Electronic coupling effects on photoinduced electron transfer in carotene-porphyrin-fullerene triads detected by time-resolved EPR

Photoinduced charge separation and recombination in a carotenoid-porphyrin-fullerene triad C-P-C60 (Bahr et al., 2000) have been followed by time-resolved electron paramagnetic resonance. The electron-transfer process has been characterized in a glass of 2-methyltetrahydrofuran and in the nematic phase of two uniaxial liquid crystals (E-7 and ZLI-1 167). In all the different media, the molecular triad undergoes two-step photoinduced electron transfer, with the generation of a long-lived charge-separated state (Ccenter dot+- P-C60center dot-) and charge recombination to the triplet state, localized in the Carotene moiety, mimicking different P-C-60(center dot-) 4 C aspects of the photosynthetic electron-transfer process. The magnetic interaction parameters have been evaluated by simulation of the spin-polarized radical pair spectrum. The weak exchange interaction parameter (J = +1.7 +/- 0.1 G) provides a direct measure of the dominant electronic Coupling matrix element V between the Ccenter dot+-P-C60(center dot-) radical pair state and the recombination triplet state C-3-P-C-60. Comparison of the estimated values of V for this triad and a structurally related triad differing only in the porphyrin bridge (octaalkylporphyrin vs tetraarylporphyrin) explains in terms of an electronic coupling effect the similar to 6-fold variation of the recombination rate induced by the modification of the porphyrin bridge as derived by kinetic experiments (Bahr et al., 2000).