4281
Organometallics 1995,14, 4281-4285
Photoinduced Electron Transfer from Polygermane to c60 Studied by Laser Flash Photolysis Akira Watanabe,: Osamu Ita,*.+ and Kunio Mochida$ Institute for Chemical Reaction Science, Tohoku University Katahira, Aoba-ku, Sendai 980-77, Japan, and Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1, Mejiro, Tokyo 171, Japan Received March 6, 1995@ Electron transfer from poly(phenylmethylgermy1ene) (PMePhGe) to photoexcited c 6 0 in benzene-acetonitrile solution has been investigated by 532 nm laser flash photolysis in the near-IR region. The transient absorption band of the c 6 0 triplet state (3C60*)appeared the absorption bands immediately after nanosecond laser exposure. With the decay of 3C60*, of the radical anion of e 6 0 and the radical cation of PMePhGe appeared in the region 900-1600 nm. The rate constant of the electron transfer from PMePhGe to 3C60* was determined to be 2.33 x lo8 M-l s-l ,which was similar to that for poly(methylphenylsily1ene). Electron transfer from the singlet state of e 6 0 (lC60*) was investigated by fluorescence quenching experiments using a picosecond fluorescence lifetime measurement system, and the quenching rate constant was determined to be 4.00 x 1O1O M-l s-l. The rate constant of intersystem crossing from 'c60" to 3C60*was determined to be 1.10 x lo9 s-l by picosecond time-resolved absorption spectroscopy using a streak camera. Electron transfer from t o PMePhGe and intersystem crossing from 'c60* to 3C60* are competitive, and the intersystem crossing is dominant in a dilute solution system. When a 355 nm laser was used as the excitation light, photochemical intermediates were produced from the direct photolysis of PMePhGe in addition to the electron transfer.
Introduction It has been reported that photoexcited fullerenes act as good electron acceptors;1-8 as electron donors, aromatic amines with n-electron-rich donors have been frequently used. Photoinduced electron transfer reactions have investigated by photochemical techniques such as transient electronic absorption spectroscopy. By these methods, it has been revealed that the electron transfer takes place via the triplet state of c 6 0 (3C60*). Photoinduced electric conductivity has been reported for polymers doped with c 6 0 . Some n-donors have been reportedg-ll as showing photoconductivity. In our previous report,12we investigated photoinduced electron transfer from poly(vinylcarbazo1e) t o 3Cs0*by the laser
' Tohoku University Katahira. t Gakushuin University. @Abstractpublished in Aduance ACS Abstracts, August 1, 1995. (1)Sension, R.: Szarka, A. Z.; Smith, G. R.: Hochstrasser, R. M. Chem. Phys. Lett. 1991,185, 179. (2) Arbogast, J. W.; Foote, C. S. J . Am. Chem. SOC.1991,113,8886. (3)Arbogast, J. W.; Foote, C . S.; Kao, M. J . Am. Chem. SOC.1992, 114,2277. (4)Biczok, L.;Linschitz, H. Chem. Phys. Lett. 1992,195,339. (5) Osaki, T.; Tai, Y.; Tazawa, M.; Tanemura, S.; Inukawa, K.; Ishiguro, K.; Sawaki, Y.; Saito, Y.; Shinohara, H.; Nagashima, H. Chem. Lett. 1993,789. (6) Nonell, S.;Arbogast, J. W.; Foote, C. S. J . Phys. Chem. 1992, 96, 4169. (7)Guldi, D. M.;Hungerbuhler, H.; Janata, E.; Asmus, K.-D. J. Chem. SOC.,Chem. Commun. 1993,84. (8) Ghosh, H.; Pal, H.; Sapre, A. V.; Mittal, J. P. J . Am. Chem. SOC. 1993,115, 11722. (9)Wang, Y. Nature 1992,356,585. (10)Wang, Y.; Herron, N.; Caspar, J. Mater. Sci. Eng. B 1993,B19, 61. (11)Yoshino, K.; Xiao, H. Y.; Nuro, K.; Kiyomatsu, S.; Morita, S.; Zakihdov, A. A,; Noguchi, T.; Ohnishi, T. Jpn. J . Appl. Phys. 1933,32, L357. (12)Watanabe, A.; Ito. 0. J . Chem. SOC.Chem. Commun. 1994, 1285.
flash photolysis method, from which the electron transfer mechanism has been revealed by following the decay of 3C60*and rise of the radical anion of c 6 0 (CSO'-) and cation radical of poly(vinylcarbazo1e) in the near-IR region. Laser flash photolysis was also successfully applied to poly(methy1methacrylate) doped with c 6 0 and an aromatic amine.13 In the case of a-conjugated polymers such as polysilanes, photoconductivities have been investigated,14J5and electron transfer between c 6 0 and polysilanes occurs in the initial step of carrier formation. We also succeeded in the observation O f C60'and the cation radical of polysilane in the near-IR region, in addition to 3C6~*.16 In this paper, we will apply this technique t o the system of polygermane and c60. The formation of c60'- and polygermane radical cation by electron transfer via 3C60* was studied by nanosecond laser flash photolysis. The possibility of electron transfer via the singlet state O f C 6 0 (1c60*) was investigated by fluorescence quenching experiments using a picosecond fluorescence lifetime measurement system.
Experimental Section CSO(99.9%)was obtained from Texas Fullerene Corp. Poly(phenylmethylgermylene), abbreviated as PMePhGe in this study, was prepared by the Kipping reaction in toluene at 110 "C using Na.I7 The molecular weight was determined to be 5020 by GPC using monodispersed polystyrene as the standard; thus, the degree of polymerization (n)is 32. CSOand (13)Gevaert, M.; Kamat, P. V. J . Phys. Chem. 1992,96,9883. (14)Wang, Y.; West, R.; Yuan. C.-H. J . Am. Chem. SOC.1993,115, 3844. (15)Kepler, R. G.; Cahill, P. A. Appl. Phys. Lett. 1933,63,1552. (16)Watanabe, A,; Ito, 0. J. Phys. Chem. 1994,98,7736. (17)Mochida, K.; Chiba, H. J . Orgnomet. Chem. 1944,473,45.
0276-733319512314-4281$09.00/0 0 1995 American Chemical Society
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Figure 1. Transient absorption spectra obtained by 532 O the presence of nm laser flash photolysis of 0.1 mM C ~ in PMePhGe (5 mM in monomer unit) in a benzene-acetonitrile ( 2 4 (v/v)) solvent mixture: (0)100 ns; (0)1000 ns. PMePhGe were dissolved in a benzene and acetonitrile solvent mixture (2/1 (v/v))deaerated with nitrogen bubbling before the measurements. The solution was excited by a Nd:YAG laser (Quanta-Ray, GCR-130,6 ns fwhm) at 355 or 532 nm. A pulsed xenon flash lamp (Tokyo Instruments, xF80-60, 15 J, 60 ps fwhm) was used for the probe beam, which was detected with a Ge-APD module (Hamamatsu Photonics, (25331-SPL) after passing through the photochemical quartz vessel (10 mm x 10 mm) and a monochromator. The Ge-APD module consists of a germanium avalanche photodiode (B28341, a high-speed current-to-voltage amplifier, and a high-voltage bias circuit on a compact board and shows high sensitivity in the frequency range 0.004-100 MHz.12J6J8 The output from Ge-APD was recorded with a digitizing oscilloscope (HP 54510B, 300 MHz) and analyzed by a personal computer (NEC, PC98). The transient absorption spectra in the UV-visible region were measured with an optical multichannel system (UNISOKU, USP-500) after the appropriate delay time. The solution UVvisible absorption spectra were recorded with a Hitachi U-3400 spectrometer. The fluorescence lifetimes were measured using an argon ion laser (Spectra-Physics, BeamLok 2060-10-SA),a pumped Tisapphire laser (Spectra-Physics, Tsunami 3950-L2S, 1.5 ps fwhm) with a pulse selector (Spectra-Physics, Model 39801, a frequency doubler (GWU-23PS),and a streak scope (Hamamatsu Photonics, C4334-01). Intersystem crossing from l C ~ to ~* 3C& was observed using a picosecond time-resolved absorption spectrometer which consists of an active/passive mode-locked Nd:YAG laser (Continuum, PY61C-10, 30 ps fwhm), optical delay lines, and a streak scope (Hamamatsu Photonics, C2830). A continuum probe light with relatively long duration (50 ns) is generated by the breakdown of Xe gas focusing the 1064 nm laser beam onto the Xe t ~ b e . All ' ~ experiments ~ ~ ~ were carried out at 20 "C.
Results and Discussion
Figure 1 shows the transient absorption spectra in the near-IR region obtained after the laser flash photolysis of c 6 0 with 532 n m light in the presence of 5 mM (in monomer units) of PMePhGe in a mixed solvent (benzene-acetonitrile, 2/1 (v/v)). The absorption band at 730 nm which appears immediately after laser exposure is attributed to 3C60*.21-24With the decay of the absorption intensity of 3C6~*,the intensities of the (18)Watanabe, A.; Ito, 0. Jpn. J . Appl. Phys. 1995, 34(1), 194. (19) Sumitani, K.; Yoshihara, K. Bull. Chem. SOC. Jpn 1982,55,85. (20) Ito, T.; Hiramatsu, M.; Hosoda, M.; Tsuchiya, Y. Rev. Sci. Instrum. 1991, 62, 1415.
0.05
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Time (ns) Figure 2. Absorption-time profiles obtained by 532 nm laser flash photolysis of 0.1 mM CSOin the presence of 5 mM PMePhGe in a benzene-acetonitrile mixed solvent (2/1 (v/v)): (A) 730 nm; (B) 1060 nm; (C) 1500 nm. absorption bands increase in the region 900-1600 nm. The absorption band at 1030 nm is a characteristic band ~ , ~ ~broad of the radical anion of c60 ( C S O * - ) . ~ The absorption band extending from 1200 to 1600 n m may be attributed to the radical cation of PMePhGe (PhMeGe*+)in analogy to the broad absorption bands of the radical cations of polysilanes in a similar r e g i ~ n . ~ ~ ? ~ ~ Such radical ions generated by the electron transfer were not observed in neat benzene. 3 C 6 ~ *is the only transient species observed by nanosecond laser flash photolysis in a nonpolar solvent, benzene. The same result was obtained in our previous study on electron transfer between c60 a n d poly(methylphenylsily1ene) (PMePhSi).16 The electron transfer from PMePhSi to 3C60* shows a clear solvent polarity effect, where the electron transfer rate increases with the solvent polarity. A similar solvent polarity effect has been reported for the electron transfer from a low-molecular-weight donor to 3C60*or 3C7~*.5,29 In Figure 2, the time profiles of the absorption bands observed under the conditions described in Figure 1are shown. The absorption intensity of 3C60* at 730 n m decays with a lifetime of ca. 500 ns in the presence of 5 (21)Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.;Alvarez, M. M.; Anz, S. J.; Whetten, R. L. J . Phys. Chem. 1991,95, 11. (22) Kajii, Y.; Nakagawa, T.; Suzuki,S.;Achiba, Y.; Obi, K.; Shibuya, K. Chem. Phys. Lett. 1991, 181, 100. (23) Sension, R. J.; Phillips, C. M.; Szarka, A. 2.; Romanow, W. J.; Macghie, A. R.; McCauley, J. P.; Smith, A. B., 111; Hochstrasser, R. M. J. Phys. Chem. 1991,95, 6075. (24) Dimitrijevic, N. M.; Kamat, P. V. J . Phys. Chem. 1992,96,4811. (25) Kato, T.; Kodama, T.; Shida, T.; Nakagawa, T.; Matsui, Y.; Suzuki, S.; Shiromaru, H.; Yamauchi, K.; Achiba, Y. Chem. Phys. Lett. 1991, 180,446. (26) Gasyna, 2.; Andrews, L.; Schatz, P. N. J . Phys. Chem. 1992, 96, 1525. (27) Ushida, K.; Kira, A.; Tagawa, S.;Yoshida, Y.; Shibata, H. Polym. Mater. Sci. Eng. 1992, 66, 299. (28) Irie, S.; hie, M. Macromolecules 1992, 25, 1766. (29) Biczok, L.; Linschitz, H. Chem. Phys. Lett. 1992, 195, 339.
Electron Transfer from Polygermane to CSO
Organometallics, Vol. 14, No. 9, 1995 4283 I
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Figure 3. First-order plots for the decay of W 6 0 * at 730 nm in the presence of PMePhGe in a benzene-acetonitrile mixed solvent (211 (v/v)). PMePhGe concentration: (a) 0 mM; (b) 0.6 mM; (c) 1.8 mM; (d) 2.9 mM; (e) 4.0 mM. Excitation was at 532 nm. Insert dependence of the pseudo-first-order rate constant (kobsd) for the decay of 3Cs0* on the concentration of PMePhGe in the monomer unit.
Scheme 1 hv (532nm) c60
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mM of PMePhGe, whereas 3C60*decays slowly in the absence of PMePhGe with a lifetime of 2300 ns. The PMePhGe'+ band a t 1500 nm increases rather smoothly with time as an exponential curve. The absorption intensity of c60'- at 1060 nm increases up to 1600 ns, accompanied by the decay of 3C60*,although the initial fast rise is seen in Figure 2B immediately after 532 nm laser pulse excitation. This suggests the existence of a direct path for the electron transfer forming c60'without passing through 3C60*. This direct path is electron transfer via 'c60*. The electron transfer process is discussed later using picosecond time-resolved spectroscopy. From the transient absorption spectra and their profiles, it is revealed that the electron transfer takes place via 3C60*with photoexcitation of c 6 0 by 532 nm light, which is not absorbed by PMePhGe (Scheme 1). The reaction rate constant of the electron transfer step (k,t) was determined by the decay rate of 3C6~* at 730 nm; in the presence of PMePhGe at a concentration greater than 0.6 mM, the decay of 3C60*obeys the firstorder kinetics as shown in Figure 3. The slopes of the first-order plots yield the first-order rate constants (kobs), which increase with the concentration of PMePhGe. The pseudo-first-order plot of kobs against [PMePhGel (concentration based on monomer repeating unit) is shown in the insert in Figure 3, where a straight line can be obtained. From the slope of the line, the second-order rate constant for the quenching reaction of 3C60*with PMePhGe was evaluated to be 2.33 x lo8M-l s-l. From the correspondence of the quenching rate of 3 C ~with ~* the rise in rates for c60'- and PMePhGe'+ as seen in
Figure 2, the quenching rate constant thus obtained is attributed t o the ket value in Scheme 1. It is well-known that the ket value depends on the electron donor ability of the counterpart of the electron transfer of c 6 0 and on solvent polarity. The ionization energy (IP) of PMePhGe was estimated to be 5.4-5.5 eV,30which is in the same range as that for polysilanes (IP = 5.6 eV);31thus, the ket value of 2.33 x lo8 M-l s-l for PMePhGe is quite similar to that for polysilane with a degree of polymerization comparable to that reported in our previous report (2.11 x lo8 M-l s-l in the same solvent).18 On the other hand, aromatic amines such as p-phenylenediamine (IP = 6.8 have k,t values of ca. 5 x lo9 M-l s - ~ ,which ~ is ca. 22 times greater than the observed k,t values for PMePhGe. The IP value for a solid polymer film measured by the AC (air counter) method is 1.0-1.5 eV lower than the usual IP value for low-molecular-weight compound^.^^ In a previous paper, we calculated the electron transfer rate constant for PMePhSi using the Rehm-Weller equation and the oxidation potential of PMePhSi (0.93 V vs Agl Ag+).16 In the case of PMePhGe, the oxidation potential has not been reported, but the IP data suggest that the oxidation potential of PMePhGe is quite similar to that of PMePhSi. The calculated value 1.60 x 1O1O M-l s-l in a benzene-acetonitrile (24)mixed solvent is 69 times greater than the observed value (ket = 2.33 x los M-l s-l). The inconsistency can be explained by considering the local concentration effect of the polymer chain in a dilute solution. In quenching experiments determining the k,t value of the polymer, the donor concentration was based on the monomer repeating units which are concentrated locally. If we imagine electron transfer between c 6 0 and a polymer chain, both of which are macromolecules, the quencher concentration becomes lower and the rate constant becomes higher by a factor of the degree of polymerization (32 for PMePhGe). The corrected value considering the local concentration effect corresponded well with the calculated value. The back-electron-transfer reaction from c60'- t o PMePhGe'+ seems to be slow; at least, back electron transfer was not observed in 1500 ns, as shown in the time profiles Of C60'- and PMePhGe*+in Figure 2. This indicates that the positive charge (hole) of PMePhGe'+ migrates quickly along the Ge-Ge main chain apart from c60'-, which decreases the chance of the hole encountering the electron. As mentioned above, the initial fast rise in Figure 2B suggests the possibility of electron transfer via 'CSO*. A fluorescence quenching experiment is effective in investigating the electron transfer quenching via 1C60*.33 Figure 4 shows the fluorescence spectra of c 6 0 in the absence and in the presence of PMePhGe, which were observed by photon counting measurements using a Ti: sapphire laser (375 nm) and a streak camera. The relative fluorescence intensity decreases in the presence of PMePhGe. The fluorescence lifetimes were measured by decay curves, and the fitting curves are shown in (30) Mochida, K.; Shimoda, M.; Kurosu, H.; Kojima, A.Polyhedron 1994,13,3039. (31)Yokoyama, K.; Tokoyama, M. Solid State Commun. 1989,70, 241. (32) Lias, S.G.;Bartmess, J. E.; Holmes, J. L.; Levin, R. D.; Libman, J. F.; Mallard, W. G. J.Phys. Chem. Rer Data, Suppl. 1988,17,Suppl. 1. (33)Williams, R.M.;Verhoeven, J. W. Chem. Phys. Lett. 1992,194, 446.
Watanabe et al.
4284 Organometallics, Vol. 14, No. 9, 1995
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Wavelength (nm) Figure 4. Fluorescence spectra of 0.1 mM c 6 0 in the absence (a) and in the presence (b) of PMePhGe (5 mM in monomer unit) in a benzene-acetonitrile 2/1 mixed solvent (2/1) obtained by excitation from the 375 nm pulse of a Ti: sapphire laser. '
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Time (ps) Figure 5. Decay curves of c60 fluorescence in the absence (a)and in the presence (b)of PMePhGe (5 mM in monomer unit) in a benzene-acetonitrile (2/1).The insert shows the Stern-Volmer plot of dro against PMePhGe concentration.
Figure 5. The fluorescence lifetime of c 6 0 in a benzeneacetonitrile (2/1) mixed solvent was determined to be 1.09 ns. The Stern-Volmer plot, zoh vs PMePhGe concentration, where zo and z are fluorescencelifetimes in the absence and in the presence of PMePhGe, is shown as an insert in Figure 5. From the slope and TO, the rate constant for quenching of electron transfer from PMePhGe t o 'c61)* was determined to be 4.00 x 1O1O M-l s-l. The electron transfer via 'c60* gives a rather large rate constant. The electron transfer process is competitive with intersystem crossing from 'c60* to 3C60*.
Figure 6 shows the absorption-time profile for 3C60* at 740 nm obtained by picosecond time-resolved spectroscopy using a streak scope. The growth of the transient absorption corresponds to the formation of 3 C 6 ~ * from 'c60* by intersystem crossing. By curve fitting as shown in Figure 6, the rate constant for intersystem crossing, kist, was determined to be 1.10 x lo9 s-'. We can estimate the ratio of electron transfer via 'c60* to that via 3 C ~ using ~ * these rate constants and considering the concentration of PMePhGe (5 mM) in the solution: the electron transfer ratios via %SO* and via 3 C 6 ~ *are 15 and 85%, respectively, for a 5 mM PMePhGe solution. The ratio of electron transfer via 'c60* increases with increasing concentration of a-conjugated polymer. In Figure 2, the fast rise of c60'- is attributable to electron transfer via 'c60*. In a condensed system such as a Cso-doped polysilane film which
Figure 7. Transient absorption spectra obtained by 355 nm laser flash photolysis of 0.1 mM c 6 0 and 5 mM PMePhGe in a benzene-acetonitrile mixed solvent (2/1): ( 0 )100 ns; (0)1000 ns. shows a high photoconductivity, electron transfer via 'CSO*may be dominant. When both c 6 0 and PMePhGe were photoexcited with 355 nm laser light, the transient absorption spectra were different from those of Figure 1, as shown in Figure 7. The most prominent difference is the decrease in the intensity of the broad absorption band longer than 1200 nm which is characteristic of PMePhGe'+ . This suggests the formation of some photochemical intermediates of PMePhGe with direct 355 nm laser exposure; intermediates such as free radicals and carbene-type germylenes quickly trap PMePhGe'+ formed by electron transfer to 3C60*. In addition, some absorption bands appear in the region 800-1000 nm, which seems to increase in place of the decay of 3C60*. The absorption bands between 800 and 1000 nm are similar to those reported for the free radicals, which are produced by the reaction between c 6 0 and alkyl radical (R); R - C S O * . ~Thus, ~ we can attribute the new absorption bands in the 800-1000 nm region to the free radicals which can be produced and polygermane by the reaction between c 6 0 (or 3C~0*) radicals. Figure 8A shows the transient spectrum obtained by the direct photolysis of PMePhGe; the absorption bands were observed at 380 and 475 nm. The transient absorption band at 380 nm can be attributed to the germy1 radicals, as shown in Scheme 2.35,36The transient absorption a t 475 nm can be considered as being due to either the Ge-centered radicals or germylenes (34)Guldi, D. M.; Hungerbuhler, H.; Janata, E.; Asmus, K.-D. J. Phys. Chem. 1993,97,11258.
Electron Transfer from Polygermane to C60
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Wavelength (nm) Figure 8. Absorption spectra obtained by 355 nm laser flash photolysis in the visible region of 5 mM PMePhGe (A) and 0.1 mM c60 and 5 mM PMePhGe (B)in a benzeneacetonitrile mixed solvent (2/1). Scheme 2 Me I Me I
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(Scheme 2).35,36The decay rates of the 380 and 475 nm bands were not prominent within 1000 ns, suggesting that these photolytic intermediates from PMePhGe are stable in benzene-acetonitrile solvent within the time scale of a few microseconds. This is in agreement with the observation for polysilanes that photolytic intermediates such as Si-centered radicals and silylenes in the polymer backbone are long-lived compared with the lowmolecular-weight ones. In Figure 8B, the transient spectra in the visible region obtained by the 355 nm laser photolysis of both c 6 0 and PMePhGe are shown; the absorption band at 740 nm due to 3C6~*was observed in addition to the photolytic intermediates at 475 nm. The absorption intensity of 3 C 6 ~ *decreases with time by accepting electrons from PMePhGe; the decay of 3C60*with 355 nm laser excitation is faster than that of 3C60*with 532 nm laser excitation in Figures 1and 2. The absorption intensities at 380 and 475 nm show slow decay, sug-
Wavelength (nm) Figure 9. Absorption spectra before (a) and after (b) irradiation of 0.1 mM c 6 0 and 5 mM PMePhGe in a benzene/acetonitrile mixed solvent (2/1) with 355 nm laser light of 20 m J energy for 5 min. gesting that photolytic intermediates of PMePhGe a t 380 and 475 nm do not play a role as electron donors to 3C~~*. On prolonged 355 nm laser irradiation of c 6 0 and PMePhGe, the color of the solution changed from purple to dark brown; the absorption spectra of the solution are shown in Figure 9. Before irradiation, a weak absorption band of c60 is seen a t 450-600 nm and intense one shorter than 375 nm includes both c60 and PMePhGe. After irradiation, a new absorption appears, extending from 375 to 700 nm. c60 itself was photostable, and no color change was observed by 355 nm light irradiation. PMePhGe itself is photounstable, but photolysis products do not show intense color. Thus, the observed absorption change may be attributed t o the reaction between c 6 0 and photochemical intermediates of PMePhGe such as that shown in Scheme 2. In conclusion, rate constants for electron transfer from PMePhGe to 3C60*,electron transfer from PMePhGe to 'c60*, and intersystem crossing from 'c60" to 3C60*were determined to be 2.33 x los M-l s-l, 4.00 x 1O1O M-l s-', and 1.10 x lo9 s-', respectively. Electron transfer via 'c60* and intersystem crossing are competitive processes. In a dilute solution of c60 containing an electron donor in a low concentration, the electron transfer proceeds mainly via 3C60*after the intersystem crossing from 'c60* to 3 C ~ ~ * .
(35) Mochida, K.; Kimijima, K.; Chiba, H.; Wakasa, M.; Hayashi,
Acknowledgment. We are grateful to Mr. Motoyuki Watanabe, Mr. Haruhisa Saito, and Mr. Musubu Koishi (Hamamatsu Photonics) for their technical support in setting up the picosecond time-resolved absorption and emission spectrometer (URAS system of the Institute for Chemical Reaction Science, Tohoku University).
(36) Watanabe, A,; Matsuda, M. Macromolecules 1992,25, 484.
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