Charge Separation in Donor-Chromophore-Acceptor Assemblies: Linkage and Driving Force Dependence of Photoinduced Electron Transfers

Article abstract of DOI:10.1021/j100017a038

A series of covalently linked Ru(bipyridine)3-donor-acceptor complexes was prepared where the donor-to-chromophore and acceptor-to-chromophore methylene chain lengths were varied.Time-resolved absorption studies were performed to elucidate intramolecular electron transfer rates.The electron donor in the above series is a phenothiazine moiety linked to a bipyridine by a (-CH2-)_p, p=3-8 chain, and the electron acceptor is an N,N'-diquaternary-2.2'-bipyridinium moiety linked to a bipyridine by a (-CH2-)_m, m=2,3,4 chain.Oxidative quenching of the Ru(bipyridine)3 metal-to-ligand charge transfer (MLCT) state followed by phenothiazine- to-ruthenium electron transfer resulted in a long-lived charge-separated state.A wavelength-dependent excitation resulted in a slowly decaying absorption which is assigned to the excited-state phenothiazine.The magnitude of this component in the transient absorption serves as an internal standard for determining relative quantum yield for formation of the charge-separated state.Marcus inverted region behavior was observed in back electron transfer.Rate constants for electron transfer from phenothiazine to Ru(III) decreased as the length of the bridging chain increased from p=4 to 8.Chain length independence of the back electron transfer rate in the series of complexes with varied chromophore-acceptor distances (m=2,3, and 4) suggests the formation of an association complex during oxidative quenching of MLCT state and argues against a ?-bond superexchange pathway for back electron transfer.