Mapping Electron Transfer Pathways in a Chromophore−Quencher Triad

Abstract

A comparative study of the photophysics of the four positional isomers (one trans and three cis) of the chromophore−quencher triad [RuII(dmb)(bpyCH2PTZ)(bpyCH2MV2+)]4+ (dmb is 4,4‘-dimethyl-2,2‘-bipyridine, bpyCH2PTZ is (4‘-methyl-4-(2,2‘-bipyridin-4-yl)methyl)phenothiazine, and bpyCH2MV2+ 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 [RuII(dmb)(bpyCH2PTZ•+)(bpyCH2MV•+)]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 ∼25% efficient. For back electron transfer from −MV+ to −PTZ+, ΔG0 = −1.14 eV for all four isomers from electrochemical measurements and yet kET varies from 4.5 × 106 to 8.7 × 106 s-1 in acetonitrile at 25 °C.