DIRECT OBSERVATION OF ULTRAFAST C-C BOND FRAGMENTATION IN A DIAMINE RADICAL-CATION

Nanosecond and picosecond transient absorption studies were carried ou t on fac-(bpy)Re-I(CO)3(DA)(+) (1), where bpy is 2,2'-bipyridine land DA is the 1,2-diamine-substituted pyridine hro-1-{p-[4-(pyridyl)methyl ]anilino}-2-piperidino- 1,2-diphenylethane. Photoexcitation of the d p i(Re) --> pi (bpy) metal-to-ligand charge-transfer excited state in 1 initiates the following sequence: (1) forward electron transfer from the 1,2-diamine unit to the photoexcited (bpy)Re(CO)(3)- chromophore t o produce the ligand-to-ligand charge transfer (LLCT) state, (bpy(.-)) Re-I(CO)(3)(DA(.+)); (2) C-C bond fragmentation of the diamine radical cation unit. Nanosecond transient absorption spectral data indicate t hat the a-amino radical which is the primary product of the C-C bond f ragmentation is produced during the 10 ns laser excitation pulse. Pico second pump-probe studies of 1 allow direct determination of the kinet ics of back electron transfer in the LLCT state (1 x 10(8) s(-1)) and for C-C bond fragmention in the diamine radical cation (3. x 10(8) s(- 1)).