Reaction center models in liquid crystals: identification of paramagnetic intermediates.

Photoinduced charge separation and recombination to the triplet state in a carotene(C) pirphyrin(P)fullerene(C60)triad have been followed by time -resolved electron paramagnetic resonance(EPR) . The electron transfer process has been studies at different temperatures in both a glassy isotropic matrix(2-MeTHF) and uniaxial liquid crystal (E-7). In both media , the triad undergoes two-step photoinduced electron transfer, with the generation of a long-lived charge separated state (C radical cation, P neutral, C60 radical anion), and charge recombination to triplet state, localized in the carotene moiety. The carotenoid triplet state is initially polarized according to the mechanism of recombination of a radical pair with singlet precursor. Both the photoinduced spin-correlated radical pair and the carotene triplet are observed in E-7 starting from the glass to the liquid crystal phase. The exchange interaction between the electrons in the radical pair(J=1.2 Gauss) has been evaluted by simulation of the EPR spectrum in the isotropic glass and comparison with the corrisponding spectrum in the oriented medium.