Increasing the Yield of Photoinduced Charge Separation through Parallel Electron Transfer Pathways

A strategy for increasing the yield of long-lived photoinduced charge separation in artificial photosynthetic reaction centers which is based on multiple electron transfer pathways operating in parallel has been investigated. Excitation of the porphyrin moiety of a carotenoid (C)-porphyrin (P)-naphthoquinone (Q) molecular triad leads to the formation of a charge-separated state C^•+-P-Q^•- with an overall quantum yield of 0.044 in benzonitrile solution. Photoinduced electron transfer from the porphyrin first excited singlet state gives C-P^•+-Q^•- with a quantum yield of ∼1.0. However, electron transfer from the carotenoid to the porphyrin radical cation to form the final state does not compete well with charge recombination of C-P^•+-Q^•-, reducing the yield. The related pentad C₃-P-Q features carotenoid, porphyrin and quinone moieties closely related to those in the triad. Excitation of this molecule gives a C^•+-P(C₂)-Q^•- state with a quantum yield of 0.073. The enhanced yield is ascribed to the fact that three electron donation pathways operating in parallel compete with charge recombination. The yield does not increase by the statistically predicted factor of three owing to small differences in thermodynamic driving force between the two compounds.