J . Phys. Chem. 1986, 90, 1465-1470
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CONDENSED PHASES AND MACROMOLECULES Effect of Ionic Polymer Environment on the Photoinduced Electron Transfer from Zinc Porphyrin to Viologen Yoshio Nosaka,* Atsushi Kuwabara, and Hajime Miyama Department of Chemistry, Technological University of Nagaoka, Kamitomioka, Nagaoka, 943-54 Japan (Received: July 30, 1985; In Final Form: November 7 , 1985)
The effect of ionic polymer environment on photoinduced electron transfer from zinc porphyrin to viologen was investigated for aqueous Nylon containing pendant viologen and quaternized amino groups. The quantum yield for the photoinduced reduction of the viologen group with zinc porphyrins was found to be variable as functions of the fraction of pendant viologen, the concentration of the electron donor, and the ionic strength of the solution. By laser flash photolysis experiments, the reaction rates for electron transfer from the excited porphyrin to the viologen group in the polymer and for back electron transfer from the reduced viologen to the oxidized porphyrin were measured. As a result, it was found that the cationic and/or hydrophobic polymer environment retards the back electron transfer, which increases the quantum yield for the formation of viologen cation radicals.
Introduction
Photoinduced charge separation in solution is an important process in the photochemical conversion and storage of solar energy.' In order to separate electron from photosensitizer, the reagent containing 4,4'-bipyridinium cation (viologen) group has been generally used as an electron-transfer catalyst. Many attempts to increase the charge separation have been reported in the systems of molecular assemblies,2-6 polyelectrolyte,' and polymeric v i o l o g e r ~ s . ~ -As ~ ~ for polymeric viologen, viologenpendant polystyrene8-I2 and polymer which contains viologen moieties in the main chain ( p o l y ~ i o l o g e n )have ' ~ ~ ~been ~ reported. By comparison with methylviologen (MV2+), the net charge separation decreased substantially for the pendant-type polymer viologen12 but the quantum yield of electron transfer increased for polyviologen because of hydrophobic i n t e r a ~ t i o n . 'In ~ these reports, ruthenium(I1) bipyridinium complex or its analogue was (1) "Photochemical Conversion and Storage of Solar Energy"; Connolly, J. S., Ed.; Academic Press: New York, 1981. (2) Graetzel, M. Ber. Bunsenges, Phys. Chem. 1980,84, 981-991. (3) Calvin, M. Photochem. Phoiobiol. 1983, 37, 349-360. (4) Kurihara, K.; Tundo, P.; Fendler, J. J . Phys. Chem. 1983, 87, 3777-3782. (5) Mandler, D.; Degani, Y.; Willner, I. J . Phys. Chem. 1984, 88, 4366-4370. (6) Fendler, J. H. J . Phys. Chem 1980,84, 1485-1491. (7) (a) Meyerstein, D.; Rabani, J.; Matheson, M. S.; Meisel, D . J . Phys. Chem. 1978,82, 1879-1885. (b) Sassoon, R. E.; Rabani, J. Isr. J . Chem. 1982, 22, 138-141. ( c ) Sassoon, R. E.; Aizebshtat, Z.; Rabani, J. J . Phys. Chem. 1985,89, 1182-1190. (8) (a) Ageishi, K.; Endo, T.; Okawara, M. J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 1085-1090; (b) Saotome, Y.; Endo, T.; Okawara, M. Mucromolecules 1983, 16, 881-884. I (9) Furue, M.; Yamanaka, S.; Phat, L.; Nozakura, S. J . Polym. Sci., Polym. Chem. Ed. 1981, 19, 2635-2646. (10) Nishijima, T.; Nagamura, T.; Matsuo, T. J . Polym. Sci., Polym. Lett. Ed. 1981, 19, 65-73. (1 1) (a) Matsuo, T.; Sakamoto, T.; Takuma, K.; Sakura, K.; Ohsako, T. J . Phys. Chem. 1981,85, 1277-1279. (b) Ohsako, T.; Sakamoto, T.; Matsuo, T. J . Phys. Chem. 1985, 89, 222-225. (12) Lee, P. C.; Matheson, M. S.; Meisel, D. Isr. J . Chem. 1982, 22, 133-137. (13) Chuaqui, C. A,; Lynn, K. R.; Williams, R. E.; Martin, S . M. Makromol. Chem. 1980, 181, 651-656. (14) Sassoon, R. E.; Gershuni, S.; Rabani, J. J . Phys. Chem. 1985, 89, 1937-1945.
0022-3654/86/2090-1465$01.50/0
employed as a photosensitizer. The attempt to study the effect of pendant photo~ensitizer"~'~ is also reported. However, the environmental effect of neighboring pendant ionic groups has not been studied. The authors synthesized a new type of polymer viologen which contains cationic neighboring pendant groups,I6 and used it for an electron-transfer catalyst,16for a matrix polymer of gel entrapment of hydrogenase," and for the stabilizing agent of colloidal platinum.18 Since the polymer viologen was obtained by the quaternization of aqueous polymer, the physicochemical properties of this polymer viologen are expected to be different from that of the styrene-type polymer viologen. In the preliminary study,I6 it was shown that the neighboring cationic groups affect the formation rate of cation radicals from the pendant viologen groups. A water-soluble metal porphyrin19 was used as the photosensitizer because of the longer lifetime of its excited state. In the present study, the effect of the polymer environment on the photoinduced electron transfer from metal porphyrin to pendant viologen group was investigated in detail by means of steady-state irradiation and laser flash photolysis. Experimental Section Polymer viologen (PV2+)used in the present study was prepared by the quaternization of dimethylamino groups of aqueous Nylon (AQN) by 1-(4-bromoethyl)-l'-methyl-4,4'-bipyridiniumor 1 (4-iodobuty1)- l'-methyL4,4'-bipyridinium in methanol solution. AQN was 100% a-dimethylaminated 6-Nylon, poly[imino(2dimethylamino- 1-oxohexamethylene)], of Toray Industries, Inc. For comparison, 63% and 52% dimethylaminated Nylons were used. The content of the pendant viologen was varied from 5% to 25% by changing the reaction time or the quaternizing reagent. The remained dimethylamine group was further quaternized by bromoethane, iodoethane, or propane sultone for the preparation (15) Kaneko, M.; Yamada, A.; Tsuchida, E.; Kurimura, Y .J . Phys. Chem. 1984, 88, 1061-1062.
(16) Miayma, H.; Nosaka, Y.; Kobayashi, T.; Kuwabara, A . J . Polym. Chem., Polym. Lett. Ed. 1983, 21, 945-949. (17) Nosaka, Y.; Kuwabara, A.; Kobayashi, T.;Miyama, H. Biofechnol. Bioeng., in press. (18) Nosaka, Y.; Kuwabara, A.; Miyama, H. J . Phorochem. 1986, 32, 143-150. (19) Darwent, J. R.; Douglas, P.; Harriman, A,; Porter, G.; Richoux, M. C. Coord. Chem. Rev. 1982, 44, 83-126.
0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 7 , 1986
Nosaka et al.
TABLE I: Compositions, Redox Potential, Spectral Properties, and Initial Photoreduction Yield for Polymer Viologens symbolQ MV2+ PV4(5) PV4(10) PV4( 12) PV4(25) PV4(5+50) PV4(5+) PV4(5-) PV4(5)-37 PV4( 5)-48
e397.
x,%
Y,%
2, %
5 10 12 25 5 5 5 5 5 0 0
95 90 88 75 45 0 0 58 47 100 0
0 0 0 0 50 95 95 0 0 0 100
R
E(V+'/V2+)* -0.685 -0.675
C2H5
C2H5 C,H,SO
