Reaction center models in liquid crystals: Identification of paramagnetic intermediates

Photoinduced charge separation and recombination to the triplet state in a carotene (C) porphyrin (P) fullerene (C ₆₀) triad have been followed by time-resolved electron paramagnetic resonance (EPR). The electron transfer process has been studied at different temperatures in both a glassy isotropic matrix (2-methyltetrahydrofuran) 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* ⁺-P-C ₆₀* ^-), and charge recombination to the 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 evaluated by simulation of the EPR spectrum in the isotropic glass and comparison with the corresponding spectrum in the oriented medium.