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J. Phys. Chem. B 1999, 103, 3812-3817
ARTICLES Optical and Electronic Properties of Polysilane Radical Anions Tsuneki Ichikawa,* Jun Kumagai, and Hitoshi Koizumi DiVision of Molecular Chemistry, Graduate School of Engineering, Hokkaido UniVersity, Sapporo 060-8628, Japan ReceiVed: August 24, 1998; In Final Form: December 18, 1998
Analyses of the electronic absorption spectra and their polarization spectra of the radical anions of permethylated hexa- and octasilane and poly(cyclohexylmethylsilane) in the mixed solvents of nonpolar methylcyclohexane and polar 2-methyltetrahydrofuran at 77 K have revealed that the orbitals of the excess electrons in the radical anions are antibonding pseudo-π composed of Si 3py atomic orbitals bisecting the Si-Si-Si planes. The near-IR and UV bands of the radical anions are assigned as being due to the excitation of an electron from the SOMO pseudo-π(3py) to pseudo-π*(3py) and from the HOMO pseudo-π(3px) to antibonding-σ(3s), respectively. The near-IR band of the polysilane radical anion shows strong solvatochromic shift, which arises from the localization of the excess electron on a part of the main chain. Localization of the excess electron is proposed to be due to Anderson localization which is induced by the fluctuation of pseudo-π conjugation arising from the irregularity of the torsional angles of the Si-Si bonds.
Introduction Polysilanes are σ-conjugated polymers composed of Si-Si skeletons and organic side chains. They are insulators with filled intramolecular valence bands and empty intramolecular conduction bands. However, because of strong σ conjugation, they have rather narrow band gaps of less than 4 eV1,2 and are converted to conductors by photoexcitation or by doping electron donors or acceptors. Conducting polysilanes thus generated can be described as shielded quantum wires, since the conducting onedimensional Si-Si skeletons are shielded with insulating organic side chains. The bulk conduction of the polymers is attained only when charge carriers can migrate through the shields. Since the organic shields are usually good insulator for conduction electrons but not for holes,3 only the holes are generally the charge carriers for bulk conduction.4-10 The migration of charge carriers along the main chains of polysilanes also plays an important role in the bulk conduction, since the electric conductivity of orientated polysilanes along the main chains is 2 orders of magnitude higher than that across the chains.8 To elucidate the static and dynamic nature of charge carriers along the main chains is therefore important for understanding the electric properties of the shielded quantum wires. Information on the static nature of the charge carriers can be obtained by analyzing the electronic absorption spectra and the ESR spectra of the radical anion and cation of polysilanes11-21 possessing unpaired excess electrons and holes in the conduction and valence bands of the silicon main chains as charge carriers, respectively. By analyzing these spectra, we have found that the charge carriers on permethylated oligosilanes are delocalized over the entire main chains, though those on polycyclohexylmethylsilane and probably on polymethylphenylsilane are localized on part of the chains composed of about six Si atoms.19-21 We suggested the Anderson localization of the carge carriers arising from the irregularities of σ-conjugation energies along * Corresponding author.
the Si-Si main chains.21 Although the irregularities were assumed to be induced by conformational disorder of the main chains, the detail of the structural disorder has not been clarified yet. Recently, Nagayama et al. found that the extension of the σ-conjugation length occurred with the elongation of polymer chains.22 This suggests that the localization arises from the fluctuation of the torsional angles of the Si-Si bonds. Elongation of the polymer chains makes the torsional angles uniform so that the σ-conjugation length increases. The above model is based on the assumption that σ-conjugation is sensitive to the torsional angles between adjacent Si atoms. However σ-conjugation is not sensitive to the torsional angle, since σ orbitals are axially symmetric about the Si-Si bonds. Understanding the relation between the localization of charge carriers and the structural deformation of polysilanes therefore necessitates detailed knowledge about the electronic structure of the charge carriers. We have suggested in previous papers that the orbitals for the holes and the excess electrons are pseudo-π composed of Si 3px and Si 3py atomic orbitals, respectively,19,20 where x is the direction parallel to the Si-Si main chain. If this is true, overlapping of adjacent 3py orbitals and therefore the energy of σ-conjugation strongly depends on the torsional angle. To assign the orbital for the hole (SOMO of the radical cation) as a pseudo-π is rather easy. The atomic orbitals at a Si atom composing the SOMO are expressed by [σSi-Si(sp3) + σSi-Si′(sp3) ]/x2 ) px, where σSi-Si(sp3) and σSi-Si′(sp3) denote hybrid orbitals composing two adjacent SiSi σ bonds. The SOMO is composed of only the Si 3px atomic orbitals so that this is a pseudo-π orbital. To assign the orbital for the excess electron (SOMO of the radical anion) as a pseudo-π is, however, not straightforward. A linear combination of antibonding Si-Si σ orbitals gives not a pseudo-π orbital but a σ-type orbital composed of [-s + py]/ x2 atomic orbitals. We therefore need further study for eluci-
10.1021/jp9834715 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/22/1999
Properties of Polysilane Radical Anions
J. Phys. Chem. B, Vol. 103, No. 19, 1999 3813
dating the orbital structure of the excess electron. In the present paper, the electronic absorption spectra and their polarization spectra of the radical anions of permethylated oligosilanes, Si6(CH3)14 and Si8(CH3)18, and that of polycyclohexylmethylsilane in the mixed solvents of nonpolar methylcyclohexane and polar 2-methyltetrahydrofuran at 77 K have been measured for elucidating the electronic structure of excess electrons on polysilanes. Experimental Section Permethylated oligosilanes, Sin(CH3)2n+2, where n is 6 and 8, were synthesized from Sim(CH3)2m+1Cl by a Wurtz-type reaction and purified by fractional distillation. Polycyclohexylmethylsilane, (SiC6H11CH3)n, was synthesized from dichlorocyclohexylmethylsilane by a Wurtz-type reaction in the mixed solvent of toluene and n-heptane (0.85:0.15) with Na-K alloy and 15-crown-5-ether under the reflux condition.23 The crude polymer was purified by repeated precipitation from the chloroform solution into methanol. The average number of Si atoms composing the polymer main chains is approximately 500. Spectroscopic grade methylcyclohexane (MCHX) was used without further purification. 2-Methyltetrahydrofuran (MTHF) was purified by fractional distillation. These solvents were vacuum distilled with Na-K alloy as a drying agent and then deaerated by freezing-pumping-thawing cycles. The mixtures of MCHX and MTHF containing about 0.01 mol/dm3 of oligosilanes or polysilane (monomer unit) in sealed high-purity quartz cells with the optical path length of 2 mm were frozen at 77 K and then irradiated in the dark with a 60Co γ-ray source. Electrons ejected from solvent molecules by γ-rays were captured by solute molecules to generate solute radical anions. Coexisting trapped electrons in the irradiated samples were eliminated before the measurements by photoillumination of the samples with near-IR light of the wavelength longer than 800 nm. The temperature of the samples was kept at 77 K throughout the experiments. The electronic absorption spectra were measured at 77 K with a Shimazu MPS 3000 spectrophotometer equipped with linear polarizers for UV-visible and the visible-near-IR regions, respectively. The absorption spectra of the radical anions are composed of two absorption bands. One is an UV band corresponding to the excitation of an electron from the HOMO to the conduction band, and the other is a near-IR band corresponding to the excitation of an excess electron in the conduction band, respectively. For determining the relative direction of the transition moments of these absorption bands, the radical anions were selectively photobleached with linearly polarized monochromatic light corresponding to one of these bands. The absorption spectra of the remaining oriented radical ions were then measured with polarized light which was parallel or perpendicular to the light for photobleaching. The relative shape of the absorption spectra after the photobleaching would not be changed if the direction of the transition moments of the two bands would be the same. Results Polarization Spectra. Figure 1 compares the absorption spectra of Si6(CH3)14- in MCHX with 5 vol % of MTHF before and after partial photobleaching of the anions with polarized (x-polarized) light corresponding to the near-IR band. The spectral intensity of the photobleached sample measured with x-polarized light is weaker than that with y-polarized light, which certifies that the anionic molecules are partially oriented after the photobleach. The relative intensity of the UV band with
Figure 1. Absorption spectra of Si6(CH3)14- in the mixed solvent of methylcyclohexane and 5 vol % 2-methyltetrahydrofuran at 77 K before and after photobleaching with polarized light of 700 nm < λ < 800 nm: (s) before photobleaching; (‚‚‚) observed with polarized light parallel to the light for photobleaching, (- - -); observed with polarized light perpendicular to the light for photobleaching.
respect to the near-IR band of the x-polarized absorption spectrum should be stronger if the direction of the UV band were different from that of the near-IR band. Observation of no such tendency indicates that the direction of the transition moment is the same for the two bands. No essential difference was observed on the direction of the transition moments between Si6(CH3)14- and Si8(CH3)18-. As shown in Figure 2, the result on (SiC6H11CH3)n- was also the same as that on Si6(CH3)14-; the direction of the transition moment for the near-IR band was the same as that for the UV band. As illustrated in Figure 3, these results support our previous conclusion that the UV band and the near-IR band of oligosilane and polysilane radical anions correspond to HOMO σ(3px) to σ*(3s) and SOMO pseudo-π(3py) to pseudo-π*(3py) transitions.19 These orbitals are composed from atomic orbitals generated by the mixing two Si-C σ * bonds and two σSi-Si*(sp3) bonds, as
(3s3p ) ) [σ y
3 Si-Si*(sp )
+ σSi-Si*′(sp3)]/2 ( [σSi-C*(sp3) + σSi-C*′(sp3)]/2
) [(s/2 - x2px/2 + py/2) + (s/2 + x2px/2 + py/2)]/2 ( [(-s/2 + py/2 + x2pz/2) + (-s/2 + py/2 - x2pz/2)]/2 (1) ) (s + py)/2 ( (-s + py)/2 The mixing may take place due to repulsive interactions between the excess electron and the other ones. The main interaction causing the delocalization of the excess electrons is therefore not σ-conjugation but pseudo-π conjugation. SolVatochromic Shift. Figures 4 and 5 show the absorption spectra of Si6(CH3)14- and (SiC6H11CH3)n- in the mixed solvents of MCHX and MTHF. The near-IR bands corresponding to the excitation of the excess electrons are blue-shifted by addition of polar MTHF, which indicates that the stabilization of the radical anions by solvation is much stronger for the ground state than for the excited state. The solvatochromic shift of the UV bands seems very small but is not clear due to strong UV absorption by neutral parent molecules. We therefore treat the spectral shift of mainly the near-IR bands. Figure 5 shows the degree of solvatochromic shift as a function of the concentration of MTHF in MCHX. The solvatochromic shift for
3814 J. Phys. Chem. B, Vol. 103, No. 19, 1999
Ichikawa et al. of the excess charge on (SiC6H11CH3)n- is the same as that of the excess electron and is confined in a part of the polymer chain. Elucidation of the orbital structure of the excess charge from the solvatochromic shift necessitates a physical model for the solvation of a one-dimensional shielded charged wire. Since the excess charge is confined within the Si-Si skeleton,3 no direct interaction between solvent molecules and the excess charge is expected. The solvation is then solely due to longrange charge-dipole interactions. Assuming that the dielectric polarization is proportional to the electric field Eg generated by an ground-state ion, the electrostatic energy Ug for the groundstate ion due to charge-dipole interaction is given by
Figure 2. Absorption spectra of (SiC6H11CH3)n- in the mixed solvent of methylcyclohexane and 5 vol % 2-methyltetrahydrofuran at 77 K before and after photobleaching with polarized light of 300 nm < λ < 400 nm. (s) before photobleaching; (‚‚‚) observed with polarized light parallel to the light for photobleaching; (- - -) observed with polarized light perpendicular to the light for photobleaching.
Ug )
∫d3 R ∫0D
Eg dD )
g
[∫
R-r Fg(r) d3r |R - r|3
e2 d3 R 32π2
∫
]
2
(2)
where refers to the dielectric constant of a solvent, r and R to the position vectors of a charge and the dielectric solvent, and Fg(r) to the distribution function of the charge. The relation of D ) Eg has been used in the above equation. Equation 2 is, however, not applicable to an excited-state ion, since the time of excitation process is not long enough for the reorientation of permanent dipoles surrounding the ion. The permanent dipoles still memorize the charge distribution of the previous ground state and create electric field Eg,perm. The electric field is then given by a sum of Eg,perm and an electric field following the instantaneous change of the charge distribution. The electrostatic energy, Ue, for the excited-state ion is defined as a sum of three energy terms. These are energies for introducing the ground-state ion in the solvent, for removing the charge of the ion without changing the direction of the oriented permanent dipoles, and for introducing the charge with a distribution function for the excited-state ion Fe(r), as Figure 3. Schematic representation of the optical transitions for oligosilane and polysilane radical anions.
Ug )
∫d3 R ∫0D
{(
)∫ [∫
1 1 op
)∫ [∫
]
[∫
∫
[∫
]
][∫ [∫ ]
(
2 R-r e2 e2 1 d3 R F (r) d3r + 2 3 e 32π |R - r| 32π2 op R-r R-r 1 d3 R F (r) d3r Fg(r) d3r 3 e |R - r| |R - r|3
)
(SiC6H11CH3)n- is much stronger than that of Si6(CH3)14-. Since the solvation energy decreases with increasing the space occupied by a charge, it is evident that the negative charge of the polysilane radical ion is not spread out over the polymer chain but is confined in a part of the chain. We have shown in the previous paper that the unpaired electron on (SiC6H11CH3)nis confined in a part of the polymer chain composed of about six Si atoms. The present result indicates that the distribution
g
2 e2 1 R-r 1 - d3 R F (r) d3r 2 3 g 16π |R - r| op 2 R-r 1 3 d R Fg(r) d3r + 3 2op |R - r| R-r R-r 3 d R F (r) d3r F (r) d3r + 3 g 3 e |R - r| |R - r| 2 R-r 1 3 d R Fe(r) d3r 3 2op |R - r|
∫
( Figure 4. Solvatochromic shift for Si6(CH3)14 in the mixed solvent of 2-methyltetrahydrofuran and methylcyclohexane at 77 K. The arrows indicate the absorption peaks.
∫d3 R ∫D0 (Eg,perm + Eg,op) dD + ∫d3 R ∫0D (Eg,perm + Ee,op) dD g
) Ug +
-
Eg dD +
g
∫
)∫ [∫
∫
] ]}
]
2
(3)
where op is the optical dielectric constant of the solvent. Solvation energy is defined as a difference of the electrostatic energies in the vacuum and in a solvent so that solvation energies for the ground- and the excited-state ion, Ss and Se, are given by
Sg )
( )∫{∫
e2 1 1 32π2 0
}
R-r Fg(r) d3r |R - r|3
2
d3 R
(2′)
Properties of Polysilane Radical Anions
J. Phys. Chem. B, Vol. 103, No. 19, 1999 3815
Figure 5. Solvatochromic shift for (SiC6H11CH3)n- in the mixed solvent of 2-methyltetrahydrofuran and methylcyclohexane at 77 K. The arrows indicate the absorption peaks.
Se )
( )∫ [∫ ( )∫ [∫
]
2 R-r e2 1 1 d3 R F (r) d3r ( )See) 2 3 e 32π 0 |R - r| 2 R-r e 1 1 d3 R Fe(r) d3r 32π2 0 |R - r|3 2 R-r 3 F (r) d r ()Sge) (3′) g |R - r|3
Figure 6. Solvation energies of ground- and first excited-state free electron in a wire of length L embedded at the center of a nonpolar cylindrical insulator with radius R and length L + R.
]
∫
The energy of solvatochromic shift ∆hν is a difference of solvation energies between ground and excited states so that
∆hν ) Sg - Se ) Sg - See + Sge
(4)
It is evident from eq 4 that solvatochromic shift cannot be expected if charge distributions are the same for a ground and an excited states. Calculation of solvatochromic shift necessitates information on the molecular shape and the ground- and excited-state charge distributions of the radical anion. We simplify the molecular shape to be a linear conducting wire with length L which is embedded at the center of a cylindrical shield with radius R and length L + R. The charge distribution can be assumed to be the same as that of a free electron on a wire,
Fn(x) )
2 2 nπ sin x L L
( )
(5)
where n is 1 for the ground state and 2 for the excited state. Figure 6 shows the solvation energies and their difference of the ground and the first excited-state free electron in the wire as a function of R/L. The solvation energy is larger for the ground state, since the charge distribution of the ground state is more localized around the center of the wire. The difference of the energies between the ground and the excited states normalized by L decreases with increasing R/L, because the difference of the charge distributions along the wire is no more important for dipoles at distant R. Comparison of the observed solvatochromic shifts with calculated ones necessitates information on the dielectric constants of the mixed solvents at 77 K. The optical dielectric constants of MTHF and MCHX can be estimated from the polarizability of the molecules (9.98 × 10-24cm3 for MTHF and 12.9 × 10-24 cm3 for MCHX) by using the Clausius-Mossotti equation. They are about 2.50 for both the solvents. The static dielectric constant of MCHX is the same as the optical one. Estimation of the static dielectric constant of MTHF in the glassy state is not easy. Decrease of the temperature causes the decrease of thermal disturbance to the orientation of the permanent dipoles, which causes the increase of the dielectric constant. However,
Figure 7. Comparison of observed (symbols) and calculated (lines) solvatochromic shifts for Si6(CH3)14-, Si8(CH3)18-, and (SiC6H11CH3)n-. The values of parameters used for the calculation are op ) ) 2.50 for methylcyclohexane, op ) 2.50 and ) 110 for 2-methyltetrahydrofuran, L ) 1.3 nm and R ) 0.4 nm for Si6(CH3)14-, L ) 1.7 nm and R ) 0.4 nm for Si8(CH3)18-, and L ) 1.3 nm and R ) 0.65 nm for (SiC6H11CH3)n-. The charge distributions of the ground and the excited anions along the main chains are assumed to be given by (2/L) sin2(πx/L) and (2/L) sin2(2πx/L), respectively.
simultaneous increase of the viscosity obstructs the orientation and the degree of the obstruction depends on both the viscosity and the strength of an electric field. We therefore determined the value in such a way that the calculated solvatochromic shifts for Si6(CH3)14- radical anions in neat MTHF reproduced the observed one. The value thus determined is MTHF ) 110. The dielectric constants of the mixed solvents are determined from the Clausius-Mossotti equation, as ( - 1)/( + 2) ) V(MTHF - 1)/(MTHF + 2) + (1 - V)(MCHX - 1)/(MCHX + 2), where V is the volume fraction of MTHF. Figure 7 compares the observed and calculated solvatochromic shifts for the near-IR bands. The values of L and R used were 1.3 and 0.4 nm, 1.7 nm and 0.4 nm, and 1.3 and 0.65 nm for Si6(CH3)14-, Si8(CH3)18-, and (SiC6H11CH3)n-, respectively. These values were estimated from the lengths and angles of Si-Si and Si-C bonds and from the molecular volumes of the oligosilanes and the polysilane.21 The calculated solvatochromic shifts reproduce the observed one for Si6(CH3)14- and Si8(CH3)18-, but not for (SiC6H11CH3)n-. The discrepancy between them cannot be reduced by increasing the value of L for (SiC6H11CH3)n-. Inspection of Figure 6 results in the conclusion that the calculated solvatochromic shift is always much smaller than that
3816 J. Phys. Chem. B, Vol. 103, No. 19, 1999
Ichikawa et al.
of Si6(CH3)14- with shorter R, as long as the same charge distribution as for the oligosilane radical anions is assumed. Increase of L from 1.3 to 2.1 nm, for example, causes the decrease of R/L and (solvation energy)/L to the same values for Si6(CH3)14-. However, because of the increase of L, the solvatochromic shift becomes 13/21 of that for Si6(CH3)14-. Discussion Although the results on the solvatochromic shift give the information not on the distribution of the excess electron but on the charge distribution, to assume them to be the same is quite reasonable. The energy of the excess electron with respect to the other ones is too large to disturb the orbitals for the other electrons. The optical and electric characteristics clarified from the present experiment are then summarized as follows. (1) The UV and the near-IR absorption bands correspond to HOMO pseudo-π(3px) to s*(3s) and SOMO pseudo-π(3px) to pseudo-π*(3py), respectively. Change of the SOMO orbital from σ*(3s3py) to pseudo-π(3py) arises from repulsive interactions between the excess electron and the other ones. (2) Delocalization of the excess electron on the Si-Si chain is induced by pseudo-π interactions. (3) The excess charge on (SiCH3C6H11)n- is not spread out over the entire main chain, but is confined on a part of the chain. (4) Near-IR excitation of the excess electron of (SiC6H11CH3)ncauses considerable change of the charge distribution, which does not take place for smaller Si6(CH3)14 and Si8(CH3)18. In this section, we discuss why the excess charge on (SiC6H11CH3)n- is localized on a part of the main chain and why the orbital of the excess electron is changed so much by the excitation. In the previous paper, we suggested that the excess electron on polysilane is localized due to Anderson localization or the fluctuation of one-dimensional potential energy. The key to understand the Anderson localization of the excess electron is its π-type interaction between adjacent Si atoms. The conjugation energy of the two 3p atomic orbital is very sensitive to the torsional angle θn,n+1 between the n-th and (n+1)-th Si atoms, and is given by
βn,n+1 ) β cos θn,n+1
(6)
where the distribution of the torsional angle can be given by a Gaussian one, as
[ ( )]
P(θn,n+1) ≈ exp -
θn,n+1 πσ
2
(7)
Using a Huckel approximation, the orbitals and the energy can be calculated from the secular equation (8).
|
R-E β2,1 0 -
β1,2 R-E β3,2 -
0 β2,3 R-E -
0 0 β3,4 -
0 0 0 -
|
- )0 -
(8)
Figures 8 and 9 show the typical wave functions of excess electrons obtained for σ ) 0.1. The distribution of β is also shown. The calculation shows that the distribution of the torsional angles causes the Anderson localization for long chains but not for short chains. This is the reason the excess electron is localized not on the oligosilanes but only on the polysilane. The degree of localization decreases with decreasing σ and is zero at σ ) 0. The Anderson localization, which is not desirable for ideal quantum wires, is therefore possible to be
Figure 8. Effect of the distribution of torsional angles on the groundand excited-state wave functions of an electron on a wire composed of 10 π-conjugated atoms. The distribution is assumed to be given by exp[-(10θ/π)2]. The Huckel approximation is used for the calculation. The values of resonance integral between adjacent atoms are also shown.
Figure 9. Effect of the distribution of torsional angles on the groundand excited-state wave functions of an electron on a wire composed of 100 π-conjugated atoms. The distribution is assumed to be given by exp[-(10θ/π)2]. The Huckel approximation is used for the calculation. The values of resonance integral between adjacent atoms are also shown.
prevented by decreasing the distribution of the torsional angles, for example, by stretching a polymer22 or by replacing the pendant groups of the polymer to bulky ones.24 Recently, Irie et al. found that the localization of the excess electron did not take place for Sin(CH3C3H7)2n+2 oligosilanes up to n ) 16. Delocalization of the excess electron over the entire chain may arise from shorter chains and also from narrow distribution of the torsional angles. As shown with a broken curve in Figure 9, the excited states are also Anderson-localized at a location totally different from that of the ground state. This may be the reason (SiC6H11CH3)nwith smaller solvation energy shows a much larger solvatochromic shift. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research from the Ministry of Education, Science and Culture, Japan. References and Notes (1) Takeda, K.; Shiraishi, K. Phys. ReV. B 1989, 39, 11028. (2) Yokoyama, K.; Yokoyama, M. Chem. Lett. 1989, 1005. (3) Kumagai, J.; Tachikawa, H.; Yoshida, H.; Ichikawa T. J. Phys. Chem. 1996, 100, 16777. (4) Kepler, R. G.; Zeigler, J. M.; Harrah, L. A.; Kurtz, S. R. Phys. ReV. B. 1987, 35, 2818.
Properties of Polysilane Radical Anions (5) Abkowitz, M. A.; Rice, M. J.; Stolka, M. Philos. Mag. 1990, 61, 25 and references therein. (6) Van de Laan, G. P.; De Haas, M. P.; Warman, J. M.; Frey, H.; Mo¨ller, M. Mol. Cryst. Liq. Cryst. 1993, 236, 165. (7) 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.; Warman, J. M. Macromolecules 1993, 26, 89. (8) Van de Laan, G. P.; De Haas, M. P.; Hummel, A.; Frey, H.; Sheiko, S.; Mo¨ller, M. Macromolecules 1994, 27, 1897. (9) Samuel, L. M.; Sanda, P. N.; Miller, R. D. Chem. Phys. Lett. 1989, 159. (10) Nakayama, Y.; Hirooka, K.; West, R. Solid State Commun. 1996, 100, 759. (11) Carberry, E.; West, R.; Glass, G. J. Am. Chem. Soc. 1969, 91, 5440. (12) Ban, H.; Sukegawa, K.; Tagawa, S. Macromolecules 1987, 20, 1775; 1988, 21, 45. (13) Ban, H.; Tanaka, A.; Hayashi, N.; Tagawa, S.; Tabata, Y. Radiat. Phys. Chem. 1989, 34, 587.
J. Phys. Chem. B, Vol. 103, No. 19, 1999 3817 (14) Irie, S.; Oka, K.; Irie, M. Maclomolecules 1988, 21, 110. (15) Irie, S.; Oka, K.; Nakao, R.; Irie, M. J. Organomet. Chem. 1990, 388, 253. (16) Irie, S.; Irie, M. Maclomolecules 1992, 25, 1766. (17) Irie, S.; Irie, M. Maclomolecules 1997, 30, 7906. (18) Ushida, K.; Kira, A.; Tagawa, S.; Yoshida, Y.; Shibata, H. Proc. Am. Chem. Soc. DiV. Polym. Mater. 1992, 66, 299. (19) Kumagai, J.; Yoshida, H.; Koizumi, H.; Ichikawa, T. J. Phys. Chem. 1994, 98, 13117. (20) Kumagai, J.; Yoshida, H.; Ichikawa, T. J. Phys. Chem. 1995, 99, 7965. (21) Ichikawa, T.; Koizumi, H.; Kumagai, J. J. Phys. Chem B 1997, 101, 10698. (22) Nagayama, N.; Ohwatari, H.; Yokoyama, M. J. Imaging Sci. Technol. 1996, 40, 304. (23) Zhang, X.-H.; West, R. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 159. (24) Kumagai, J.; Yoshida, H.; Ichikawa, T. Nucleonica 1997, 42, 465.
