MATERIALS CHEMISTRY COMMUNICATIONS Electrochemical doping of a-ethyl-disubstituted oligothiophenes and electrical conductivities of the resulting radical-cation salts Naoki Noma, Kazuhiro Kawaguchi, Ichiro Imae, Hideyuki Nakano and Yasuhiko Shirota” Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565, Japan Electrochemical doping of a-ethyl-disubstituted oligothiophenes and the electrical conductivity of the resulting radical-cation salts have been studied. 5,5”‘-Diethyl- 2,2’:5’,2”:5”,2’”-quaterthiophene(DEt4T) and 5,5””-diethyl- 2,2’:5””:5”,2”’:5”’,2’”’-quinquethiophene(DEt5T) are found to contrast with each other in the stoichiometry and electrical properties of the resulting solid radical-cation salts. The DEt4T and DEt5T radical-cation salts with a doping extent of ca. 100 and 50%, respectively, produced by electrochemical doping exhibited room-temperature conductivities of 9 x 10-8 and 5 x S cm-’ with activation energies of 0.54 and 0.11 eV, respectively.Oligothiophenes linked at the a-position of the thiophene ring have recently received attention not only as model compounds for electrically conducting polythiophenes but also as a new class of organic 7c-electron systems. Unsubstituted and subs tit uted oligo thiophenes have been synthesized,1-3 and their 0ptica1,~ electro~hemical,~-~ electrical8-” and photoelec- trical’’v’2 properties have been studied. With regard to the electrical conductivities of oligothio- phenes, both charge-transfer complexes and chemically doped materials have been studied.Charge-transfer complexes of the unsubstituted thiophene trimer or tetramer with TCNQ have been reported to exhibit room-temperature conductivities of 10-10-10-9 S cm-’ .8 Electrical conductivities of the unsubsti- tuted thiophene tetramer and pentamer doped with iodine have also been reported to be ca. 10’ S cm-’; however, unsub- stituted oligothiophenes undergo doping-induced coupling reaction to produce higher oligomers.’ Single crystals of the a-methyl-disubstituted thiophene tetramer doped with I,, NOBF, or NOPF6 have been reported to exhibit conductivities of 10-2-10-1 S cr”;’’ however, the stoichiometry of these complexes is not clear. Electrochemical doping is of interest in that definitely ident- ified radical-ion salts with dopants derived from support- ing electrolytes can be prepared.We report here the first ex- ample of electrochemical doping of oligothiophenes and the electrical conductivities of the resulting solid radical-cation salts. a-E th yl-dis ubs ti t uted oligo thiophenes, 5,5”’-die thyl- 2,2’:5’,2”:5”,2”’-quaterthiophene(DEt4T) and 5,5””-diethyl-2,2’:5’,2“:5”,2”’:5”’,2’”’-quinquethiophene(DEtST), where the coupling reaction of the resulting radical cations was expected to be prevented due to the presence of the two a-ethyl groups, were investigated and were found to contrast with each other in the second anodic oxidation process, and in the stoichi- ometry and electrical properties of the resulting solid radical- cation ~a1ts.l~ DEt4T DEt5T DEt4T and DEt5T were synthesized in diethyl ether by Grignard coupling reactions of 2-bromo-5-ethylthiophene with 5,5’-dibromo-2,2’-bithiophene, and 2-bromo-5-ethylthiophene with 5,5”-dibromo-2,2’:5’,2”-terthiophene,respectively.DEt4T: mp 183°C. Elemental analysis: Found: C, 61.99; H, 4.59; s, 33.22%. Calc. for C20H18S4: c, 62.13; H, 4.69; s, 33.18%. DEt5T: mp 246°C. Elemental analysis: Found: C, 61.26; H, 4.31; S, 34.19%. Calc. for C24H20S5: C, 61.50; H, 4.30; S, 34.20%. Fig. 1 shows cyclic voltammograms for the anodic oxidations of DEt4T and DEt5T in dichloromethane. Whereas the anodic oxidation processes of the unsubstituted thiophene tetramer and pentamer are irreversible, those of DEt4T and DEt5T are reversible owing to the presence of the ethyl group at the a-position of each terminal thiophene ring.The oxidation poten- tials of DEt4T and DEt5T were determined to be 0.58 and 0.52 V us. Ag/Ag+ (0.01 mol dmP3), respectively. DEt4T and DEt5T were found to contrast with each other in the 0 +0.5 +1.0 EN(vs. AS/Ag+, 0.01mol dm4) Fig. 1 Cyclic voltammograms of a, DEt4T (1.0x mol dm-j) and b, DEt5T (3.0 x lop4mol dm-3) in dichloromethane in the presence of Bu4NC104 (sweep rate: 100 mV s-’) J. Muter. Chem., 1996, 6(1), 117-118 117 was ca. 2: 1, as determined from elemental analysis. That is, the anodically oxidized species of DEt4T and DEt5T are regarded as a simple salt with a doping extent of ca.loo%, ,.:.*.a9 .*' 'b.,-' 400 500 600 700 wavelengthtnm Fig.2 Electronic absorption spectra of a, neutral DEt4T; b, DEt4T radical-cation salt; c, neutral DEt 5T; and d, DEt5T radical-cation salt in the solid state second anodic oxidation process. Whereas the second oxi- dation process of DEt4T was irreversible, that of DEt5T was reversible; a second anodic wave and a corresponding cathodic wave were observed at 0.82 (Epa)and 0.75 V (Epc).This result can be compared with the literature data for the second oxidation process of a thiophene tetramer capped at both the CI-and /?-positions of the terminal thiophene rings which was reversible, although the second oxidation process of a trimer was irreversible.6 Based on the information obtained by cyclic voltammetry, electrochemical doping of DEt4T and DEt5T was carried out by controlled-potential anodic oxidation at 0.65 and 0.55 V us.Ag/Ag+ (0.01 mol dm-3) for dichloromethane solutions of DEt4T and DEtST, respectively, for 24 h. In the case of DEt4T, the solution turned dark green when the electrolysis started, and black powders were deposited onto the surface of the working electrode. In the case of DEtST, however, there was no deposition onto the working electrode, but black powders were precipitated in the solution. The black powders of the anodically oxidized DEt4T and DEt5T were washed with fresh dichloromethane. They were identified as radical-cation salts with a perchlorate anion as a dopant, as characterized by various spectroscopies and elemen- tal analysis.The IR absorption spectra of the electrochemically oxidized DEt4T and DEt5T show strong bands at ca. 1100 cm-' due to the dopant perchlorate anion. As Fig. 2 shows, the electro- chemically oxidized DEt4T and DEt5T show new broad electronic absorption bands assignable to the DEt4T and DEt5T radical cations in the wavelength region from 650 to 750nm (A-ca. 700nm) and from 700 to 850nm (Amax ca. 790 nm), respectively. The presence of unpaired electrons in the solid radical- cation salts of DEt4T and DEt5T was also confirmed by electron paramagnetic resonance spectroscopy. Sharp signals with g values of 2.004 and 2.003, and peak-to-peak linewidths of 4.4 and 2.3 G were observed for electrochemically oxidized DEt4T and DEtST, respectively.A striking difference in the stoichiometry of the resulting radical-cation salts was observed for DEt4T and DEt5T. While the mole ratio of oligothiophene : dopant for the DEt4T radical- cation salt was ca. 1 : 1, that for the DEt5T radical-cation salt and a complex salt with a doping extent of ca. So%, respect- ively. The electronic absorption spectral data are also in support of this; i.e. whereas the n-n* absorption band in the wavelength region from 420 to 540nm, which is due to the neutral thiophene tetramer, becomes significantly smaller in intensity for the electrochemically oxidized DEt4T relative to the neutral DEt4T, the n-n* absorption band in the wavelength region from 420 to 580nm, which is due to the neutral thiophene pentamer, still remains in the case of the electro- chemically oxidized DEt5T.The difference in the stoichiometry between DEt4T and DEt5T is thought to be caused probably by the difference in the mode of the deposition of the oxidized species, because coulometry in a thin cell exhibited one-electron oxidation for both DEt4T and DEt5T. Electrical conductivity of radical-cation salts of DEt4T and DEt5T was measured for pellet samples by a two-probe dc method in the temperature range 20-70 "C.The radical-cation salts of DEt4T with a doping extent of ca. 100% and DEt5T with a doping extent of ca. 50% exhibited room-temperature conductivities of (7-9) x S cm-' and (2-5) x S cm-', with activation energies of 0.54 and 0.11 eV, respect- ively. The four orders of magnitude difference in electrical conductivity between DEt4T and DEt5T radical-cation salts is attributed to the difference in the extent of doping; i.e., the on-site coulombic repulsion in the complex salt of DEt5T is much lower than that in the simple salt of DEt4T.The present study presents the first example of electrochemi- cal doping of oligothiophenes, showing a striking difference in the electrical conductivity of the resulting radical-cation salts between DEt4T and DEt5T. It will be of interest to investigate further the correlation between molecular structure and electri- cal properties of electrochemically doped oligothiophenes with different n-conjugation lengths.This work was supported in part by Grants-in-Aid for scientific research, nos. 06226244 and 06453155, from the Ministry of Education, Science, and Culture of Japan. References 1 J. Kagan and S. K. Arora, J. Org. Chem., 1983,48,4317. 2 J. Nakayama, T. Konishi and M. Hoshino, Heterocycles, 1988, 27, 1731. 3 W. ten Hoeve and H. Wynberg, J. Am. Chem. SOC., 1991,113,5887. 4 D. Fichou, G. Horowitz, B. Xu and F. Garnier, Synth. Met., 1990, 39,243. 5 Z. Xu, D. Fichou, G. Horowitz and F. Garnier, J. Electroanal. Chem., 1989,267,339. 6 P. Bauerle, U. Segelbacher, A. Maier and M. Mehring, J. Am. Chem. SOC.,1993,115,10217. 7 J. Guay, P. Kasai, A. Diaz, R. Wu, J. M. Tour and L. H. Dao, Chem. Muter., 1992,4, 1097. 8 S. Hotta and K. Waragai, Synth. Met., 1989,32, 395. 9 E. E. Havinga, I. Rotte, E. W. Meijer, W. ten Hoeve and H. Wynberg, Synth. Met., 1991,4143,473. 10 S. Hotta and K. Waragai, J. Muter. Chem., 1991,1, 835. 11 Y. Kuwabara, K. Miyawaki, K. Nawa, N. Noma and Y. Shirota, Nippon Kagakukaishi, 1992,1168. 12 N. Noma, T. Tsuzuki and Y. Shirota, Ado. Muter., 1995,7,647. 13 K. Kawaguchi, I. Imae, H. Nakano, N. Noma and Y. Shirota, Polym. Prepr., Jpn., 1993,42,2842. Communication 5/05331E; Received 9th August, 1995 118 J. Muter. Chem., 1996, 6(1), 117-118