Mapping electron transfer pathways in a chromophore-quencher triad

A comparative study of the photophysics of the four positional isomers (one trans and three cis) of the chromophore-quencher triad [Ru-II(dmb)(bpyCH(2)PTZ)(bpyCH(2)MV(2+))](4+) (dmb is 4,4'-dimethyl-2,2'-bipyridine, bpyCH(2)PTZ is (4'-methyl-4-(2,2'-bipyridin-4-yl)methyl)phenothiazine, and bpyCH(2)MV(2+) is (4'-methyl-4-(2,2'-bipyridin-4-yl)methyl)-1'-methyl-4,4'-bipyridinium cation) has been undertaken, Following metal-to-ligand charge transfer (MLCT) excitation by laser flash photolysis at 460 or 532, nm, the redox-separated states [Ru-II(dmb)(bpyCH(2)PTZ(.+))(bpyCH(2)MV(.+))]4+ are formed rapidly (< 5 ns), Quenching of MLCT emission occurs with near unit efficiency for all four isomers. For the trans and cis3 isomers, formation of the redox-separated state is similar to 25% efficient. For back electron transfer from -MV+ to -PTZ(+), Delta G0=-1.14 eV for all four isomers from electrochemical measurements and yet k(ET) varies from 4.5 x 10(6) to 8.7 x 10(6) s(-1) in acetonitrile at 25 degrees C.