Emission decay measurements under various excitation intensities and time-resolved absorption spectroscopy were applied to investigate the bimolecular reactions of the triplet lowest excited metal-to-ligand charge-transfer state ( 3 MLCT) in the neat crystals or doped crystals with energy acceptor. The rate of hopping to the nearest-neighbor site in the energy migration of 3 MLCT in [Ru(bpy) 3 ](PF 6 ) 2 was determined to be (3.5 ± 0.5) × 10 8 s -1 from the dependence of emission decay rates on the doping concentration of Os 2+ and the rate constant of triplet−triplet (T−T) annihilation in the emission decay for the single-crystal or transparent mixed-crystal ground with KCl. The rate constants of hopping to the nearest-neighbor sites obtained for the crystals of [Ru(bpy) 3 ]X 2 (X = Cl -, ClO 4 -, and PF 6 - ) showed a distance dependence. The diffusion constants of triplet exciton in the crystals were also estimated. Time-resolved absorption spectroscopy revealed that T−T annihilation produced the one-electron transferred products. The rate of electron transfer between 3 MLCT in the crystal of [Ru(bpy) 3 ](PF 6 ) 2 was estimated to be ≥5 × 10 10 s -1 by time-resolved absorption spectroscopy.
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Ikeda et al. (2000) studied this question.