Radical Cation Probes for Photoinduced Intramolecular Electron Transfer in Metal−Organic Complexes

Abstract

Two transition metal complexes of the type fac-(bpy)ReI(CO)3(DA)+ (where bpy = 2,2‘-bipyridine and DA is a pyridine ligand that is substituted with a 1,2-diamine electron donor) have been prepared. The 1,2-diamine serves as a “reactive donor ligand” owing to its propensity to undergo rapid C−C bond fragmentation when activated by single electron transfer oxidation. Photoexcitation of the diamine complexes affords a ligand-to-ligand charge transfer (LLCT) state via intramolecular electron transfer quenching of a metal-to-ligand charge transfer (MLCT) state, [(bpy)ReI(CO)3(DA)]+ + hν → [(bpy•-)ReII(CO)3(DA)]+*(MLCT) → [(bpy•-)ReI(CO)3(DA•+)]+*(LLCT). Photochemical product and quantum efficiency studies indicate that the diamine reactive donor ligand undergoes photoinduced C−C bond fragmentation with high efficiency, presumably via the radical cation (DA•+) which is present in the LLCT excited state. Laser flash photolysis allows direct detection of the metal complex based radicals that are formed by C−C bond fragmentation. Quantitative kinetic information gathered through luminescence, laser flash photolysis, and quantum yield studies allows estimation of the rates for formation of the LLCT state by forward electron transfer (kFET), decay of the LLCT state by back electron transfer (kBET), and the rate of diamine radical cation bond fragmentation in the LLCT state (kBF). The relationship between these kinetic parameters and the driving force for electron transfer and bond fragmentation as well as the structure of the reactive donor ligands is discussed