C-C BOND FRAGMENTATION AS A PROBE FOR PHOTOINDUCED INTRAMOLECULAR ELECTRON-TRANSFER

Photochemical and photophysical studies are reported for the complex, fac-(bpy)Re-I(CO)(3)(DA)(1+) (e-1), where bpy is 2,2'-bipyridine and D A is a ''reactive donor ligand'' that contains a vicinal diamine funct ionality. Photoexcitation of e-l into the dst (Re) --> pi (bpy) metal -to-ligand charge transfer (MLCT) excited state leads to formation of the ligand-to-ligand charge transfer (LLCT) state, (bpy(.-))Re-I(CO)( 3)(DA(.+))(1+), via diamine ligand-to-Re electron transfer. In the LLC T state, the reactive donor ligand radical cation undergoes an exceedi ngly rapid heterolytic C-C bond fragmentation reaction to produce an i minium ion and an cr-amino radical fragment. Steady-state photochemica l studies reveal that in air-saturated solution the only products obse rved 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 fr agmentation 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 em ission lifetime data on the diamine complex and a suitable non-donor-s ubstituted model complex and (2) C-C bond fragmentation is irreversibl e. This analysis affords lower limits for the rates of back electron t ransfer and bond fragmentation (1.5 x 10(8) and 1.0 x 10(8) s(-1), res pectively).