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 [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 ∼25% efficient. For back electron transfer from −MV + to −PTZ +, Δ G 0 = −1.14 eV for all four isomers from electrochemical measurements and yet k ET varies from 4.5 × 10 6 to 8.7 × 10 6 s -1 in acetonitrile at 25 °C.