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Radical anions of polysilane derivatives studied by pulse radiolysis
Radiation Physics and Chemistry 60 (2001) 411–415
Radical anions of polysilane derivatives studied by pulse radiolysis S. Seki, Y. Yoshida, S. Tagawa* The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Abstract The electronic structure of polysilane charged radicals has been studied by transient absorption spectroscopy using nano-second pulse radiolysis technique. Radical anions of polysilanes with symmetric alkyl, asymmetric alkyl, and phenyl substituents displayed a near-UV absorption band with a 0–0.5 eV red shift from absorptions ascribed to exciton states. A near-IR transient absorption was also observed with the same kinetics as the UV band. Electron transfer from polysilane radical anions to pyrene was observed with rate constants from 4.5 109 to 7.3 109 M1 s1. Extinction coefficients (eðÞ) of the radical anions were determined to be 8.5 104–1.6 105 M1 cm1. The value of eðÞ and band width of the transient absorption depended on the substitution of the Si main chains. The degree of electron delocalization was estimated based on the 1-D polaron model, and showed a clear relationship with the extinction coefficient e of the band-gap transition and the viscosity index a of the polymers. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Polysilane; Pulse radiolysis; Radical anions; Polaron
1. Introduction Silicon skeleton polymers (for a review, see Miller and Michl, 1989) have attracted considerable attention because of their interesting physical properties (Kepler et al., 1987; Fujino, 1987; Stolka et al., 1987; Kajzar et al., 1986). Recent experimental results and theoretical calculations on polysilanes have suggested that their conjugated s bonding in a silicon skeleton (s conjugated system) is responsible for their interesting physical properties (Rice and Phillpot, 1987; Takeda et al., 1987). Ionic species of Si-based molecules have been studied for cyclic and linear polysilanes (Carberry et al., 1969; West and Carberry, 1975; Kira et al., 1979). Cyclic oligosilanes form radical anions by reduction with alkali metal, and also form radical cations by oxidation with *Corresponding author. Tel.: +81-6-6879-8502; fax:+81-66876-3287. E-mail addresses: [email protected] (S. Seki), [email protected] (S. Tagawa).
AlCl3. The ionic species have an unpaired electron delocalized over the silicon skeleton. Charged molecules of polysilanes were produced by a pulse radiolysis technique, and displayed chromophores at near-UV and IR regions with very high extinction coefficients (Ban et al., 1987; 1988). The transient spectra suggest that an excess electron or a hole is delocalized over a conjugated segment in a molecule. This indicates that the conjugated molecular orbital (s-conjugation) is responsible for the absorption spectra of the ionic species. Solid-state polysilanes become p-type semiconductors in the presence of strong electron acceptors, suggesting that the polymers essentially have a semiconducting path for the carriers along a main chain (West et al., 1981). Thus, an ionized molecule simulates the conduction of holes or electrons on the silicon chain. The oneelectron-theory model has been extended to tetrahedrally bonded polymers such as polysilanes, and predicted small electron–lattice interaction (Rice and Phillpot, 1987). The polaron model provides a better interpretation of hole transport in polysilanes; however, a direct
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observation of the state has not been carried out yet. Thus, the transient spectroscopy by the pulse radiolysis technique is useful to elucidate the polaron states on s conjugated systems. The present paper describes the transient absorption spectra of radical anions of polysilanes with symmetric alkyl, asymmetric alkyl, and phenyl substituents. The spectra were obtained by the nano-second pulse radiolysis technique with a wide wavelength range from 300 to 1600 nm. Their optical properties were quantitatively discussed in relation with the structure of Si main chain. The binding energy of excess electrons and holes is estimated on the basis of the polaron model, leading to an elucidation of the degree of delocalization of excess electrons on Si main chains.
2. Experimental section Polysilanes were synthesized by the conventional Kipping method from the dichlorosilane monomers. All chlorosilanes were doubly distilled products from Shin-Etsu Chemical Co. Ltd. Polymerization reactions were carried out in an Ar atmosphere, in 100 ml of dry toluene which was refluxed with sodium for 10 h and distilled before use. The monomer was added into the reaction vessel and mixed with the sodium dispersion during the time listed in Table 1. The sodium microdispersion in toluene was purchased from Acros Co. LTD. The polymers obtained showed good solubility in toluene, THF, 2-methyltetrahydrofuran (MTHF), chloroform and dichloromethane. Polymer solutions were precipitated in iso-propylalcohol (IPA) after filtration through a 0.45 mm PTFE filter to roughly eliminate NaCl, and the precipitates were dried under vacuum. The toluene solutions of these polymers were transferred into a separatory funnel, washed with water to eliminate the remaining NaCl, and precipitated twice
with toluene-IPA and tetrahydrofuran (THF)-methanol. The amounts of residual Cl atoms were confirmed to be less than 0.1% in all polysilanes by elemental analysis. The structure of the polymers was characterized by 1H NMR using JEOL EX-270 and EX-600 NMR spectrometer at 270 MHz. The molecular weight distributions in all the polymers were measured with a Shimadzu C-R3A gel permeation chromatography (GPC) system with polystyrene calibration standards. Glass transition temperatures were measured with a Perkin-Elmer DSC-7 system. The UV–visible absorption spectra were recorded using a Shimadzu UV-3100 PC spectrophotometer. The photoluminescence spectra were measured with a Perkin-Elmer LS-50B spectrofluorometer. The pulse radiolysis measurements were performed with an L-band electron linear accelerator at the Radiation Laboratory of the Institute of Scientific and Industrial Research, Osaka University. Details of the apparatus have been described elsewhere (Seki et al., 1999). All the polysilanes were dissolved into THF at 0.02– 0.1 mol dm3 conc. (base mol unit). The THF solutions were deaerated in suprasil quartz cells having a 2 cm optical path.
3. Results and discussion Fig. 1 shows the transient absorption spectra of PS7 radical anions obtained by nano-second pulse radiolysis. Two strong absorptions were observed in the near-UV and IR regions. The transition energy of the IR band was already confirmed by low-temperature matrix experiment to be 0.5–0.6 eV (Seki et al., 1999). The kinetic traces in the near-UV and IR overlap each other, suggesting that the transient absorption originates from a single reactive intermediate, i.e. the radical anion of PS7. This result also implies the presence of an interband level occupied by an excess electron or a hole. The
Table 1 Side-chain substituents, UV absorption maxima and extinction coefficients, molecular weights, and synthetic details of the polysilane derivatives studied Polymers
Entry
lmax (nm)
e M1 cm1
Mwa ( 104)
Mnb ( 104)
Reaction time (h)
Yield (%)
CH3, C3H7 CH3, n-C6H13 CH3, c-C6H11 CH3, n-C12H25 n-C6H13, n-C6H13 n-C4H9, n-C4H9 CH3, C6H5 CH3, –C2H4–C6H5
PS1 PS2 PS3 PS4 PS5 PS6 PS7 PS8
306 306 320 310 318 314 339 305
5800 6000 7500 4900 9500 8800 9200 6500
2.5 1.4 6.2 11.5 8.3 7.5 2.3 1.8
1.4 1.1 3.9 3.1 3.1 2.5 1.3 1.1
5 5 10 24 10 10 4 12
24 15 20 9 30 18 38 12
a b
Weight average molecular weight. Number average molecular weight, respectively, determined by using polystyrene calibration standards.
S. Seki et al. / Radiation Physics and Chemistry 60 (2001) 411–415
413
near-UV and IR absorptions was estimated to be about 4.0 eV, which is in good agreement with theoretical calculations. The transient absorptions could be scavenged by the presence of pyrene (Py), leading to anion radicals of Py. All the polysilanes clearly showed the electron transfer reaction, as displayed in Fig. 2 for PS7. The kinetic trace in the UV region indicates the presence of reactive intermediates having relatively longer lifetime than that of radical anions. The species may be attributed to the neutral silyl radicals which are produced by the direct interaction of incident electrons with polysilane molecules, and have been reported to show an intense absorption at 360 nm (Shizuka et al., 1988; Watanabe and Matsuda, 1992; Peinado et al., 1992; Tsutsui et al., 1998). The reaction schema can be written as k
PS þ Py ! PS þ Py ;
Fig. 1. Transient absorption spectra of radical anions of PS7 formed by pulse radiolysis in THF at 0.1 M conc.(base mol unit). Diamonds and circles indicate the optical density observed 4 and 250 n after pulses, respectively. Superimposed figures denote kinetic traces at the wavelength indicated by arrows.
level was already revealed to be formed by the interaction between an excess electron and 1-D Si backbone phonons (Rice and Phillpot, 1987; Seki et al., 1999). The sum of the transition energy of the
ð1Þ
where PS, Py, and X denote polysilane, pyrene, and counterion, respectively, and k1 and k2 are rate constants. Concentration units of polysilanes and their radical anions were defined as mole of Si units and mole of added electrons, respectively. The value of k and extinction coefficient eðÞ were determined at room temperature on the basis of e for pyrene radical anions. Table 2 summarizes the values of k1 and eðÞ observed for a variety of polysilanes. Fig. 3 plots the UV transient absorption of PS7 vs. transition energy together with the Gaussian fits. Bleaching of the steady-state UV absorption band (exciton absorption) was simultaneously observed in the transient spectroscopy, which gave the degree of delocalization: ndel of an excess
Fig. 2. Transient absorption spectra of the radical anion of PS7 formed by pulse radiolysis in THF at 0.1 M conc. (base mol unit) with 0.005 M pyrene (a). The spectra (b) denote an enlarged view at UV–Vis region. Circles, squares, and diamonds indicate the optical density observed 4, 100, and 250 ns after pulses, respectively.
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Table 2 UV absorption maxima, extinction coefficients, rate constant for electron transfer to pyrene, and degree of delocalization of the radical anions of the polysilane derivatives studied Entry
lmax (nm)
eðÞa ( 105 M1 cm1)
k ( 109 M1 s1)
ndel b
PS1 PS2 PS3 PS4 PS5 PS6 PS7 PS8
358 360 366 358 359 358 369 355
1.1 1.1 1.3 0.85 1.3 1.2 1.6 1.1
4.9 5.6 7.3 6.2 6.9 5.9 6.5 4.5
5.5 2.1 6 2.3 8.5 2.6 4.5 1.7 11 2.9 9.5 2.7 13 3.4 6.5 2.4
a
Extinction coefficient per radical anion. Average degree of delocalization estimated as the number of Si units. b
Fig. 3. UV absorption of PS7 as a function of the transition energy. Solid line denotes a Gaussian fit to the transient absorption.
Fig. 4, indicating a monotonous increase of ndel with increase in euv . Recently, Fujiki (1996) reported an empirical relationship between euv and the viscosity index a which reflected the geometrical structure of the polymer main chain. The following empirical formula was obtained for the relation between a and euv : euv ¼ 1130 e2:9a :
ð3Þ
The values of ndel are plotted vs. a in Fig. 5, giving a clear relationship between the values of ndel and a. This indicates that an excess electron will be highly delocalized on a Si chain the conformation of which is tightly locked by the side chains. A Si skeleton is regarded to take a ‘‘stiff’’-like structure when the value of a equals 1 which is the highest value of polysilanes used in the present study. To date, several groups reported the synthesis of polysilanes having a ‘‘rod’’-like backbone
Fig. 4. Degree of electron delocalizaton on Si chains as a function of the extinction coefficient of the main-chain sconjugated system in polysilanes.
electron on a silicon skeleton as follows: ndel ¼
DBl OD eðÞ ; DOD
euv
ð2Þ
where DBl OD and DOD denote the observed changes in the optical density of the UV transient absorption and the bleaching, respectively, euv is the extinction coefficient at the exciton absorption maxima of the polysilanes. The degree of delocalization was calculated to be 5.5–13 Si units for the polysilanes as summarized in Table 2. Kumagai et al. (1994) reported the degree of electron delocalization on poly(c-hexylmethylsilane): PS3 as 7 Si units by the EPR spectroscopy of PS3 , which was consistent with the value in the present study. The degree of delocalization is plotted as a function of euv in
Fig. 5. Degree of electron delocalizaton on Si chains as a function of the viscosity index of the polysilanes. The value of viscosity index a was estimated using the experimental equation suggested by Fujiki, 1996.
S. Seki et al. / Radiation Physics and Chemistry 60 (2001) 411–415
conformation (a 1:5). It is predicted that an excess electron should be delocalized and highly mobile in the polysilanes. 4. Conclusion The electronic states of excess electrons on polysilane chains were investigated by nano-second pulse radiolysis. Transient absorption spectroscopy was carried out for radical anions of polysilanes with a variety of substitution patterns. The radical anions of polysilanes displayed near-UV and IR absorption maxima at ca. 3.1–3.5 eV and 0.5–1 eV, respectively. They were ascribed to the inter-band and the sub-band transition of polaron states. The extinction coefficients were calculated from kinetic traces of electron transfer reactions between polysilane radical anions and pyrene, simultaneously giving the rate constants of the reaction. The degree of electron delocalization rapidly increased with increasing euv and a. This indicates that excess electrons and holes are highly delocalized in polysilanes with linear Si chains. Acknowledgements The authors acknowledge Dr. T. Kozawa, Mr. T. Yamamoto, and Mr. H. Habara at ISIR Osaka University for the experimental support. They also acknowledge Dr. A.D. Trifunac, Dr. K.R. Cromack, and Dr. D. Wurst at Argonnne National Laboratory, and Prof. K. Ishigure and Prof. K. Asai at the University of Tokyo for their useful advise. This work was supported by a Grant-in-aid for scientific research from the Ministry of Education, Science and Culture. References Ban, H., Sukegawa, K., Tagawa, S., 1987. Pulse radiolysis study on organopolysilane radical anions. Macromolecules 20, 1775. Ban, H., Sukegawa, K., Tagawa, S., 1988. Side-chain effects on ultraviolet absorption of organopolysilane radical anion. Macromolecules 21, 45. Carberry, E., West, R., Glass, G.E., 1969. Cyclic polysilanes. IV. Anion radicals and spectroscopic properties of the permethylcyclopolysilanes. J. Am. Chem. Soc. 91, 5446.
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Fujiki, M., 1996. A Correlation between global conformation of polysilane and UV absorption characteristics. J. Am. Chem. Soc 118, 7424. Fujino, M., 1987. Photoconductivity in organopolysilanes. Chem. Phys. Lett. 136, 451. Kajzar, F., Messier, J., Rosilo, C., 1986. Nonlinear optical properties of thin films of polysilane. J. Appl. Phys. 60, 3040. Kepler, R.G., Zeigler, J.M., Harrah, L.A., Kurtz, S.R., 1987. Photocarrier generation and transport in s-bonded polysilanes. Phys. Rev. B 35, 2818. Kira, M., Bock, H., Hengge, E., 1979. Radical ions. XXVI. Radical anions of perphenylcyclopolysilanes. J. Organomet. Chem. 164, 277. Kumagai, J., Yoshida, H., Kuizumi, H., Ichikawa, T., 1994. Electronic structure of oligosilane radical anions as studied by electron spin resonance and electronic absorption spectroscopy. J. Phys. Chem 98, 13117. Miller, R.D., Michl, J., 1989. Polysilane high polymers. Chem. Rev. 89, 1359. Peinado, C., Wollny, T., Schnabel, W., 1992. On the degradation of polysilanes in solution. flash photolysis and pulse radiolysis studies. Radiat. Phys. Chem. 40, 101. Rice, M.J., Phillpot, S.R., 1987. Polarons and bipolarons in a model tetrahedrally bonded homopolymer. Phys. Rev. Lett. 58, 937. Seki, S., Yoshida, Y., Tagawa, S., Asai, K., 1999. Electronic structure of radical anions and cations of polysilanes with structural defects. Macromolecules 32, 1080. Shizuka, H., Tanaka, H., Tonokura, K., Murata, K., Hiratsuka, H., Ohshita, J., Ishikawa, M., 1988. Absorption, emission and reaction kinetics of dimethylsilylene. Chem. Phys. Lett. 143, 225. Stolka, M., Yuh, H.J., McGrane, K., Pai, D.M., 1987. Hole transport in organic polymers with silicon backbone (polysilylenes). J. Polym. Sci. Polym. Chem. Ed. 25, 823. Takeda, K., Fujino, M., Seki, K., Inokuchi, H., 1987. Skeletonside-group interaction in organopolysilane. Phys. Rev. B 36, 8129. Tsutsui, S., Sakamoto, K., Kira, M., 1998. Bis(diisopropylamino)silylene and its dimer. J. Am. Chem. Soc. 120, 9955. Watanabe, A., Matsuda, M., 1992. Photodegradation of alkyland aryl-substituted polysilanes studied by flash photolysis. Macromolecules 25, 484. West, R., Carberry, E., 1975. Permethylpolysilanes: Silicon analogs of hydrocarbons. Science 189, 179. West, R., David, L.D., Djurovich, P.I., Stearly, K.L., Srinivasan, K.S.V., Yu, H., 1981. Phenylmethylpolysilanes: formable silane copolymers with potential semiconducting properties. J. Am. Chem. Soc. 103, 7352.
Radical anions of polysilane derivatives studied by pulse radiolysis S. Seki, Y. Yoshida, S. Tagawa* The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Abstract The electronic structure of polysilane charged radicals has been studied by transient absorption spectroscopy using nano-second pulse radiolysis technique. Radical anions of polysilanes with symmetric alkyl, asymmetric alkyl, and phenyl substituents displayed a near-UV absorption band with a 0–0.5 eV red shift from absorptions ascribed to exciton states. A near-IR transient absorption was also observed with the same kinetics as the UV band. Electron transfer from polysilane radical anions to pyrene was observed with rate constants from 4.5 109 to 7.3 109 M1 s1. Extinction coefficients (eðÞ) of the radical anions were determined to be 8.5 104–1.6 105 M1 cm1. The value of eðÞ and band width of the transient absorption depended on the substitution of the Si main chains. The degree of electron delocalization was estimated based on the 1-D polaron model, and showed a clear relationship with the extinction coefficient e of the band-gap transition and the viscosity index a of the polymers. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Polysilane; Pulse radiolysis; Radical anions; Polaron
1. Introduction Silicon skeleton polymers (for a review, see Miller and Michl, 1989) have attracted considerable attention because of their interesting physical properties (Kepler et al., 1987; Fujino, 1987; Stolka et al., 1987; Kajzar et al., 1986). Recent experimental results and theoretical calculations on polysilanes have suggested that their conjugated s bonding in a silicon skeleton (s conjugated system) is responsible for their interesting physical properties (Rice and Phillpot, 1987; Takeda et al., 1987). Ionic species of Si-based molecules have been studied for cyclic and linear polysilanes (Carberry et al., 1969; West and Carberry, 1975; Kira et al., 1979). Cyclic oligosilanes form radical anions by reduction with alkali metal, and also form radical cations by oxidation with *Corresponding author. Tel.: +81-6-6879-8502; fax:+81-66876-3287. E-mail addresses: [email protected] (S. Seki), [email protected] (S. Tagawa).
AlCl3. The ionic species have an unpaired electron delocalized over the silicon skeleton. Charged molecules of polysilanes were produced by a pulse radiolysis technique, and displayed chromophores at near-UV and IR regions with very high extinction coefficients (Ban et al., 1987; 1988). The transient spectra suggest that an excess electron or a hole is delocalized over a conjugated segment in a molecule. This indicates that the conjugated molecular orbital (s-conjugation) is responsible for the absorption spectra of the ionic species. Solid-state polysilanes become p-type semiconductors in the presence of strong electron acceptors, suggesting that the polymers essentially have a semiconducting path for the carriers along a main chain (West et al., 1981). Thus, an ionized molecule simulates the conduction of holes or electrons on the silicon chain. The oneelectron-theory model has been extended to tetrahedrally bonded polymers such as polysilanes, and predicted small electron–lattice interaction (Rice and Phillpot, 1987). The polaron model provides a better interpretation of hole transport in polysilanes; however, a direct
0969-806X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 0 ) 0 0 4 1 5 - 1
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S. Seki et al. / Radiation Physics and Chemistry 60 (2001) 411–415
observation of the state has not been carried out yet. Thus, the transient spectroscopy by the pulse radiolysis technique is useful to elucidate the polaron states on s conjugated systems. The present paper describes the transient absorption spectra of radical anions of polysilanes with symmetric alkyl, asymmetric alkyl, and phenyl substituents. The spectra were obtained by the nano-second pulse radiolysis technique with a wide wavelength range from 300 to 1600 nm. Their optical properties were quantitatively discussed in relation with the structure of Si main chain. The binding energy of excess electrons and holes is estimated on the basis of the polaron model, leading to an elucidation of the degree of delocalization of excess electrons on Si main chains.
2. Experimental section Polysilanes were synthesized by the conventional Kipping method from the dichlorosilane monomers. All chlorosilanes were doubly distilled products from Shin-Etsu Chemical Co. Ltd. Polymerization reactions were carried out in an Ar atmosphere, in 100 ml of dry toluene which was refluxed with sodium for 10 h and distilled before use. The monomer was added into the reaction vessel and mixed with the sodium dispersion during the time listed in Table 1. The sodium microdispersion in toluene was purchased from Acros Co. LTD. The polymers obtained showed good solubility in toluene, THF, 2-methyltetrahydrofuran (MTHF), chloroform and dichloromethane. Polymer solutions were precipitated in iso-propylalcohol (IPA) after filtration through a 0.45 mm PTFE filter to roughly eliminate NaCl, and the precipitates were dried under vacuum. The toluene solutions of these polymers were transferred into a separatory funnel, washed with water to eliminate the remaining NaCl, and precipitated twice
with toluene-IPA and tetrahydrofuran (THF)-methanol. The amounts of residual Cl atoms were confirmed to be less than 0.1% in all polysilanes by elemental analysis. The structure of the polymers was characterized by 1H NMR using JEOL EX-270 and EX-600 NMR spectrometer at 270 MHz. The molecular weight distributions in all the polymers were measured with a Shimadzu C-R3A gel permeation chromatography (GPC) system with polystyrene calibration standards. Glass transition temperatures were measured with a Perkin-Elmer DSC-7 system. The UV–visible absorption spectra were recorded using a Shimadzu UV-3100 PC spectrophotometer. The photoluminescence spectra were measured with a Perkin-Elmer LS-50B spectrofluorometer. The pulse radiolysis measurements were performed with an L-band electron linear accelerator at the Radiation Laboratory of the Institute of Scientific and Industrial Research, Osaka University. Details of the apparatus have been described elsewhere (Seki et al., 1999). All the polysilanes were dissolved into THF at 0.02– 0.1 mol dm3 conc. (base mol unit). The THF solutions were deaerated in suprasil quartz cells having a 2 cm optical path.
3. Results and discussion Fig. 1 shows the transient absorption spectra of PS7 radical anions obtained by nano-second pulse radiolysis. Two strong absorptions were observed in the near-UV and IR regions. The transition energy of the IR band was already confirmed by low-temperature matrix experiment to be 0.5–0.6 eV (Seki et al., 1999). The kinetic traces in the near-UV and IR overlap each other, suggesting that the transient absorption originates from a single reactive intermediate, i.e. the radical anion of PS7. This result also implies the presence of an interband level occupied by an excess electron or a hole. The
Table 1 Side-chain substituents, UV absorption maxima and extinction coefficients, molecular weights, and synthetic details of the polysilane derivatives studied Polymers
Entry
lmax (nm)
e M1 cm1
Mwa ( 104)
Mnb ( 104)
Reaction time (h)
Yield (%)
CH3, C3H7 CH3, n-C6H13 CH3, c-C6H11 CH3, n-C12H25 n-C6H13, n-C6H13 n-C4H9, n-C4H9 CH3, C6H5 CH3, –C2H4–C6H5
PS1 PS2 PS3 PS4 PS5 PS6 PS7 PS8
306 306 320 310 318 314 339 305
5800 6000 7500 4900 9500 8800 9200 6500
2.5 1.4 6.2 11.5 8.3 7.5 2.3 1.8
1.4 1.1 3.9 3.1 3.1 2.5 1.3 1.1
5 5 10 24 10 10 4 12
24 15 20 9 30 18 38 12
a b
Weight average molecular weight. Number average molecular weight, respectively, determined by using polystyrene calibration standards.
S. Seki et al. / Radiation Physics and Chemistry 60 (2001) 411–415
413
near-UV and IR absorptions was estimated to be about 4.0 eV, which is in good agreement with theoretical calculations. The transient absorptions could be scavenged by the presence of pyrene (Py), leading to anion radicals of Py. All the polysilanes clearly showed the electron transfer reaction, as displayed in Fig. 2 for PS7. The kinetic trace in the UV region indicates the presence of reactive intermediates having relatively longer lifetime than that of radical anions. The species may be attributed to the neutral silyl radicals which are produced by the direct interaction of incident electrons with polysilane molecules, and have been reported to show an intense absorption at 360 nm (Shizuka et al., 1988; Watanabe and Matsuda, 1992; Peinado et al., 1992; Tsutsui et al., 1998). The reaction schema can be written as k
PS þ Py ! PS þ Py ;
Fig. 1. Transient absorption spectra of radical anions of PS7 formed by pulse radiolysis in THF at 0.1 M conc.(base mol unit). Diamonds and circles indicate the optical density observed 4 and 250 n after pulses, respectively. Superimposed figures denote kinetic traces at the wavelength indicated by arrows.
level was already revealed to be formed by the interaction between an excess electron and 1-D Si backbone phonons (Rice and Phillpot, 1987; Seki et al., 1999). The sum of the transition energy of the
ð1Þ
where PS, Py, and X denote polysilane, pyrene, and counterion, respectively, and k1 and k2 are rate constants. Concentration units of polysilanes and their radical anions were defined as mole of Si units and mole of added electrons, respectively. The value of k and extinction coefficient eðÞ were determined at room temperature on the basis of e for pyrene radical anions. Table 2 summarizes the values of k1 and eðÞ observed for a variety of polysilanes. Fig. 3 plots the UV transient absorption of PS7 vs. transition energy together with the Gaussian fits. Bleaching of the steady-state UV absorption band (exciton absorption) was simultaneously observed in the transient spectroscopy, which gave the degree of delocalization: ndel of an excess
Fig. 2. Transient absorption spectra of the radical anion of PS7 formed by pulse radiolysis in THF at 0.1 M conc. (base mol unit) with 0.005 M pyrene (a). The spectra (b) denote an enlarged view at UV–Vis region. Circles, squares, and diamonds indicate the optical density observed 4, 100, and 250 ns after pulses, respectively.
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Table 2 UV absorption maxima, extinction coefficients, rate constant for electron transfer to pyrene, and degree of delocalization of the radical anions of the polysilane derivatives studied Entry
lmax (nm)
eðÞa ( 105 M1 cm1)
k ( 109 M1 s1)
ndel b
PS1 PS2 PS3 PS4 PS5 PS6 PS7 PS8
358 360 366 358 359 358 369 355
1.1 1.1 1.3 0.85 1.3 1.2 1.6 1.1
4.9 5.6 7.3 6.2 6.9 5.9 6.5 4.5
5.5 2.1 6 2.3 8.5 2.6 4.5 1.7 11 2.9 9.5 2.7 13 3.4 6.5 2.4
a
Extinction coefficient per radical anion. Average degree of delocalization estimated as the number of Si units. b
Fig. 3. UV absorption of PS7 as a function of the transition energy. Solid line denotes a Gaussian fit to the transient absorption.
Fig. 4, indicating a monotonous increase of ndel with increase in euv . Recently, Fujiki (1996) reported an empirical relationship between euv and the viscosity index a which reflected the geometrical structure of the polymer main chain. The following empirical formula was obtained for the relation between a and euv : euv ¼ 1130 e2:9a :
ð3Þ
The values of ndel are plotted vs. a in Fig. 5, giving a clear relationship between the values of ndel and a. This indicates that an excess electron will be highly delocalized on a Si chain the conformation of which is tightly locked by the side chains. A Si skeleton is regarded to take a ‘‘stiff’’-like structure when the value of a equals 1 which is the highest value of polysilanes used in the present study. To date, several groups reported the synthesis of polysilanes having a ‘‘rod’’-like backbone
Fig. 4. Degree of electron delocalizaton on Si chains as a function of the extinction coefficient of the main-chain sconjugated system in polysilanes.
electron on a silicon skeleton as follows: ndel ¼
DBl OD eðÞ ; DOD
euv
ð2Þ
where DBl OD and DOD denote the observed changes in the optical density of the UV transient absorption and the bleaching, respectively, euv is the extinction coefficient at the exciton absorption maxima of the polysilanes. The degree of delocalization was calculated to be 5.5–13 Si units for the polysilanes as summarized in Table 2. Kumagai et al. (1994) reported the degree of electron delocalization on poly(c-hexylmethylsilane): PS3 as 7 Si units by the EPR spectroscopy of PS3 , which was consistent with the value in the present study. The degree of delocalization is plotted as a function of euv in
Fig. 5. Degree of electron delocalizaton on Si chains as a function of the viscosity index of the polysilanes. The value of viscosity index a was estimated using the experimental equation suggested by Fujiki, 1996.
S. Seki et al. / Radiation Physics and Chemistry 60 (2001) 411–415
conformation (a 1:5). It is predicted that an excess electron should be delocalized and highly mobile in the polysilanes. 4. Conclusion The electronic states of excess electrons on polysilane chains were investigated by nano-second pulse radiolysis. Transient absorption spectroscopy was carried out for radical anions of polysilanes with a variety of substitution patterns. The radical anions of polysilanes displayed near-UV and IR absorption maxima at ca. 3.1–3.5 eV and 0.5–1 eV, respectively. They were ascribed to the inter-band and the sub-band transition of polaron states. The extinction coefficients were calculated from kinetic traces of electron transfer reactions between polysilane radical anions and pyrene, simultaneously giving the rate constants of the reaction. The degree of electron delocalization rapidly increased with increasing euv and a. This indicates that excess electrons and holes are highly delocalized in polysilanes with linear Si chains. Acknowledgements The authors acknowledge Dr. T. Kozawa, Mr. T. Yamamoto, and Mr. H. Habara at ISIR Osaka University for the experimental support. They also acknowledge Dr. A.D. Trifunac, Dr. K.R. Cromack, and Dr. D. Wurst at Argonnne National Laboratory, and Prof. K. Ishigure and Prof. K. Asai at the University of Tokyo for their useful advise. This work was supported by a Grant-in-aid for scientific research from the Ministry of Education, Science and Culture. References Ban, H., Sukegawa, K., Tagawa, S., 1987. Pulse radiolysis study on organopolysilane radical anions. Macromolecules 20, 1775. Ban, H., Sukegawa, K., Tagawa, S., 1988. Side-chain effects on ultraviolet absorption of organopolysilane radical anion. Macromolecules 21, 45. Carberry, E., West, R., Glass, G.E., 1969. Cyclic polysilanes. IV. Anion radicals and spectroscopic properties of the permethylcyclopolysilanes. J. Am. Chem. Soc. 91, 5446.
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