RADICAL-CATION PROBES FOR PHOTOINDUCED INTRAMOLECULAR ELECTRON-TRANSFER IN METAL-ORGANIC COMPLEXES

Two transition metal complexes of the type fac-(bpy)Re-I(CO)(3)(DA)(+) (where bpy = 2,2'-bipyridine and DA is a pyridine ligand that is subs tituted 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 elec tron transfer oxidation. Photoexcitation of the diamine complexes affo rds a ligand-to-ligand charge transfer (LLCT) state via intramolecular electron transfer quenching of a metal-to-ligand charge transfer (MLC T) state, [(bpy)Re-I(CO)(3)(DA)](+) + h nu --> [(bpy(.-))Re-II(CO)(3)( DA)](+)(MLCT) --> [(bpy(.-))Re-I(CO)(3)(DA(.+))](+)*(LLCT). Photochem ical product and quantum efficiency studies indicate that the diamine reactive donor ligand undergoes photoinduced C-C bond fragmentation wi th high efficiency, presumably via the radical cation (DA(.+)) which i s present in the LLCT excited state. Laser flash photolysis allows dir ect detection of the metal complex based radicals that are formed by C -C bond fragmentation. Quantitative kinetic information gathered throu gh luminescence, laser flash photolysis, and quantum yield studies all ows estimation of the rates for formation of the LLCT state by forward electron transfer (k(FET)), decay of the LLCT state by back electron transfer (k(BET)), and the rate of diamine radical cation bond fragmen tation in the LLCT state (k(BF)). The relationship between these kinet ic parameters and the driving force for electron transfer and bond fra gmentation as well as the structure of the reactive donor ligands is d iscussed.