FT-EPR study of spin-correlated radical pairs formed by electron transfer quenching of porphyrin triplets in micellar solution

The spin-correlated radical pairs (SCRPs) formed by photoinduced electron t ransfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to quinones in micelles of the cationic surfactant cetyltrimethylammonium chloride (CT AC) were studied by means of Fourier transform EPR (FT-EPR). It is shown th at variation of the power of the microwave pulse allows the separation of E PR signals arising from SCRPs and free radicals. The measured kinetics of r adical formation can be accounted for in terms of a statistical model takin g into account the non-uniform distribution of the solutes over the micelle s. The rate constant of electron transfer quenching (k(q)) of the ZnTPPS tr iplet state by duroquinone (DQ) is found to be 1.05 x 10(6) s(-1) The FT-EP R measurements gave information also on the kinetics of the homogeneous ele ctron transfer reaction DQ(-) + DQ --> DQ + DQ(-) in CTAC solution in which the DQ(-) anion radicals were generated by light-induced electron transfer from ZnTPPS. It is found that the dependence of the rate of this reaction on quinone concentration deviates from the linear relationship found in hom ogeneous solutions. A statistical model is proposed to account for the data . Based on this model, the rate constant of the self-exchange reaction (k(e x)) is 4.1 x 10(6) s(-1). From results obtained with duroquinone and benzoq uinone as accepters, it is concluded that ZnTPPS is located at the micelle/ water interface. (C) 1998 John Wiley & Sons, Ltd.