METAL TO LIGAND CHARGE-TRANSFER PHOTOCHEMISTRY OF RE(I)-ALKYL COMPLEXES

A preliminary study has been carried out concerning the photochemistry and photophysics of a series of (bpy)Re(I)(CO)3-R complexes, where bpy is 2,2'-bipyridine and R=-CH3, -CH2Ph and -Ph. Photolysis of the complexes in to the lowest energy absorption band (436 nm) leads to a highly efficient Re-C bond homolysis reaction for R=-CH3 and -CH2Ph. Absorption and low-temperature emission studies of the complexes suggest that the lowest excited state is based on dpi (Re)--> pi* (bpy) metal to ligand charge transfer (MLCT). Nanosecond transient absorption studies confirm that the MLCT state is formed after near-UV excitation of (bpy)Re(CO)3-Ph; however, only the reactive intermediate which is formed as a result of Re-C bond homolysis is observed for (bpy)Re(CO)3-CH3 and (bpy)Re(CO)3-CH2Ph. The results are interpreted by a photochemical model in which the bond homolysis reaction occurs either directly from an unrelaxed MLCT excited state or from a spectroscopically silent state which is reached during non-radiative relaxation of the Franck-Condon MLCT state.