On the effect of the nature of the bridge on oxidative or reductive photoinduced electron transfer in donor-bridge-acceptor systems

Photoinduced electron transfer is a topical issue in chemistry. In multicomponent donor-bridge-acceptor systems, electron transfer is usually discussed within the frame of superexchange theory, which takes into account electronic coupling mediated by virtual states involving bridge orbitals. However, the schematization used for superexchange in thermal electron transfer processes is not suitable to immediately understand some intriguing aspects of photoinduced charge separation and recombination processes, which are only uncovered by analyzing the virtual states involved in forward and backward excited-state electron transfer. In particular, for oxidative photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward charge separation can efficiently mediate charge recombination via the hole-transfer superexchange route, whereas for reductive photoinduced electron transfer, a low-energy virtual state which cannot mediate the forward process can efficiently mediate charge recombination via electron-transfer superexchange. As a consequence, to obtain long-lived charge-separated states upon oxidative photoinduced electron transfer in donor-bridge-acceptor systems it is preferable to avoid easy-to-oxidize bridges, whereas easy-to-reduce bridges should better be avoided in reductive photoinduced charge separation. These considerations, exemplified by the analysis of some literature cases, can be useful hints for the design of long-lived charge-separated states.