The formation of triplet chlorophyll and carotenoid by radical pair recombination have been observed in the reaction centers of photosystems of bacteria and higher plants. This is an important process for the photoprotection of the reaction centers, for the dissipation of excessive energy by non_radiative decay of carotenoid triplet. Triplet generation by the same mechanism in an artificial system has rarely been observed, only a few cases were reported in donor_acceptor triad supermolecules. This is probably the first time report of the simulation of the generation of triplet by back electron transfer using dye_sensitized TiO 2 solar cell reaction. Triplet states have been observed in all_ trans _retinoic acid sensitized TiO 2 colloid during the recombination of the trapped electron with the retinoic acid radical cation after photoexcitation. The intermediates were characterized by ns time_resolved spectroscopy.
