Temperature-Dependent Rates of Electron Transfers and Intersystem Crossing on the Laser Excitation of Ligand-Bridged Ru(II) and Co(III) Compounds

Photoinduced intramolecular electron transfers within ligand-bridged Ru(II) and Co(III) compounds of [(bpy) 2 Ru(L)Co(bpy) 2 ] 5+ ( a, b, and c ) were examined in butyronitrile by means of laser photolysis kinetic spectroscopy, where L is a bridging tetradentate ligand, 2,6-bis(2-pyridyl)benzodiimidazole for a, 2,2‘-bis(2-pyridyl)bibenzimidazole for b, and 1,5-bis[2-(2-pyridyl)benzimidazoyl]pentane for c . The temperature dependence of the rates of electron transfer and intersystem crossing is analyzed by taking the temperature dependence of the Gibbs energy change and reorganization energy into account. A transient difference absorption band at 420 nm observed after the picosecond laser excitation of a and b is not ascribed to the formation of excited Ru(II) moiety but to the reaction product ( 2 Co(II)) of the Co(III) moiety, which decayed to form a long-lived species of 4 Co(II) within 1 ns below 270 K. The inner-sphere reorganization of the intersystem crossing of 2 Co(II) → 4 Co(II) is estimated to be 0.6 eV from the temperature dependence of the transition rate. The electron transfer from the excited MLCT state of the Ru(II) moiety to Co(III) for c occurred within 1 ns at 300 K. The inner-sphere reorganization energy (0.6 eV) of the electron transfer was determined from the temperature-dependent electron transfer rate observed. The back electron transfer from 4 Co(II) to 2 Ru(III) was much slower than the initial electron transfer. It is not the small electronic coupling between 4 Co(II) and 2 Ru(III) but the negative entropy change (−1.3 meV/K) of Co(II) oxidation that is responsible for the small frequency factor of the electron transfer process. The inner-sphere organization energy of the back electron transfer was determined (1.1−1.3 eV) from the temperature-dependent rate.