Ultrafast time-resolved UV-visible and infrared absorption spectroscopy of binuclear rhenium(I) polypyridyl complexes in solution

Four binuclear rhenium(I) complexes of the form [Re(CO)(3)Cl](2)BL, where BL is one of the bridging N-4-donor ligands 2,3-di(2-pyridyl)quinoxaline, 2,3-di(2-pyridyl)-6-methylquinoxaline, 2,3-di(2-pyridyl)pyrido[2,3-b]pyrazine, or 2,3-di(2-pyridyl)pyrido[3,4-b]pyrazine, have been studied by femtosecond time-resolved UV-visible and infrared absorption spectroscopies. Photolysis at 606 nm, within the metal-to-ligand charge-transfer (MLCT) absorption band of each complex, produces transient features with lifetimes which vary from 100 to 1800 ps, depending on the structure of the bridging ligand. These transient species are assigned to the (MLCT)-M-3 excited states of the complexes. A detailed analysis of the time-resolved infrared spectra reveals that the three principal nu(CO) bands of the ground state, which arise from two Re(CO)(3)Cl groups in similar environments, split into two sets of three nu(CO) bands in the (MLCT)-M-3 excited states. This splitting pattern is attributed to asymmetric charge distribution in the excited state, in which one rhenium center is oxidized and the other is a spectator which senses the reduction of the bridging ligand; i.e., the excited state is a class II mixed-valence state, Re+(BL-)Re. Changes in the transient spectra which occur within ca, 5 ps, and which are observed most clearly as broadening to lower wavenumber of the excited-state infrared bands, are attributed to vibrational relaxation in the excited state. The variation in excited-state lifetime with bridging ligand structure is attributed to changes in the rate constants for nonradiative decay, which correspond qualitatively with changes in the energy of the excited state in accordance with the "energy-gap law" for nonradiative relaxation.