Direct Observation of Ultrafast C-C Bond Fragmentation in a Diamine Radical Cation

Abstract

Nanosecond and picosecond transient absorption studies were carried out on fac-(bpy)Re{sup 1}(CO){sub 3}(DA){sup +} (1), where bpy is 2,2{prime}-bipyridine and DA is the 1,2-diamine-substituted pyridine erythro-l-[p-[4-(pyridyl)methyl]-anilino]-2piperidino-1, 2-piperidino-1,2-diphenylethane. Photoexcitation of the d{pi} (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){sub 3}{sup -} chromophore to produce the ligand-to-ligand charge transfer (LLCT) state, (bpy{sup -})Re{sup 1}(CO){sub 3}(DA{sup +}); (2) C-C bond fragmentation of the diamine radical cation unit. Nanosecond transient absorption spectral data indicate that the {alpha}-amino radical which is the primary product of the C-C bond fragmentation is produced during the 10 ns laser excitation pulse. Picosecond pump-probe studies of 1 allow direct determination of the kinetics of back electron transfer in the LLCT state (1 x 10{sup 8} s{sup -1}) and for C-C bond fragmentation in the diamine radical cation (3 x 10{sup 8} s{sup -1}). 36 refs., 2 figs.