© Copyright 1996 by the American Chemical Society
VOLUME 100, NUMBER 42, OCTOBER 17, 1996
LETTERS Interchain Migration of Electrons and Holes in Polysilanes Jun Kumagai, Hiroto Tachikawa, Hiroshi Yoshida, and Tsuneki Ichikawa* Department of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060, Japan ReceiVed: April 3, 1996; In Final Form: June 11, 1996X
The measurements of the ESR and optical absorption spectra of polysilane radical ions revealed that the hole in the radical cation is delocalized onto the side chains, whereas the excess electron in the radical anion is confined within the silicon main chain. This observation suggests that the side chains hinder the interchain hopping of the charge for the anion but not the cation. Therefore the hole is the primary charge carrier in polysilanes.
Polysilanes are σ-conjugated polymers composed of a silicon main chain and organic side chains. Recently they have attracted much attention because of their potential applications as one-dimensional conductors, photoresists and radiation resists, optical displays, nonlinear optical materials, and high-density optical data storage materials.1,2 Although polysilanes are insulators with bandgaps of ca. 4 eV,3,4 they can be converted to semiconductors either by photoexcitation or by doping electron donors or acceptors. Charge transport in photoexcited or charge-injected polysilanes has been extensively studied using time-of-flight5,6 or timeresolved microwave conductivity techniques.7-9 It has been established from these experiments that thermally activated10 and field-assisted hole hopping is responsible for the charge transport. The mobility of the holes is as high as 10-8 m2/V s, while the mobility of the electrons is a few orders of magnitude lower. However a plausible postulate for why only the hole is mobile has not been given. Polysilanes may be considered as potentially conductive main chains surrounded by insulating side chains. The holes and probably the conducting electrons are highly mobile within the silicon main chain and can migrate very rapidly from one end of the chain to the other.9 However, since the size of a typical polymer molecule is less than 10-6 m, interchain charge hopping X
Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01004-0 CCC: $12.00
is necessary for macroscopic conduction. It is therefore possible that the side chains limit the interchain hopping of the conducting electrons but apparently not the holes. In this letter, we provide some evidence to the validity of this assumption by comparing the ESR and optical absorption spectra of both polysilane radical cations and anions by using a radiation-chemical matrix-isolation technique.11,12 Figure 1 compares the ESR spectra at 77 K of both the radical anion and cation of tetradecamethylhexasilane in 2-methyltetrahydrofuran and in Freon matrixes, respectively. The line width for the radical anion primarily arises from the anisotropy of the ESR g factors (gxx * gyy * gzz).11 The spectrum of the radical cation is more than 5 times broader than that of the radical anion. This is caused by the stronger hyperfine interaction between the unpaired electron and the protons of the side-chain methyl groups. The stronger hyperfine interaction arises from the delocalization of the hole onto the methyl substituents. The stronger hyperfine interaction could also conceivably arise from the localization of the unpaired electron near some specific methyl group. However, a continuous decrease in the ESR line width with increasing catenation of permethyloligosilanes CH3(SiCH3CH3)nCH3, (2.65, 2.56, 2.32, and 2.14 mT for n ) 2, 4, 5, and 6, respectively) suggests that the unpaired electron is delocalized all over the chain for the oligomers.12 The marked difference of the line width was also observed © 1996 American Chemical Society
16778 J. Phys. Chem., Vol. 100, No. 42, 1996
Letters
Figure 2. Optical absorption spectra of the radical anion (top) and cation (bottom) of both poly(cyclohexylmethylsilane) (solid lines) and poly(methylphenylsilane) (broken lines). The matrixes used were the same as those for the ESR measurement. Figure 1. ESR spectra of tetradecamethylhexasilane radical anion (A) and cation (B) and poly(cyclohexylmethylsilane) radical anion (C) and cation (D). The anion spectra were obtained in 2-methyltetrahydrofuran, while the cations were measured in a Freon mixture.
for the radical ions of both poly(cyclohexylmethylsilane) and poly(methylphenylsilane) with the average number of Si atoms per unit chain of 80 and 82, respectively. Shown in Figure 1 are the ESR spectra of the radical anion and cation of poly(cyclohexylmethylsilane). The ESR spectra of poly(methylphenylsilane) radical ions were approximately the same as those from poly(cyclohexylmethylsilane) radical ions. The apparent ESR line width of the polymeric radical anion is less than that of the oligomeric one, which arises not from the delocalization of the unpaired electron all over the entire main chain but from the axial symmetry of the g tensor. A recent electron spinecho envelope modulation analysis of the polymeric radical anion revealed that the unpaired electron is confined to a polymer segment composed of about six silicon atoms.13 The unpaired electron in the polymeric radical cation is also confined to a polymer segment composed of about six silicon atoms, so that the hyperfine interactions are still detectable.12 The lack of a detectable hyperfine interaction in the polysilane radical anion therefore suggests that the excess electron in the intramolecular conduction band is not delocalized onto the side chains so that the side chains act as good intermolecular insulators for the electron. On the other hand, since the hole is delocalized onto the side chains, it can migrate to an adjacent main chain via the side chains. The optical absorption spectra of polysilane radical ions also support the validity of the above-mentioned mechanism of charge migration. The absorption spectra are composed of two bands; a UV band corresponding to the HOMO f LUMO transition for the parent neutral polymer and a near-infrared band corresponding to the excitation of an electron from or to the SOMO orbital of the radical ions.11,12 Figure 2 compares the absorption spectra of the radical anion and cation of both poly(cyclohexylmethylsilane) and poly(methylphenylsilane). The near-infrared band of the radical anion is scarcely influenced by the substitution of the side chains from the cyclohexyl group to the phenyl group. On the other hand, the near-infrared band of the radical cation is blue-shifted by the substitution. This
indicates that the SOMO of the radical anion is confined within the main chain, whereas that of the radical cation is spread to the side chains. The delocalization of the hole onto the side chains may arise from the electron-donating nature of the side-chain substituents. This could facilitate the migration of the hole from one chain to another. On the other hand, since the side-chain substituents are not necessarily much electrophilic, the conducting electron for the radical anion cannot easily migrate to an adjacent polymer chain via the side chain. It is therefore expected that the electron mobility can be increased by the presence of substituents with the electron affinity much higher than that of the phenyl substituent. Acknowledgment. This work was supported by grant-inaid for Scientific Research from the Ministry of Education, Science and Culture, Japan, and by General Sekiyu Research and Development Encouragement and Assistance Foundation. References and Notes (1) Miller, R. D.; Michl, J. Chem. ReV. 1989, 89, 1359 and references therein. (2) Brus, L. J. Phys. Chem. 1994, 98, 3575 and references therein. (3) Takeda, K.; Shiraishi, K. Phys. ReV., B 1989, 39, 11028. (4) Yokoyama, K.; Yokoyama, M. Chem. Lett. 1989, 1005. (5) Kepler, R. G.; Zeigler, J. M.; Harrah, L. A.; Kurtz, S. R. Phys. ReV. B 1987, 35, 2818. (6) Abkowitz, M. A.; Rice, M. J.; Stolka, M. Philos. Mag. 1990, 61, 25 and references therein. (7) Van de Laan, G. P.; De Haas, M. P.; Marman, J. M.; Frey, H.; Mo¨ller, M. Mol. Cryst. Liq. Cryst. 1993, 236, 165. (8) Frey, H.; Mo¨ller, M.; De Haas, M. P.; Zenden, N. J. P.; Schouten, P. G.; Van de Laan, G. P.; De Haas, M. P.; Marman, J. M. Macromolecules 1993, 26, 89. (9) Van de Laan, G. P.; De Haas, M. P.; Hummel, A.; Frey, H.; Scheiko, S.; Mo¨ller, M. Macromolecules 1994, 27, 1897. (10) Samuel, L. M.; Sanda, P. N.; Miller, R. D. Chem. Phys. Lett. 1989, 159. (11) Kumagai, J.; Yoshida, H.; Koizumi, H.; Ichikawa, T. J. Phys. Chem. 1994, 98, 13117. (12) Kumagai, J.; Yoshida, H.; Ichikawa, T. J. Phys. Chem. 1995, 99, 7965. (13) Ichikawa, T.; Kumagai, J.; Kurai, T.; Yoshida, H., to be published.
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