CHARGE SEPARATION IN LINKED RU(BIPYRIDINE)3-DONOR-ACCEPTOR COMPLEXES

Time-resolved emission studies have been performed on a series of covalently linked Ru(bipyridine)3-donor-acceptor complexes. In these complexes the donor is a phenothiazine moiety linked to a bipyridine by a (-CH2-)4 chain, and the acceptor is an N,N'-diquaternary-2,2'-bipyridinium moiety, linked to a bipyridine by a (-CH2-)2 chain. Excitation to the Ru(bipyridine)3 metal-to-ligand charge-transfer (MLCT) state leads to a long-lived charge-separated state. The results are compared to those obtained in the analogous Ru(bipyridine)3-acceptor complexes. The results can be understood in terms of a model in which the initial step in the formation of the charge-separated state is oxidative quenching of the MLCT state, followed by phenothiazine to ruthenium electron transfer. The charge separation occurs efficiently in dichloroethane, but not in acetonitrile. This result is discussed in terms of the relative rates of acceptor-to-ruthenium reverse electron transfer in the two solvents.