Photochemistry in heterogeneous media: A Fourier transform-EPR study

Abstract

Fourier transform-electron paramagnetic resonance (FT-EPR) is a novel spectroscopic technique that is ideally suited for the study of transient free radicals generated in photochemical reactions. The salient features of the application of FT-EPR in this field of research are the following. (1) The high spectral resolution of the technique makes it possible to identify free radicals. (2) The time evolution of spectra can be monitored over a time regime extending from nanoseconds to milliseconds. Hence, spectral data can give rate constants of radical formation and decay. (3) Signal intensities can reflect chemically induced electron polarization effects that provide information on the mechanism of radical formation. (4) Measurement of spin-relaxation rates gives an insight into molecular motion. In this paper the application of the technique in a time-resolved study of photoinduced electron transfer in micellar solutions and in the pores of silica gel is presented. Measurements on micellar solutions concern electron transfer from photoexcited tetrasulfonated zinc tetraphenylporphyrin to duroquinone. The study of silica gel systems focused on electron transfer from zinc tetraphenylporphyrin to benzoquinone. The results of the investigation show that the technique can provide valuable insights into photochemical reactions in heterogeneous media. The time evolution of the spectra provides information on the formation and decay of radical pairs and free radicals. The data reflect the spatial distribution of reactants and products. Linewidths and relaxation data give information on the location and motional freedom of free radicals.