Time-resolved electron spin resonance spectra of linked radical pairs on laser excitation of zinc tetraphenylporphyrin-viologen systems

Abstract

Photoinduced electron transfer from the zinc tetraphenylporphyrin moiety to the viologen unit in linked compounds in reversed micelles generated radical pairs, which afforded chemically induced dynamic electron polarization (CIDEP) spectra at room temperature. The CIDEP spectra varied with the number of methylene groups (n) in the alkyl spacer of the linked compounds. In the case of longer spacers (n = 6 and 8), common E/A/E/A patterns were observed throughout the measurement (0.7-5 [mu]s). The signal pattern could be simulated by a formula for the well-known spin-correlated radical pair mechanism, and the corresponding exchange coupling (J) was evaluated ([minus]0.4 mT). In the case of ZP4V, the CIDEP signals rapidly changed with elapsed time (0.5-3 [mu]s) from the absorptive to emissive mode. The observation was explained on the basis of a strong contribution of the triplet mechanism in the photogeneration of the radical pair in the early period of measurement. 15 refs., 2 figs.