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 bon...