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

Abstract

Photoinduced intramolecular electron transfers within ligand-bridged Ru(II) and Co(III) compounds of [(bpy)2Ru(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 (2Co(II)) of the Co(III) moiety, which decayed to form a long-lived species of 4Co(II) within 1 ns below 270 K. The inner-sphere reorganization of the intersystem crossing of 2Co(II) → 4Co(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 4Co(II) to 2Ru(III) was much slower than the initial electron transfer. It is not the small electronic coupling between 4Co(II) and 2Ru(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.