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

Abstract

The excited-state dynamics and photochemistry of [Re(R)(CO)3(dmb)] (R=Me, Et); dmb=4,4′-dimethyl-2,2′-bipyridine) in CH2Cl2 have been studied by time-resolved visible absorption spectroscopy on a broad time scale ranging from approximately 400 fs to a few microseconds, with emphasis on the femtosecond and picosecond dynamics. It was found that the optically prepared Franck-Condon 1MLCT (singlet metal-to-ligand charge transfer) excited state of [Re(R)(CO)3(dmb)] undergoes femtosecond branching between two pathways (≤400 fs for RMe; approximately 800 fs for REt). For both methyl and ethyl complexes, evolution along one pathway leads to homolysis of the Re-R bond via a 3SBLCT (triplet σ-bond-to-ligand charge transfer) excited state, from which [Re(S)(CO)3(dmb)]. and R. radicals are formed. The other pathway leads to an inherently unreactive 3MLCT state. For [Re(Me)(CO)3(dmb)], the 3MLCT state lies lowest in energy and decays exclusively to the ground state with a lifetime of approximately 35 ns, thereby acting as an excitation energy trap. The reactive 3SBLCT state is higher in energy. The quantum yield (0.4 at 293 K) of the radical formation is determined by the branching ratio between the two pathways. [Re(Et)(CO)3(dmb)] behaves differently: branching of the Franck-Condon state between two pathways still occurs, but the 3MLCT excited state lies above the dissociative 3SBLCT state and can decay into it. This shortens the 3MLCT lifetime to 213 ps in CH2Cl2 or 83 ps in CH3CN. Once populated, the 3SBLCT state evolves toward radical photoproducts [Re(S)(CO)3(dmb)]. and Et.. Thus, population of the 3MLCT excited state of [Re(Et)(CO)3(dmb)] provides a second, delayed pathway to homolysis. Hence, the quantum yield is unity. The photochemistry and excited-state dynamics of [Re(R)(CO)3(dmb)] (R=Me, Et) complexes are explained in terms of the relative ordering of the Franck-Condon, 3MLCT, and 3SBLCT states in the region of vertical excitation and along the Re-R reaction coordinate. A qualitative potential energy diagram is proposed.