Counter Ion Effect on Photoinduced Electron Transfer Reaction between Ruthenium Complexes

Quenching experiments by photoinduced electron transfer between a charged donor and a neutral acceptor were carried out in acetonitrile, dichloromethane and mixed solvents of acetonitrile and dichloromethane. Tris(2, 2'-bipyridine) ruthenium(Ⅱ) ([Ru(bpy₃]^(2+)) which has 2+ charge and dicyanobis (2, 2'-bipyridine) ruthenium(Ⅱ) (Ru(bpy)₂(CN₂) which has no charge were used as electron donors, and a series of tris(β-diketonato) ruthenium (Ⅲ) was used as acceptor. In dichloromethane, [Ru(bpy₃]^(2+) and its counter ions (ClO₄-) form ion pair. In the estimate of ΔG of electron transfer, the electrostatic potential between counter ions and product ion pair produced by electron transfer must be taken into account. A similar effect of counter ions was found in mixed solvents of 10, 30, 50, 70 and 90% acetonitrile ratio in volume. The effect of counter ion on ΔG became smaller with the increase in acetonitrile ratio. The result in mixed solvents suggests that [Ru(bpy₃]^(2+) and its counter ions form ion pair even in 90% acetonitrile solution.