Chem. Mater. 1996, 7, 58-68
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Poly(3,4=ethylenedithiathiophene). A New Soluble Conductive Polythiophene Derivative Chenggang Wang,? Jon L. Schindler,*Carl R. Kannewurf,S and Mercouri G. Kanatzidis*??>g Department of Chemistry and the Center for Fundamental Materials Research, Michigan State University, East Lansing, Michigan 48824, and Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208-3118 Received June 7,1994. Revised Manuscript Received September 22, 1994@
A new polythiophene derivative has been synthesized by both chemical and electrochemical oxidative polymerization of the monomer 3,4-ethylenedithiathiophene(EDTT). Both methods (NMP) and partly produce a polymer which is completely soluble in 1-methyl-2-pyrrolidinone soluble in tetrahydrofuran (THF) and chloroform. The FT-IR spectra of the neutral polymer indicate a regular structure formed via a,a coupling of thiophene rings. The new polymer shows two absorption bands at 341 nm and 413-419 nm in NMP solution in the UV-vis region. Photoexcitation of the polymer in dilute NMP solution results in a broad band luminescence with peak at ca. 552 nm. The redox behavior of the polymer was studied by cyclic voltammetry in 0.1 M ( B d ) C 1 0 4 acetonitrile solution. The average molecular weights have been determined by gel permeation chromatography (GPC) to be M, 3.03 x lo3 and M , 4.75 x lo3 for the chemically and electrochemically synthesized polymer, respectively. Electron spin resonance data are reported. Thermal gravimetric analysis studies show that the new polymer is stable in nitrogen up to 276 “C. The chemically doped (with FeCL-) polymer and the electrochemically doped (with c104-)polymer show electrical conductivity of 0.1 and 0.4 S/cm at room temperature, respectively. These results are compared to some previously characterized polythiophenes.
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Introduction Polythiophenes and their derivatives have been investigated intensely because of their interesting electrical and electronic properties and relatively good environmental stabi1ity.l Significant experimental and theoretical effort2has been focused on the modification of their chemical structures, in order to alter their electronic structures and improve their electrical properties, environmentalstability, and processability. For example, substituting long alkyl chains on the 3-position of thiophene affords a series of processable and highly conductive poly(3-alkylthiophene~).~These properties t Michigan State University.
*8 A.Northwestern University P. Sloan Foundation Fellow 1991-93 and Camille and Henry Dreyfus Teacher Scholar 1993-95. Abstract published in Advance ACS Abstracts, November 1,1994. (1)For reviews, see: (a) Roncali, J. Chem. Rev.1992,92,711-738. (b) Yamamoto, T. Progr. Polym. Sci. 1992, 17, 1153-1205. (c) Tourillon, G.,Skotheim, T. J., Eds. Handbook of Conducting Polymers; Marcel Dekker: New York, 1986;Vol. I, pp 293-350. (d) Gustafsson, G.; Inganas, 0.; Salanek, W. R.; Laasko, J.; Loponen, M.; Taka, T.; bsterholm, J.-E.; Stubb, H.; Hjertberg, T. In Conjugated Polymers; Bredas, J. L., Silbey, R., Eds.; Kluwer Academic Publishers: Netherlands, 1991;pp 315-362. (2)Experimental: (a)Souto-Maior, R. M.; Hinkelmann, K.; Eckert, H.; Wudl, F. Macromolecules 1990, 23, 1268-1279. (b) Faid, K.; Cloutier, R.; Leclerc, M. Macromolecules 1993,26, 2501-2507. (c) Roncali, J.; Gorgues, A.; Jubault, M. Chem. Muter. 1993,5 , 14561464. (d) Ritter, S.K.; NoRle, R. E.; Ward, A. E. Chem. Muter. 1993, 5,752-754. (d) Yamamoto, T.;Sanechica, K.; Yamamoto, A. J . Polym. Sci., Polym. Lett. Ed. 1980,18,9-12. (e) Yamamoto, T.; Sanechica, K.; Yamamoto, A. Bull. Chem. SOC.Jpn. 1983, 56, 1503-1507. Theoretical: (a) Hong, S. Y.;Maqmick, D. S. Macromolecules 1992, 25,4652-4657. (b) Bredas, J. L.; Themans, B.; Andre, J. M.; Heeger, A. J.; Wudl, F. Synth. Met. 1986,11,343-352. (c) Otto, P.;Ladik, J . Synth. Met. 1990, 15, 327-335. (d) Quattrochi, C.; Lazzaroni, R.; Bredas, J. L.; Zamboni, R.; Taliani, C. Macromolecules 1993,26,12601264. @
0897-4756l95I2807-0058$09.00l0
have been attributed to the higher solubility of polymers with long alkyl chains and suppression of unfavorable 2,4’-couplings that interrupt the conjugation system along the polymer backbone. Furthermore, using regiochemically defined oligothiophenes can lead to more regular conjugated polymers, due to the reduced number of 2,4-mi~linkages.~ Fusing a benzene ring on the 3,4position of thiophene leads to a low bandgap (1.0 eV) “transparent”conducting p~ly(isothianaphthene).~ “his happens because the more stable planar quinoid form of the thiophene is favored with the presence of the benzene ring.6 Recently, the synthesis of nearly 100% head-to-tail poly(alky1thiophene) has been reported, which shows great improvement of conjugation chain length and high electrical c~nductivity.~ In addition to the numerous studies on poly(3-alkylthiophenes),polymers generated from 3,kdialkylthiophene monomers also attracted considerable interest due to the fact that no possibility exists for a,/3’ or ,4,pcoupling of monomers during the polymerization process. This approach has (3)(a) Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F. Macromolecules 1987,20,212-215.(c) Leclerc, M.; Diza, F.; Wegner, G. Mukromol. Chem. 1989, 190, 3105. (d) Sato, M.; Tanaka, S.; Kaeriyama, K J. Chem. Soc., Chem. Commun. 1986,873-874. (4)(a)Roncali,J.; Garnier, F.; Lemaire, M.; Garreau, R. Synth. Met. 1986,15,323-331. (b) Delabouglise, D.;Hmyene, M.; Horowitz, G.; Yassar, A.; Garnier, F.Adu. Muter. 1992,4,107-110. ( 5 ) (a) Wudl, F.; Kobayashi, M.; Heeger, A. J. J . Org. Chem. 1984, 49,3382-3384. (b) Kobayashi, M.; Colaneri, N.; Boysel, M.; Wudl, F.; Heeger, A. J . J . Chem. Phys. 1985,82,5717-5723. (6)(a)Nayak, K.;Marynick, D. S. Macromolecules 1990,23,22372245. (b) Lee, Y. S.; Kertesz, M.; Elsenbaumer, R. L. Chem. Muter. 1990,2,526-530. (7)(a) McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L. J . Org. Chem. 1993, 58, 904-912. (b) McCullough, R. D.; Williams. S. P. J . Am. Chem. SOC.1993. 115. 11608-11609. (c) McCullough, R. D.; Tristam-Nagle, S.; Williams; 5.P.; Lowe. R. D.. Jayaraman, M. J . Am. Chem. SOC.1993,115,4910-4911.
0 1995 American Chemical Society
A New Soluble Conductive Polythiophene Derivative
been drastically limited by the large steric hindrance of the two substituents,s which forces the polymer backbone out of planarity and results in loss of conjugation and reduced electrical conductivity. Cyclization between 3,4 positions of thiophene considerably reduces the steric h i n d r a n ~ e .Recently, ~ we reported the synthesis and properties of the new polythiophene derivative poly(3’,4’-dibutyl-2,2’:5’,2”-terthiophene)[poly(DBTT)I,1° as another approach to reduce the steric hindrance and yield an ordered, soluble, conjugated polymer. In the poly(DBTT) backbone, every dibutylsubstituted thiophene unit is separated by two nonsubstituted thiophene units (acting as steric diluents). The reduced number of side chains provides more space for the extension of the two butyl groups, minimizes extensive steric effects, and thus leads to a more coplanar conjugated polymer. In solution and in the solid state the poly(DBTT1appears to possess one of the longest chain conjugation lengths among polythiophenes. Relative to monosubstituted polythiophenes, disubstituted polythiophenes are less well explored. To date, there have been only a few reports on the study of mercapto-substituted polythiophenes, in addition to alkyl, alkoxy and mixed alkyl, alkoxy disubstituted polythiophenes.ll Poly[(3-methylmercapto)thiophenel (PMMT)has been prepared both chemically and electrochemicallysaJ2 and shows conductivity in the order of 10-1-10-2 S/cm. Poly[3-(ethylmercapto)thiophenel (PEMT) and poly[3,4-bis(ethylmercaptoXhiophenel(PBEMT) have been synthesized via a nickel-catalyzed Grignard coupling reaction of the corresponding 2,5-dihalogeno m0n0mers.l~Both polymers are soluble in common organic solvents and semiconducting and lo-’ S/cm, respectively) in the oxidized state. However, both the 3-(ethy1mercapto)thiophene (EMT) and 3,4-bis(ethy1mercapto)thiophene (BEMT) monomers fail to be electrochemically polymerized because neither monomer has significant positive spin density a t both a-carbons on the thiophene rings according to the theoretical calculations. l3 Recently, a new polythiophene derivative, poly(3,4-ethylenedioxythiophene)(PEDT),has been briefly reported.14 PEDT can be synthesized by either chemical or electrochemical polymerization of the corresponding 3,4-ethylenedioxythiophene(EDT)monomer and results in the conductivity of 15-19 and 200 S/cm, respectively. Unfortunately, PEDT is insoluble and infusible, which limits its further characterization. When a solution of the 3,4-ethylenedioxythiophene (EDT) monomer reacts with a thin layer of poly(viny1 acetate) containing an iron(II1) salt, a transparent film (8)(a) Gourillon, G.; Garnier, F. J.Electroanal. Chem. 1984,161, 51-58. (b) Roncali, J.; Garreau, R.; Yassar, A.; Marque, P.; Garnier, F.; Lemaire, M. J. Phys. Chem. 1987,91,6706-6714. (9)Roncali, J.; Garnier, F.; Garreau, R.; Lemaire, M. J.Chem. SOC., Chem. Commun. 1987,1500-1502. (10)Wang, C.;Benz, M.; LeGoff, E.; Schindler, J. L.; AlbrittonThomas, J.; Kannewurf, C.R.; Kanatzidis, M. G. Chem. Mater. 1994, 6,401-411. (11)(a) Feldhues, M.; Kampf, G.; Litterer, H.; Mecklenburg, T.; Wegener, P. Synth. Met. 1989,28,C487-C493. (b) Daoust, G.; Leclerc, M. Macromolecules 1991,24,455-459. (12) (a)Elsenbaumer, R. L.; Jen, K. Y.; Oobodi, R. Synth. Met. 1986, 15, 169-174. (b) Tanaka, S.;Sato, M.; Kaeriyama, K. Synth. Met. 1988,25,277-288. (13)(a) Ruiz, J. P.; Gleselman, M. B.; Nayak, K.; Marynick, D. S.; Reynolds, J. R. Synth. Met. 1989,28, C481-C486. (b) Ruiz, J. P.; Nayak, K.; Marynick, D. S.; Reynolds, J. R. Macromolecules 1989,22, 1231-1238. (c) Tasi, E. W.; Basak, S.; Ruiz, J. P.; Reynolds, J . R.; Rajeshwar, K. J. Electrochem. SOC.1989,136,3683-3689. (14)(a) Heywang, G.;Jonas, F. Adu. Mater. 1992,4,116-118.(b) Jonas, F.; Schrader, L. Synth. Met. 1991,41-43,831-836.
Chem. Mater., Vol. 7,No.1, 1995 59
(1pm thikness) of PEDT has been claimed t o form. In this paper we report on the synthesis and characterization of the sulfur analog of PEDT, i.e., poly(3,4-ethylenedithiathiophene) [poly(EDTT)l,which represents a new type of fused-ring mercapto-disubstituted polythiophene. In contrast to PEDT, the poly(EDTT1 is completely soluble in NMP and partly soluble in THF, CHCl3, and other common organic solvents. This advantage lends this polymer to a more detailed spectroscopic, physicochemical, and charge-transport characterization than PEDT. The properties of poly(EDTT) are compared with some known related polythiophenes.
Experimental Section l,$-Dibromoethane, n-butyllithium, carbon disulfide, and hydrazine hydrate were used as received from Aldrich Chemicals Inc. 3,4-Dibromothiophene was used as received from Lancaster Synthesis. Tetra-n-butylammonium perchlorate was purchased from GFS Chemicals Inc. and used without W h e r purification. Acetonitrile (HPLC grade) and l-methyl2-pyrrolidinone (NMP) (HPLC grade) were used as received. Other solvents were distilled and degassed prior t o use. All reactions were performed under an atmosphere of nitrogen or argon with either standard Schlenk or drybox techniques. Thieno[3,4-cZl-1,3-dithiole-2-thione. A solution of 3,4dibromothiophene (4.95 g, 20.5 mmol) in anhydrous diethyl ether (30 mL) was cooled to -78 “C (dry ice and acetone bath) under nitrogen. To this stirred precooled solution, n-butyllithium (8.2 mL, 20.5 “01, 2.5 M in hexane) was added via syringe. The solution was stirred for 0.5 h, and then sulfur (0.66 g, 20.6 mmol) was added and stirred for 1 h. Another portion of n-butyllithium (8.2 mL, 20.5 mmol, 2.5 M in hexane) was added via syringe and stirred for another 0.5 h. To the reaction mixture, additional sulfur (0.66 g, 20.6 mmol) was added and stirred for an additional 1h. Finally, the mixture was allowed to come to room temperature, and the solvent was removed under vacuum to get a yellow solid. To the yellow solid, 2 N sodium hydroxide solution (50 mL) and carbon disulfide (20 mL) were added. The mixture was refluxed under nitrogen for 6 h and then allowed to stand at room temperature overnight. The excess carbon disulfide was removed under vacuum, and the dark reaction mixture was filtered and washed with 2 x 30 mL of water t o give a yellow solid. Recrystallization of the solid from dichloromethane-hexane (15(v/v))gave 0.85 g (22%yield) of thieno[3,4-d]-l,3-dithiole2-thione as amber needles (lit.1533% yield); mp 142 “C; GCMS (in ether solution) (mh)(re1 intensity) 190 (loo), 146 (71.5), 126 (15.6), 82(10). ‘H NMR (CDC13) 6 7.21 (9, 2H). 3,4-Ethylenedithiathiophene(EDIT).Potassium metal (0.284 g, 7.28 mmol) was added in one portion into stirred, freshly distilled, methanol (75 mL) under nitrogen. After the potassium dissolved completely, thieno[3,4-d]-1,3-dithiole-2thione (0.380 g, 2.0 mmol) was added to the solution. The reaction mixture was left to react for 1-2 h a t 50 “C under nitrogen, and gave a clear yellow solution. To this yellow solution, l,2-dibromoethane (0.2 mL, 2.32 mmol) was added via syringe. After 24 h a t room temperature, the methanol was removed under dynamic vacuum, and anhydrous ether (30 mL) was added. A yellow liquid of 3,4-ethylenedithiathiophene was obtained in 87.6% yield (0.305 g ) when the ether was removed from the yellow filtered extract. GC-MS (mlz)(re1 intensity) 174 (87), 159 (1001, 146 (381, 82 (22). lH NMR (CDC13) 6 6.95 (s,2H, aromatic), 3.21 (s,4H, methylene). I3C NMR (CDC13) 6 125.08 (aromatic, ,B-C),118.06 (aromatic, a-C), 27.94 (methylene); FT-IR (KBr pellet) 3091(m), 2956(m), 2915(m), 1472(m), 1411(m), 1385(w), 1324(m), 1287(m), 857(s), and 772(s). UV-vis (CHC13),A = 283 nm. Poly(3,4-ethylenedithiathiophene) poly(EDIT)-C. Chemical Polymerization. Anhydrous FeCl3 (0.57g, 3.5 mmol) was dissolved in 80 mL of CHsCN and stirred for 10 min. To this red-orange solution, a solution of 3,4-ethylene(15) Chiang, L.-Y.; Shu, P.; Holt, D.; Cowan, D. J.Org. Chem. 1983, 48,4713-4714.
Wang et al.
60 Chem. Mater., Vol. 7, No. 1, 1995 dithiathiophene (0.15 g, 0.86 mmol) in 20 mL of CH&N was added dropwise. A dark-green precipitate fomred immediately. The reaction mixture was stirred for about 24 h under nitrogen at room temperature. The dark-green solid was filtered through a glass frit and washed with 3 x 20 mL of fresh CH3CN and dried under vacuum for 24 h. An amount of 0.076 g of product was obtained in doped form (-0.25 FeCLper repeat unit). The neutral form of the polymer was obtained by first Soxhlet extraction of the green solid with methanol, followed by acetone, and then treated with 20 mL of NzH4sHz0 for 12 h. The resultant mixture was filtered and Soxhlet-extracted again with methanol and then dried under vacuum for 12 h. Brown neutral poly(3,4-ethylenedithiathiophene) [poly(EDTT)-C]was obtained in 43% yield (0.065 g). Elemental analysis by EDS showed almost no impurity of Fe and C1(
