Superexchange-mediated charge separation and charge recombination in covalently linked donor-bridge-acceptor systems

Evidence is presented in support of the concept that electron transfer ( ET) between a pair of chromophores may take place efficiently over large distances (> 10 Angstrom) by the mediation of an intervening saturated hydrocarbon medium. For example, ET is found to take place on a sub-nanosecond timescale through saturated norbornylogous bridges greater than 13 Angstrom in length, by a superexchange (through-bond coupling) mechanism. The dependence of the ET dynamics on the bridge length and configuration are consistent with the operation of a superexchange mechanism. The distinction between molecular wire behaviour and superexchange-mediated ET is made. The distance dependence of ET dynamics through different types of bridges - saturated and unsaturated hydrocarbon bridges, proteins, and duplex DNA - is discussed and explained. Strategies for prolonging the lifetimes of charge-separated states are explored and discussed. In general, long-lived charge-separated species have been generated using giant multichromophoric systems in which the charges are separated by large distances, often exceeding 20 Angstrom. In contrast, it is shown that very long-lived charge-separated states, possessing the triplet multiplicity, may be generated using short 'dwarf' dyads, in which the charges are less than 6 Angstrom apart. Charge recombination in these species is slowed by the difference in electron spin multiplicity between the charge-separated state and the ground state.