Time-resolved EPR investigation of charge recombination to a triplet state in a carotene-diporphyrin triad

Photoinduced charge separation and recombination were investigated in a triad consisting of a carotenoid (C), a tetraarylporphyrin (P) and a tris(heptafluoropropyl) porphyrin (PF), C-P-P-F, by means of time-resolved electron paramagnetic resonance. The electron transfer process was studied in a glass of 2-methyltetrahydrofuran at 10 K, in the crystalline phase at 150K and in the liquid nematic phase of the uniaxial LC E-7 at 295 K, and in the nematic phase of the LC ZLI-1167 at 300 K. In all the different media and in the different phases, the molecular triad undergoes two-step photoinduced electron transfer, with the generation of a long-lived charge-separated state (C center dot+-P-P-F(center dot-)), and charge recombination to the triplet state, localized in the carotene moiety C-3-P-P-F. Low-temperature charge separation and triplet recombination are common features of both fullerene-based and diporphyrin molecular triads, proving that the large delocalized pi-electron system of the porphyrin electron acceptor leads to low total reorganization energy and low sensitivity to solvent stabilization of the radical ions in a similar way as for fullerene systems.