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Conjugated Polymers via Electrochemical Polymerization of Thieno[3,4-b]thiophene (T34bT) and 3,4-Ethylenedioxythiophene (EDOT) Venkataramanan Seshadri, Linda Wu, and Gregory A. Sotzing* Department of Chemistry and the Polymer Program, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, Connecticut 06269 Received March 10, 2003. In Final Form: August 8, 2003 Herein the electrochemical polymerizations of thieno[3,4-b]thiophene (T34bT) and 3,4-ethylenedioxythiophene (EDOT) and the electrochemical copolymerizations of T34bT and EDOT carried out in tetrabutylammonium perchlorate and tetrabutylammonium hexafluorophosphate electrolyte are compared utilizing cyclic voltammetry, chronocoulometry, and chronogravimetry. Simultaneous polymerization of T34bT and EDOT produced a copolymer, as confirmed by vis-NIR spectroscopy. The simultaneous polymerization of T34bT and EDOT exhibits a peak for monomer oxidation intermediate to the monomer oxidation peaks for each of the single component monomer systems. Furthermore, the copolymer exhibited tremendous stability to n-doping in comparison to either of the homopolymers of T34bT or EDOT. Polymerization rates as determined via chronogravimetry were found to be highest for EDOT with the lowest rate being obtained for the copolymerization of T34bT and EDOT. All polymers exhibited high anion dominancy (>80%) with the highest value, ∼92%, exhibited by PT34bT. Doping levels ranging from 27 to 38% were obtained for polymers containing T34bT. PEDOT was determined to exhibit doping levels of approximately 15%.
Introduction One significant research topic within the field of intrinsically conducting polymers (ICPs) is the preparation of polymers that are both optically transparent and highly conductive in the doped state and have the capability of being both easily p- and n-doped. The quest for very stable low band gap ICPs has been the prime research of several groups due to their potential application in many device applications such as electrochromics,1 type III supercapacitors,2 and all polymeric batteries.3 Some examples of low band gap polymers include poly(isothianaphthene)4 and both vinylene and cyanovinylene containing polymers.5 Other low band gap ICPs have been reviewed elsewhere.6 Poly(3,4-ethylenedioxythiophene) (PEDOT) has been classified as a low band gap conducting polymer and has gained popularity among the ICP research community. Thin films of PEDOT are sky-blue in the oxidized p-doped state and have been reported to have high environmental stability. Polymers from thieno[3,4-b]thiophene (T34bT) have been reported to have band gaps approximately half that of PEDOT. Although thieno[3,4-b]thiophene (T34bT) was first prepared by Wynberg in 1967,7 there has been no report of any ICP being synthesized from it until recently. Theoretically, it was proposed that the polymer (1) Pielartzik, H.; Heuer, H.; Wehrmann, R.; Bieringer, T. Kunststoffe 1999, 89, 135. (2) Bolognesi, A.; Catellani, M.; Destri, S.; Zamboni, R.; Taliani, C. J. Chem. Soc., Chem. Commun. 1988, 246. (3) Novak, P.; Muller, K.; Santhanam, K. S. V.; Haas, O. Chem. Rev. 1997, 97, 207. (4) (a) Kobayashi, M.; Colaneri, N.; Boysel, M.; Wudl, F.; Heeger, A. J. J. Chem. Phys. 1985, 82, 5717. (b) Colerneri, N.; Kobayashi, M.; Heeger, A. J.; Wudl, F. Synth. Met. 1986, 14, 45. (c) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382. (5) (a) Martinez, M.; Reynolds, J. R.; Basak, S.; Black, D. A.; Marynick, D. S.; Pomerantz, M. J. Polym. Sci. B 1988, 26, 911. (b) Ho, H. A.; Brisset, H.; Frere, P.; Roncali, J. J. Chem. Soc., Chem. Commun. 1995, 2309. (6) Roncali, J. Chem. Rev. 1997, 97, 173. (7) Wynberg, H.; Zwanenburg, D. J. Tetrahedron Lett. 1967, 9, 761.
produced by coupling through the 4 and 6 positions of T34bT should have a very low band gap,8 and this was demonstrated only recently by Pomerantz9 and Ferraris10 in their polymerizations of 2-substituted T34bTs. Recently, we have reported initial investigations on the electrochemical polymerization of thieno[3,4-b]thiophene.11 T34bT has three open R-positions for oxidative coupling and, hence, when polymerized could produce a branched or cross-linked polymer (PT34bT) with conjugation through the branch/cross-link junctions. We have reported PT34bT to have a very low band gap (∼0.85 eV) as measured by the onset for the π to π* transition and to be stable upon oxidative redox switching. Furthermore, we have demonstrated PT34bT to be stable to p-doping and moderately stable to redox cycling through the n-doped state. PT34bT is highly transparent in the oxidized conducting state and light-blue in the neutral insulating state. Various copolymers consisting of EDOT have been previously reported.12 Most of them have been modified EDOT. For example, EDOT covalently bound to moieties with selective ion or biomolecules binding ability.13 Polymers prepared from these are useful in sensor applications. In the field of commodity polymers, several copolymers have been prepared by mixing two or several monomers and polymerizing by addition or step polymerization techniques, for example, styrene-co-maleic anhydride, vinyl acetate-co-methyl methacrylate, and so forth. These copolymers are relatively easy to characterize, as most of these are soluble. Within the field of conducting (8) Hong, S. Y.; Marynick, D. S. Macromolecules 1992, 25, 4652. (9) Pomerantz, M.; Gu, X.; Zhang, S. X. Macromolecules 2001, 34, 1817. (10) Neef, C. J.; Brotherston, I. D.; Ferraris, J. P. Chem. Mater. 1999, 11, 1957. (11) Sotzing, G. A.; Lee, K. Macromolecules 2002, 35, 7281-7286. (12) (a) Beouch, L.; Tran Van, F.; Stephan, O.; Vial, J. C.; Chevrot, C. Synth. Met. 2001, 122, 351. (b) Stephan, O.; Tran-Van, F.; Chevrot, C. Synth. Met. 2002, 131, 31. (13) (a) Zong, K.; Reynolds, J. R. J. Org. Chem. 2001, 66, 6873. (b) Kros, A.; Van Hovell, S. W. F. M.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. Adv. Mater. 2001, 13, 1555.
10.1021/la0344128 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/27/2003
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polymers, there have been only a few reports of simultaneous electrochemical polymerizations due to limited characterization techniques, owing to their poor solubility in known organic solvents.14 Herein we report a comparative electrochemical polymerization study between EDOT and T34bT. Furthermore, we report the simultaneous electrochemical polymerization of these two monomers to produce stable, highly optically transparent n-dopable polymers. Electrochemical p- and n-doping experiments have been utilized to determine the band gap, and this was verified utilizing vis-NIR spectroscopy. The electrochemical quartz crystal microbalance (EQCM) was used to determine the ion transport behavior of thin films of the homopolymers and polymers prepared via simultaneous polymerization of EDOT and T34bT. Furthermore, polymerization kinetics, measured as mass of polymer repeat unit deposited per unit time in the presence of different electrolytes, were determined using the EQCM. The EQCM was also utilized to determine ion transport behavior in addition to the dopant levels of the polymers. Experimental Section Chemicals. Ethyl acetate was purchased from Fisher Scientific and used without further purification. Acetonitrile (ACN) was purchased from J.T. Baker Inc. and was distilled over calcium hydride prior to use. Tetra(n-butyl)ammonium hexafluorophosphate (TBAPF6) was purchased from Acros and used without further purification. Tetra(n-butyl)ammonium perchlorate (TBAP) was prepared by the addition of 70% perchloric acid (J.T. Baker Inc.) solution to a slight molar excess of an aqueous solution of tetra(n-butyl)ammonium bromide (Purchased from Acros and used without further purification). Warning: Perchloric acid and organic perchlorates are known to be explosive when heated. Furthermore, perchloric acid is a strong oxidizing acid and should never be stored or mixed with organics. Proper safety and precautions are necessary when handling both perchloric acid and organic perchlorates. TBAP thus prepared was recrystallized from ethyl acetate and dried in vacuo at room temperature for 2 days before use. 3,4-Ethylenedioxythiophene (EDOT) was procured from Aldrich and distilled under reduced pressure. T34bT was synthesized and purified according to our previously reported procedure.11 Instrumentation. A CH Instruments 660A potentiostat was used for all the electrochemical polymerizations carried out on button working electrodes, while a CH Instruments 430 potentiostat equipped with an oscillator circuit was used for the EQCM studies. Optical studies on the polymer were performed on a Perkin-Elmer Lambda 900 UV-vis-NIR spectrophotometer. General Electrochemistry. Electrochemical polymerizations were carried out in a conventional three electrode cell using either Pt or Au button working electrodes of 2 mm diameter, a 1 cm2 platinum flag counter electrode, and a nonaqueous Ag/0.01 M Ag+ (silver wire in 0.1 M TBAP or 0.1 M TBAPF6 in ACN) reference electrode. The reference electrode was calibrated to be 0.456 V versus the standard hydrogen electrode (SHE) using a ferrocene standard solution. Electrochemical polymerizations were carried out using cyclic voltammetry. Solutions used for the electrochemical studies were prepared from freshly distilled ACN and contained 10 mM monomer and 0.1 M electrolyte. For copolymerization studies, the concentration of each monomer was 5 mM to give a total monomer concentration of 10 mM. After the deposition of the ICPs onto the button working electrodes, the ICPs were washed with ACN and then cycled in a monomer free electrolyte solution during electrochemical characterizations of the polymers. EQCM Studies. Polished quartz crystals were purchased from International Crystal Manufacturing. The crystals had a resonant frequency of approximately 7.995 MHz and were coated with a 0.201 in. diameter key-electrode on either side of the crystal. (14) Latonen, R. M.; Kvarnstorm, C.; Ivaska, Ari. Electrochim. Acta 1999, 44, 1933.
Seshadri et al. Scheme 1. Oxidative Polymerization of (A) EDOT and (B) T34bT, and (C) Simultaneous Polymerization of EDOT and T34bTa
a (i) Electrochemical oxidative polymerization. (ii) Electrochemical polymer reduction.
The key-electrode comprised of a 1000 Å thick gold coating with a 100 Å chromium underlay was soldered to leads for electrical contact that were sealed away from the solution. A 1 cm2 platinum flag was used as the counter electrode, and a nonaqueous Ag/0.01 M Ag+ was used as the reference electrode. The monomers were polymerized onto polished gold-coated quartz crystals by stepping to a potential of 1.15 V (vs Ag/Ag+) for 3 s and then stepping to -1.0 V (vs Ag/Ag+). Deposited polymers were then washed with ACN before carrying out electrochemical characterization in monomer-free electrolyte solution. Optical Studies. Indium doped tin oxide (ITO) coated glass (dimensions 7 mm × 50 mm × 0.7 mm) with a nominal resistance of 100 Ω was purchased from Delta Technologies. ICPs were deposited electrochemically onto ITO by potential cycling through the potentials previously mentioned in the General Electrochemistry section. The polymer thus grown was in the neutral form at the end of the preparation and was further treated with hydrazine to ensure complete reduction of the ICPs to the neutral form before spectra were obtained. Elemental Analysis. The copolymers were electrochemically prepared on stainless steel plates (3 in. × 3 in.) at a constant potential of 1.15 V (vs Ag/Ag+) for approximately 5 min from a 250 mL of stock solution containing acetonitrile, 0.1 M electrolyte (TBAPF6 or TBAP), 5 mM EDOT, and 5 mM T34bT. The copolymer thus obtained on the stainless steel plate was further reduced at -1 V for 2 min and washed with acetonitrile. The copolymer was then removed from the electrode and dried under vacuum (0.1 Torr) overnight. Elemental analysis was carried out at Galbraith Laboratories Inc. For the copolymer prepared using TBAP as the electrolyte: C, 46.82; S, 33.97; H, 2.59; N, 10) and was further confirmed using rapid redox switching (five switches per second). Low nitrogen content of less than 0.5% from the elemental analysis of the neutral conjugated copolymers supports the high anion dominancy calculated from the chronogravimetry and concurrent chronocoulometry experiments. For example, the percent nitrogen calculated from the percent cation (tetrabutylammonium) retained in the neutral copolymers switched in TBAP and TBAPF6 using chronogravimetry was found to be 0.32% and 0.25%, respectively. The doping level was calculated from the fraction of charge passed to dope the polymer by a constant potential to the number of moles of polymer repeat unit deposited
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Figure 7. Vis-NIR spectrum of (A) neutral PEDOT deposited over PT34bT and (B) neutral copolymer containing T34bT and EDOT obtained from electrochemical polymerization in (s) TBAP and (- - -) TBAPF6.
on the working electrode. PT34bT and PT34bT-co-EDOT were determined to have maximum doping levels of 33.0 and 38.6%, whereas PEDOT had a maximum doping level of 15% for the two electrolytes studied. Earlier, Chevrot et al. had determined the doping level of PEDOT prepared from acetonitrile and TBAP to be 30%. It should be noted that the method for determination of the doping level reported by Chevrot was not gravimetric but solely determined from the charge passed during cyclic voltammetry.22 The doping level of 15% reported here for PEDOT is very close to the value of 20% reported by Efimov et al.23 Vis-NIR Spectroscopy. The band gap of the copolymers was calculated from the onset of the π to π* transition obtained from the vis-NIR spectrum of the neutral polymer. To test that the simultaneous polymerization of T34bT and EDOT does not yield a mixture of the two homopolymers, the individual polymers were grown on ITO as two layers, one atop the other, and the vis-NIR spectrum in the neutral state of these two polymers was recorded (Figure 7A). The spectrum of this layered structure shows two peaks corresponding to the λmax of each homopolymer. As is evident from the spectrum, the two peaks corresponding to the homopolymers are well separated and the addition of these two spectra in any ratio does not yield a single transition at an intermediate value. Figure 7B shows the onset and peak for the π to π* transition for (22) Randriamahazaka, H.; Noel, V.; Chevrot, C. J. Electroanal. Chem. 1999, 472, 103. (23) Efimov, I.; Winkels, S.; Schultze, J. W. J. Electroanal. Chem. 2001, 499, 169.
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the copolymers produced using TBAP and TBAPF6 as the dopant. T34bT and EDOT copolymerize, since the spectra obtained from the polymers resulting from the simultaneous polymerizations of T34bT and EDOT show a single broad transition without any shoulders. The onset of the π to π* transition was measured to be 1.06 eV (1170 nm) with a λmax at 1.65 eV (750 nm) for copolymers produced using perchlorate as the dopant, while for the copolymer produced using hexafluorophosphate as the dopant it was measured to be 1.19 eV (1040 nm) with a λmax at 1.91 eV (650 nm). The shift in λmax for the polymers produced using two different electrolytes indicates that the electrolyte plays an important role in the optical properties of the copolymer being formed. Although the copolymers were formed using the same molar ratio of monomers in solution, the copolymer composition need not be the same as that of the monomer feed. Since these polymers are insoluble, the elemental composition of the copolymer was calculated using elemental analysis, the results of which are presented in the Experimental Section. The carbon content is also comprised of the cation, tetrabutylammonium, which has 16 carbon atoms per mole. Thus, from the dopant levels and percent cations transported as determined from gravimetry and coulometry, the percent nitrogen in the neutral copolymer was determined to be 0.32% or 0.25% when TBAP or TBAPF6 was used as the electrolyte, respectively. From the percent nitrogen, the percent carbon from the electrolyte was calculated to be 4.34% or 3.47% for copolymers prepared using TBAP or TBAPF6 as the electrolyte, respectively. After subtracting these amounts of carbon, the ratio of carbon to sulfur was calculated to be 1.25 or 1.34 when TBAP or TBAPF6 was used as the electrolyte. EDOT has only one sulfur atom per repeat unit compared to two sulfurs for T34bT. Thus, the EDOT/T34bT percent composition in the copolymers as calculated from the C/S ratio was determined to be 20.1:79.9 (TBAP) and 32.3:67.7 (TBAPF6). As observed, the copolymer containing more T34bT results in a lower band gap. These copolymers most likely are random given the nature of the polymerization. Conclusion In accordance with vis-NIR spectroscopy, copolymers are produced from the simultaneous oxidative electrochemical polymerizations of T34bT and EDOT. These copolymers exhibit λmax values intermediate to those of both PT34bT and PEDOT, thereby making them low band gap materials. Furthermore, the optical properties of these polymers are highly dependent upon the electrolyte used during polymerization. PT34bT-co-EDOT was found to exhibit unprecedented stability to n-doping, as indicated by the stability in the current response upon 15 redox cycles. In general, polymers consisting of T34bT showed approximately 15% higher doping levels and approximately 10% more anion dominancy than polymers solely consisting of EDOT. The rate of EDOT polymerization was two times higher than that of T34bT polymerization. Furthermore, deposition rates obtained for the copolymerization of EDOT and T34bT were found to be inferior to the deposition rates resulting from either of the homopolymerizations of EDOT or T34bT. The EDOT/ T34bT percent compositions in the copolymers prepared from TBAP and TBAPF6 electrolyte solutions were calculated from elemental analysis of the neutral copolymers as 20.1:79.9 and 32.3:67.7, respectively. The copolymer prepared from TBAP contains, on average, more T34bT and, as such, exhibited a maximum absorbance at a longer wavelength.
Electrochemical Polymerization of T34bT and EDOT
Acknowledgment. The authors would like to thank Professor James Rusling for use of his EQCM and would like to thank both the Petroleum Research Fund (Type G)
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and the National Science Foundation, DMR REU, for partial support of this work. LA0344128
