.46 V vs. SCE scanning cathodically and a peak at -0.7 V scanning anodically. The LiClO4-switched films have peaks at -0.60 V and -0.89 V scanning cathodically and anodically respectively. This yields an estimated E1/2 for the PBEDOT-DCF film switched in TBAP of -0.6 V and -0.75 V for the film switched in LiClO4 which is comparable, but not identical, to the CV-determined E1/2 of -0.6 V in TBAP. DPV is a simple electrochemical technique to estimate band gap because of the definition in the onsets compared to CV (see Chapter 2.7) and these DPV results suggest a band gap of ca. 0.55 eV if the difference between the onsets of the DPV traces are considered and the reduction is assumed to be due to n-type doping. PBTh-DCF electrochemistry in ACN/TBAP indicates the same relatively anodic (ca. -0.5 V) reductive cyclic voltammetric process as PBEDOT-DCF. Figure 4-7 shows these CV results and the DPV results. Qualitatively, the ratio between the p-type doping process in the CV compared to the n-type doping process in the CV is reversed compared to the analogous regions of in the DPV. This polymer is generally unstable and even though the oxidative process can be pushed to more anodic potentials, the response quickly degrades. Spectroelectrochemistry is often the most reliable determinant of band gap because only one peak onset must be estimated and it is not necessary to determine whether a particular peak is due to the band gap or a mid-gap state. Figure 4-7 shows the spectroelectrochemistry for PBEDOT-DCF in ACN/[0.1 M TBAP] degassed with Ar. This

141

6

A. PBTh-DCF CV

NC

4

CN

S

S

x

J mA cm-2

2 0 -2 -4 15

B. PBTh-DCF DPV

Jdiff mA cm-2

10 5 0 -5 -10 -15 -1.0

-0.5

0.0

0.5

1.0

1.5

Potential /V vs SCE Figure 4-7. PBTh-DCF electrochemistry in TBAP/ACN. (A) CV of PBTh-DCF as deposited from ACN/TBAP galvanostatically (10 µA, 250 sec). (B) DPV results. film was prepared by potentiostatic deposition of BEDOT-DCF at 1.17 V vs. SCE until a charge of 25 mC cm-2 had passed (in this case 50 to 60 seconds). The film was rinsed well in both toluene and ACN. At +0.37 V vs. SCE or below, the polymer adopts a reproducible spectrum characteristic of the neutral form where a gently sloping trace to near 2 eV (620

142 nm) followed by a rapid rise to a peak at 2.7 eV (460 nm) and another rise to a peak at 3.7 eV (335 nm) is evident. As the potential is stepped up in 0.5 V increments, the peak at 2.7 eV attributed to the π to π* transition decreases and eventually becomes a trough at the same energy while a peak at 2.2 eV (560 nm) grows in. This can also be visualized as a shifting of what was originally the π to π* to lower energy and absorbance. The peak at 3.7 eV (335 nm) undergoes almost no change in absorbance during potential switching and is attributed to monomer absorbance (λmax = 339 nm) remaining in the polymer. This can be due to one of several things. The most likely explanations for this are absorbance due to trapped monomer in the film or secondly, to states or chromophores that do not delocalize in the polymer and disappear. The rinsing in a good solvent for the monomer (toluene) presumably takes care of the trapped monomer in a film this thin (as no absorbances due to monomer have been observed in other films in this work following the same rinsing procedure) but this possibility cannot be entirely ruled out. Duplication of this spectroelectrochemical experiment produced identical results. Attempts to electrosynthesize redox active films of PBTh-DCF on ITO were unsuccessful as an optically inactive brown film similar in color to PBEDOT-DCF was created at long deposition times where it was necessary to raise the potential more anodic of 1.5 V in order to get any polymer to deposit in less than 20 minutes. Colorimetry on the same PBEDOT-DCF film used for spectroelectrochemistry indicates that the film switches from a brown neutral state 0.17 V (near the Voc) to a purple oxidized state at 1.17 V. Figure 4-8 shows this data for two PBEDOT-DCF films, Film 1 being thicker than Film 2 (25 mC cm-2) since Film 1 was deposited to a charge density of nearly 35 mC cm-2. A line drawn from the color coordinates of the light source to the locus

143 1.4 O

1.2

O

NC

CN

O

S

O

S

x

PBEDOT-DCF 1.0

Absorbance

0.8

0.6

0.4

1.27 V

0.2

0.0

0.37 V 0.5 V increments E vs SCE

-0.2 1

2

3

4

Energy /eV

Figure 4-8. PBEDOT-DCF spectroelectrochemistry in ACN/[0.1 M TBAP] degassed in Ar. PBEDOT-DCF was stepped from 0.37 V vs. SCE to 1.27 V vs. SCE in 0.5 V increments. of the CIE 1931 color scale indicates that the color purity of the neutral polymer is qualitatively greater than that of the doped form as the neutral form is nearer to the locus traced in the figure. Spectroelectrochemical or colorimetry measurements are unavailable for the reduced forms of this polymer due to lack of a sufficient cell to exclude O2 and H2O. Qualitative observation of a polymer film on ITO viewed through the dry box case indicates

144

520

O

530

0.8

540

510

NC

CN

O

S

O

S

x

PBEDOT-DCF

550 560

0.6 500

CIE y

O

570

0.17 V vs SCE

580

5000 K Source

0.4

590 600 610

Film 2 Film 1

490

780

1.17 V vs SCE

0.2 480 470 380

0.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CIE x Figure 4-9. Colorimetry of PBEDOT-DCF in ACN/[0.1 M TBAP]. Film 1 was deposited to 35 mC cm-2, Film 2 was deposited to 25 mC cm-2 from 4:6 ACN:toluene [0.1 M TBAP]. Colors are shown for each terminal film state for Film 1. that there is no color change in this polymer even when poising the polymer at a potential consistent with the unstable second reduction. As discussed in Chapter 2, spectroelectrochemical results coupled with in situ conductance data are the most reliable way to establish the energy levels and band gap of a polymer electrochemically. Figure 4-10 shows the electrochemical results obtained on a 5 µm Pt IME electrode. PBEDOT-DCF was deposited potentiostatically at the IME

145

6

Current /µA

4

80 70 60 50 40 30 20 10 0 -10

Deposition on IME

Drain Current /µA

8

2

A

B Gate Off

EGate = 1.27 V vs SCE 0

200 400 600 800 1000 Time /Seconds

0 -2 O

-4

O

NC

S

-6

CN

O

O

S

x

PBEDOT-DCF

Pseudo Conductivity /mS

TBAP TEAP

8 6

C

C1

C2

4 40 µS 2 0 TEAP

-1.5

2 µS

C3

-1.0

-0.5

0.0

0.5

1.0

1.5

Potential /V vs SCE

Figure 4-10. PBEDOT-DCF electrochemistry on an IME and in situ conductance in ACN. (A) Deposition on Pt-IME (5 mm) at a gate potential of 1.27 V vs. SCE as shown in (B). (C) In situ conductance results for reduction in TBAP (C1), p-type doping in TBAP (C2) and reduction in TEAP (C3). Experimental conditions are described in Chapter 2. working electrode analogous to the gate electrode in Figure 2-10 at a polymerization potential of 1.27 V vs. SCE in Figure 4-6 B. The polymerization potential was established from Elim in the Pt button galvanostatic growth data (ca. 1.0 V vs. SCE). This potential was then applied for 300 seconds, if no drain current was observed between the two IMEs

146 having an offset potential of 10 mV, the polymerization potential was increased by 50 mV and the 300 second trial deposition was repeated. This process was continued until drain current (Edrain = 10 mV) was detected and reached approximately 50 µA indicating the film had shorted the electrodes to a sufficient conductance to avoid lead resistance. The gate potential was then allowed to reach open circuit while the drain current was measured for another 200 seconds. This drain current typically jumps when the gate potential is removed and then quickly plateaus. It should be noted that this behavior occurs in only this family of monomers of all the monomers studied in this work. Figure 4-10 A shows the CV obtained on the IME at a scan rate of 20 mV sec-1. The CV was started at a potential where the polymer was neutral and scanned cathodically first which is the reason for the pre-peak in the p-type doping wave (See Chapter 3). This CV correlates quite well with the electrochemical results obtained on a Pt button (Figure 4-6) both in the shape and intensity of the electrochemical waves. Figure 4-10 C show several in situ conductance results. The order these results are presented in is described by the arrows at the top of Figure 4-10 C. The polymer as deposited was broken in over 20 double potential square wave potential steps encompassing both the p-type doped and reduced forms. The polymer was then returned to a potential where the neutral state predominates (-0.2 V) and in situ conductance data was collected first for the reductive process in TBAP, then the p-type doping process in TBAP, followed by the reductive process in TEAP. TEAP was selected as an alternate electrolyte since it contains a smaller cation that maintains the same low electrophilicity as TBA+. Additionally, CV switching and DPV results in TEAP are comparable to those in TBAP while the LiClO4 results show a substantial decrease in the current and differential current compared to the TBAP standard. The initial TBAP results for reduction (Figure 4-

147 10 C1) indicate a small relative peak in conductance near -0.6 V and a large conductance peak near -1.15 V. Both of these peaks are symmetrical and quite different in shape than typical oxidative p-type doping waves. The p-type doping conductance profile for PBEDOT-DCF (Figure 4-10 C2) mirrors the typical sigmoidal shape seen in other conducting polymers throughout this work where there is a sharp onset and rise to peak after which the conductance slowly drops. In order to avoid over-oxidation of the polymer film, the experiment is reversed when this drop in conductance is evident from the decreasing slope of the i-V line. The return path for PBEDOT-DCF nearly exactly mirrors the forward path. Once the conductance returned to that of the neutral form, the polymer was rinsed with ACN/[0.1 M TEAP] and cycled ten times (neutral form to reduced form) in this electrolyte to remove any remaining TBAP. conductance results in TEAP (Figure 410 C3) indicate substantially lower conductance than seen in the TBAP results over the same potential regime. However, similarly shaped symmetrical peaks at identical potentials to those in TBAP are observed, the first of these corresponding almost exactly to the stable polymer reduction observed in Figure 4-6. The reduction in conductance magnitude may be due to polymer breakdown or may be the result of electron-counter ion effects that limit charge carrier mobility. Previous results on cyano-containing polymers (Chapter 3) indicate that the peak magnitude of the reductive event on the forward scan decreases slightly in magnitude on subsequent experiment repetition. This is not likely to be the cause of the scale change from 40 µS (Figure 4-10 C1) to 2 µS (Figure 4-10 C3) observed on the change from TBAP to TEAP. Additionally, it should be noted that the method for determining conductance used throughout this work is excellent for establishing relative polymer conductance at equilibrium states but does not contain information on absolute

148 conductance (S cm-1) because the data is reported as conductance in Siemens (S). However, the amount of charge passed during the double potential square wave break-in and a visible estimate of the amount of polymer on the electrode allows an estimation of the relative conductance of the polymer compared to others in this work. Using such a qualitative approach it is apparent this family of polymers has the lowest conductance in the p-type doped state of any polymer in this work. This is especially true of PBTh-DCF (data not shown) for which it is difficult to deposit films with suitably high conductance for this experiment. Despite this, the data for PBTh-DCF indicates a remarkable similarity to PBEDOT-DCF in conductance onset (ca. 0.6 V vs. SCE compared to 0.5 V for PBEDOTDCF). What is clear from the polymer electrochemistry is that it is very un-polymerlike. Specifically, the peaks are more similar to those of a quasi-reversible monomer diffusing from solution to an electrode rather than an electroactive polymer undergoing structural changes concomitant to charge carrier formation. An investigation of the monomer electrochemistry was undertaken to understand the differences.

4.4 Monomer Electrochemistry The discrepancies between the spectroelectrochemically deduced band gap and the electrochemically determined band gap led to an investigation of the monomer electrochemical properties. The edge of the HOMO was accurately established from the onset for conductance in Figure 4-9. The edge of the LUMO should then be Eσ onset - Eg = 0.5 V - 1.8 V = -1.3 V vs. SCE. However, there are two electrochemical couples that occur more anodic of this region with the more cathodic of these two couples leading to polymer degradation. Scanning more cathodic of these two couples leads to an inactive polymer.

149 Background scans on the electrolyte used indicate no redox activity and the redox couple at -0.6 V can be repeatedly accessed as described in Section 4-2 and has a scan rate dependence consistent with an electrode adhered redox couple. These data suggest that there is an electroactive component from the monomer remains in the polymer upon polymerization. 0.8

0.4

J /mA cm-2

0.2

J /mA cm-2

0.6

0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 2

O

B (Me)3Si

O

NC

CN

S

O

O

S

Si(Me)3

BEDOT-DCF A 4 6 8 10 12 (Scan Rate /mV sec-1)1/2

14

0.0

a1

-0.2 a2 -0.4

-0.6

-0.8 -1.5

-1.0 Potential /V vs SCE

-0.5

0.0

Figure 4-11. Monomer electrochemistry (CV) and scan rate dependence for BEDOT-DCF. (A) CV at 20 mV sec-1 starting at -0.2 V saving second of two scans. (B.) Scan rate dependence for redox couple a1 over the range 10 mV sec-1 to 200 mV sec-1 viewed as the square root of scan rate. 4:6 ACN:toluene [0.1 M TBAP], 10 mM BEDOT-DCF.

Analysis of the scan rate dependence for monomer reduction can establish whether the electroactive species is electrode confined. The CV for the monomer BEDOT-DCF with TMS solubilizing groups does indeed show two nearly reversible redox couples as

150 shown in Figure 4-11 A. The scan rate dependence for the more anodic of the two redox couples (Figure 4-11 a1) in this figure is shown in Figure 4-11 B. The linear dependence of the peak currents for these processes vs. the square root of scan rate indicate that the redox active species is diffusing to the electrode rather than being confined at the electrode surface. Similar attempts to repeatedly scan over the range encompassing a2 in Figure 4-11 results in a fouled electrode. There is no color observed on the electrode surface at the conclusion of this experiment but the electrode activity is drastically suppressed for all redox processes. Figure 4-10 a1 has an E1/2 of -0.59 V and a ∆E of 150 mV while a2 has an E1/2 of -1.22 V and ∆E = 160 mV. 1.0 NC (Me)3Si

CN

S

S

A

Si(Me)3

0.5 BTh-DCF a1 0.0

J /mA cm-2

a2 -0.5 2.0

-1.0

J /mA cm-2

1.5

-1.5

200 mV/sec 150 mV/sec 100 mV/sec

C

B

1.0 10 mV/sec

0.5 0.0 -0.5 -1.0 -1.5 -2.0 2

-2.0 -2.0

4 6 8 10 12 (Scan Rate /mV sec-1)1/2

-1.5

14

-1.0 Potential /V vs SCE

-0.5

Figure 4-12. Monomer electrochemistry (CV) and scan rate dependence for BTh-DCF

0.0

151 The electrochemical characteristics of the BTh-DCF monomer are simultaneously superior in quality (∆E, linear scan rate dependence vs. the square root of scan rate) and nearly identical in terms of redox potentials and stability of the second redox couple to BEDOT-DCF. Figure 4-12 A shows the CV of these two redox couples accessed sequentially. Redox couple a1 has an E1/2 of -0.54 V and ∆E of 68 mV, exactly that of the theoretical peak-to-peak separation for a reversible system. a2 has an E1/2 of -1.17 V and ∆E = 98 mV. Additionally, for a2, the oxidation back to the mono-anion shows some flattening that indicates the BTh-DCF-2 anion may be the species involved in the degradation and electrode fouling. Figure 4-12 B shows the scan rate dependence graphically while Figure 4-12 C shows the linearity of the peak current vs. square root of scan rate.

4.5 Conclusions The monomers and polymers described in this chapter combine several molecular fragments which are excellent in particular applications of polymer optics and electronics because of their redox properties. Namely, the fluorenone core has excellent photo- and electroluminescent properties, EDOT has excellent electrochemical polymerization properties and the dicyanomethylene group is a highly effective electron poor fragment used to tune similar materials for low band gap applications. However, the sum of these parts does not make a family of materials that excels in these areas. Fluorescence measurements on the monomer and qualitative observations on the neutral polymer indicate ineffective light fluorescence for both. The characteristic ease of polymerization typical of EDOT containing molecules is not evident for these systems.

152 PBEDOT-DCF

-2.0 O

O

NC

S

CN

O

O

S

x

-1.5 Hypothetical LUMO/Conduction Band

2nd CN Reduction

Potential /V vs SCE

-1.0

-0.5

1.8 V (Spectral Band Gap)

1st CN Reduction

0

+0.5

HOMO/Valence Band

+1.0

Figure 4-13. Energy level diagram for PBEDOT-DCF Additionally, it appears that the band gap mixing described in Figure 3-2 is occurring in these systems but is not sufficient to bring the LUMO low enough in energy to cause the non-delocalized dicyanomethylene chromophore higher in energy than the two CN reductions. This, coupled with the high band gap of PBEDOT-DCF, is likely the reason why only redox conductance is observed in this polymer rather than a conductance profile characteristic of electronic type conductivity and doping. Figure 4-13 shows schematically the energy levels of the HOMO and LUMO relative to the two CN reductions observed in both the monomer and polymer. This indicates that, as the potential is scanned cathodically of open circuit, an electron is injected into each CN group on every molecule in the film. Upon further cathodic scanning, the hypothetical electrons that would delocalize and populate the conduction band would then be injected forcing there to be two to three

153 electrons on every repeat unit. While this is feasible and fairly common for solution electrode reactions, it seems highly unlikely that such a high charge concentration in a film would give rise to any modicum of stability. The conclusions this leaves with the designer of conjugated polymeric materials is that while it is possible to drastically shift the bands using alternating donors and acceptors, there needs to be some attempt made to match the energy levels. This concept seems to work with the cyanovinylenes in Chapter 3 because at least one band from the mixing polymer can be drawn inside the band gap of the original polymer. In the fluoreneones, the LUMOs for PEDOT and poly(fluorenones) are nearly the same (more cathodic than -2 V) and the HOMO of the original poly(fluorenone) would be more anodic of PEDOT.

4.6 Experimental Section General. Melting points were determined using a capillary apparatus and are uncorrected. The yields are given on pure products and were not optimized. The 1H NMR spectra were recorded at 300 MHz and the 13C NMR spectra were recorded at 75 MHz on a Varian VXR-300S and Varian Gemini-300 spectrometers respectively. If not otherwise stated CDCl3 was used as a solvent and Me4Si (δ=0 ppm) as internal standard. Elemental analyses were performed by Robertson Microlit Laboratories, Inc., Madison, New Jersey. Analytical thin layer chromatography tests (TLC) were carried out on Whatman silica gel plates (silica gel 60Å, fluorescent indicator UV254, layer thickness 250 µm) and visualized by UV lamp at 254 nm and in an iodine chamber. n-Butyllithium (1.6 M solution in hexanes), thiophene (99+%), trimethylsilyl chloride (99+%), trimethyltin chloride (1.0 M solution in tetrahydrofuran), 2,7-dibromofluoren-9-one (96%), dichlorobis(triphenylphosphine)palladium(II) (99.99%), malononitrile (99%) and cellulose microcrystalline

154 powder were from Aldrich and were used as received. 3,4-Ethylenedioxythiophene (EDOT) from Aldrich was distilled under reduced pressure prior to use. Pyridine from Fisher Scientific (certified A.C.S.) was used as received. Tetrahydrofuran (THF) used was freshly distilled from potassium-sodium benzophenone ketyl. Toluene used was freshly distilled from sodium. General procedure for the trimethylsilylation reaction of thiophene and EDOT to prepare 3 and 4. n-Butyllithium solution in hexanes (62.5 mL, 0.100 mole) was added via syringe to a stirred solution of thiophene derivative 1 or 2 (0.100 mole) in tetrahydrofuran (100 mL) at -78ºC under argon. Stirring was continued at this temperature for 2 h when trimethylsilyl chloride (13.3 mL, 0.105 mole) was added via syringe. The reaction mixture was stirred at -78ºC for 1 h, was allowed to reach room temperature and then stirred for 2 h at room temperature. All volatile materials were distilled off (in the case of product 3 under atmospheric pressure and in the case of product 4 under reduced pressure) and water (100 mL) was added to the residue. See below for individual differences in the isolation and purification of the products. Trimethyl(2-thienyl)silane (3). Prepared from thiophene (1) (8.41 g, 0.100 mole). The whole lot was obtained after addition of water to the crude mixture was extracted with diethyl ether (3 x 40 mL), the combined extracts were washed with water (40 mL portions) until neutral and dried over anhydrous magnesium sulfate. From the etheral extracts diethyl ether was distilled off under atmospheric pressure and the product was fractionated under reduced pressure giving a colorless liquid. Yield 12.48 g (80%), b.p. 68-69ºC/33 mm Hg (lit. 159-160ºC/748 mm Hg). 1H NMR [δ, ppm; J, Hz]: 7.58 (1H, dd, J5-4=4.6, J5-3=0.8, 5H), 7.26 (1H, dd, J3-4=3.3, J3-5=0.8, 3-H), 7.17 (1H,dd, J4-5=4.6, J4-3=3.3, 4-H), 0.32 (9H,

155 s, with satellites 2J(29Si-1H)=6.7, CH3Si-H). 13C NMR [reference CDCl3 (δ=77.0 ppm); δ, ppm; J, Hz]: 140.0 (2-C), 133.9, 130.3, 128.1, 0.0 (with satellites 1J(29Si-13C)=53.5, CH3Si-C). [3,4-(Ethylenedioxy)-2-thienyl]trimethylsilane (4). Prepared from 3,4(ethylenedioxy)thiophene (2) (14.22 g, 0.100 mole). From the suspension obtained after addition of water an insoluble solid was filtered, washed on a filtration funnel with water (3 x 30 mL), dried at room temperature under reduced pressure in a dessicator over anhydrous calcium chloride and fractionated under reduced pressure giving a colorless solid. Yield 19.35 g (90%), b.p. 97-101ºC/2.2 mm Hg, m.p. 48.5-52ºC. For C9H14O2SSi calculated C 50.43%, H 6.58%, S 14.96%; found C 50.39%, H 6.57%, S 14.90%. 1H NMR [δ, ppm; J, Hz]: 6.53 (1H, s, 5-H), 4.12-4.20 (4H, m, CH2-H), 0.28 (9H, s, with satellites 2J(29Si-1H)=6.9,

CH3Si-H). 13C NMR [reference CDCl3 (δ=77.0 ppm); δ, ppm; J, Hz]:

147.2 (3- or 4-C), 142.6 (3- or 4-C), 111.2 (2-C), 104.7 (5-C), 64.5 (CH2-C), 64.4 (CH2C), -0.8 (with satellites 1J(29Si-13C)=53.8, CH3Si-C). General procedure for the trimethylstannylation reaction of the trimethylsilyl derivatives 3 and 4. n-Butyllithium in hexanes (31.3 mL, 0.0500 mole) was added via syringe to a stirred under argon solution of trimethylsilyl derivative 3 or 4 (0.0500 mole) in tetrahydrofuran (100 mL) at -78ºC. Stirring at this temperature was continued for 2 h and after that trimethyltin chloride solution in tetrahydrofuran (52.5 mL, 0.0525 mole) was added via syringe. The reaction mixture was stirred at -78ºC for 1 h, then it was allowed to reach room temperature and stirred for further 2 h. From this reaction mixture, all volatile materials were removed under reduced pressure and water (50 mL) was added to the obtained residue. From this suspension, an insoluble solid was filtered, washed with water

156 (3 x 15 mL), dried at room temperature under reduced pressure in a dessicator over anhydrous calcium chloride and fractionated under reduced pressure giving a colorless solid. Trimethyl(5-trimethylstannyl-2-thienyl)silane (5). Prepared from trimethyl(2thienyl)silane (3) (7.62 g, 0.0500 m). Yield 13.41 g (84%), b.p. 87-88ºC/0.6 mm Hg, m.p. 52-54ºC. For C10H20SSiSn calculated C 37.64%, H 6.32%, S 10.05%; found C 37.61%, H 6.25%, S 9.83%. 1H NMR [δ, ppm; J, Hz]: 7.40 (1H, d, J3-4=3.1, with satellites 4 117,119

Sn-1H)=4.2 appeared as doublets coupled by J=3.1, 3-H), 7.29 (1H, d, J4-3=3.0,

J(

with satellites 3J(117Sn-1H)=23.0 and 3J(119Sn-1H)=23.9 appeared as doublets coupled by J=3.1, 4-H), 0.37 (9H, s, with satellites 2J(117Sn-1H)=55.0 and 2J(119Sn-1H)=57.6, CH3SnH), 0.32 (9H, s, with satellites 2J(29Si-1H)=6.8, CH3Si-H).13C NMR [reference CDCl3 (δ=77.0 ppm); δ, ppm; J, Hz]: 146.0 (with satellites 1J(29Si-13C)=65.3, 2-C), 143.0 (with satellites 1J(117Sn-13C)=363.0 and 1J(119Sn-13C)=380.2, 5-C), 136.1 (with satellites 2 117,119

J(

Sn-13C)=35.5, 4-C), 134.9 (with satellites 3J(117,119Sn-13C)=48.1, 3-C), 0.1 (with

satellites 1J(29Si-13C)=53.8, CH3Si-C), -8.2 (with satellites 1J(117Sn-13C)=356.2 and 1J(119Sn-13C)=372.2,

CH3Sn-C).

[3,4-(Ethylenedioxy)-5-trimethylstannyl-2-thienyl]trimethylsilane (6). Prepared from [3,4-(ethylenedioxy)-2-thienyl]trimethylsilane (4) (10.72 g, 0.0500 mole). Yield 16.13 g (86%), b.p. 121-124ºC/1.2 mm Hg, m.p. 66-72ºC. For C12H22O2SSiSn calculated C 38.22%, H 5.88%, S 8.50%; found C 38.24%, H 5.68%, S 8.23%. 1H NMR [δ, ppm; J, Hz]: 4.11-4.19 (4H, m, CH2-H), 0.34 (9H, s, with satellites 2J(117Sn-1H)=55.8 and 2J(119Sn-1H)=58.5, CH3Sn-H), 0.28 (9H, s, with satellites 2J(29Si-1H)=6.9, CH3Si-H). 13C

NMR [reference CDCl3 (δ=77.0 ppm); δ, ppm; J, Hz]: 148.7 (with satellites

157 2 117,119

J(

Sn-13C)=14.9, 4-C), 147.9 (with satellites 3J(117,119Sn-13C)=35.5, 3-C), 118.0 (2-

C), 114.8 (with satellites 1J(117Sn-13C)=349.3 and 1J(119Sn-13C)=366.9, 5-C), 64.5 (CH2C) -0.7 (with satellites 1J(29Si-13C)=53.8, CH3Si-C), -8.7 (with satellites, 1J(117Sn13

C)=359.6 and 1J(119Sn-13C)=375.6, CH3Sn-C). General procedure for the preparation of

Bis(trimethylsilylthienyl)fluorenones 7 and 8. A solution of the thiophene under argon, 5 or EDOT, 6 (0.01050 mole) in toluene (10 mL) was added via syringe to a stirred suspension of 2,7-dibromofluoren-9-one (1.69 g, 0.00500 mole) and dichlorobis(triphenylphosphine)palladium(II) (0.175 g, 0.00025 mole) in toluene (40 mL). The resulting suspension was stirred at room temperature for 10 min. and then heated at 100ºC. Progress of the reaction was monitored by thin layer chromatography (chloroform–hexane 3:1 v/v). After the reaction was completed the reaction mixture was cooled to room temperature, filtered through cellulose powder to remove catalyst and from the filtrate toluene was removed under reduced pressure. To the obtained residue a mixture toluene–hexane 1:4 v/ v (20 mL) was added and an insoluble solid was filtered and recrystallized yielding the product as a red solid. See below for individual differences in time of reaction and recrystallization solvent. 2,7-Bis(5-trimethylsilyl-2-thienyl)fluoren-9-one (7). Prepared from trimethyl(5trimethylstannyl-2-thienyl)-silane (5) (3.35 g, 0.01050 mole)) by heating for 5 h. Yield 2.10 g (86%) after recrystallization from toluene, m.p. 223-225ºC. For C27H28OS2Si2 calculated C 66.34%, H 5.77%, S 13.12%; found C 66.22%, H 5.87%, S 12.99%. 1H NMR [δ, ppm; J, Hz]: 7.81(2H, d, J1(8)-3(6)=1.4, 1- and 8-H), 7.61 (2H, dd, J3(6)-4(5)=7.8, J3(6)-1(8)=1.7, 3and 6-H), 7.31-7.38 (4H, m, 4-, 5- and thienyl 3- or thienyl 4-H), 7.18 (2H, d, J3-4 or J4-

158 3=3.5, 13C

thienyl 3- or thienyl 4-H), 0.35 (18H, s, with satellites 2J(29Si-1H)=6.8, CH3Si-H).

NMR [reference CDCl3 (δ=77.0 ppm); δ, ppm; J, Hz]: 193.1 (C=O), 148.1, 142.6,

140.8, 135.2, 135.1, 134.9, 131.6, 124.9, 121.4, 120.6, -0.2 (with satellites 1J(29Si13

C)=53.8, CH3Si-C). 2,7-Bis[5-trimethylsilyl-3,4-(ethylenedioxy)-2-thienyl]fluoren-9-one (8).

Prepared from [3,4-(ethylene-dioxy)-5-trimethylstannyl-2-thienyl]trimethylsilane (6) (3.96 g, 0.01050 mole) by heating for 4 h. Yield 2.36 g (78%) after recrystallization from a mixture toluene-hexane 1:1 v/v, m.p. 207.5-209ºC. For C31H32O5S2Si2 calculated C 61.56%, H 5.33%, S 10.60%; found C 61.36%, H 5.20%, S 10.61%. 1H NMR [δ, ppm; J, Hz]: 8.05 (2H, d, J1(8)-3(6)=1.5, 1- and 8-H), 7.78 (2H, dd, J3(6)-4(5)=8.1, J3(6)-1(8)=1.7, 3and 6-H), 7.42 (2H, d, J4(5)-3(6)=8.1, 4- and 5-H), 4.27-4.35 (4H, m, CH2-H), 4.19-4.27 (4H, m, CH2-H), 0.32 (18H, s, with satellites 2J(29Si-1H)=6.8, CH3Si-H). 13C NMR [reference CDCl3 (δ=77.0 ppm); δ, ppm; J, Hz]: 193.8 (C=O), 147.6, 141.9, 139.5, 134.7, 134.1, 131.3, 121.5, 121.0, 120.2, 109.7 (thienyl 5-C), 64.5 (CH2-C), 64.2 (CH2-C), -0.7 (with satellites 1J(29Si-13C)=53.9, CH3Si-C). General procedure for the preparation of Dicyanomethylenebis(trimethylsilylthienyl)fluorenes 9 and 10. A solution of malononitrile (0.13 g, 0.002 mole) in pyridine (2 mL) was added dropwise to a stirred solution of the ketone 7 or 8 (0.001 mole) in pyridine (30 mL) at room temperature. Stirring at this temperature was continued for 24 h and after pouring of the reaction mixture into water (100 mL) a solid precipitated. It was filtered, washed on a filtration funnel with water (3 × 10 mL), dried at room temperature under reduced pressure in a dessicator over

159 anhydrous calcium chloride and recrystallized yielding the product as a purple solid. See below for individual differences in recrystallization solvent. 9-Dicyanomethylene-2,7-bis(5-trimethylsilyl-2-thienyl)fluorene (9). Prepared from 2,7-bis(5-trimethylsilyl-2-thienyl)fluoren-9-one (7) (0.49 g, 0.001 mole). Yield 0.48 g (89%) after recrystallization from ethyl acetate, m.p. 267-269ºC. For C30H28N2S2Si2 calculated C 67.12%, H 5.26%, N 5.22%, S 11.94%; found C 66.90%, H 5.33%, N 5.20%, S 11.90%.1H NMR [δ, ppm; J, Hz]: 8.42 (2H, s, 1- and 8-H), 7.58 (2H, dd, J3(6)-4(5)=8.0, J3(6)-1(8)=1.4, 3- and 6-H), 7.27-7.41 (4H, m, 4-, 5- and thienyl 3- or thienyl 4-H), 7.17 (2H, d, J3-4 or J4-3=3.5, thienyl 3- or thienyl 4-H), 0.35 (18H, s, CH3Si-H).13C NMR [reference CDCl3 (δ=77.0 ppm); δ, ppm; J, Hz]: 160.7 (9-C), 147.4, 141.6, 140.4, 135.4, 135.2, 134.9, 131.6, 125.3, 123.7, 120.9, 113.1 (CN-C), 76.3 (dicyanomethylene-C), -0.1 (with satellites 1J(29Si-13C)=53.8,

CH3Si-C).

9-Dicyanomethylene-2,7-bis[5-trimethylsilyl-3,4-(ethylenedioxy)-2thienyl]fluorene (10). Prepared from 2,7-bis[5-trimethylsilyl-3,4-(ethylenedioxy)-2thienyl]fluoren-9-one (8) (0.60 g, 0.001 m). Yield 0.53 g (82%) after recrystallization from a mixture toluene-hexane 1:1 v/v, m.p. 260.5-262.5ºC. For C34H32N2O4S2Si2 calculated C 62.54%, H 4.94%, N 4.29%, S 9.82%; found C 62.53%, H 4.97%, N 4.29%, S 9.87%. 1H NMR [δ, ppm; J, Hz]: 8.83 (2H, d, J(1(8)-3(6)=1.2, 1- and 8-H), 7.56 (2H, dd, J3(6)-4(5)=7.9, J3(6)-1(8)=1.6, 3- and 6-H), 7.25 (2H, d, J4(5)-3(6)=8.1, 4- and 5-H), 4.27-4.35 (4H, m, CH2H), 4.18-4.27 (4H, m, CH2-H), 0.33 (18H, s, CH3Si-H). 13C NMR [reference CDCl3 (δ=77.0 ppm); δ, ppm; J, Hz]: 160.9 (9-C), 147.8, 140.0, 139.3, 134.5, 133.9, 130.2, 123.5, 120.2, 120.1, 113.2 (CN-C), 110.0 (thienyl 5-C), 75.6 (dicyanomethylene-C), 64.4 (CH2C), 64.2 (CH2-C), -0.7 (with satellites 1J(29Si-13C)=53.9, CH3Si-C).

CHAPTER 5 POLY(ALKYLENEDIOXYPYRROLES): AQUEOUS COMPATIBLE CONDUCTING POLYMERS WITH LOW FORMAL REDOX POTENTIALS 5.1 Introduction Chapters 3 and 4 detail the importance of VB and CB control for impacting band gap and n-type doping properties. In that work, substantial benefit was realized by appending a 3,4-ethylenedioxy group to a simple heterocycle such as thiophene to form EDOT. This change shifts the reduction potential to more cathodic potentials making these polymers easier to oxidize and thus more difficult to reduce. To this point there have been only cursory examinations of the effect the 3,4-ethylenedioxy addition has on poly(pyrrole’s) electronic properties. Poly(pyrroles) have been extensively studied1 and are interesting for a variety of applications. The most notable of these are applications that involve PPy’s high conductivity2 and ability to be electrodeposited and switched extensively in aqueous environments. The aqueous compatibility opens up an array of interesting biological applications involved in human health ranging from detection of DNA mismatches3 and human lymphocyte antigen groups, to the tissue engineering of nerve cell guidance channels. Korri-Youssoufi and Garnier appended an oligonucleotide

1. Street, G. B. Handbook of Conducting Polymers, Skotheim, T. A., Ed.; Dekker: New York, 1966; Vol 1. pp. 265-290. 2. Diaz, A. F.; Bargon, J. Handbook of Conducting Polymers, 1986, Skotheim, T. A., Ed.; Dekker: New York, Vol. 1; pp. 81-115. 3. Schafer, A. J.; Hawkins, J. R. Nature Biotech. 1998, 16, 33-39. 160

161 (5’CCT AAG AGG GAG TG3’) to a spacer attached to the three position of PPy (Figure 51 A) and found that it was electroactive in aqueous solution containing a variety of biological buffers including non-complimentary salmon DNA.4 Incubation with varying concentrations of the complementary strand (5’CAC TCC CTC TTA GG3’) and a noncomplimentary strand (5’GGT GAT AGA AGT ATC3’) of DNA indicates the oligonucleotide hybridization of the complimentary strand causes an easily observable change in the CV. This effect is based on the conformational modifications required upon doping while the noncomplimentary strand, which is incapable of hybridization, results in no change in the polymer electrochemistry. Saint-Aman and coworkers synthesized poly(pyrroles) functionalized at the N position with glucose (Figure 5-1 B) with the goal of preparing materials with enantioselective recognition properties that are capable of performing chiral electrosynthesis.5 While only transiently stable in aqueous perchlorate systems, this polymer is enantioselective to (1S)-(+)-10- and (1R)-(-)-10-camphorsulfonic acid and polymerization is inhibited in the presence of the (S) enantiomer. Also from Garnier’s laboratories, PPy functionalized in the 3 position with a chiral dipeptide (Figure 5-1 C, glycyl-D-phenylalanine, Gly-D-Phe) that was selected for its specificity and binding capacity toward carboxypetidase A and trypsin was used for enantioselective switching. The presence of either of these enzymes during electrochemical switching (CV) leads to an increase in polymer oxidation potential, Ep,m.6 Tripathy and colleagues found that the third major class of biomolecule, DNA, can be incorporated into PPy films and have performed 4. Korri-Youssoufi, H.; Garnier, F.; Srivastava, P.; Godillot, P.; Yassar, A. J. Am. Chem. Soc. 1997, 119, 7388-7389. 5. Moutet, J.-C.; Saint-Aman, E.; Tran-Van, F.; Angibeaud, P.; Utille, J.-P. Adv. Mater. 1992, 4, No. 7/8, 511-513.

162 imaging experiments on these films.7 From these works, it is clear that even rather delicate biomolecules including enzymes8 (horseradishperoxidase, glucose oxidase), histone proteins9 and photodynamic proteins (bacteriorhodopsin) are stable to some degree in a PPy matrix. A. Specific DNA Recognition Based on Oligonucleotide-PPy O N O

O

O O

OH

O

O

HN CCT-AAG-AGG-GAG-TG O

1) Electropolymerization 0.6

+ 0.4

N H

2) H2N-CCT-AAG-AGG-GAG-TG

N H

N 0.6 N H H

B. Glucose-Pyrrole Chiral Electrodes for Enantioselective Recognition

0.4

C. Enzyme Recognition with PPy-Bioactive Peptides OH O

OCOCH3 OCOCH3

H3COCO

* HN O

O OCOCH3 O

O

NH O

N x

N H

x

Figure 5-1. Examples of biologically relevant pyrrole modifications. (A) Garnier’s specific recognition of DNA by CV detection in the presence of various concentrations of complimentary and noncomplimentary hybrids. (B) N-substituted glucose pyrrole is enantioselective for camphorsulfonic acid. (C) dipeptide modified Py is capable of binding and recognizing carboxypeptidase A.

6. Garnier, F.; Korri-Youssoufi, H.; Srivastava, P.; Yassar, A. J. Am. Chem. Soc. 1994, 116, 8813-8814. 7. Pande, R.; Ruben, G. C.; Lim, J. O.; Tripathy, S.; Marx, K. A. Biomaterials, 1998, 19, 1657-1667. 8. Selampinar, F.; Akbulut, U.; Ozden, M. Y.; Toppare, L. Biomaterials, 1997, 18(17), 1163-1168 9. Prezyna, L. A.; Qiu, Y.-J.; Reynolds, J. R.; Wnek, G. E. Macromolecules, 1991, 24, 5283-5287.

163 Langer has found that PPy can noninvasively control the shape and growth of mammalian cells10 and advanced the applications of PPy to tissue engineering in his seminal paper on the stimulation of neurite outgrowth using PPy in an applied electric field.11 These studies suggest the ability to apply a field to a suitable cell substrate, while simultaneously being able to control hydrophilicity, will revolutionize tissue engineering. Subsequent studies have extended this work to endothelial cell attachment and growth using a PPy-heparin composite.12 The change from the general use of PTh to PEDOT when a stable CP is desired has been gradually occurring since about 1996. As of 2001, researchers active in developing new conjugated materials recognize that EDOT is a superior monomer in almost every way (except cost) compared to Th. It easily electropolymerizes but is relatively stable as a monomer, and the polymers are remarkably stable and structurally homogenous due to the blocked 3 and 4 positions which preclude β couplings. Switching is more facile in PEDOT than PTh in terms of both low polymer oxidation potential and switching speed. Finally, PEDOT is stable to air and water in its oxidized, conducting state where PTh is more stable in the neutral, insulating state (Figure 1-7). PPy based materials, however, even when prepared using optimized conditions, are poorly defined in that there is a significant amount of α-β coupling.13 The presence of these defect sites along the polymer backbone decreases

10. Wong, J. Y.; Langer, R.; Ingber, D. E. Proc. Natl. Acad. Sci. USA 1994, 91, 32013204. 11. Schmidt, C. E.; Shastri, V. R.; Vacanti, J. P.; Langer, R. Proc. Natl. Acad. Sci. USA 1997, 94, 8948-8953. 12. Garner, B.; Hodgson, A. J.; Wallace, G. G.; Underwood, P. A. J. Mater. Sci.: Mater. Med. 1999, 10, 19-27.

164 its effective conjugation length, induces structural disorder, limits the electrochemical response, and is implicated as the primary site of polymer breakdown due to overoxidation, attenuating the electrochemical switching life-time.14 Additionally, oxidized PPy is unstable to reduction by even relatively weak reducing agents. Regardless of the polymer eventually used, it is clear that future advances in CP based biosensors, controlled release devices15 and eventually implantable devices16 require a material that is redox active, stable as a conductor, compatible in aqueous environments and biocompatible. Figure 5-2 illustrates this as a puzzle that is simultaneously multi-tiered where the redox active, aqueous compatible polymer must be well understood for the advanced applications to be possible. The dioxy substitution pattern adds electron density to the aromatic heterocycle, reduces both the monomer and polymer oxidation potential (thus reducing propensity towards over-oxidation), and tends to lower the electronic band gap of the π system. Specifically, the reduction potential of PTh (E1/2 > +0.5 V vs. SCE) is reduced to approximately 0.0 V in poly(3,4-ethylenedioxythiophene) (PEDOT), and the band gap is concurrently lowered from 2.0 to 1.6 eV. We hypothesized that if this principle of a bridged 3,4-dioxy substituent was applied to form a poly(3,4-alkylenedioxypyrrole), the expected decrease in the polymer's E1/2 would yield an increased stability in the doped (conducting)

13. Pfluger, P.; Street, G. B. J. Chem. Phys. 1984, 80, 544. 14. Zotti, G.; Schiavon, G.; Zecchin, S. Synth. Met. 1995, 72, 275. 15. Pyo, M.; Maeder, G.; Kennedy, R. T.; Reynolds, J. R. J. Electroanal. Chem. 1994, 368, 329. 16. Peppas, N. A.; Langer, R. Science 1994, 263, 1715-1720.

165 TISSUE ENGINEERING & IMPLANTABLE DEVICES

DEVICES: BIOSENSORS &CONTROLLED RELEASE

LI BI M

PA

TI

AQUEOUS COMPATIBILITY BI

O

C

O

REDOX ACTIVITY

TY

STABLE CONDUCTORS

Figure 5-2. Requirements for tissue engineering and implantable devices. Biologically relevant devices for biosensors and controlled release must be based on a foundation of redox active, stable conductors with aqueous and biological compatibility. form. Relative to pyrrole, the monomer's oxidation potential would decrease, and the blocked 3- and 4-positions would preclude α-β coupling leading to a polymer repeat unit with fewer defects. This chapter addresses the synthesis, electrochemical characterization as a measure redox activity, and stability in aqueous media of the poly(3-4alkylenedioxypyrroles) PXDOPs.

5.2 Monomer Synthesis and Polymer Electrosynthesis The synthetic conditions necessary to prepare the simplest monomer (Figure 5-3), 3,4-ethylenedioxypyrrole (EDOP, n = 0, thus R1, R2 n.a.) have been reported17 with the

17. Merz, A.; Schropp, R.; Dötterl, E. Synthesis 1995, 795.

166 only mention of polymer properties appearing in the context of a photographic imaging agent.18 Treatment of dimethyl-N-benzyl-3,4-dihydroxypyrrole-2,5-dicarboxylate with a variety of dibromo- or dimesylalkanes in the presence of base gave the corresponding dicarboxyl-3,4-alkylenedioxypyrrole intermediates. After subsequent deprotection of the benzyl group in the presence of H2/Pd(C), hydrolysis in 1 N NaOH and decarboxylation in triethanolamine at 180 °C, a series of parent 3,4-alkylenedioxypyrrole monomers was obtained.19 N-substitution was easily effected by treatment of the parent pyrrole with NaH in THF followed by alkylation. In addition to EDOP, ProDOP (n = 1, R1, R2 = H) is the other standard parent monomer in this study and will be compared to EDOP as a potential replacement for Py as the monomer of choice for future bio-oriented experiments. EtO

OEt OH

O

K2CO3, CH3OH NH2

2

MeO2C

OMe

Br

O N Bz

OH

NaOCH3 CO2Me

CH3OH, reflux 8h, 56 %

H3CO2C

CO2CH3

N Bz

R1 R2

O

n

X

X

K2CO3 dry DMF 110 ºC 10 h R1 R2 n

O

O

R1 R2 n

N(CH2CH2OH)3

O

60-65 %

HO2C

N H

n

48 h, 95 %

O

180 ºC, 5-10 min. N H

R1 R2

(1) H2, Pd/C AcOH, 70 ºC

CO2H

(2) 2 N NaOH (aq.) RT, 12 h 95 %

O H3CO2C

O N Bz

CO2CH3

Figure 5-3. Synthesis of PXDOP monomers.

Single crystals of some of the XDOPs were obtained and the crystal structures for EDOP and ProDOP are depicted in Figure 5-3. Densities in the single crystal are 1.384 g

18. Savage, D. J.; Schell, B. A.; Brady, B. K. U.S. Patent 5 665 498, 1997 19. Thomas, C. A.; Zong, K.; Schottland, P.; Reynolds, J. R. Adv. Mater. 2000 12(3), 222225.

167 cm-3 and 1.405 g cm-3 respectively. The pyrrole ring fragment of EDOP is more planar than that of ProDOP with the C2-N-C5-C4 torsional angle (numbered as described in Figure 113) being 0.4° for EDOP and 1.1° for ProDOP. It has been postulated that the slight strain in the propylenedioxy ring vs. the ethylenedioxy ring is responsible for the slightly less electron rich character observed in ProDOP (126.6°) compared to EDOP (123.7°) (the ideal being 120°).20 Whatever the reason, neither monomer is stable in the crystalline state at room temperature, the solids turning dark brown and eventually black over the course of days to weeks. ProDOP is considerably more stable than EDOP but both monomers require storage in the freezer and sublimation (0.01 mm Hg) before use when purity was critical. Solution stability is decreased to days at the most when any oxygen is present, the solutions turning blue then a deep purple as the monomer is oxidized to form oligomers. In contrast to Chapters 3 and 4 where planarity was critical, the torque in the seven membered propylenedioxy bridge of ProDOP when viewed from the side should not be considered as a disadvantage in this monomer over EDOP. Recent studies from the Reynolds group have indicated that the open morphology provided by propyl and gem-dimethylpropyl linkers in PProDOTs facilitates switching.21 Propyl linkers also impart a unique spectral feature in the UV-Vis-NIR spectra of conjugated polymers known as vibronic bands where multiple peaks are present and taken to be a sign of order in the polymer. Despite the propensity to spontaneously polymerize in air, a variety of electrochemical methods are routinely used to polymerize XDOPs on Pt, ITO and glassy 20. Schottland, P.; Zong, K.; Gaupp, C. L.; Thomas, C. A.; Giurgiu, I.; Hickman, R.; Abboud, K. A.; Reynolds, J. R. Macromolecules 2000, 33(19) 7051-7061. 21. Kumar, A.; Welsh, D. M.; Morvant, M. C.; Abboud, K.; Reynolds, J. R. Chem. Mater. 1998, 10, 896-902.

168

A. EDOP top and side (below) views

B. ProDOP top and side (below) views

Figure 5-4. Crystal structures of EDOP and ProDOP when viewed from above and from the side. carbon electrodes. Compared to PPy, it is easier to obtain smooth, homogenous films of PEDOP or PProDOP on ITO, even to the naked eye. Propylene carbonate (PC), a viscous, high dielectric solvent widely used in the battery industry and a supporting electrolyte of tetraethylammonium p-toluenesulfonic acid (tosylate, OTs) (TEAOTs) was found to be the ideal solvent-electrolyte combination for galvanostatic, potentiostatic or scanning potential deposition. Other solvents tried were acetontrile with several dopant ions ranging from perchlorate, hexafluorophosphate, tetrafluorborate to triflate and bis(trifluoromethane)sulfonylimide (3M salt) and γ-butyrolactone as solvent, like PC, also common in the battery industry. This ideal solvent finding is no surprise given that high quality films are also observed with PPy under these same conditions and it appears that there is a special plasticizing effect in tosylate-pyrroles systems. The notable difference

169 between the XDOPs studied here and PPy are that no conditions have been found yet that allow the polymerization of an XDOP to proceed electrochemically in water using any of the above dopant anions in addition to dodecylbenzene sulfonate and Na+ poly(styrenesulfonate) (PSS). To standardize experiments comparing the qualities of PPy, PEDOP and PProDOP based on film thickness, the film thickness vs. deposition charge profiles for several different dry films were investigated as prepared in PC based electrolytes (Figure 5-5). The results are that for a given charge density, PProDOP is thicker than PEDOP which is thicker than PPy. PPy and PEDOP are linear over the entire charge density range collected but PProDOP has at least two different linear regimes. The first of these spans the thin film range until the deposition charge density is ca. 100 mC cm-2 where the membrane thickness is comparable to that of PEDOP. Between ca. 100 mC cm-2 and 300 mC cm-2 the film deposited is thicker per given coulomb than any of the other polymers. It appears there may be an even faster increase in film thickness above this charge density but data is limited. Assuming comparable polymerization yields, polymer density is the most obvious factor affecting these values where PProDOP would be the least dense of the polymers and PPy the densest. Most CPs are assumed to have a density near 1.5 g cm-3 but crude float tests were unable to pin down any differences in polymer film density between the above polymers.

5.3 Polymer Electrochemistry The polymer electrodepositions by CV for PEDOP and PProDOP in PC are shown in Figures 5-6 and 5-7 respectively. These processes are typical of CPs where as the potential is scanned anodically, a peak corresponding to monomer oxidation is observed.

170 5 PProDOP PEDOP PPy 4 H N

Film Thickness /µm

x O

3

O

H N x O

O

2 H N x

1

0 0

100

200 300 Charge / mC cm-2

400

500

Figure 5-5. Film thickness vs. deposition charge for PPy, PEDOP and PProDOP. Approximately 0.9 cm2 films were deposited on ITO from [10 mM] monomer [0.1 M] TEAOTs in PC at 0.1 mA cm-2 for a varying amount of time until a desired charge was reached. Thickness is reported as measured across a scratch on a profilometer. Upon scan direction reversal, no current peak is seen indicating that the oxidized monomer has coupled to form dimer, trimer, and eventually oligomer and polymer. As the potential is scanned cathodically, the current arising from the reduction of the oxidized polymer to the neutral polymer is observed. Upon reversal of the potential scan, the polymer p-type doping process is observed followed by subsequent monomer oxidation, all at higher currents than in previous scans. This indicates oxidation on an electrode with a higher surface area. The alternative electrochemical deposition techniques (galvanostatic and potentiostatic) used in Chapters 3 and 4 are also applicable here and are preferable to CV

171 deposition because they are simpler and it is easier to measure the charge passed during polymerization and the resulting film thickness. A. PEDOP Deposition

H N x O

O

0.2 mA

B. PEDOP CV 1st and 3000th Scan (Inset)

0.5 mA

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Potential /V vs SCE

Figure 5-6. PEDOP deposition and aqueous electrochemistry. (A) Electrochemical deposition from PC showing even cycles scans 4 through 20. (B) PEDOP switching in aqueous buffer initially and after 3000 scans indicates that it retains ca. 95% of its charge storage capacity.

After electrosynthesis in PC, films were washed with deionized water and [0.1 M] KCl-H2O buffered to pH 7.2 with Na2HPO4 (phosphate buffered KCl). They were then broken in over 20 double potential step cycles in Ar degassed solvent with an Ar blanket to remove PC and OTs and to acclimate the polymer to chloride influx and egress. The CVs

172 H N

A. PProDOP Deposition

x O

O

0.5 mA B. PProDOP CV 1st and 3000th Scan (Inset)

0.5 mA

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Potential /V vs SCE

Figure 5-7. PProDOP deposition and aqueous electrochemistry. (A) Electrochemical deposition from PC showing scans 2 through 21. (B) Cyclic voltammetry as a measure of stability indicates that PPy retains much (90%) of its charge storage capability after 3000 scans in aqueous buffer. in aqueous KCl buffer for PEDOP and PProDOP are shown in part B of Figures 5-6 and 57 respectively. They indicate that the switch from organic solvent and electrolyte to the aqueous system cathodically shifts the E1/2 of each polymer in a significant manner. In fact, the E1/2s observed for these polymers are among the lowest ever observed for any CP. Repeated cycling of each of these polymers indicates they are extremely stable to switching. Each figure shows the polymer shortly after it was deposited when it had less

173 than a 10 scan history on the outside. The inner CVs show the behavior of the polymers 3000 scans after the scan shown. Analysis of the charge stored in each cycle indicates that PEDOP retains 96% of its electroactivity while PProDOP retains 90% of its electroactivity after 3000 cycles. By contrast, PPy under the same conditions retains a lower but still respectable 80% of its charge storage capability.22 Because of the similarity in polymer oxidation potentials and optical properties (Section 5.4) of PEDOP and PProDOP, and because of the relative difficulty in handling EDOP as a monomer, PProDOP is likely to be the candidate for next generation materials and its properties will be exclusively discussed below. Additionally, the ability to symmetrically modify the propyl bridge in PProDOP lends this polymer to more facile functionalization such as shown in Figure 5-1. Unless noted, all properties investigated for PEDOP that rely on polymer redox potential are identical to PProDOP further rationalizing this decision. The in situ conductance of PProDOP (Figure 5-8), synthesized and characterized under identical electrochemical conditions to the CVs above, indicates that the conductance onset for p-type doping is quite cathodic at -0.65 V vs. SCE compared to any other CP as shown in Figure 5-8. Furthermore, unlike other pyrrole containing polymers studied by other groups, the polymer remains conducting even at potentials very anodic of the onset for conductance. In these other materials, often a window of conductivity is observed followed by a decay to low conductivities. While the method of in situ conductance measurement used in this work is unable to determine absolute conductivities, free standing films of PProDOP are routinely in the range of 50-80 S cm-1 (relative to PPy under the same

22. (a) Yamato, H.; Ohwa, M.; Wernet, W. J. Electroanal. Chem. 1995, 397, 163-170. (b) Pyo, M.; Reynolds, J. R.; Warren, L. F.; Marcy, H. O. Synth. Met. 1994, 68, 71-77.

174 conditions which is 250 S cm-1) at low current densities on glassy carbon electrodes. Despite free standing film conductivities representing an entirely different aspect of conductivity than these solution measurements, the trend is probably preserved and PProDOP films are slightly less conducting than PPy and perhaps an order of magnitude less than PEDOT. Of note also is the extremely low potential needed to polymerize the monomer used for conductance measurements, +0.63 V. This affords the possibility of using many more compounds as pendant groups attached to ProDOP monomers or including interesting easily oxidized dopant ions while retaining compatibility with the low oxidation potential.

5.4 Polymer Spectral Characteristics Spectroelectrochemistry of PEDOP and PProDOP in aqueous KCl buffer are shown in Figure 5-9. Both polymers switch repeatedly in aqueous buffer solution and undergo clean state to state transitions as observed by the relatively sharp isosbestic points. In both part A and B of Figure 5-9, it is clear that complete reduction of these polymers is difficult even at potentials nearing water reduction (ca. -1 V) as spectral signatures of doping are evident near 1.5 eV for PEDOP and 1 eV for PProDOP. Films showing distinct electrochromic response ( PPy0+ DTTox (Figure 5-10). This reaction does not proceed for oxidized PEDOP. Identical results are obtained by reaction with GSH.

25. (a) Lamoureux, G. V.; Whitesides, G. M. J. Org. Chem. 1993, 58, 633. (b) Lees, W. J.; Whitesides, G. M. J. Org. Chem. 1993, 58, 642. 26. Millis, K. K.; Weaver, K. H.; Rabenstein, D. L. J. Org. Chem. 1993, 58, 4144.

178

A. Potential Ladder for Postulated Pyrrole Reduction -0.8

RED

B. Summary of Observed Reactions H N

OX

H N

Potential /V vs. SCE

x O

x

O

O

O

-0.7 PProDOP

PProDOP+

-0.6

DTTred

DTTox

-0.5

GSH

GSSG

PPy

PPy+

PProDOP+

PProDOP

X HO

SH SH

HO

-0.4

HO

S S

HO

DTTred

DTTox

H N

H N

C. Glutathione Structures O

OH H N

H2N O

O

O N H SH

OH

OH

O

H N

H2N

O

O O HO

S

O

GSH

O

S H N

x

OH

N H

PPy

x

PPy+

O NH2

N H HO

O

GSSG

Figure 5-10. Explanation for the reaction with PPy+ and DTT. (A) Potential ladder indicating that PPy+ reacts with both GSH and DTTred to form GSSG or DTTox and PPy. (B) Summary of observed reaction with DTT indicating that PProDOP does not react. (C) Structures of oxidized and reduced glutathione. Smooth films of PEDOP and PProDOP are easily deposited (when compared to the deposition characteristics of PPy) on ITO and are electrochromic, switching between shades of highly transmissive blue/gray in the oxidized p-doped states (E = +0.85 V, x = 0.34, y = 0.38) and red/orange in the neutral state (E = -0.80 V, x = 0.52, y = 0.45).27 Figure 5-11 shows the colorimetry results relative to the CIE 1931 standard observer for PProDOP undergoing p-type doping-dedoping in aqueous buffer. The colors exhibited are identical 27. x and y represent true color values as referenced to the CIE 1931 color table. True color values are reported as measured with a Minolta CS-100 Chromameter and referenced to the CIE color table. See Granström, M.; Berggren, M.; Pede,D.; Inganäs, O.; Andersson, M. R.; Hjertberg, T.; Wenneström, O. Supramol. Sci. 1997, 4, 27.

179 to those observed in PC-OTs solvent-electrolyte and are very similar to PEDOP, which has a more red-colored reduced state. y 0.9 520

H N

515

0.8

x

525 530

O

O

535 540

510

545

0.7

550 555

505

560

0.6

565 570

500

575

0.5

0.4

580 585 590 595 600 λd, neutral = 584

495 λd, p-type = 481 +0.07 V vs. SCE

0.3

0.2

610 620 630 650

-0.93 V vs. SCE

490

485 480

0.1

0.0 0.0

475 470 460 380

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

x

Figure 5-11. Colorimetry of PProDOP in aqueous buffer. The star represents the CIE 1931 coordinates of the light source through ITO and aqueous KCl buffer (pH 7.2). The color transitions from a highly pure orange in the neutral state at -0.93 V to a highly transparent blue/gray at +0.07 V.

180 5.5 Electrochemical Quartz Crystal Microbalance Studies on Ion Transfer Much of the interest in aqueous compatible CPs that are able to be repeatedly switched is for applications in controlled molecular release where the rate and period of bioactive molecules supplied to a reservoir can be controlled.28 This interest is largely due to the need to target increasingly potent drugs to the area of the body where they are physiologically needed, dodging possible side effects due to unintended systemic reactions. CPs are interesting for these applications because of the ability to alter their redox states in small increments to allow controlled ion transport, all while preserving the ease of structural modification and molecular tuning inherent in organic materials. An alternative to polymeric active release is the system developed by Langer utilizing an array of wells containing a pharmacophore held in place by a thin Au film which can be electrochemically removed.29 Alternatives to active release are either the cleavage of a biologically inert polymer containing a drug molecule (poly(lactic acid-co-glycolic acid), PLGA),30 or the swelling of an inert polymer such as PEO/PEG containing water soluble drug molecules.31 An elegant alternative to the PLGA approach is to incorporate a difunctional drug such as

28. (a) Park, K., Ed. Controlled Drug Delivery, Challenges and Strategies; American Chemical Society: Washington, DC, 1997. (b) Langer, R. Science 1990, 249, 1527. 29. Langer, R. Acc. Chem. Res. 2000, 33, 94-101 30. (a) Miyajima, M.; Koshika, A.; Okada, J.; Ikeda, M. J. Control. Rel. 1999, 60, 199209. (b) Langer, R. J. Control. Rel. 1999, 62, 7-11. 31. (A) Chandra, R.; Rustgi, R. Prog. Polym. Sci. 1998, 23, 1273-1335. (B) Moriyama, K.; Ooya, T.; Yui, N. J. Control. Rel. 1999, 59, 77-86. (C) Peppas, N. A.; Keys, K. B.; Torres, Lugo, M.; Lowman, A. M. J. Control. Rel. 1999, 62, 81-87. (D) Breitenbach, A.; Li, Y. X.; Kissel, T. J. Control. Rel. 2000, 64, 167-178.

181 salicylate into a polymer pro-drug where the degradation process itself releases the active drug, a process Uhrich has used to implement anesthetic sutures.32 A. Cationic Pharmacophores for Controlled Release HO

HO

HO OH

OH HO

HO

HO NH3

DOPAMINE

NH2

NH2 CH3 EPINEPHRINE

METAPROTERENOL

B. Anionic Pharmacophores for Controlled Release O

NH2 CO2

N

-

OH

O O O NaO P O P O P O ONa OH OH

N O

H

N N

O NaO P O ONa

OH OH SALICYLATE

ATP

CH3

N

O

N

O N3 AZTMP

Figure 5-12. Pharmacophores for controlled release. (A) cationic and anionic (B) molecules of pharmaceutical interest which are capable of being released from a conjugated polymer.

The relative simplicity and generality of using CPs to release charged pharmacophores still remains an attractive option for actives release and the use of PPy to this end has been well established for salicylate and ferrocyanide,33 glutamate,34 and ATP.35 Figure 5-12 shows several molecules which are interesting as dopants in CPs. A

32. Uhrich, K. E.; Cannizzaro, S. M.; Langer, R. S.; Shakesheff, K. M. Chem. Rev. 1999, 99, 3181-3198. 33. Chang, A. C.; Miller, L. L. J. Electroanal. Chem., 1988, 247, 173. 34. Zinger, B.; Miller, L. L. J. Am. Chem. Soc. 1984, 106, 6861. 35. (a) Pyo, M.; Maeder, G.; Kennedy, R. T.; Reynolds, J. R. J. Electroanal. Chem. 1994, 368, 329. (b) Pyo, M.; Reynolds, J. R. Chem. Mater. 1996, 8, 128.

182 complete understanding of the mass transport properties of a CP release candidate must be understood before a device is designed around it. The mass transport characteristics for the PPy p-type to neutral transition are anion dominant. That is, upon charge neutralization the compensating anions are released from the film. The mass transport dominance of PPy can be reversed by polymerization using an anionic polymeric counterion such as PSS. In this composite, the polymer is deposited in the oxidized state with sulfonate ions trapped in the film acting as counterions. Charge neutralization of the polymer leaves vacant anionic sites that are charge compensated by the diffusion of solution cations in the polymer membrane making the polymer cation dominant. EQCM is typically used to probe these mass transport dependencies as it allows measurement of mass changes on a surface while an electrochemical potential is applied. EQCM data for PProDOP deposited from PC-OTs and switched in aqueous KCl buffer is shown in Figure 5-13. Similar to PPy, the mass transport for PProDOP-Cl is anion dominant. Starting from p-type doped polymer at 0 V vs. SCE, the polymer film contains chloride ions. A cathodic potential scan neutralizes the polymer and the anions formerly used for charge compensation are released into solution with a concomitant mass decrease of the polymer. Because the redox potential of PProDOP is near where water is reduced, ca. -1 V vs. SCE, the polymer cannot be scanned very cathodic of its reduction potential and the mass transfer is not observed to plateau. On scan reversal, the polymer is still neutral and Cl- is still expelled until near the E1/2, where Cl- ions are taken up again. Attempts to switch the mass transport dominance of PProDOP by electrochemical deposition in the presence of PSS were unsuccessful. Since ProDOP cannot be deposited from aqueous solution and NaPSS is not soluble in PC, a soluble form of PSS had be

183

SOLUTION

O O

H NH N

O

H N

O

H N

H N

H N

H N

O O O O O O O O O O O O H N

H N

H N

O

H N

H N H N

H N O

O

O

x

x O

H

N O

O

H x

O

O

HN O

O

H N

O

N

O

O

O

O

H N

H N

N

O O

O HN

HN

N

O

H

H

O

O H NH N

N

O

N

H

H

H

O

O

O O

O

O O

O H N

O

O

H N

H N

O

O

H N

O O

H N

O O

O

H N H N

H NH N

O

O

O

H N

O

OHN

H N

O

O

O O

O

O

H N

O

O

O

O

O

O O

O HN

O

H N

O

O O

O

O

O O

O H NO

HN

H N

H N

H N

O O HN

O O HN

O O

O

O

O

O H N

H N

O

O

O

O

H N

O

O

H N

H N

H N

HN

O O

O

HN

Pt on QUARTZ

Pt on QUARTZ

Mass Increases

H N

H N

x

x O

O

O

H N

O

O

H N

O

H N

O

O O

O

H N

O

O O

O

O

O

N O

H

O

H N

H N O

H N

O

O

H N

N

H N

O O

H N O

x O

N

H

O

O

O

H

O

N

O H NO

O

H N O

O

O

O

O O HN

H N

H N O

O

x O

H

H

H N

O

N

N

H N

H N O

H N

O

O

O

H N

O

HO

O

O

O

OHN

O

H N

O N

O

O

O

O

O O

O

O

HN x

O

H

O

O

O

H N O

N

O O

O

H

O

O

O H

O

O

N

O

O

H N

H O O N

O

O

O

O

H N

O

O

O

OH N

O

O H N

H N

H N

O

H N

O

O

O O O

N O

H N O

O H N

HN

O

O

O

HN

H N O

O

x O

O

O

H N

O

H

O

x O

N

O

O

O

x O

O

H N O

H

O

O

O O

O

O

O

O

H N

O

O

O

H N

H N O

N

O

O

H

H N

N

H N

O

H N

O

O

O

O HN

O

x

O H N

O

H N O

O O

O

O

O

HN

O HHN N

H N

H N O

H

O

O

O

HN

H N

O

N

O

H N

O

O

O

O

H N

H

O

O

O

O

O H N

O

N

H N

H N O

O

O

H N

H N H N

O

O

H N O

O

O

O

O

O

O

O

x O

O

O

O

O

O

O

H N

H N O

O

O

O

O

x O

O

O

O

H N

H O O N

O

POLYMER H N

O

H N

O

H N

O

HO

O

O

O

H N

O N

H NH N

O

O

O

O

O H N

H N

H N O

H

O

O

O

O

O HN

H N O

N

OH N

O

O

O HN

H N O

H N

O

O

H N

O

x O

O

H N O

H

O

O

O HN

O

O

H N

O

H N

O

O

H N

N

H N

O

H N

H N

O

O HN

O

x

O H N

O

O O

O

O

HN

O HHN N

O

O

HN

O

O

H N

H N O

O

O

O

O

O H N

O

O

O

H N

H N H N

O

O

O

O

H N O

H N

POLYMER

O

SOLUTION

O

O

O

J /mA cm-2 Mass /ng

0.1 mA cm-2

-1.0

4000 ng

-0.5

0.0

0.5

Potential /V vs SCE Figure 5-13. EQCM and CV in aqueous buffer for PProDOP on Pt quartz crystal. Exposed crystal area was approximately 0.96 cm2. PProDOP was deposited to a charge density of 100 mC cm-2 in PC as described previously. After rinsing in aqueous buffer, and potential square wave break in, CV mass transport data was collected at 25 mV sec-1. synthesized. This is accomplished by exchange of Na+ for TEA+ cations through a membrane. TEAPSS is indeed PC soluble and electroactive films are achieved by electrochemical deposition as above for PProDOP-Cl. The CV for this composite looks similar to that of PProDOP-OTs. However, these films did not undergo mass changes in the potential range over which the polymer dopes.

184 5.6 Conclusions PProDOP was investigated as a replacement candidate for PPy in aqueous systems of biological interest. It excels compared to PPy in all areas examined except for cost and compatibility with water during electrosynthesis. The electrochemical stability and charge storage capacity for PProDOP and PEDOP under repeated potential cycling in aqueous solution indicates that both of these polymers retain higher electroactivity than PPy. PProDOP and PEDOP undergo similar color transitions and are both nearly colorless in the p-type doped state making them ideal for optical observation of film confined species of biological interest. The high stability in the conducting state precludes the strongest biological reductants from neutralizing either PProDOP or PEDOP and the conductivity is sufficient and constant over the entire stable p-type doped range. Furthermore, the mass transfer characteristics with standard anionic dopants are similar to PPy. Since the ProDOP monomer is more stable than EDOP, it is suggested as the starting point for development of aqueous compatible systems.

5.7 Experimental Section General. PC, Py, KCl, TEABr, NaPSS and TEAOTs were purchased from Aldrich. All were used as received except for Py which was stored over KOH and passed through a short alumina (Brockman Activity I) column (gravity eluted in a disposable pipet) immediately prior to use. Millipore 18 MΩ water was used throughout. ProDOP and EDOP were prepared by Kyukwan Zong using the method described in the literature and were vacuum sublimed to a white powder, and used even if slightly gray. Monomer was stored in a freezer (vials degassed before closure) in the dark. Solutions of EDOP and ProDOP were prepared by degassing PC [0.1 M] TEAOTs prior to adding it to weighed monomer

185 in a volumetric flask. Flasks were stored in a refrigerator or freezer immediately upon use and were discarded after a week of use or when the color became more than slightly yellow (almost always less than a week). Electrochemistry, spectroelectrochemistry, colorimetry, and in situ conductance were performed as described in Chapters 2 and 3. Profilometry. A Dektak 3030 profilometer was used for film thickness measurements in collaboration with Roberta Hickman. Films were prepared from [10 mM] monomer [0.1 M] PC-OTs solutions and deposited at 0.1 mA until a desired charge density was reached. The films were rinsed in acetone and allowed to air dry, or if not measured immediately, stored in a vacuum dessicator. The film was scored in the middle with a piece of broken ITO36 and the profilometry measurement spanned the gap. The data was leveled across the gap and the surface roughness was calculated on the polymer while the film thickness was taken from the average film height to the average depth of the score. Aqueous NaCl buffer preparation. 1.46 g NaCl, and 3.0 g NaH2PO4 were added to a 100mL volumetric flask. The flask was filled with 18 MΩ water and the solution was adjusted to pH 7.2 with ca. 10 % w/w NaOH. KCl buffer was prepared in an analogous fashion. Reduction experiments. PEDOP and PProDOP were galvanostatically electrodeposited from [10 mM] monomer [0.1 M] PC at 0.1 mA cm-2 on an ITO slide until the current density reached 25 mC cm-2 (250 seconds for this ITO size). The films were rinsed in acetone and allowed to air dry. PPy was deposited under identical conditions except to 50 mC cm-2 charge density. 10 mM DTT (7.7 mg in 5 mL water) and GSH (15.3 36. Several scoring implements were used including those ranging in hardness from wooden dowels to razor blades but the broken ITO was found to give flattest region in the score.

186 mg in 5 mL) solutions were prepared. Polymer films (duplicates) were dipped in each solution for 30 minutes and the color observed and compared to a third film which was dipped in water. EQCM. The UF (Don Cameron) designed cell was used on an EG&G (now PerkinElmer) model QCA 917 as described in Chapter 2. The QCA probe and crystal were mounted vertically and the crystal sealed with poly(butadiene) o-rings. Monomer solution (standard [10 mM] monomer, [0.1 M] TEAOTs) in PC was added and leads attached. An open circuit baseline was observed for 10 minutes in both the 20 kHz V-1 (used for deposition) and 2 kHz V-1 (used for observing polymer) ranges of the EQCM. When the crystal stabilized, as determined by minimal change during an open circuit experiment, the voltage output of the QCA was normalized to ca. +5 V so that a decrease in frequency (mass increase) range of 15 V X 20 kHz V-1 = 300 kHz total was available for observation. Polymer was deposited galvanostatically at 100 µA for 196 seconds (A = 0.196 cm2) making a 100 mC cm-2 film (the frequency usually changing by 200 kHz). The monomer solution was then removed and the electrode was rinsed in the cell with 5 X 2 mL portions of Ar degassed KCl buffer. If the water solution leaked, the crystal was removed from the cell and the solvent swollen o-rings were replaced with fresh ones upon remounting. The polymer was then subjected to a CV to determine the potential limits for switching (current autoranging was turned off) and broken in by 20 double potential square waves of 20 seconds each over the CV determined limits. The mass changes were measured on the 2 kHz V-1 range which was zeroed prior to collection. Electrochemical data was then collected and the polymer treated as if it was broken in.

187 TEAPSS. (Effective molecular weight, 313.5 g/eq.) 10 g NaPSS (Mw 70,000) and 30 g TEABr were dissolved in 250 mL water with heating and stirring. This solution was loaded into a wet, knotted on one end, spectra/Por dialysis membrane (MWCO 3500, ca. 9 inches before filling) and the other end knotted. This tube was then treated as follows at room temperature in a bucket. 48 h in distilled water (tube became taut). 48 h in a 500 mL water solution containing 25 g TEABr (done in a narrow vacuum dewar with stirring to limit solvent usage). 48 h in distilled water changed every 12 h on average. A small portion was then tested for Br- content with AgNO3. No precipitate was observed and the control TEABr solution tested positive. The water solution bumped excessively on the rotovap and was instead dried in a crystallization dish in a fume hood, redissolved in EtOH and dried in vacuo overnight to a yellow solid to yield 9.8 g isolated and dried.

APPENDIX A CRYSTALLOGRAPHIC INFORMATION FOR CYANOVINYLENE MONOMERS

X = N or S N1

S1

X2

C12

C11

C6

C8

C2 C3

C4

C10

C7

C5

C9

O4 C15

O3 O1

C16

O2 C13

C14

Note: In thiophene containing monomers, O1, O2, C13, C14 and/or O3, O4, C15, C16 may be skipped.

Figure A-1. Numbering system for cyanovinylene monomer crystal structures. BThCNV, Th-CNV-EDOP and EDOT-CNV-EDOP were determined by Khalil Abboud. ThCNV-EDOT and EDOT-CNV-Th were determined by Peter Steel.

∑ ( F0 – Fc ) R1 = -------------------------------------∑ F0 [ w ( F0 – Fc ) ] ∑ ----------------------------------------2 ∑ [ w ( F20 ) ] 2

wR2 =

2 2

1⁄2

1 , w = --------------------------------------------------------------------------------------2 2 2 [ σ ( F 0 ) + ( 0.0370 × p ) + 0.31 × p ]

 w ( F 2 – F 2 ) 2 0 c  ∑ S = -----------------------------------------(n – p)

1⁄2

2

2

[ max ( F 0, 0 ) + 2 × F c ] , p = -----------------------------------------------------3

188

189 Crystal data and structure refinement for BTh-CNV. Empirical formula

C11H7NS2

Formula weight

217.30

Temperature

173(2) K

Wavelength

0.71073 Å

Crystal system

Orthorhombic

Space group

Pbca

Unit cell dimensions

a = 9.2545(7) Å

α = 90°

b = 18.090(2) Å

β = 90°

c = 23.917(2) Å

γ = 90°

Volume

4004.2(6) Å3

Z

16

Density (calculated)

1.442 Mg/m3

Absorption coefficient

0.485 mm-1

F(000)

1792

Crystal size

0.40 x 0.15 x 0.04 mm3

Theta range for data collection

1.70 to 27.49°

Index ranges

– 7 ≤ h ≤ 11, – 23 ≤ k ≤ 10, – 30 ≤ l ≤ 10

Reflections collected

11624

Independent reflections

4224 [R(int) = 0.0545]

Completeness to theta = 27.49°

92.0 %

Absorption correction

Integration

Max. and min. transmission

0.9869 and 0.9571

Refinement method

Full-matrix least-squares on F2

Data / restraints / parameters 2

4224 / 68 / 343

Goodness-of-fit on F

1.014

Final R indices [I>2sigma(I)]

R1 = 0.0560, wR2 = 0.1262 [2600]

R indices (all data)

R1 = 0.1026, wR2 = 0.1531

Largest diff. peak and hole

0.484 and -0.354 e.Å-3

190 Table A-1. Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 x 103) for BTh-CNV. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x

y

z

U(eq)

S1

4489(2)

1302(1)

7015(1)

32(1)

C2

4922(5)

1009(3)

7693(2)

27(1)

S2

8379(2)

-927(1)

8028(1)

38(1)

C3

4160(18)

1371(8)

8110(7)

129(6)

C4

3180(20)

1903(9)

7832(6)

50(4)

C5

3248(11)

1924(5)

7266(4)

31(3)

C6

5978(4)

432(2)

7790(2)

24(1)

C7

6672(4)

46(2)

7386(2)

27(1)

C8

7701(5)

-542(3)

7416(2)

27(1)

C9

8353(9)

-918(4)

6945(3)

41(2)

C10

9277(19)

-1458(10)

7131(6)

53(5)

C11

9449(14)

-1501(6)

7692(5)

46(3)

N1

6567(13)

216(6)

8838(2)

45(2)

C12

6269(7)

297(4)

8372(2)

33(1)

S1A

8101(7)

-800(3)

6917(2)

48(2)

C2A

7748(19)

-574(9)

7604(5)

26(5)

S2A

4185(5)

1353(2)

8104(2)

27(1)

C3A

8470(40)

-1063(18)

7961(11)

129(6)

C4A

9300(40)

-1605(16)

7629(12)

34(8)

C5A

9350(50)

-1460(20)

7074(12)

32(9)

C6A

6698(15)

-10(7)

7760(6)

45(4)

C7A

5930(16)

422(7)

7421(7)

48(4)

C8A

4882(17)

1001(9)

7491(5)

25(4)

C9A

4250(30)

1428(14)

7040(9)

41(2)

C10

3310(50)

1950(20)

7249(12)

80(18)

C11A

3170(50)

1970(20)

7812(11)

21(7)

191 Table A-1. (Continued) Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 x 103) for BTh-CNV. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x

y

z

U(eq)

N1A

6350(40)

69(17)

8832(7)

34(6)

C12A

6560(30)

102(11)

8360(6)

35(6)

S21

680(2)

236(1)

9250(1)

43(1)

C22

1416(8)

-622(3)

9404(3)

26(2)

S22

-1353(3)

-2875(2)

9309(2)

36(1)

C23

2729(11)

-612(5)

9555(4)

43(3)

C24

3336(11)

180(5)

9535(4)

46(2)

C25

2310(10)

664(4)

9389(3)

39(2)

C26

577(5)

-1290(2)

9366(2)

15(1)

C27

-918(5)

-1319(2)

9384(2)

17(1)

C28

-1889(7)

-1918(3)

9349(3)

24(1)

C29

-3480(20)

-1880(13)

9345(9)

187(9)

C30

-4062(13)

-2532(6)

9311(5)

42(2)

C31

-3124(15)

-3126(8)

9285(10)

38(4)

N21

2179(11)

-2443(5)

9222(5)

30(2)

C32

1477(8)

-1930(4)

9294(3)

29(2)

S21A

-3411(4)

-1823(2)

9341(2)

44(1)

C22A

-1623(12)

-2123(6)

9328(6)

35(4)

S22A

2991(5)

-528(2)

9577(2)

43(1)

C23A

-1530(30)

-2812(11)

9324(15)

43(3)

C24A

-2990(30)

-3204(14)

9310(20)

53(10)

C25A

-4050(20)

-2719(9)

9298(11)

36(5)

C26A

-324(14)

-1676(8)

9346(7)

91(5)

C27A

-109(17)

-993(8)

9378(7)

90(5)

C28A

1078(13)

-476(7)

9381(7)

32(4)

C29A

720(30)

298(15)

9273(15)

187(9)

192 Table A-1. (Continued) Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 x 103) for BTh-CNV. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x

y

z

U(eq)

C30A

1814(16)

700(9)

9286(6)

40(4)

C31A

3120(20)

388(8)

9488(7)

30(4)

N21A

2050(30)

-2472(15)

9214(15)

85(10)

C32A

1130(13)

-2056(7)

9282(7)

30(4)

193 Table A-2. Bond lengths [Å] for BTh-CNV. Bond

Bond Distance (Å)

Bond

Bond Distance (Å)

S1-C5

1.716(7)

S21-C25

1.727(9)

S1-C2

1.752(6)

S21-C22

1.736(7)

C2-C3

1.386(15)

C22-C23

1.268(12)

C2-C6

1.450(6)

C22-C26

1.438(7)

S2-C11

1.644(11)

S22-C31

1.702(12)

S2-C8

1.738(6)

S22-C28

1.805(7)

C3-C4

1.483(17)

C23-C24

1.540(13)

C4-C5

1.356(12)

C24-C25

1.337(14)

C6-C7

1.355(5)

C26-C27

1.385(6)

C6-C12

1.439(6)

C26-C32

1.437(7)

C7-C8

1.430(6)

C27-C28

1.409(7)

C8-C9

1.446(9)

C28-C29

1.47(2)

C9-C10

1.373(12)

C29-C30

1.30(2)

C10-C11

1.354(14)

C30-C31

1.382(16)

N1-C12

1.157(7)

N21-C32

1.145(7)

S1A-C5A

1.71(2)

S21A-C25A

1.728(15)

S1A-C2A

1.725(13)

S21A-C22A

1.742(12)

C2A-C3A

1.40(2)

C22A-C23A

1.248(18)

C2A-C6A

1.457(15)

C22A-C26A

1.450(14)

S2A-C11A

1.62(2)

S22A-C31A

1.675(14)

S2A-C8A

1.725(13)

S22A-C28A

1.834(13)

C3A-C4A

1.48(3)

C23A-C24A

1.52(2)

C4A-C5A

1.35(2)

C24A-C25A

1.31(2)

C6A-C7A

1.331(14)

C26A-C27A

1.253(13)

C6A-C12A

1.454(15)

C26A-C32A

1.519(14)

C7A-C8A

1.438(15)

C27A-C28A

1.443(15)

C8A-C9A

1.451(18)

C28A-C29A

1.46(2)

194 Table A-2. (Continued) Bond lengths [Å] for BTh-CNV. Bond

Bond Distance (Å)

Bond

Bond Distance (Å)

C9A-C10

1.37(2)

C29A-C30A

1.25(3)

C10-C11A

1.36(2)

C30A-C31A

1.415(18)

N1A-C12A

1.149(16)

N21A-C32A

1.148(14)

195 Table A-3. Bond angles[°] for BTh-CNV. Atoms

Bond Angle (°)

Atoms

Bond Angle (°)

C5-S1-C2

91.6(3)

C25-S21-C22

91.0(4)

C3-C2-C6

124.6(8)

C23-C22-C26

123.2(7)

C3-C2-S1

114.0(8)

C23-C22-S21

115.1(6)

C6-C2-S1

121.4(4)

C26-C22-S21

121.7(5)

C11-S2-C8

93.3(4)

C31-S22-C28

89.6(6)

C2-C3-C4

107.2(12)

C22-C23-C24

110.7(8)

C5-C4-C3

115.8(11)

C25-C24-C23

111.0(9)

C4-C5-S1

111.4(7)

C24-C25-S21

112.2(6)

C7-C6-C12

121.1(4)

C27-C26-C32

123.5(5)

C7-C6-C2

125.2(4)

C27-C26-C22

124.7(5)

C12-C6-C2

113.8(5)

C32-C26-C22

111.8(5)

C6-C7-C8

131.5(4)

C26-C27-C28

131.7(4)

C7-C8-C9

126.0(6)

C27-C28-C29

127.0(10)

C7-C8-S2

125.5(4)

C27-C28-S22

124.4(5)

C9-C8-S2

108.5(4)

C29-C28-S22

108.6(10)

C10-C9-C8

110.1(8)

C30-C29-C28

111.8(17)

C11-C10-C9

115.7(9)

C29-C30-C31

116.6(14)

C10-C11-S2

112.2(7)

C30-C31-S22

113.4(9)

N1-C12-C6

176.2(8)

N21-C32-C26

178.0(9)

C5A-S1A-C2A

94.9(11)

C25A-S21A-C22A

91.8(9)

C3A-C2A-C6A

127.2(15)

C23A-C22A-C26A

120.2(15)

C3A-C2A-S1A

110.0(13)

C23A-C22A-S21A

112.0(13)

C6A-C2A-S1A

122.4(10)

C26A-C22A-S21A

127.8(9)

C11A-S2A-C8A

96.1(11)

C31A-S22A-C28A

89.0(8)

C2A-C3A-C4A

110.(2)

C22A-C23A-C24A

113.9(18)

C5A-C4A-C3A

115.(2)

C25A-C24A-C23A

110.(2)

C4A-C5A-S1A

109.1(18)

C24A-C25A-S21A

111.8(16)

196 Table A-3. (Continued) Bond angles[°] for BTh-CNV. Atoms

Bond Angle (°)

Atoms

Bond Angle (°)

C7A-C6A-C12A

118.3(13)

C27A-C26A-C22A

133.1(14)

C7A-C6A-C2A

127.6(13)

C27A-C26A-C32A

108.2(13)

C12A-C6A-C2A

114.0(12)

C22A-C26A-C32A

118.6(11)

C6A-C7A-C8A

135.8(14)

C26A-C27A-C28A

139.5(16)

C7A-C8A-C9A

125.1(14)

C27A-C28A-C29A

116.7(15)

C7A-C8A-S2A

128.4(11)

C27A-C28A-S22A

134.6(10)

C9A-C8A-S2A

106.5(11)

C29A-C28A-S22A

108.1(13)

C10-C9A-C8A

110.4(18)

C30A-C29A-C28A

112.(2)

C11A-C10-C9A

117.(2)

C29A-C30A-C31A

117.8(19)

C10-C11A-S2A

110.2(19)

C30A-C31A-S22A

112.3(13)

N1A-C12A-C6A

168.(3)

N21A-C32A-C26A

166.(2)

197 Crystal data and structure refinement for EDOT-CNV-Th. Empirical formula

C13H9NO2S2

Formula weight

275.33

Temperature

153(2) K

Wavelength

0.71073 Å

Crystal system

Orthrhombic

Space group

Pna2(1)

Unit cell dimensions

a = 18.606(3) Å

α= 90°

b = 12.0690(18) Å

β= 90°

c = 5.5511(8) Å

γ = 90°

Volume

1246.6(3) Å3

Z

4

Density (calculated)

1.467 Mg/m3

Absorption coefficient

0.418 mm-1

F(000)

568

Crystal size

0.71 x 0.09 x 0.05 mm3

Theta range for data collection

2.76 to 26.41° – 22 ≤ h ≤ 23, – 14 ≤ k ≤ 15, – 4 ≤ l ≤ 6

Index ranges Reflections collected

13685

Independent reflections

2126 [R(int) = 0.0389]

Completeness to theta = 26.41°

98.9 %

Max. and min. transmission

0.9794 and 0.7555

Refinement method

Full-matrix least-squares on F2

Data / restraints / parameters 2

2126 / 17 / 182

Goodness-of-fit on F

1.051

Final R indices [I>2sigma(I)]

R1 = 0.0318, wR2 = 0.0698

R indices (all data)

R1 = 0.0420, wR2 = 0.0738

Absolute structure parameter

0.02(8)

Largest diff. peak and hole

0.208 and -0.199 e.Å-3

198 Table A-4. Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2x 103) for EDOT-CNV-Th. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x

y

z

U(eq)

S(1)

6914(1)

8914(1)

5305(2)

38(1)

S(2)

5490(1)

6791(1)

13531(3)

40(1)

S(2')

6505(4)

4941(5)

13610(14)

29(1)

O(1)

8059(1)

6429(1)

7572(4)

42(1)

O(2)

8720(1)

7547(1)

3494(4)

51(1)

N(1)

5508(1)

8770(2)

9486(5)

52(1)

C(2)

7111(1)

7794(2)

7153(5)

31(1)

C(3)

7753(1)

7330(2)

6468(5)

34(1)

C(4)

8073(1)

7870(2)

4467(5)

37(1)

C(5)

7685(1)

8740(2)

3656(6)

42(1)

C(6)

6612(1)

7471(2)

9058(5)

31(1)

C(7)

6700(1)

6575(2)

10498(6)

32(1)

C(8)

6246(1)

6178(2)

12403(5)

32(1)

C(9)

6359(4)

5195(5)

13749(14)

28(1)

C(9')

5618(8)

6498(11)

13670(30)

42(1)

C(10)

5840(1)

4976(2)

15535(6)

43(1)

C(11)

5357(1)

5800(2)

15508(6)

47(1)

C(12)

5992(1)

8183(2)

9328(5)

36(1)

C(13)

8817(2)

6329(2)

6939(7)

53(1)

C(14)

8920(2)

6450(2)

4291(7)

58(1)

199 Table A-5. Bond lengths [Å] for EDOT-CNV-Th. Bond

Bond Distance (Å)

Bond

Bond Distance (Å)

S(1)-C(5)

1.715(3)

C(2)-C(6)

1.461(4)

S(1)-C(2)

1.736(2)

C(3)-C(4)

1.418(4)

S(2)-C(11)

1.642(3)

C(4)-C(5)

1.351(4)

S(2)-C(8)

1.710(2)

C(6)-C(7)

1.354(4)

S(2')-C(10)

1.635(4)

C(6)-C(12)

1.446(3)

S(2')-C(8)

1.705(3)

C(7)-C(8)

1.435(4)

O(1)-C(3)

1.372(3)

C(8)-C(9)

1.417(6)

O(1)-C(13)

1.458(3)

C(8)-C(9')

1.417(6)

O(2)-C(4)

1.376(3)

C(9)-C(10)

1.408(7)

O(2)-C(14)

1.445(3)

C(9')-C(11)

1.409(7)

N(1)-C(12)

1.149(3)

C(10)-C(11)

1.341(4)

C(2)-C(3)

1.373(3)

C(13)-C(14)

1.489(5)

200 Table A-6. Bond angles[°] for EDOT-CNV-Th. Atoms

Bond Angle (°)

Atoms

Bond Angle (°)

C(5)-S(1)-C(2)

92.47(13)

C(9)-C(8)-C(7)

125.6(3)

C(11)-S(2)-C(8)

93.03(13)

C(9')-C(8)-C(7)

139.4(4)

C(10)-S(2')-C(8)

91.2(2)

C(9)-C(8)-S(2')

11.1(3)

C(3)-O(1)-C(13)

111.1(2)

C(9')-C(8)-S(2')

106.0(3)

C(4)-O(2)-C(14)

111.4(2)

C(7)-C(8)-S(2')

114.6(2)

C(3)-C(2)-C(6)

130.2(2)

C(9)-C(8)-S(2)

106.9(3)

C(3)-C(2)-S(1)

109.75(19)

C(9')-C(8)-S(2)

11.9(3)

C(6)-C(2)-S(1)

120.04(18)

C(7)-C(8)-S(2)

127.54(17)

O(1)-C(3)-C(2)

124.1(2)

S(2')-C(8)-S(2)

117.9(2)

O(1)-C(3)-C(4)

122.7(2)

C(10)-C(9)-C(8)

115.3(4)

C(2)-C(3)-C(4)

113.2(2)

C(11)-C(9')-C(8)

118.8(5)

C(5)-C(4)-O(2)

123.8(2)

C(11)-C(10)-C(9)

108.2(3)

C(5)-C(4)-C(3)

113.2(2)

C(11)-C(10)-S(2')

121.2(3)

O(2)-C(4)-C(3)

123.0(2)

C(9)-C(10)-S(2')

13.3(3)

C(4)-C(5)-S(1)

111.3(2)

C(10)-C(11)-C(9')

102.7(4)

C(7)-C(6)-C(12)

120.6(2)

C(10)-C(11)-S(2)

116.6(2)

C(7)-C(6)-C(2)

124.3(2)

C(9')-C(11)-S(2)

13.9(3)

C(12)-C(6)-C(2)

115.1(2)

N(1)-C(12)-C(6)

177.8(3)

C(6)-C(7)-C(8)

129.1(2)

O(1)-C(13)-C(14)

110.7(3)

C(9)-C(8)-C(9')

95.0(5)

O(2)-C(14)-C(13)

111.0(3)

201 Crystal data and structure refinement for Th-CNV-EDOT. Empirical formula

C13H9NO2S2

Formula weight

275.33

Temperature

148(2) K

Wavelength

0.71073 Å

Crystal system

Monoclinic

Space group

P2(1)/n

Unit cell dimensions

a = 4.942(1) Å

α= 90°

b = 10.082(2) Å

β= 90.430(10)°

c = 23.879(3) Å

γ = 90°

Volume

1189.7(4) Å3

Z

4

Density (calculated)

1.537 Mg/m3

Absorption coefficient

0.438 mm-1

F(000)

568

Crystal size

0.88 x 0.12 x 0.08 mm3

Theta range for data collection

2.19 to 23.98° 0 ≤ h ≤ 5, –0 ≤ k ≤ 11, – 27 ≤ l ≤ 27

Index ranges Reflections collected

2134

Independent reflections

1872 [R(int) = 0.0391]

Completeness to theta = 23.98°

99.9 %

Max. and min. transmission

0.9658 and 0.6990

Refinement method

Full-matrix least-squares on F2

Data / restraints / parameters 2

1872 / 0 / 163

Goodness-of-fit on F

0.757

Final R indices [I>2sigma(I)]

R1 = 0.0394, wR2 = 0.0682

R indices (all data)

R1 = 0.0829, wR2 = 0.0738

Largest diff. peak and hole

0.237 and -0.258 e.Å-3

202 Table A-7. Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2x 103) for Th-CNV-EDOT. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x

y

z

U(eq)

S(1)

-3185(2)

2138(1)

2415(1)

39(1)

S(2)

3516(2)

1093(1)

427(1)

35(1)

O(3)

4139(4)

4375(2)

1261(1)

27(1)

O(4)

7707(4)

4317(2)

307(1)

30(1)

N(1)

-111(7)

-1355(3)

1023(1)

48(1)

C(2)

-2879(7)

880(3)

1934(1)

24(1)

C(3)

-4678(6)

-113(3)

2043(1)

28(1)

C(4)

-6283(7)

146(4)

2517(1)

35(1)

C(5)

-5683(7)

1321(4)

2756(2)

39(1)

C(6)

-843(7)

937(3)

1489(1)

24(1)

C(7)

674(6)

2002(3)

1366(1)

24(1)

C(8)

2708(6)

2202(3)

957(1)

22(1)

C(9)

4267(6)

3322(3)

902(1)

22(1)

C(10)

6038(6)

3292(3)

438(1)

21(1)

C(11)

5862(7)

2144(3)

152(1)

33(1)

C(12)

-438(7)

-321(4)

1211(1)

29(1)

C(15)

5286(7)

5550(3)

1013(1)

33(1)

C(16)

7937(7)

5280(3)

746(1)

34(1)

203 Table A-8. Bond lengths [Å] for Th-CNV-EDOT. Bond

.

Bond Distance (Å)

Bond

Bond Distance (Å)

S(1)-C(5)

1.698(4)

C(2)-C(6)

1.471(4)

S(1)-C(2)

1.718(3)

C(3)-C(4)

1.411(4)

S(2)-C(11)

1.705(3)

C(4)-C(5)

1.348(5)

S(2)-C(8)

1.738(3)

C(6)-C(7)

1.343(4)

O(3)-C(9)

1.367(3)

C(6)-C(12)

1.446(5)

O(3)-C(15)

1.443(4)

C(7)-C(8)

1.421(4)

O(4)-C(10)

1.360(4)

C(8)-C(9)

1.373(4)

O(4)-C(16)

1.432(4)

C(9)-C(10)

1.417(4)

N(1)-C(12)

1.147(4)

C(10)-C(11)

1.347(4)

C(2)-C(3)

1.365(4)

C(15)-C(16)

1.487(4)

204 Table A-9. Bond angles[°] for Th-CNV-EDOT Atoms

Bond Angle (°)

Atoms

Bond Angle (°)

C(5)-S(1)-C(2)

91.76(18)

C(9)-C(8)-C(7)

125.7(3)

C(11)-S(2)-C(8)

92.35(16)

C(9)-C(8)-S(2)

109.1(2)

C(9)-O(3)-C(15)

111.1(2)

C(7)-C(8)-S(2)

125.2(3)

C(10)-O(4)-C(16)

113.0(2)

O(3)-C(9)-C(8)

123.4(3)

C(3)-C(2)-C(6)

128.0(3)

O(3)-C(9)-C(10)

122.6(3)

C(3)-C(2)-S(1)

110.7(2)

C(8)-C(9)-C(10)

114.1(3)

C(6)-C(2)-S(1)

121.2(3)

C(11)-C(10)-O(4)

125.0(3)

C(2)-C(3)-C(4)

112.8(3)

C(11)-C(10)-C(9)

112.1(3)

C(5)-C(4)-C(3)

112.3(3)

O(4)-C(10)-C(9)

122.9(3)

C(4)-C(5)-S(1)

112.4(3)

C(10)-C(11)-S(2)

112.4(3)

C(7)-C(6)-C(12)

121.5(3)

N(1)-C(12)-C(6)

175.9(4)

C(7)-C(6)-C(2)

125.1(3)

O(3)-C(15)-C(16)

112.1(3)

C(12)-C(6)-C(2)

113.3(3)

O(4)-C(16)-C(15)

111.9(3)

C(6)-C(7)-C(8)

131.5(3)

205 Crystal data and structure refinement for EDOT-CNV-EDOP. Empirical formula

C15H12N2O4S

Formula weight

316.33

Temperature

173(2) K

Wavelength

0.71073 Å

Crystal system

Triclinic

Space group

P-1

Unit cell dimensions

a = 8.0521(4) Å

α= 107.617(1)°

b = 9.5708(5) Å

β= 94.428(1)°

c = 10.3402(5) Å

γ = 113.677(1)°

Volume

677.29(6) Å3

Z

2

Density (calculated)

1.551 Mg/m3

Absorption coefficient

0.260 mm-1

F(000)

328

Crystal size

0.49 x 0.19 x 0.18 mm3

Theta range for data collection

2.12 to 27.49°

Index ranges

– 10 ≤ h ≤ 10, – 12 ≤ k ≤ 11, – 9 ≤ l ≤ 13

Reflections collected

4519

Independent reflections

3002 [R(int) = 0.0241]

Completeness to theta = 27.49°

96.0 %

Absorption correction

Integration

Max. and min. transmission

0.9626 and 0.9092

Refinement method

Full-matrix least-squares on F2

Data / restraints / parameters 2

3002 / 0 / 200

Goodness-of-fit on F

1.032

Final R indices [I>2sigma(I)]

R1 = 0.0352, wR2 = 0.0949 [2716]

R indices (all data)

R1 = 0.0386, wR2 = 0.0979

Extinction coefficient

0.003(2)

Largest diff. peak and hole

0.316 and -0.248 e.Å-3

206 Table A-10. Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 x 103) for EDOT-CNV-EDOP. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x

y

z

U(eq)

S1

2340(1)

4027(1)

4923(1)

30(1)

O1

96(2)

4963(1)

8063(1)

29(1)

O2

-1033(2)

1476(1)

6711(1)

36(1)

O3

2515(2)

10477(1)

10477(1)

34(1)

O4

3281(2)

13652(1)

10247(1)

29(1)

N1

4687(2)

8143(2)

4999(2)

36(1)

N2

3528(2)

10737(1)

7263(1)

26(1)

C2

1900(2)

5279(2)

6312(1)

23(1)

C3

707(2)

4340(2)

6930(1)

23(1)

C4

154(2)

2632(2)

6269(2)

26(1)

C5

916(2)

2277(2)

5161(2)

32(1)

C6

2709(2)

7042(2)

6640(1)

23(1)

C7

2550(2)

8130(2)

7772(2)

26(1)

C8

3041(2)

9817(2)

8105(2)

25(1)

C9

2934(2)

10856(2)

9338(2)

25(1)

C10

3329(2)

12368(2)

9223(2)

24(1)

C11

3694(2)

12271(2)

7932(2)

27(1)

C12

3771(2)

7647(2)

5706(1)

26(1)

C13

-1536(2)

3698(2)

8220(2)

32(1)

C14

-1279(2)

2199(2)

8080(2)

37(1)

C15

2142(3)

11714(2)

11426(2)

37(1)

C16

3471(3)

13421(2)

11547(2)

34(1)

207 Table A-11. Bond lengths [Å] for EDOT-CNV-EDOP. Bond

Bond Distance (Å)

Bond

Bond Distance (Å)

S1-C5

1.7126(16)

C6-C7

1.3650(19)

S1-C2

1.7325(13)

C6-C12

1.4382(19)

O1-C3

1.3682(16)

C7-C8

1.4182(19)

O1-C13

1.4468(17)

C7-H7A

0.9500

O2-C4

1.3684(17)

C8-C9

1.3927(19)

O2-C14

1.443(2)

C9-C10

1.3965(19)

O3-C9

1.3614(17)

C10-C11

1.372(2)

O3-C15

1.4434(17)

C11-H11A

0.9500

O4-C10

1.3744(16)

C13-C14

1.497(2)

O4-C16

1.4342(19)

C13-H13A

0.9900

N1-C12

1.1472(19)

C13-H13B

0.9900

N2-C11

1.3658(18)

C14-H14A

0.9900

N2-C8

1.3892(17)

C14-H14B

0.9900

N2-H2A

0.8800

C15-C16

1.510(2)

C2-C3

1.3739(19)

C15-H15A

0.9900

C2-C6

1.4571(18)

C15-H15B

0.9900

C3-C4

1.4243(19)

C16-H16A

0.9900

C4-C5

1.358(2)

C16-H16B

0.9900

C5-H5A

0.9500

208 Table A-12. Bond angles[°] for EDOT-CNV-EDOP. Atoms C5-S1-C2

Bond Angle (°)

Atoms

Bond Angle (°)

93.12(7)

C11-C10-O4

128.99(13)

C3-O1-C13

111.22(11)

C11-C10-C9

107.48(12)

C4-O2-C14

111.95(11)

O4-C10-C9

123.51(13)

C9-O3-C15

110.34(11)

N2-C11-C10

107.99(12)

C10-O4-C16

108.64(11)

N2-C11-H11A

126.0

C11-N2-C8

110.19(11)

C10-C11-H11A

126.0

C11-N2-H2A

124.9

N1-C12-C6

177.03(16)

C8-N2-H2A

124.9

O1-C13-C14

111.21(13)

C3-C2-C6

129.76(12)

O1-C13-H13A

109.4

C3-C2-S1

109.60(10)

C14-C13-H13A

109.4

C6-C2-S1

120.60(10)

O1-C13-H13B

109.4

O1-C3-C2

124.05(12)

C14-C13-H13B

109.4

O1-C3-C4

122.77(12)

H13A-C13-H13B

108.0

C2-C3-C4

113.18(12)

O2-C14-C13

111.46(13)

C5-C4-O2

123.81(13)

O2-C14-H14A

109.3

C5-C4-C3

113.25(13)

C13-C14-H14A

109.3

O2-C4-C3

122.93(13)

O2-C14-H14B

109.3

C4-C5-S1

110.84(11)

C13-C14-H14B

109.3

C4-C5-H5A

124.6

H14A-C14-H14B

108.0

S1-C5-H5A

124.6

O3-C15-C16

111.98(13)

C7-C6-C12

119.07(12)

O3-C15-H15A

109.2

C7-C6-C2

124.42(12)

C16-C15-H15A

109.2

C12-C6-C2

116.50(12)

O3-C15-H15B

109.2

C6-C7-C8

129.76(13)

C16-C15-H15B

109.2

C6-C7-H7A

115.1

H15A-C15-H15B

107.9

C8-C7-H7A

115.1

O4-C16-C15

111.78(13)

N2-C8-C9

105.46(12)

O4-C16-H16A

109.3

209 Table A-12. (Continued) Bond angles[°] for EDOT-CNV-EDOP. Atoms

Bond Angle (°)

Atoms

Bond Angle (°)

N2-C8-C7

128.76(13)

C15-C16-H16A

109.3

C9-C8-C7

125.48(13)

O4-C16-H16B

109.3

O3-C9-C8

126.10(12)

C15-C16-H16B

109.3

O3-C9-C10

125.01(12)

H16A-C16-H16B

107.9

C8-C9-C10

108.88(12)

APPENDIX B CRYSTALLOGRAPHIC INFORMATION FOR FLUORENONE MONOMERS

Figure B-1. Numbering system for fluorenone monomer crystal structures.

∑ ( F0 – Fc ) R1 = -------------------------------------∑ F0 [ w ( F0 – Fc ) ] ∑ ----------------------------------------2 ∑ [ w ( F20 ) ] 2

wR2 =

2 2

1⁄2

1 , w = --------------------------------------------------------------------------------------2 2 2 [ σ ( F 0 ) + ( 0.0370 × p ) + 0.31 × p ]

 w ( F 2 – F 2 ) 2 0 c  ∑ S = -----------------------------------------(n – p)

1⁄2

2

2

[ max ( F 0, 0 ) + 2 × F c ] , p = -----------------------------------------------------3

210

211 Crystal data and structure refinement for BEDOT-DCF. Empirical formula

C40H38N2O4S2Si2

Formula weight

731.02

Temperature

173(2) K

Wavelength

0.71073 Å

Crystal system

Monoclinic

Space group

C2/c

Unit cell dimensions

a = 41.234(2) Å

α= 90°

b = 9.4131(4) Å

β= 109.159(1)°

c = 20.0598(8) Å

γ = 90°

Volume

7354.9(5) Å3

Z

8

Density (calculated)

1.320 Mg/m3

Absorption coefficient

0.254 mm-1

F(000)

3072

Crystal size

0.21 x 0.18 x .07 mm3

Theta range for data collection

2.06 to 27.50°

Index ranges

– 41 ≤ h ≤ 53, – 10 ≤ k ≤ 12, – 25 ≤ l ≤ 25

Reflections collected

27056

Independent reflections

8381 [R(int) = 0.0890]

Completeness to theta = 27.50°

99.2 %

Absorption correction

Integration

Max. and min. transmission

0.9833 and 0.9537

Refinement method

Full-matrix least-squares on F2

Data / restraints / parameters

8381 / 0 / 458

Goodness-of-fit on F2

1.041

Final R indices [I>2sigma(I)]

R1 = 0.0509, wR2 = 0.1234 [5750]

R indices (all data)

R1 = 0.0812, wR2 = 0.1423

Extinction coefficient

0.00031(7)

Largest diff. peak and hole

0.472 and -0.411 e.Å-3

212 Table B-2. Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 x 103) for BEDOT-DCF. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x

y

z

U(eq)

S1

-946(1)

12243(1)

4782(1)

28(1)

Si1

-1582(1)

12576(1)

5259(1)

27(1)

O1

-806(1)

8180(2)

4900(1)

32(1)

O2

-1384(1)

9025(2)

5324(1)

33(1)

N1

-328(1)

5720(2)

4428(1)

36(1)

C2

-746(1)

10675(2)

4688(1)

24(1)

C3

-906(1)

9571(2)

4899(1)

25(1)

C4

-1184(1)

9995(2)

5120(1)

25(1)

C5

-1247(1)

11431(2)

5089(1)

26(1)

C6

-469(1)

10668(2)

4383(1)

23(1)

C7

-334(1)

9374(2)

4234(1)

25(1)

C8

-83(1)

9410(2)

3914(1)

23(1)

C9

38(1)

10706(2)

3734(1)

24(1)

C10

-86(1)

11980(3)

3898(1)

29(1)

C11

-337(1)

11951(3)

4222(1)

29(1)

C12

102(1)

8241(2)

3701(1)

23(1)

C13

75(1)

6828(3)

3821(1)

28(1)

C14

-153(1)

6245(2)

4161(1)

29(1)

C15

-1068(1)

7204(3)

4936(2)

34(1)

C16

-1222(1)

7652(3)

5486(2)

34(1)

C17

-1775(1)

13713(3)

4468(2)

45(1)

C18

-1372(1)

13771(3)

6012(2)

42(1)

C19

-1908(1)

11443(3)

5444(2)

56(1)

S1'

1320(1)

9702(1)

2361(1)

31(1)

Si1'

1955(1)

8652(1)

1994(1)

30(1)

O1'

807(1)

6214(2)

2226(1)

32(1)

213 Table B-2. (Continued) Atomic coordinates (x 104) and equivalent isotropic displacement parameters (Å2 x 103) for BEDOT-DCF. U(eq) is defined as one third of the trace of the orthogonalized Uij tensor. x

y

z

U(eq)

O2'

1418(1)

5823(2)

1836(1)

33(1)

N1'

437(1)

4891(3)

3482(2)

73(1)

C2'

986(1)

8662(3)

2433(1)

26(1)

C3'

1030(1)

7302(3)

2232(1)

25(1)

C4'

1329(1)

7121(3)

2035(1)

26(1)

C5'

1521(1)

8320(3)

2080(1)

30(1)

C6'

735(1)

9237(2)

2733(1)

25(1)

C7'

551(1)

8343(3)

3036(1)

26(1)

C8'

329(1)

8914(2)

3355(1)

24(1)

C9'

286(1)

10403(2)

3374(1)

23(1)

C10'

464(1)

11296(2)

3068(1)

28(1)

C11'

684(1)

10712(3)

2749(1)

29(1)

C14'

276(1)

5766(3)

3624(2)

45(1)

C15'

955(1)

4840(3)

2187(2)

36(1)

C16'

1135(1)

4844(3)

1641(2)

35(1)

C17'

2285(1)

8607(4)

2886(2)

51(1)

C18'

1944(1)

10463(3)

1628(2)

48(1)

C19'

2055(1)

7314(4)

1411(2)

56(1)

C20

-1877(1)

12466(3)

1325(2)

52(1)

C21

-2060(1)

12840(3)

651(2)

48(1)

C22

-2246(1)

14079(3)

516(2)

45(1)

C23

-2247(1)

14957(3)

1066(2)

46(1)

C24

-2065(1)

14587(4)

1744(2)

55(1)

C25

-1877(1)

13334(4)

1878(2)

55(1)

214 Table B-3. Bond lengths [Å] for BEDOT-DCF. Bond

Bond Distance (Å)

Bond

Bond Distance (Å)

S1-C2

1.729(2)

S1'-C2'

1.732(2)

S1-C5

1.737(2)

S1'-C5'

1.734(3)

Si1-C19

1.848(3)

Si1'-C18'

1.851(3)

Si1-C18

1.854(3)

Si1'-C19'

1.856(3)

Si1-C17

1.863(3)

Si1'-C17'

1.858(3)

Si1-C5

1.868(2)

Si1'-C5'

1.878(3)

O1-C3

1.372(3)

O1'-C3'

1.376(3)

O1-C15

1.439(3)

O1'-C15'

1.444(3)

O2-C4

1.379(3)

O2'-C4'

1.373(3)

O2-C16

1.442(3)

O2'-C16'

1.439(3)

N1-C14

1.144(3)

N1'-C14'

1.151(4)

C2-C3

1.369(3)

C2'-C3'

1.372(3)

C2-C6

1.464(3)

C2'-C6'

1.461(3)

C3-C4

1.418(3)

C3'-C4'

1.421(3)

C4-C5

1.373(3)

C4'-C5'

1.366(4)

C6-C11

1.404(3)

C6'-C7'

1.400(3)

C6-C7

1.411(3)

C6'-C11'

1.406(3)

C7-C8

1.388(3)

C7'-C8'

1.385(3)

C8-C9

1.408(3)

C8'-C9'

1.415(3)

C8-C12

1.479(3)

C9'-C10'

1.382(3)

C9-C10

1.386(3)

C10'-C11'

1.386(3)

C9-C9'

1.461(3)

C15'-C16'

1.511(4)

C10-C11

1.392(4)

C20-C21

1.361(5)

C12-C13

1.362(3)

C20-C25

1.378(5)

C12-C8'

1.479(3)

C21-C22

1.372(4)

C13-C14'

1.434(4)

C22-C23

1.379(4)

C13-C14

1.439(4)

C23-C24

1.366(5)

215 Table B-4. Bond angles[°] for BEDOT-DCF. Atoms

Bond Angle (°)

Atoms

Bond Angle (°)

C2-S1-C5

94.85(11)

C10-C11-C6

121.8(2)

C19-Si1-C18

111.30(16)

C13-C12-C8’

'127.0(2)

C19-Si1-C17

111.44(16)

C13-C12-C8

126.6(2)

C18-Si1-C17

107.49(14)

C8'-C12-C8

106.37(19)

C19-Si1-C5

109.49(12)

C12-C13-C14’

123.0(2)

C18-Si1-C5

108.86(12)

C12-C13-C14

124.1(2)

C17-Si1-C5

108.17(12)

C14'-C13-C14

113.0(2)

C3-O1-C15

112.28(19)

N1-C14-C13

176.8(3)

C4-O2-C16

112.48(19)

O1-C15-C16

111.3(2)

C3-C2-C6

130.3(2)

O2-C16-C15

111.2(2)

C3-C2-S1

108.71(18)

C2'-S1'-C5’

94.69(12)

C6-C2-S1

120.92(18)

C18'-Si1'-C19’

110.84(17)

C2-C3-O1

123.4(2)

C18'-Si1'-C17’

108.55(16)

C2-C3-C4

113.8(2)

C19'-Si1'-C17’

110.27(17)

O1-C3-C4

122.8(2)

C18'-Si1'-C5’

106.83(13)

C5-C4-O2

122.9(2)

C19'-Si1'-C5’

111.19(13)

C5-C4-C3

115.1(2)

C17'-Si1'-C5’

109.06(14)

O2-C4-C3

122.0(2)

C3'-O1'-C15’

111.90(19)

C4-C5-S1

107.57(18)

C4'-O2'-C16’

112.08(19)

C4-C5-Si1

134.43(19)

C3'-C2'-C6’

130.3(2)

S1-C5-Si1

117.93(13)

C3'-C2'-S1’

108.49(18)

C11-C6-C7

119.0(2)

C6'-C2'-S1’

120.96(18)

C11-C6-C2

120.4(2)

C2'-C3'-O1’

123.0(2)

C7-C6-C2

120.6(2)

C2'-C3'-C4’

113.9(2)

C8-C7-C6

118.9(2)

O1'-C3'-C4’

123.2(2)

C7-C8-C9

121.4(2)

C5'-C4'-O2’

123.3(2)

C7-C8-C12

130.5(2)

C5'-C4'-C3’

114.8(2)

216 Table B-4. (Continued) Bond angles[°] for BEDOT-DCF. Atoms

Bond Angle (°)

Atoms

Bond Angle (°)

C9-C8-C12

108.2(2)

O2'-C4'-C3’

121.9(2)

C10-C9-C8

119.9(2)

C4'-C5'-S1’

108.14(19)

C10-C9-C9’

131.3(2)

C4'-C5'-Si1’

133.0(2)

C8-C9-C9’

108.8(2)

S1'-C5'-Si1’

'118.53(15)

C9-C10-C11

118.9(2)

C7'-C6'-C11

'118.5(2)

C7'-C6'-C2’

121.0(2)

C10'-C11'-C6’

121.9(2)

C11'-C6'-C2’

120.4(2)

N1'-C14'-C13

178.1(3)

C8'-C7'-C6’

120.1(2)

O1'-C15'-C16’

110.8(2)

C7'-C8'-C9’

120.3(2)

O2'-C16'-C15’

110.7(2)

C7'-C8'-C12

131.8(2)

C21-C20-C25

120.0(3)

C9'-C8'-C12

107.9(2)

C20-C21-C22

120.5(3)

C10'-C9'-C8’

120.1(2)

C21-C22-C23

119.9(3)

C10'-C9'-C9

131.2(2)

C24-C23-C22

120.0(3)

C8'-C9'-C9

108.7(2)

C23-C24-C25

119.9(3)

C9'-C10'-C11’

119.1(2)

C20-C25-C24

119.7(3)

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BIOGRAPHICAL SKETCH Christopher A. Thomas was born in Santa Barbara county, California in 1974 where he lived for four years. After an eight year hiatus in Austin, Texas, he returned to Southern California to attend high school in Thousand Oaks and undergraduate school at UCLA. After obtaining a B.S. in chemistry in 1996 he moved to North Central Florida to study with John Reynolds at the University of Florida.

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