C-C Bond Fragmentation as a Probe for Photoinduced Intramolecular Electron Transfer

Abstract

Photochemical and photophysical studies are reported for the ~omplex,fac-(bpy)Re’(CO)3(DA)~+ (e-l), where bpy is 2,2’-bipyridine and DA is a “reactive donor ligand” that contains a vicinal diamine functionality. Photoexcitation of e-1 into the dn (Re) -. n* (bpy) metal-to-ligand charge transfer (MLCT) excited state leads to formation of the ligand-to-ligand charge transfer (LLCT) state, *(bpy’-)Re1(CO)3(DA’+)]+, via diamine ligand-to-Re electron transfer. In the LLCT state, the reactive donor ligand radical cation undergoes an exceedingly rapid heterolytic C-C bond fragmentation reaction to produce an iminium ion and an a-amino radical fragment. Steady-state photochemical studies reveal that in air-saturated solution the only products observed arise from C-C bond fragmentation, which implies that under these conditions bond fragmentation is irreversible. Laser flash photolysis studies indicate that the a-amino radical that is produced by bond fragmentation in the LLCT state absorbs strongly in the near-UV region. A kinetic analysis is carried out under the assumptions that: (1) the quantum yield for formation the LLCT state can be derived from MLCT emission lifetime data on the diamine complex and a suitable non-donor-substituted model complex and (2) C-C bond fragmentation is irreversible. This analysis affords lower limits for the rates of back electron transfer and bond fragmentation (1.5 x lo8 and 1.0 x lo8 s-l, respectively).