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Structural effects in alkyl and alkoxy-substituted polythiophenes
Synthetic Metals, 41-43 (1991) 529-532
529
$TRLICTI,IRAL EFFECTS IN ALKYL AND ALKOXY-SUBSTITUTED PQLYTHIOPHENE$
M. LECLERC and G. DAOUST Department of Chemistry, University of Montreal, Montreal, Quebec, H3C 3J7, (Canada)
ABSTRACT Different alkyl and alkoxy polythiophene derivatives have been synthesized in good yields (4070%) by chemical polymerization using iron(III) chloride as oxidizing agent. The influence of the bulkiness and the length of the side chains on the electrical and optical properties was principally analyzed. It has been found that only monosubstituted thiophenes give soluble and electroactive polymers in the alkyl series whereas a 3,4-disubstituted structure is required for preparing processable conducting poly(alkoxythiophenes).
INTRODUCTION In the last few years, many studies have been devoted to the development of processable conducting polymers. It has been recently shown that polymerization of long-chain 3alkylthiophenes gives soluble, conducting polymers [1,2]. This substitution in the 3-position also improves the regularity of the polythiophene backbone but structural defects (2-4' couplings, branched structures) are still present in these polymers [3]. The use of 3,4disubstituted thiophenes is an attracting possibility for preparing regular conducting polymers but steric interactions between the substituents and the polymer backbone have to be minimized to get high electrical conductivities [4,5]. Previous studies on poly(3,4-dimethylthiophene) and poly(3,4-cycloalkylthiophenes) [6,7] have shown that slight modifications in the position and]or the bulkiness of the substituents may have a strong influence on both conjugation length and conductivity of the resulting materials. In order to obtain more information concerning the steric hindrance occurring in substituted polythiophenes, various alkyl and alkoxy polythiophene derivatives have been synthesized and characterized. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
530
EXPERIMENTAL SECTION 3-Alkoxythiophenes, 3-alkoxy-4-methylthiophenes and 3,4-dialkoxythiophenes were prepared from 3-bromothiophene, 3-bromo-4-methylthiophene and 3,4-dibromothiophene respectively, following conditions reported by Bryce et al. [8]. Alkylation of these bromo derivatives by Grignard reagents [9] gave the corresponding alkyl-substituted thiophenes. All monomers were polymerized by chemical oxidation using anhydrous iron(III) chloride according to a procedure similar to that of Sugimoto et al. [3,10]. Undoped polymers were obtained by reduction with an aqueous solution of hydrazine. Chemical doping was performed by soaking the neutral polymers in a nitromethane solution of iron(Ill) chloride (0,1 M). Four-probe conductivity measurements were carried out at room temperature in air. Molecular weights were determined by gel permeation chromatrography in tetrahydrofuran from a calibration curve of polystyrene standards. For low molecular weight samples, molecular weights were also calculated from the relative surface of the peaks of chain ends in the proton NMR spectra. UV-visible absorption measurements were taken from neutral polymer films cast on quartz plates. RESULTS AND DISCUSSION Chemical oxidation of different alkoxy and alkyl-substituted thiophenes (see Table I) by iron(Ill) chloride gave blue-black doped polymers in relatively good yields (40-70%). A complete reduction of the polymers was obtained by using an aqueous solution of hydrazine and after this treatment, all neutral polymers were found to be soluble in common organic solvents such as chloroform, tetrahydrofuran and methylene chloride. The good solubility and stability of these materials allowed a careful characterization of their chemical structure. As reported in Table I, we obtained high molecular weight materials with all alkyl-substituted Table I. Physical properties of alkyl and alkoxy-substituted polythiophenes. Polymer
Poly(3-hexylthiopbene) Poly(3--decylthiophene) Poly(3-octyl-4-methylthiophene) Poly(3,4-dihexylthiophene) Poly(3-butoxythiophene) Poly(3-butoxy-4-methylthiophene) Poly(3-octyloxy-4-methylthiophene) Poly(3-butoxy-4-methoxythiophene) Poly(3,4-dibutoxythiophene)
Mn a
Imax
Conductivity
x l 0 -3
nm
S/cm
16 55 21 23 1,5 (3,5) 6,4 (8,7) 21 2,7 (9,1) 2,0
505 505 325 315 530 545 545 500 460
2x 101 3x101 1x 10-5 < 1x 10-5 8x10 -4 2x10° lxl0° 5x10 -4 l x l 0 -5
a) Number-average molecular weights in brackets have been determined by NMR.
531
polythiophenes while only poly(3-octyloxy-4-methylthiophene) exhibited a number-average molecular weight higher than 104 in the alkoxy series. However, it seems that the molecular weight of the alkoxy-substituted polythiophenes determined from a calibration curve of polystyrene standards are underestimated. Most of the polymers had a polydispersity (Mw/Mn) between 3,0 and 5,0. Table I also shows that the UV-visible absorption maximum of the 3,4-disubstituted poly(alkylthiophenes) is strongly blue shifted in comparison with the monosubstituted polymers and that can be explained by a non-planar conformation of the polymer backbone resulting from strong steric interactions between the substituents and the thiophene rings. These non-planar conformations and bulky substituents lead to poor interchain contacts by keeping the chains away from one another, thereby reducing the probability of charge carder hopping [5]. Decreasing the steric hindrance in the vicinity of the polymer backbone, by using alkoxy substituents (i.e. poly(3-alkoxy-4-methylthiophenes)),led to the synthesis of highly conjugated and conducting disubstituted polythiophenes. The possibility of a nearly planar conformation for the poly(3-alkoxy-4-methylthiophenes)can be related to the smaller Van der Waal's radius of an oxygen atom (1,4/~) than that of a methylene group (2,0/~).
However, the presence of two
long substituents on each repeat unit (i.e. poly(3,4-dialkoxythiophenes)) induces a decrease of both conjugation length and electrical conductivity probably due to steric interactions between the substituents which induce a twisted conformation and increase the interchain distance [11]. Steric hindrance is minimized in poly(3-butoxythiophene) but a low conductivity has also been observed in this material. This low conductivity can be attributed to a low molecular weight and an irregular chemical structure as previously reported for poly(3-methoxythiophene) [12]. These analyses clearly indicate that a 3,4-disubstituted structure is necessary for obtaining high electrical conductivities with the alkoxy-substituted polythiophenes whereas only monosubstituted polyalkylthiophenes exhibit a good electrical transport. The presence of an alkoxy group in the 3-position seems to decrease the selectivity of the 0t-ct' couplings during the polymerization processes and therefore, the 4-position has to be blocked by a substituent as small as possible to get a regular and highly conjugated chemical structure. Similar effects can also be observed in recent studies on electrochemically prepared alkoxy-substituted polythiophenes [ 13-15].
CONCLUSIONS We found that a regular ~x-ct' linked backbone and a co-planar or at least nearly planar conformation are important criterions for a good electrical conductivity. Poly(3-alkoxy-4methylthiophenes) meet both requirements and therefore, high electrical conductivities have been observed in these materials. Moreover, the structural parameters defined in this study may be used for designing new processable and electroactive polymers.
532
ACKNOWLEDGEMENTS This work was supported by grants from the National Sciences and Engineering Council of Canada (NSERC) and the Department of Education of Province of Quebec (FCAR programs). The poly(3-alkylthiophenes) were prepared at the MPI f'drPolymefforschung in the laboratories of Prof. G. Wegncr. REFERENCES 1) 2) 3) 4) 5) 6) 7) 8)
M.A. Sato, S. Tanaka and K. Kaeriyama, J. Chem. Soc.. Chem. Commun., (1986) 873 K.Y. Jen, G.G. Miller and R.L. Elsenbaumer, J. Chem. So~,, Chem. Commun.. (1986) M. Leclerc, F.M. Diaz and G. Wegner, Makromol. Chem.. 190 (1989) 3105 A.F. Diaz, J. Castillo, K.K. Kanazawa, J.A. Logan, M. Salmon and O. Fajardo, J. Electroanal. Chem.. 133 (1982) 233 M. Leclerc and R.E. Prud'homme, Macromolecules. 20 (1987) 2153 J. Roncali, F. Gamier, R. Garreau and M. Lemaire, J. Chem.Soc.. Chem. Commun.. (1987) 1500 J. Riihe, A. Berlin and G. Wegner, Shrinker Set. Solid State Sci.. 91 (1989) 348 M.R. Bryce, A.D. Chissel, N.M.R. Smith, D. Parker and P. Kathirgamanathan, Svnth.
Met.. 26 (1988) 193 9) K. Tamao, S. Kodama, I. Nakajima and M. Kumada, Tetrahedron. 38 (1982) 3347 10) R. Sugimoto, S. Takeda, H.B. Gu and K. Yoshino, Chem. Exoress. 1 (1986) 63511 11) J.P. Ruiz, K. Nayak, D.S. Marynick and J.R. Reynolds, .M~cromolecules. 22 (1989) 1231 12) A.C. Chang, R.L. Elsenbaumer and L.L. Miller, ~ Chem.. 236 (1987) 239 13) M. Feldhues, G. Kampf, H. Litterer, T. Mecklenburg and P.Wegener, Synth, Metals. 28 (1989) C487 14) T. Yamamoto, A. Kashiwasaki and K. Kato, Makiomol. Chem.. 190 (1989) 1649 15) T, Hagiwara, M. Yamaura, K. Sato, M. Hirasaka and K. Iwata, Synth. Metals. 32 (1989) 367.
529
$TRLICTI,IRAL EFFECTS IN ALKYL AND ALKOXY-SUBSTITUTED PQLYTHIOPHENE$
M. LECLERC and G. DAOUST Department of Chemistry, University of Montreal, Montreal, Quebec, H3C 3J7, (Canada)
ABSTRACT Different alkyl and alkoxy polythiophene derivatives have been synthesized in good yields (4070%) by chemical polymerization using iron(III) chloride as oxidizing agent. The influence of the bulkiness and the length of the side chains on the electrical and optical properties was principally analyzed. It has been found that only monosubstituted thiophenes give soluble and electroactive polymers in the alkyl series whereas a 3,4-disubstituted structure is required for preparing processable conducting poly(alkoxythiophenes).
INTRODUCTION In the last few years, many studies have been devoted to the development of processable conducting polymers. It has been recently shown that polymerization of long-chain 3alkylthiophenes gives soluble, conducting polymers [1,2]. This substitution in the 3-position also improves the regularity of the polythiophene backbone but structural defects (2-4' couplings, branched structures) are still present in these polymers [3]. The use of 3,4disubstituted thiophenes is an attracting possibility for preparing regular conducting polymers but steric interactions between the substituents and the polymer backbone have to be minimized to get high electrical conductivities [4,5]. Previous studies on poly(3,4-dimethylthiophene) and poly(3,4-cycloalkylthiophenes) [6,7] have shown that slight modifications in the position and]or the bulkiness of the substituents may have a strong influence on both conjugation length and conductivity of the resulting materials. In order to obtain more information concerning the steric hindrance occurring in substituted polythiophenes, various alkyl and alkoxy polythiophene derivatives have been synthesized and characterized. 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
530
EXPERIMENTAL SECTION 3-Alkoxythiophenes, 3-alkoxy-4-methylthiophenes and 3,4-dialkoxythiophenes were prepared from 3-bromothiophene, 3-bromo-4-methylthiophene and 3,4-dibromothiophene respectively, following conditions reported by Bryce et al. [8]. Alkylation of these bromo derivatives by Grignard reagents [9] gave the corresponding alkyl-substituted thiophenes. All monomers were polymerized by chemical oxidation using anhydrous iron(III) chloride according to a procedure similar to that of Sugimoto et al. [3,10]. Undoped polymers were obtained by reduction with an aqueous solution of hydrazine. Chemical doping was performed by soaking the neutral polymers in a nitromethane solution of iron(Ill) chloride (0,1 M). Four-probe conductivity measurements were carried out at room temperature in air. Molecular weights were determined by gel permeation chromatrography in tetrahydrofuran from a calibration curve of polystyrene standards. For low molecular weight samples, molecular weights were also calculated from the relative surface of the peaks of chain ends in the proton NMR spectra. UV-visible absorption measurements were taken from neutral polymer films cast on quartz plates. RESULTS AND DISCUSSION Chemical oxidation of different alkoxy and alkyl-substituted thiophenes (see Table I) by iron(Ill) chloride gave blue-black doped polymers in relatively good yields (40-70%). A complete reduction of the polymers was obtained by using an aqueous solution of hydrazine and after this treatment, all neutral polymers were found to be soluble in common organic solvents such as chloroform, tetrahydrofuran and methylene chloride. The good solubility and stability of these materials allowed a careful characterization of their chemical structure. As reported in Table I, we obtained high molecular weight materials with all alkyl-substituted Table I. Physical properties of alkyl and alkoxy-substituted polythiophenes. Polymer
Poly(3-hexylthiopbene) Poly(3--decylthiophene) Poly(3-octyl-4-methylthiophene) Poly(3,4-dihexylthiophene) Poly(3-butoxythiophene) Poly(3-butoxy-4-methylthiophene) Poly(3-octyloxy-4-methylthiophene) Poly(3-butoxy-4-methoxythiophene) Poly(3,4-dibutoxythiophene)
Mn a
Imax
Conductivity
x l 0 -3
nm
S/cm
16 55 21 23 1,5 (3,5) 6,4 (8,7) 21 2,7 (9,1) 2,0
505 505 325 315 530 545 545 500 460
2x 101 3x101 1x 10-5 < 1x 10-5 8x10 -4 2x10° lxl0° 5x10 -4 l x l 0 -5
a) Number-average molecular weights in brackets have been determined by NMR.
531
polythiophenes while only poly(3-octyloxy-4-methylthiophene) exhibited a number-average molecular weight higher than 104 in the alkoxy series. However, it seems that the molecular weight of the alkoxy-substituted polythiophenes determined from a calibration curve of polystyrene standards are underestimated. Most of the polymers had a polydispersity (Mw/Mn) between 3,0 and 5,0. Table I also shows that the UV-visible absorption maximum of the 3,4-disubstituted poly(alkylthiophenes) is strongly blue shifted in comparison with the monosubstituted polymers and that can be explained by a non-planar conformation of the polymer backbone resulting from strong steric interactions between the substituents and the thiophene rings. These non-planar conformations and bulky substituents lead to poor interchain contacts by keeping the chains away from one another, thereby reducing the probability of charge carder hopping [5]. Decreasing the steric hindrance in the vicinity of the polymer backbone, by using alkoxy substituents (i.e. poly(3-alkoxy-4-methylthiophenes)),led to the synthesis of highly conjugated and conducting disubstituted polythiophenes. The possibility of a nearly planar conformation for the poly(3-alkoxy-4-methylthiophenes)can be related to the smaller Van der Waal's radius of an oxygen atom (1,4/~) than that of a methylene group (2,0/~).
However, the presence of two
long substituents on each repeat unit (i.e. poly(3,4-dialkoxythiophenes)) induces a decrease of both conjugation length and electrical conductivity probably due to steric interactions between the substituents which induce a twisted conformation and increase the interchain distance [11]. Steric hindrance is minimized in poly(3-butoxythiophene) but a low conductivity has also been observed in this material. This low conductivity can be attributed to a low molecular weight and an irregular chemical structure as previously reported for poly(3-methoxythiophene) [12]. These analyses clearly indicate that a 3,4-disubstituted structure is necessary for obtaining high electrical conductivities with the alkoxy-substituted polythiophenes whereas only monosubstituted polyalkylthiophenes exhibit a good electrical transport. The presence of an alkoxy group in the 3-position seems to decrease the selectivity of the 0t-ct' couplings during the polymerization processes and therefore, the 4-position has to be blocked by a substituent as small as possible to get a regular and highly conjugated chemical structure. Similar effects can also be observed in recent studies on electrochemically prepared alkoxy-substituted polythiophenes [ 13-15].
CONCLUSIONS We found that a regular ~x-ct' linked backbone and a co-planar or at least nearly planar conformation are important criterions for a good electrical conductivity. Poly(3-alkoxy-4methylthiophenes) meet both requirements and therefore, high electrical conductivities have been observed in these materials. Moreover, the structural parameters defined in this study may be used for designing new processable and electroactive polymers.
532
ACKNOWLEDGEMENTS This work was supported by grants from the National Sciences and Engineering Council of Canada (NSERC) and the Department of Education of Province of Quebec (FCAR programs). The poly(3-alkylthiophenes) were prepared at the MPI f'drPolymefforschung in the laboratories of Prof. G. Wegncr. REFERENCES 1) 2) 3) 4) 5) 6) 7) 8)
M.A. Sato, S. Tanaka and K. Kaeriyama, J. Chem. Soc.. Chem. Commun., (1986) 873 K.Y. Jen, G.G. Miller and R.L. Elsenbaumer, J. Chem. So~,, Chem. Commun.. (1986) M. Leclerc, F.M. Diaz and G. Wegner, Makromol. Chem.. 190 (1989) 3105 A.F. Diaz, J. Castillo, K.K. Kanazawa, J.A. Logan, M. Salmon and O. Fajardo, J. Electroanal. Chem.. 133 (1982) 233 M. Leclerc and R.E. Prud'homme, Macromolecules. 20 (1987) 2153 J. Roncali, F. Gamier, R. Garreau and M. Lemaire, J. Chem.Soc.. Chem. Commun.. (1987) 1500 J. Riihe, A. Berlin and G. Wegner, Shrinker Set. Solid State Sci.. 91 (1989) 348 M.R. Bryce, A.D. Chissel, N.M.R. Smith, D. Parker and P. Kathirgamanathan, Svnth.
Met.. 26 (1988) 193 9) K. Tamao, S. Kodama, I. Nakajima and M. Kumada, Tetrahedron. 38 (1982) 3347 10) R. Sugimoto, S. Takeda, H.B. Gu and K. Yoshino, Chem. Exoress. 1 (1986) 63511 11) J.P. Ruiz, K. Nayak, D.S. Marynick and J.R. Reynolds, .M~cromolecules. 22 (1989) 1231 12) A.C. Chang, R.L. Elsenbaumer and L.L. Miller, ~ Chem.. 236 (1987) 239 13) M. Feldhues, G. Kampf, H. Litterer, T. Mecklenburg and P.Wegener, Synth, Metals. 28 (1989) C487 14) T. Yamamoto, A. Kashiwasaki and K. Kato, Makiomol. Chem.. 190 (1989) 1649 15) T, Hagiwara, M. Yamaura, K. Sato, M. Hirasaka and K. Iwata, Synth. Metals. 32 (1989) 367.