Ultrafast dynamics of photochemical radical formation from [Re(R)(CO)(3)(dmb)] (R = Me, Et; dmb=4,4 '-dimethyl-2,2 '-bipyridine): A femtosecond time-resolved visible absorption study

The excited-state dynamics and photochemistry of [Re(R)(CO)(3) (dmb)l (R=Me , Et): dmb=4,4'-dimethyl-2.2'-bipyridine) in CH2Cl2 have been studied by ti me-resolved visible absorption spectroscopy on a broad time scale ranging f rom approximately 400 fs to a few microseconds, with emphasis on the femtos econd and picosecond dynamics. It was found that the optically prepared Fra nck - Condon (MLCT)-M-1 (singlet metal-to-ligand charge transfer) excited s tate of [Re(R)(CO)(3)(dmb)] undergoes femtosecond branching between two pat hways (less than or equal to 400 fs. for R = Me: approximately 800 fs for R = Et). For both methyl and ethyl complexes, evolution along one pathway le ads to homolysis of the Re-R bond via a (SBLCT)-S-3 (triplet sigma-bond-to- ligand charge transfer) excited state, from which [Re(S)(CO)(3)(dmb)](.) an d R-. radicals are formed. The other pathway leads to an inherently unreact ive (MLCT)-M-3 state. For [Re(Mc)(CO)(3)(dmb)], the (MLCT)-M-3 state lies l owest in energy and decays exclusively to the ground state with a lifetime of approximately 35 ns, thereby acting as an excitation energy trap. The re active (SBLCT)-S-3 state is higher in energy. The quantum yield (0.4 at 293 K) of the radical formation is determined by the branching ratio between t he two pathways. [Re(Et)(CO),(dmb)] behaves differently: branching of the F ranck-Condon state between two Franck pathways still occurs, but the 3MLCT excited state lies above the dissociative (SBLCT)-S-3 state and can decay i nto it. This shortens the (MLCT)-M-3 lifetime to 213 ps in CH2C2 or 83 ps i n CH3CN. Once populated, the (SBLCT)-S-3 state evolves toward radical photo products [Re(S)(CO)(3)(dmb)](.) and Et-.. Thus Population of the (MLCT)-M-3 excited state of [Re(Et)(CO)(3)(dmb)] provides a second, delayed pathway t o homolysis. Hence, the quantum yield is unity. The photochemistry and exci ted state dynamics of [Rc(R)(CO),(dmb)] (R - Me, Et) complexes are explaine d in terms of the relative ordering of the Franck-Condon, (MLCT)-M-3, and ( SBLCT)-S-3 states in the region of vertical excitation and along the Re-R r eaction coordinate. A qualitative potential energy diagram is proposed.