J. Phys. Chem. 1987, 91, 6706-6714
6706
Effects of Steric Factors on the Electrosynthesis and Properties of Conducting Poly(3-alkyithiophenes) J. Roncali,*t R. Garreau,t A. Yassar,? P. Marque,? F. Gamier,? and M. Lemairet Laboratoire de Photochimie Solaire, CNRS ER 241, 94320 Thiais, France, and Laboratoire de Chimie Organique, CNAM, UA 1103, 75003 Paris, France (Received: June 23, 1987)
Two series of thiophene monomers 3-substituted with linear and branched alkyl chains have been synthesized, and their electropolymerizationhas been investigated together with the properties of the resulting polymers. An analysis of the effects of the electrosynthesis conditions by visible absorption spectroscopy shows that the mean conjugation length presents an increasing dependence on the applied electrical conditions as the length of the alkyl chain increases. In the case of linear alkyl substituents (R3 = CH, to C18H37),the length of the alkyl chain affects slightly the polymerization reaction but produces important modificationson the structure and on the properties of the polymers. Thus, the mean conjugation length and the electrochemical reversibility admit an optimum for the polymers containing 7-9 carbons in the alkyl chain while the polymer films remain highly conductive up to R3 = CloH21.In contrast, the steric hindrance to conjugation caused by branched alkyl substituents totally inhibits the polymerization (isopropyl) or leads to weakly conjugated polymers (isobutyl). It is shown that the distance between the thiophene ring and the branched alkyl chain is the determining parameter which governs the aptitude for electropolymerization and the properties of the resulting polymer.
Introduction Only a few years after their first electrochemical synthesis, electrogenerated conducting polymers and particularly poly(pyrrole)' and poly(thiophene) derivatives2 rank among the most widely studied conducting polymers. Within this short period a considerable number of theoretical and experimental works have been devoted to the study of the electronic properties and conduction mechanisms of these polymers3 and to the development of their multiple possible applications based on (i) the conducting properties of the doped state, like for instance antistatic coatings and electromagnetic shieldings, (ii) the semiconducting properties of the undoped state, namely microelectronic and photovoltaic device^,^ and (iii) the electrochemical reversibility of the doping process, namely energy storage5 and modified electrodes including electrochromic devices,6 electrocatalytic system^,^ and selective electrodes and membranes.8 The electrochemical synthesis which leads to the direct grafting of the polymer onto an electrode surface appears as a decisive advantage for the realization of modified electrodes. The realization of such electrodes based on conducting polymers implies the introduction of a function in these polymers, and different approaches have been proposed for this purpose: electrochemical inclusion of an anionic dopant with catalytic properties,7a* electrodeposition of metallic aggregates7d and also composite structures9 allowing the introduction of a specific function via the alloyed polymer,8b-10and/or the immobilization of nonelectroactive species." Among the different methods of functionalization, the direct electropolymerization of tailor-made functionalized monomers appears as the most adequate to achieve a control at a molecular level of the specific activity of an electrode and allows the design of highly specific systems like electrodes for selective complexation or asymmetric electrosynthesis.I2 Despite its elegant simplicity, this approach appears in fact as a challenging task since several prerequisites have to be fulfilled concerning (i) the structure of the monomer, (ii) its aptitude to undergo electropolymerization, and (iii) the conservation of the conductivity and of the electrochemical reversibility of the polymer. A high conductivity appears necessary for two reasons. On the one hand, high conductivity will ensure fast and efficient electron transfers and allow the realization of electrodes with larger specific areas. On the other hand, it seems reasonable to assume that a link exists between conductivity and electrochemical stability and that a large Ohmic resistance of the polymer could lead to a faster degradation of the electrode material under cycling. The limited conductivity and electrochemical stability reported for N-sub-
stituted poly(pyrro1es) functionalized with bipyridyl, anthraquinones, or chiral substituents seems to support this hypothes i ~ The . substitution ~ ~ ~ ~on the ~ P-position ~ of poly(thiophene) could be an interesting alternate route for electrode functionalization. As a matter of fact, the introduction of a methyl group in the /3-position produces an important enhancement of conductivity in contrast to poly(3-methylpyrrole) which is less conductive than poly(pyrr~le).'~ Moreover, poly(thi0phenes) are more stable in their undoped state than p o l y ( p y r r ~ l e ) . ' ~ JThe ~ pres(1) Diaz, A. F.;Kanazawa, K.; Gardini, G. P. J . Chem. SOC.,Chem. Commun. 1979, 635. (2) (a) Diaz, A. Chem. Scr. 1981, 27, 145. (b) Tourillon, G.; Garnier, F. J. Electroanal. Chem. 1982, 135, 173. (c) Waltman, R. J.; Bargon, J.; Diaz, A. F. J . Phys. Chem. 1983, 87, 1459. (3) (a) Bredas, J. L.; Chance, R. R.; Silvey, R. Mol. Cryst. Liq. Cryst. 1981, 77, 319. (b) Bredas, J. L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J . A m . Chem. Soc. 1983,105, 6555. (c) Chung, T. C.; Kaufman, J. H.; Heeger, A. J.; Wudl, F. Phys. Rev. B: Condens. Matter 1984, 30, 702. (d) Buhks, E.;Hodge, I. M. J . Chem. Phys. 1985,83, 5976. (4) (a) White, H.S.; Kittlesen, G. P.; Wrighton, M. S. J . A m . Chem. SOC. 1984,106, 5375. (b) Glenis, S.;Horowitz, G.; Tourillon, G.; Garnier, F. Thin Solid Films 1984, 111, 93. (c) Koezuka, H.; Tsumura, A,; Ando, T. Synth. Met. 1987, 18, 699. (5) (a) Kaufman, J. H.; Chung, T. C.; Heeger, A. J.; Wudl, F. J . Electrochem. Soc. 1984, 131, 9, 2092. (b) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J . Appl. Phys. 1983, 22, 9, L567. (c) Yamamoto, T. J . Chem. Soc., Chem. Commun. 1981, 187. (6) (a) Garnier, F.;Tourillon, G.; Gazard, M.; Dubois, J. C. J . Electroanal. Chem. 1983, 148, 299. (b) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J. Appl. Phys. 1983, 22, L412. (7) (a) Bull, R. A.; Fan, F. R.; Bard, A. J. J . Electrochem. SOC.1983, 130, 7, 1636. (b) Okabayashi, K.;Ikeda, 0.;Tamura, H. J . Chem. SOC.,Chem, Commun. 1983,684. (c) Noufi, R.J. Electrochem. SOC.1983,130, 10,2126. (d) Tourillon, G.; Garnier, F. J . Phys. Chem. 1984, 88, 5281. (8) (a) Burgmayer, P.; Murray, R. W. J. Am. Chem. SOC.1982,104,6139. (b) Iyoda, T.;Othani, A.; Shimidzu, T.; Honda, K. Chem. Lett. 1986, 687. ( c ) Zinger, B.;Miller, L. L. J . Am. Chem. Soc. 1984, 106, 6861. (9) (a) De Paoli, M.; Waltman, R. J.; Diaz, A. F.; Bargon, J. J . Chem. Soc., Chem. Commun. 1984, 1015. (b) Niwa, 0.; Tamamura, T. J . Chem. SOC.,Chem. Commun. 1984. (c) Roncali, J.; Garnier, F. J . Chem. SOC., Chem. Commun. 1986, 783. (10) Fan, F. R.;Bard, A. J. J . Electrochem. SOC.1986, 133, 301. (11) Mizutani, F.; Iijima, S. I.; Tanabe, Y.; Tsuda, K. J . Chem. SOC., Chem. Commun. 1985, 1728. (12) (a) Komori, T.; Nonaka, T. J . Am. Chem. SOC.1984, 106,2656. (b) Salmon, M.; Bidan, G. J. Electrochem. SOC.1985, 132, 8 , 1897. (c) Lemaire, M.; Delabouglise, D.; Garreau, R.; Roncali, J. In Proceedings of ISEOS, Kurashiki, Japan, Oct 31-Nou 3, 1986. (13) (a) Bidan, G.; Deronzier, A.; Moutet, J. C. J . Chem. SOC.,Chem. Commun. 1984, 1185. (b) Cosnier, S.;Deronzier, A.; Moutet, J. C. J. Electroanal. Chem. 1985, 193, 193. (c) Audebert, P.;Bidan, G.; Lapkowski, M. J . Chem. SOC.,Chem. Commun. 1986, 887. (14) Street, G. B.; Clarke, T. C.; Geiss, R. H.; Lee, V. Y.; Nazzal, A.; Pfluger, P.; Scott, J. C. J . Phys., Colloq. 1983, 44, C3-599.
Laboratoire de Photochimie Solaire. 'Laboratoire de Chimie Organique.
0022-3654187 , ,I209 1-6706SO 1.5010 0 1987 American Chemical Societv I
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Properties of Conducting Poly(3-alkylthiophenes) ervation of the conductivity of poly(thiophenes) functionalized with bulky substituents requires a detailed knowledge of both the electronic and the steric factors compatible with the obtention of highly conducting and stable polymers. The electronic factors involved in the electropolymerization of heterocycles have been discussed extensively by Waltman and Bargon, who have shown that the relative stability of the radical cation produced by the anodic oxidation of the monomer is the essential parameter determining the possibility for a given monomer to electropolymerize.16 Within the anodic potential range delimitated by electronic factors, the possibility to electropolymerize a given substituted monomer is mainly determined by steric parameters. When applied to the field of electrogenerated conducting polymers, the general concept of steric effects includes in fact a wide variety of situations in which total inhibition of the polymerization and effects concerning only the properties of the polymer represent the two extreme cases. Thus, prior to undertaking the synthesis and the polymerization of monomers containing bulky functionalized groups, we have carried out a systematic analysis of the various effects of the steric factors. For this purpose, a large series of new thiophene monomers 3-substituted with linear and branched alkyl substituents have been synthesized. The electropolymerization of these monomers has been analyzed, and the resulting polymers have been characterized by UV-visible spectroscopy, cyclic voltammetry, scanning electron microscopy, and conductivity measurements. Alkyl substituents have been selected on the basis of the high conductivity of poly(3-methylthiophene)17 which suggests interesting possibilities of functionalization via an alkyl chain. Furthermore, the similar electronic effects of the various alkyl substituents allow a clear discrimination of steric factors. In the first part of this work, the effects of the lengthening of linear alkyl chain are discussed. It is shown that this parameter has little effect on the polymerization reaction but affects essentially the structure and the properties of the resulting polymers. Thus, the mean conjugation length and the electrochemical reversibility admit an optimum for the polymers containing 7-9 carbon atoms in the alkyl chain. These effects are accompanied by strong modifications of the morphology of the polymer films that remain highly conductive. In contrast, the steric hindrance attendant to branched alkyl substituents totally inhibits the polymerization or leads to weakly conjugated polymers. It is shown that this steric hindrance to conjugation can be neutralized by the introduction of a spacer containing two methylene groups between the branched alkyl chain and the thiophene ring. Experimental Section Synthesis of the Monomers. 3-Ethylthiophene (C2T), 3propylthiophene (C3T), 3-butylthiophene (C4T), 3-amylthiophene (C5T), 3-heptylthiophene (C7T) 3-octylthiophene (CST), 3nonylthiophene (C9T), 3-decylthiophene (ClOT), 3-tetradecylthiophene (C14T), 3-octadecylthiophene (C18T), 3-isobutylthiophene (iC4T), and 34soamylthiophene (iC5T) have been prepared according to the method described by Kumada et a1.18 3-Isopropylthiophene (iC3T) has been synthesized by reduction of 2-(3-thienyl)-2-propano1.lg 3-Methylthiophene, solvents, and electrolytes have been purified according to already published procedures. Determination of the Oxidation Potential of the Monomers. The oxidation potentials of the various monomers have been measured by cyclic voltammetry in acetonitrile containing 0.5 M LiC104. Low monomer concentration (0.01 M) and high scan rates (500 mV/s) were used in order to prevent polymerization. The anode was a platinum disk electrode ( S = 0.07 cm2) polished (15) Tourillon, G.; Garnier, F. J . E/ectrochem. SOC.1983, 130, 10, 2042. (16) (a) Waltman, R. J.; Bargon, J. Tetrahedron 1984,40, 20,3963. (b) Waltman, R. J.; Bargon, J. Can. J. Chem. 1986, 64, 76. (17) (a) Sato, M.; Tanaka, S.;Kaeriyama, K. Synth. Met. 1986, 14, 279. (b) Roncali, J.; Garnier, F. N o w . J. Chim. 1986, 10, 237. (18) Hayashi, T.; Konishi, M.; Fukushima, M.; Mise, T.; Kagobani, M.; Tajika, M.; Kumada, M. J. Org. Chem. 1981, 46, 4481. (19) Lemaire, M.; Garreau, R.; Garnier, F.; Roncali, J. N o m . J. Chim., in press.
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
6707
with 0.05-fim diamond paste before each experiment. A platinum wire was used as counter electrode, and a saturated calomel electrode (SCE) was used as reference. Electrochemistry. For electrochemical characterization, the polymers were synthesized in the above-described three-electrode cell from a reaction medium involving 0.2 M monomer (except for C14T and C18T for which a concentration of 0.1 M was used for solubility reasons) and 0.02 M electrolyte: tetrabutylammonium perchlorate or hexafluorophosphate in nitrobenzene. The solutions were degassed by argon bubbling prior to polymerization which was performed at 5 OC under an argon atmosphere. The polymers were grown on platinum disk electrodes (S = 0.07 cm2) under galvanostatic conditions by using deposition charges of 100 mC/cm2 which corresponds to films thicknesses ranging from 2000 A for C1T to 1 hm for ClST, estimated from values measured on thicker films. After the polymerization, the anode was rinsed with acetonitrile and transferred in another three-electrode cell containing 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile. Cyclic voltammetric experiments were performed with a PAR 175 potentiostat/galvanostat equipped with a PAR 173 universal programmer and a PAR 179 plug-in digital coulometer. Visible Absorption Spectroscopy. The films for absorption spectroscopy have been prepared from the same reaction medium on indium-tin oxide (ITO) electrodes (2 X 1 cm), applying current densities of 0.5-10 mA/cmZ. An aluminum foil was used as the cathode in that case. After the synthesis, the films were rinsed with hexane, dried in an argon flow, and electrochemicallyundoped in 0.1 M Bu4NPF6in acetonitrile. Absorption spectra were recorded on a Cary 2 19 spectrophotometer. Spectroelectrochemical experiments were performed in a 1 X 1 cm quartz cuvette which was inserted into the spectrophotometer by means of micrometric positioners. For these experiments, a Pt wire was used as cathode and an Ag wire (+0.1 V/SCE under our conditions) served as quasi-reference. Conductivity Measurements. Thicker films for conductivity measurements were prepared with similar procedures using deposition charges of l C/cm.* As-grown polymer films were rinsed with hexane and dried under vacuum at ambient temperature. The film thicknesses were determined with a Sylvac P 100 thickness monitor, and dc conductivities were measured by a standard four-probe technique. Results and Discussion ( a ) Effect of the Length of the Alkyl Chain on the Properties of Poly(thiophenes) 3-Substituted by Linear Alkyl Chains Visible Absorption Spectroscopy. It has already been shown that the conductivity and the electrochemical properties of poly(3methylthiophene) are strongly dependent on the current density and monomer concentration used for the s y n t h e s i ~ . ' ~ These .~~ results suggest the existence of an optimum monomer concentration/current density ratio; however, since this optimum is not necessarily the same for every new monomer, the relevance of structure/properties relationships based on results obtained under unoptimized conditions could be questionable. Previous works have shown that a strong correlation exists between the mean conjugation length determined by UV-visible, IR, and Raman absorption spectroscopy and the conductivity and electrochemical properties of poly(thiophenes).*' Since visible absorption spectroscopy gives information on the mean degree of conjugation, this technique can thus be used as a tool to analyze the effects of synthesis conditions on the properties of the polymers without requiring the quantities of monomer needed by conductivity measurements. Therefore, in order to determine the electrical conditions leading to the most conjugated and thus most conductive ~
~
(20) Roncali, J.; Garnier, F.; Lemaire, M.; Garreau, R. Synth. Met. 1987, 18, 139. (21) (a) Diaz, A. F.; Crowley, J.; Gardini, G. P.; Torrance, J. B. J . Electroana/. Chem. 1981, 121, 355. (b) Roncali, J.; Garnier, F.; Lemaire, M.; Garreau, R. Synth. Met. 1986, 15, 323. (c) Cao, Y.; Guo, D.; Pang, M.; Qian, R. Synth. Met. 1987, 28, 189. (d) Furukawa, Y.; Akimito, M.; Harada, I. Synth. Met. 1987, 18, 151.
Roncali et al.
6708 The Journal of Physical Chemistry, Vol. 91, No. 27, I987
TABLE I: Visible Absorption Spectroscopic Data of Undoped Poly(thiophenes) 3-Substituted by Linear Alkyl Chains“
polymer PClT
current density, mA/cm2 0.5 1 2
5 PC2T PC3T PC4T
PC5T
I
PC7T
PC8T
uw
5W
600
5
A,nm
Figure 1. Visible absorption spectra of PClT films synthesized at various current densities on I T 0 with a 100 mC/cm2deposition charge: (a) 0.5, (b) 1, (c) 2, and (d) 5 mA/cm2.
PC9T
PC 1OT
PC14T
PC18T 05
10 2 2 1 2 5 10 0.5 1 2 5 10 1 2 5 10 0.5 1 2 10 0.5 1 2 5 10 0.5 1 2 5 10 0.5 1 2 5 10 1 2 3
potential, V/SCE
opt density
absorp max, nm
1.50 1.65 2.05 2.75 3.70 2.05 2.07 1.68 2.00 2.62 3.80 1.60 1.72 2.02 2.68 3.85 1.70 2.02 2.80 3.86 1.69 1.76 2.12 3.00 3.92 1.68 1.80 2.18 2.95 3.93 1.69 1.82 2.20 3.00 3.95 1.75 1.84 2.20 2.98 4.00 1.78 2.25 4.00
0.96 1.02 1.05 1.10 1.01 0.83 0.80 0.43 0.76 0.93 0.90 0.21 0.33 0.38 0.79 0.98 0.28 0.47 0.70 0.87 0.15 0.26 0.38 0.73 0.76 0.34 0.49 0.53 0.65 0.75 0.38 0.69 0.78 0.89 0.9 1 0.32 1.oo 0.96 0.95 0.67 0.72 0.94 0.92
518 521 525 525 523 470 472 480 486 488 480 484 500 510 512 514 440 455 492 495 438 482 498 513 517 440 514 518 525 542 426 476 485 498 502 450 460 46 1 462 460 456 440 418
” Deposition charge 100 mC/cm2 on I T 0 electrodes
Figure 2. Visible absorption spectra of PC9T films synthesized at various cumnt densities on IT0 with a 100 mC cm2deposition charge: (a) 0.5, (b) 1, (c) 2, (d) 5, and (e) 10 mA/cm .
I
polymers, we have analyzed the effect of the applied current density on the spectroscopic properties of the different poly( 3alkylthiophenes). For this purpose, the polymers have been prepared as thin films on I T 0 electrodes by using a constant deposition charge of 100 mC/cm2 and current densities ranging from 0.5 to 10 mA/cm2. The lower and upper limits of the current density and potential have been conditioned by the quality of the polymer films obtained with the extreme values of the electrical parameters. Low current density results in inhomogeneous deposition of the polymer onto the I T 0 electrode and in poor adhesion. On the other hand, current densities exceeding 10
mA/cmZ, which corresponds in our case to potentials in the range of 4 V/SCE, produce a decrease of the polymer yield and in some cases cause damages to the IT0 coating. Figure 1 shows as an example the absorption spectra of undoped P C l T films synthesized by applying current densities of 0.5, 1, 2, and 5 mA/cm2. These spectra show that raising the current density produces a double effect, an increase of the optical density of the polymer films together with a slight bathochromic shift of the absorption maximum. The comparison with the spectra obtained under the same conditions on PC9T (Figure 2) shows that these effects are noticeably enhanced with this polymer. When the current density is increased from 1 to 10 mA/cm2, the optical density increases from 0.4 to 0.75 and the absorption maximum shifts bathochromically from 468 to 542 nm. This very important red shift indicates that the mean conjugation length of the polymer is considerably enhanced when the current density is increased. On the other hand, the large variations of optical density show that the applied current density strongly affects the polymer yield. The term of polymer yield as employed here concerns the quantity of polymer which is effectively deposited on the electrode. As a consequence, this yield differs from the faradaic yield which must take into account the soluble oligomers produced during the polymerization and also the polymer fraction which does not remain attached on the electrode, as occurs with the lowest current densities. With these reserves, the variations of the optical density indicate that the polymer yield admits a optimum which depends
Properties of Conducting Poly(3-alkylthiophenes)
The Journal of Physical Chemistry, Voi. 91, No. 27, 1987 6709
TABLE 11: Cyclic Voltammetric Data of Poly(thiophenes) 3-Substituted by Linear Alkyl Chains in 0.1 M ByNPF,/CH3CNa
polymer
PClT PC2T PC3T PC4T PCST PC7T PC8T PC9T PClOT PC 14T PCllT
E,,, V/SCE monomer polymer
E,,
calcd doping
V/SCE 0.66
0.75 0.80
0.70 0.75
1.84 1.84 1.84
0.85
0.78
0.99
0.91
1.01
0.94
1.84 1.84 1.84
1.02
0.93
1.03
0.9 1
level, 7% 18 15 16 19 17 16 16 16 17
1.08
0.94
16
1.85
1.10
0.92
15
1.82 1.82 1.83
1.84
0.66 0.72
0.64
'Deposition charge 100 mC/cm2 on Pt electrodes (S = 0.07 cm2), scan rate SO mV/s.
on the applied current density. The spectroscopic data obtained with the different polymers synthesized in the various electrical conditions are listed in Table I. These results show that a similar behavior is observed with all the polymers of the series. Considering the optical density and the red shift of the absorption maximum, the optimum value of the current density is found in every case in the range of 5-10 mA/cm2 except for the last terms PC14T and PC18T, for which the optimum is found for lower current densities. This latter result could be due to the lower monomer concentration used in that case. A comparison of the results obtained with the different polymers shows that the amplitude of the red shift and of the variation of optical density produced by a same increase of current density augments considerably with the length of the alkyl chain. Thus, for PC7T to PClOT low current densities (0.5-1 mA/cmZ) lead to very low polymer yields and low degrees of conjugation. These results show that the polymer yield and the conjugation are increasingly dependent on the applied electrical conditions as the length of the alkyl chain increases. A possible interpretation could involve the diffusion rate of the monomers which is expected to decrease as their size increases. As a consequence, higher current densities might be necessary to maintain the polymerization rate and to limit the formation of oligomers that have been shown to have deleterious effects on the mean conjugation length.'7b,20,21bA comparison of the absorption maxima of the various polymers shows that under optimized conditions the absorption maximum which shifts toward the short wavelengths for PC2T to PC4T increases and reaches an optimum in the 520-540-nm region for PC8T-PC9T and decreases again for P C l 4 T and PC 18T. This red shift of the absorption maximum is accompanied by an enhancement of the shoulder at 600 nm which produces a broadening of the absorption band in the long-wavelength region. Thus, the width at half-height increases from 190 nm for P C l T to 210 nm for PC9T. These results indicate that the mean degree of conjugation admits an optimum for the polymers containing 8-9 carbons in the alkyl chain, and furthermore, the broadening of the absorption band suggests that the polymers contain an increasing proportion of long conjugated chains. Thus, the mean degree of conjugation of poly(3-alkylthiophenes) can be enhanced by the introduction of an alkyl chain containing 8-10 carbons. On the other hand, the increasing dependence of the polymer yield and of the mean conjugation length on the applied electrical conditions also emphasizes the necessity of the optimization of the electrosynthesis conditions. As it will be shown, these results have important consequences on the electrochemical properties of the polymers and on their conductivity and solubility. Electrochemistry. Table I1 lists the electrochemical data obtained by cyclic voltammetry on the various polymers films prepared by using a charge of 100 mC/cm2 on Pt. As could be expected, the different monomers have almost the same oxidation potential, in agreement with the similar inductive effect of the various alkyl substituents. A potential sweep at 50 mV/s performed in the synthesis media containing the different monomers shows that the polymerization occurs at 1.6-1.65 V/SCE. A slight
Figure 3. Cyclic voltammograms of polymers films on Pt electrodes (S = 0.07 cm2)recorded in 0.1 M Bu,NPF,/CH,CN (deposition charge 100 mC/cm2, scan rate SO mV/s): (a) PCZT, (b) PC9T, (c) PC18T.
increase of the onset potential is observed for C14T and C18T (1.7 V/SCE) which may be due to the lower monomer concentration used. These results indicate that the length of the alkyl chain exerts only a limited effect on the polymerization reaction. Figure 3 shows as representative examples the cyclic voltammograms of PCZT, PC9T, and PC18T films prepared by using a 100 mC/cm2 deposition charge under the conditions determined by visible absorption spectroscopy. These curves and the data in Table I1 show that the lengthening of the alkyl chain produces two major effects on the electrochemical behavior of the polymers. On the one hand, the potential of the anodic current peak, E,,, increases steadily from 0.66 V/SCE for P C l T to 1.10 V/SCE for PC18T, and on the other hand, the symmetry of the voltammogram increases from P C l T to PC9T and decreases then for longer alkyl chains. In addition, the voltammograms reveal the presence of a prewave at ca. 0.5 V/SCE, the intensity of which increases with the length of the alkyl chain up to PClOT. Table I1 also shows that the doping level of the polymers, calculated from the coulometric data,22is nearly constant throughout the polymer series. In the case of conducting polymers, the interpretation of cyclic voltammograms (CV) is rather delicate since the shape of the CV curves depends on the electrolyte used for cycling,23the relative contribution of the capacitive charging current, and also the distribution of the different lengths of conjugated segments in the polymer. Furthermore, in the case of poly( 3-alkylthiophenes), the situation is rendered even more complicated by the decreasing proportion of electroactive sites in the polymer as the length of the alkyl chain increases. This factor results in modifications of the thickness and morphology of the polymer films as well as in changes in their capacitance behavior. Figure 4a-c reproduces SEM pictures obtained on films of respectively PC2T, PC8T, and PC14T prepared by using a charge of 1 C/cm2. A comparison of these micrographs shows that the lengthening of the alkyl chain produces strong modifications in the morphology of the polymers. Whereas PCZT and PC3T form very compact films similarly to PClT, the following terms of the series show less regular and more porous structures. Thus, the films contain aggregates of increasing dimensions as the length of the alkyl chain increases (Figure 4b,c). This result suggests that, between the extreme cases P C l T and PC18T, the porosity and hence the surface-to-volume ratio of the polymer electrode could admit an optimum. These important modifications of the morphology of the polymer films and their increasing content of insulating material as the length of the alkyl chain augments could also induce changes in the capacitive behavior of the polymers. Previous works on electrogenerated poly(pyrro1e) have shown that, in its oxidized form, this polymer acts as a porous metal electrode with a high surface-to-volume ratio and a large double-layer ~ a p a c i t a n c e . ~This ~ large capacitance which appears as current plateaus following the oxidation ( 2 2 ) Genies, E. M.; Pernaut, J. M. Synth. Met. 1987/85, IO, 117. (23) (a) Diaz, A. F.; Castillo, J. I.; Logan, J. A.; Lee, W. Y . J . Electroanal. Chem. 1981,129, 115. (b) Marque, P.; Roncali, J.; Gamier, F. J . Electroanal. Chem. 1987, 218, 107. (24) Bull, A. R.; Fan, F. R.; Bard, A . J. J . Electrochem. Soc. 1982, 129, 5 , 1013.
6710 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
Roncali et al.
t
Figure 5. DCVA response recorded at 510 nm of a PClT film prepared by using a deposition charge of 100 mC/cm2 on IT0 (scan rate 10 mV/s).
Figure 6. DCVA response recorded at 530 nm of a PC9T film prepared by using a deposition charge of 100 mC/cm2 on I T 0 (scan rate 10 mV/s).
I
Figure 4. scanning electron micrograpns or polymers rilms prepareu by using a deposition charge of 1 C/cm2 on ITO: (a, top) PC2T, (b, middle) PC8T, (c, bottom) PC 14T.
peak renders difficult the interpretation of cyclic voltammog r a m ~ ? ~A J ~qualitative picture of the variation of the capacitive current with the length of the alkyl chain is provided by derivative cyclic voltabsorptometry (DCVA). As it has been shown already, the presentation of the absorbance vs applied potential in a derivative format leads to a response which is morphologically similar to cyclic voltammetry but presents the advantage to be insensitive to nonfaradaiccharge-consuming processes. Figure 5 and 6 show the DCVA responses of PClT and PC9T films on IT0 electrodes. The DCVA responses have been obtained by recording the variations of absorbance at the absorption maximum of the undoped (25) Feldberg, S. W. J. Am. Chem. Soc. 1984, 206,4671. (26) Tanguy, J.; Mermillod, N.; Hoclet, M. Synth. Met. 1987, 28, 7.
form of the polymers during a voltammetric cycle performed at low scan rate (10 mV/s). In the case of PCIT, the shapes of the CV curve and of the DCVA response are similar and the charging capacitive current which occurs essentially beyond the anodic peak produces a slight increase of the apparent potential of the anodic current peak in the CV curve from 0.5 to 0.6 V/Ag. This behavior is in good agreement with previous works on p ~ l y ( p y r r o l e ) . ~ ~ - ~ ~ In the case of PC9T, a quite different behavior is observed. The DCVA curve in Figure 6 confirms the presence of two distinct components already observed in the voltammogram in Figure 3; however, the relative intensities of these two waves are inverted when compared with the CV curve. The main component of the anodic peak is now observed at 0.45 V vs Ag instead of 1.02 V/SCE in the CV curve. This value, which is lower than that of PClT, is in agreement with the longer mean conjugation length of PC9T indicated by the spectroscopic data. Moreover, the presence of two components in both the CV and the DCVA curves is consistent with the absorption spectrum of PC9T, which presents an enhancement of the shoulder at 600 nm. These results confirm that the capacitive behavior of the polymers is strongly modified by the length of the alkyl chain which renders more and more difficult the interpretation of the voltammograms. These changes in the capacitive behavior of the polymers could account for the increase of the E p value with the length of the alkyl chain. A further comparison of the CV and DCVA curves of PC9T shows that the high symmetry observed in the voltammogram is confirmed by the DCVA response. This result suggests a better electrochemical reversibility for PC9T. A more general illustration of this conclusion is provided by Figure 7 which represents the variation of the ratio of the anodic and cathodic peak currents I p / I p c as a function of the number of carbon atoms in the alkyl chain. These data show that I p / I p c is minimal when the alkyl
Properties of Conducting Poly(3-alkylthiophenes)
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6711
&I
TABLE 111: DC Conductivity of Free-Standing Films of Poly(thiophenes) 3-Substituted by Linear Alkyl Chains 2 .
0
polymer
I
PClT PC2T PC3T PC4T
D 0 0
0 0
0
0
0
I -
PC5T PC7T PC8T
0
5
10
15
N
Figure 7. Variation of the ratio of the anodic and cathodic peak currents &/IF as a function of the number of carbons in the alkyl chain.
chain contains between 7 and 9 carbons which suggests that the electrochemical reversibility admits an optimum around this length of the alkyl chain. Preliminary stability tests have been performed in order to confirm this improved electrochemical reversibility. A thin PC8T film deposited on Pt by using a charge of 50 mC/cm2 has been submitted to potential steps between 0 and 1 V/SCE in 0.1 M LiC104/CH3CN with a 50-ms pulse width. This pulse width corresponds to the limit of visual detection of the contrast between the doped and the undoped forms. After 1.6 X lo6 switches, the charge exchanged by the filtn during a voltammetric cycle still represents 80% of the charge exchanged before cycling while this value is only of 50% in the case of a P C l T film prepared and analyzed under the same conditions.*' This result confirms the improvement of the electrochemical reversibility of PC8T when compared to PClT. In summary, the lengthening of the alkyl chain produces important modifications in the morphology of the polymers films and in their capacitive behavior. These effects could account for the increase of E,, with the length of the alkyl chain. On the other hand, our results show that the electrochemical reversibility of the polymers is noticeably improved when the alkyl chain contains 7-9 carbons. This optimum of the reversibility could result from the conjunction of several factors related to the length of the alkyl chain, e.g., enlargement of the surface-to-volume ratio, dilution of the number of electroactive sites in the polymer, and changes in the capacitive behavior of the films. These various aspects are still under investigation in our laboratory and will be reported in more detail in a future publication. Electrical Conductivity of the Polymers. The conductivity of the various polymers has been determined on films synthesized on I T 0 electrodes by using deposition charges of 1 C/cm2. As already indicated, the lengthening of the alkyl chain produces strong modifications in the morphology of the polymer films. On a macroscopic scale and for thicknesses in the range of a few micrometers, the films appear more fibrous and their consistence more elastic as the length of the alkyl chain increases. Consequently, as-grown polymer films retain increasing quantities of solvent and become more adherent onto the electrode. Another interesting consequence of this spongelike structure is that the as-grown films of polymers containing long alkyl chains appear translucent even when their dry thickness is in the range of 8-10 pm while PMeT films are opaque beyond 1 pm. Table I11 lists the thicknesses and conductivity values of the various polymers. As could be expected, the thickness of the films prepared with a constant deposition charge increases rapidly with the length of (27) Lemaire, M.; Roncali, J.; Garnier, F.; Garreau, R.; Hannecart, E. French Patent 86.04744, April 4, 1986. (28) Bancroft, E. E.; Sidwell, J. S.; Blount, H. N. Anal. Chem. 1981,53,
1390.
(29) Sato, M.; Tanaka, S.; Kaeriyama, K. J. Chem. Soc., Chem. Commun. 1986, 873. (30) Jen, K. Y.; Miller, G. G.; Elsenbaumer, R. L. J . Chem. Soc., Chem. Commun. 1986, 1346.
PC9T PClOT PC14T PC18T
current density, mA/cm2 5 5 5 2 5 5 2 5 10 2 5 10 5 10 2 4 2 4 1 2
thickness, wm
conductivity, S/cm
1.8-2.1 2-2.4 2.2-2. I 2.6-3
250 180 160 50 150 140 45 65 110 40 70 90 85 100 40 70 7 16 1 2
3-3.3 6-7 7-8 8-10 9-1 1 12-1 5 15-20
the alkyl chain. In order to confirm the conclusions of the spectroscopic study, the effect of the current density on the electrical conductivity of the polymers has been briefly analyzed. The conductivity values obtained with the various polymers show a close agreement with the spectroscopic results, and they confirm that the highest conductivities are obtained in every case with the current density which leads to the most conjugated polymer. Thus, the conductivity of PC4T, which is of 50 S/cm for 2 mA/cm2, in agreement with recent works,31increases by a factor of 3 when the polymer is prepared at the optimal current density of 5 mA/cm2. A comparison of the conductivities of the different polymers shows that despite the presence of long alkyl chains, the polymers remain highly conductive; thus, the conductivity of PClOT is still in the range of 70 S/cm even though the electroactive part of the polymer, e.g. the polythiophene backbone, represents only one-third of the polymer weight. The conductivity decreases then more abruptly for PC14T and PC18T. This decrease of the conductivity with the length of the alkyl chain is probably due to more resistive interchain contacts caused by the presence of an increasing proportion of insulating material in the polymer. However, the decrease of the conductivity appears relatively limited for polymers up to PClOT. A possible interpretation of this high conductivity is that the more resistive interchain contacts caused by the lengthening of the alkyl chain are partially compensated by the enhancement of the mean degree of conjugation observed up to PC 1OT. Solubility of Poly(3-alkylthiophenes).Solubility is a highly desirable property for conducting polymers for both practical applications and fundamental studies. Some of the above-described polymers, namely PC4T, PCST, and PC18T, have been recently prepared by several groups who reported that poly(thiophenes) containing long alkyl substituents were readily soluble at room temperature in solvents like chloroform or THF.29-31In these works, the polymers were prepared whether chemically30 or electrochemically by using a constant current density of 2 mA/cm2. Taking into account the strong dependence of the mean conjugation length on the applied electrical conditions, it was interesting to investigate the solubility of these polymers when prepared under optimized electrical conditions. Two polymers have been chosen as examples, PC8T and PC14T. These polymers have been prepared as thick films by using current densities of respectively 5 and 3 mA/cm2. Extensive polymerization using 50-60 C yielded after rinsing with hexane, reduction by 24-h immersion in aqueous ammonia, and drying at 50 OC under vacuum 60 mg of PC8T and 102 mg of PC14T. These two (31) Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F. Macromolecules 1987, 20, 212.
6712 The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
Roncali et al.
TABLE IV: Visible Absorption Spectroscopic Data (in nm) of the Soluble Fraction of PCST and PC14T PCXT PC14T
acetone chloroform chlorobenzene polymer film on I T 0
360" 43 5 440
360" 430
517
460
435
"Shoulder. polymers were then refluxed for 24 h successively in acetone, chloroform, and chlorobenzene. After extraction with these three solvents and drying, the weight loss was 3% for PC8T and 10% for PCl4T which demonstrates that when operating in optimized conditions, the solubility of these polymers is very limited. Table IV lists the visible spectroscopic data of the various fractions resulting from the extraction of the two polymers, compared with those of the corresponding undoped polymer films prepared at the optimal current densities. The shift to longer wavelengths observed from acetone to chlorobenzene shows that chlorobenzene extracts longer conjugated chains in both cases. The chlorobenzene fraction shows an absorption maximum at 440 nm for PC8T and 435 nm for PC14T. This small difference suggests the presence of slightly longer conjugated segments in the PC8T chlorobenzene fraction. These absorption maxima are in good agreement with = 440 nm)29and the results published for PC12T solutions (A, = 435 nm).3' However, a comparison for PC6T solutions (A,, of the spectra of the chlorobenzene fractions with those of the corresponding undoped polymers films reveals a difference of 77 nm for PC8T and 25 nm for PC14T. These differences suggest a lower degree of conjugation in the soluble compounds than in the corresponding polymers. In summary, the length of the alkyl chain affects only slightly the polymerization reaction but produces important modifications in the structure and properties of the polymers. The mean conjugation length can be enhanced by introducing alkyl chains containing 7-9 carbons while concurrently the electrochemical reversibility admits an optimum for these polymers. This high degree of conjugation explains the relatively low decrease of the conductivity when the length of the alkyl chain increases. These results could be interpreted by an improved long-range order caused by lipophilic interactions between alkyl chains beared by adjacent monomer units. Such interactions would promote the cisoid structure which has been previously demonstrated in the crystalline domains of PC1T.32 Finally, these results together with the solubility tests also outline the necessity of optimizing the electrosynthesis conditions when dealing with new functionalized monomers. ( 6 ) Effects of Steric Hindrance on the 0-Position. Additional arguments consistent with the cisoide structure of 3-substituted poly(thiophenes) have been gained by the study of the steric hindrance on @-position. Previous works concerning thiophenes 3-4-disubstituted by methyl or ethyl groups have demonstrated that the steric hindrance caused by disubstitution leads to poorly conjugated polymer^.'^^^^ However, since the coplanarity of adjacent thiophene rings is similarly hindered in both the cisoide and the transoide conformations, these results alone give no information about the most probable structure. In order to obtain further informations about this question and especially about the steric factors compatible with further functionalization, the effects of steric hindrance on the 0-position have been analyzed. For this purpose, taking the simplest bulky group, namely the isopropyl group as a probe, we have investigated the effect of the proximity of this substituent to the thiophene ring, on the polymerization reaction, and on the properties of the resulting polymers. A comparative study of the polymerization of iC3T, iC4T, iCST, and their homologues substituted with linear alkyl chains has been carried out, and the resulting polymers have been characterized by using the above-described procedures. (32) Gamier, F.; Tourillon, G.; Barraud, J. Y . ;Dexpert, H.J . Muter. Sci. 1985, 20, 2687.
(33) Tourillon, G.; Gamier, F. J . Electroanal. Chem. 1984, 161, 51.
Figure 8. Current vs potential curves corresponding to the potentiostatic polymerization of (a) C4T, CST, iC5T; (b) iC4T; and (c) iC3T. TABLE V Visible Absorption Spectroscopic Data of Undoped Poly(thiophenes) 3-Substituted by Branched and Linear Alkyl Chains"
polymer PiC4T PC4T PiC5T PC5T
current density, mA/cm2 1 2 5 5 1 2 5 5
potential, V/SCE 1.90 2.24 3.05 2.62 1.80 2.10 2.80 2.68
opt density 0.08
0.27 0.41 0.93 0.30 0.58 0.85 0.90
absorp max, nm 390 426 432 488 490 510 510 510
'Deposition charge 100 mC/cm2 on I T 0 electrodes.
Electropolymerization. Figure 8 shows the current potential curves corresponding to the potentiostatic electropolymerization of iC3T, iC4T, and iC5T compared with the I / V curve of their linear homologues. In the case of linear alkyl chains, the current increases sharply at 1.6 V/SCE, indicating that the polymerization is fast. In contrast, in the case of iC3T, no increase of the current is observed even when the potential is raised to 2.5 V/SCE. Despite several attempts using monomer concentrations ranging from 0.2 to 0.5 M and potentiostatic as well as galvanostatic conditions, the polymerization of iC3T remained impossible. This negative result shows that the steric hindrance attendant to the isopropyl group is sufficient to totally inhibit the polymerization. The shape of the iC4T curve shows that an overpotential is required to achieve the polymerization. This result clearly shows the persistence of the steric hindrance on the polymerization reaction of this compound. Finally, the fact that the iCST polymerization curve is identical with that of monomers containing linear alkyl polymerization curve is identical with that of monomers containing linear alkyl chains reveals that the steric effect of the branched alkyl chain on the polymerization is neutralized when the branched chain is separated from the thiophene ring by two methylene groups. These results clearly show that the possibility for a given substituted monomer to undergo electropolymerization is largely determined by steric parameters. Visible Absorption Spectroscopy. Table V lists the visible absorption spectroscopy data obtained on undoped PC4T, PiC4T, PCST, and PiC5T films synthesized on I T 0 electrodes at various current densities. These results show that as for polymers containing linear alkyl chains, the current density greatly determines both the polymer yield and the mean conjugation length. As previously observed, the optimum current densities are found in the range of 5-10 mA/cm2. A comparison of the spectroscopic data of PiC4T and PC4T shows that the optical densities are much lower for PiC4T, which indicates a lower polymerization yield. Furthermore, the strong difference between the absorption maxima at 432 and 486 nm for respectively PiC4T and PC4T reveals a much shorter mean conjugation length in PiC4T. On the other hand, the similar results obtained with PiC5T and PCST (A,,, = 510 nm in both cases) show that the mean conjugation length is no longer affected by the steric hindrance in the case of PiCST. Electrochemistry. Figure 9 shows the cyclic voltammograms of PiC4T and PiC5T compared respectively with those of their linear homologues. These curves and the electrochemical data
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987 6713
Properties of Conducting Poly(3-alkylthiophenes) en
Figure 9. Comparison of the cyclic voltammogramsof poly(thiophenes) 3-substituted by linear and branched alkyl chains recorded in 0.1 M Bu4NPF6/CH3CN[deposition charge 100 mC/cm2 on Pt (S = 0.07 cm2),scan rate 50 mV/s]: (a) PC3T, (b) PC4T, (c) PiC4T, (d) PCST, (e) PiC5T. TABLE VI: Electrochemical Data of Poly(thiophenes) 3-Substituted bv Branched and Linear Alkyl Chains' E,* E,, calcd doping V/SCE V/SCE polymer level, 76 Ip/Ipc
PC4T PiC4T PCST PiCST
0.80 0.94 0.85 0.90
0.75 0.84 0.78 0.85
19 7 18 21
1.35 2.20 1.25 1.40
'Electrolytic medium, 0.1 M Bu4NPF6/CH3CN;deposition charge, 100 mC/cm2 on Pt (S= 0.07 cm2);scan rate, 50 mV/s. TABLE VII: DC Conductivity of Poly(thi0phenes) $Substituted by Branched and Linear Alkvl Chains
polymer PC4T PiC4T PCST PiCST
current density, mA/cm2 5 5 5 5 4 5 8
deposition charge, C/cm2 1
1.25 2.1 1 1.25 1.1 1
thickness, Pm 2.8 7 12 3.3 8.5 7 5
conductivity, S/cm 150 10 8 140 70 75 40
listed in Table VI show that the anodic potential peak of PiC4T occurs at 0.94 V/SCE, which is 140 mV higher than that of PC4T. Furthermore, the oxidation wave of PiC4T is much sharper than that of PC4T. The data in Table VI reveal that these effects are accompanied by a lower electrochemical reversibility and a lower calculated doping level. With the assumption of a similar capacitance behavior, the higher oxidation potential of PiC4T suggests a shorter mean conjugation length, in agreement with the spectroscopicdata, while the sharper oxidation wave could be attributed to a narrower distribution of conjugated chain lengths in this polymer. A comparison of the voltammogramsof PiCST and PCST and of the corresponding data in Table VI shows that the difference between the oxidation potentials of the two polymers is only 50 mV and that the doping levels are comparable. These results confirm, as already indicated by the polymerization curves and by the spectroscopic results, that the steric hindrance to conjugation can be neutralized by introducing a spacer containing an adequate number of methylene groups. Conductivity. Table VI1 lists the values of the electrical conductivity of the different polymers films. PCi4T forms freestanding films with a conductivity in the range of 10 S/cm. The important difference between the conductivities of PiC4T and PC4T is in good agreement with the steric hindrance to conjugation revealed by the electrochemical and the spectroscopic results. Furthermore, PiC4T films are about twice thicker than PC4T films which reveals a much more disordered structure in PiC4T. A similar thickness difference is observed between PiCST and PCST films. However, the conductivities of these two polymers differ only by a factor of 2 which corresponds exactly
-Figure 10. CPK models of the cisoide conformation of poly(thiophenes)
3-substituted by branched alkyl chains. The substituent is represented on one thiophene ring only for sake of clarity. (top) PiC5, (middle) PiC4T, (bottom) PiC3T.
to the ratio of the thicknesses of the films produced with the same deposition charge. Since the spectral and electrochemical data reveal a similar mean degree of conjugation in PCST and PiCST, the differences in the conductivity of these two polymers can be attributed to more resistive interchain contacts caused by differences in the packing arrangement of the polymer chains. This latter result shows that a branched alkyl chain sufficiently remote from the thiophene ring to not affect the conjugation represents a convenient means to modulate the density of the polymer. The progressivity of the effects observed on iC3T, PiC4T, and PiCST suggests an increasing torsion angle34between adjacent monomer units as the branched alkyl chain draws nearer to the thiophene ring. An examination of CPK molecular models (Figure 10) shows that such a progressive torsion angle between adjacent monomers is only observed in the case of a cisoide structure, whereas in the case of a transoide structure, the steric hindrance appears almost equivalent in iC3T, PiC4T, and PiCST. This information together with the experimental results obtained from the two series of polymers provides further arguments supporting the presence of the cisoide conformation in 3-substituted poly(thiophenes).
Conclusion The effects of steric parameters on the electrosynthesis and properties of poly(3-alkylthiophenes) including linear and branched alkyl chains have been analyzed. On the basis of the correlation (34) Bredas, J. L.;Street, G. B.;
1985,83, 1323.
Themans, B.; Andre, J. M.J. Chem.Phys.
6714
The Journal of Physical Chemistry, Vol. 91, No. 27, 1987
existing between the absorption features of the polymers and their mean degree of conjugation and conductivity, a preliminary analysis of the effects of the electrosynthesis conditions was performed. This analysis provided the experimental conditions leading in every case to materials of optimized conjugation and conductivity. The pertinence of this approach has been demonstrated by the increasing dependence of the conjugation of the polymers on the electrosynthesis conditions as the length of the alkyl substituent increases. In the case of linear alkyl substituents, the length of the alkyl chain has little effect on the aptitude for electropolymerization but affects essentially the structure and the properties of the resulting polymers. Thus, the mean conjugation length and the electrochemical reversibility can be markedly improved by the introduction of alkyl chains containing 7-9 carbons. This enhancement of the conjugation can explain the high conductivity of these polymers. In contrast, the analysis of the steric hindrance on the @-positionshows that when directly grafted on the thiophene ring, a branched alkyl chain totally inhibits the polymerization. This steric hindrance to conjugation related to the torsion angle between adjacent monomer units can be neutralized by introducing a spacer containing an adequate number of methylene groups.
Roncali et al. However, the branched alkyl chain still affects the interchain contacts and the conductivity. These two sets of results provide further arguments supporting a cisoide structure of substituted poly(thiophenes), and they represent a first step toward the definition of a "functionalization space" compatible with the preservation of the conductivity and electrochemical reversibility of functionalized poly(thiophene) electrodes. As a first example of the practical applications of these concepts, we have recently described the first optically active poly(thiophenes) with conductivities of 1-15 S/cm.'*" Works aiming at the design of new functionalized poly(thiophenes) are now in progress in our laboratory. Registry No. PClT, 616-44-4; PCZT, 1795-01-3; PC3T, 1518-75-8; PC4T, 34722-01-5; PCST, 102871-31-8; PC7T, 65016-61-7; PCST, 65016-62-8; PC9T, 65016-63-9; PClOT, 65016-55-9; PC14T, 11085166-6; PCl8T, 104934-54-5; PClT (homopolymer), 84928-92-7; PCZT, 90451-70-0; PC3T, 110851-61-1; PC4T, 98837-51-5; PCST, 11085162-2; PC7T, 110851-63-3; PCBT, 104934-51-2; PC9T, 110851-64-4; PClOT, 110851-65-5; PC14T, 110851-67-7; PC18T, 104934-55-6; PiC4T, 110851-68-8; P E S T , 110851-70-2; iC4T, 79428-75-4; iC5T, 110851-69-9; Bu4NPF6, 3109-63-5; Bu4NC104, 1923-70-2; Sn02, 18282-10-5; In, 7440-74-6; Pt, 7440-06-4.
