Solvent effect on the photoinduced electron-transfer reactions between dicyanobis(polypyridine)ruthenium(II) complexes and tris(β-diketonato)ruthenium(III) complexes

Abstract

The solvent effect on photoinduced electron-transfer reactions has been investigated by use of cis-dicyanobis(polypyridine)ruthenium(II) Ru(L)₂(CN)₂[L: 2,2′-bipyridine (bpy) and 1,10-phenanthroline (phen)], as electron donors and a series of tris(β-diketonato)ruthenium(III), Ru(β-diketonato)₃, as electron acceptors. Luminescence of Ru(L)₂(CN)₂ was quenched by Ru(β-diketonato)₃via an oxidative electron-transfer reaction. The ΔG dependence of the quenching rate constants (k_q) of Ru(bpy)₂(CN)₂ was found to be the same as that of Ru(phen)₂(CN)₂ in 1,2-dichloroethane. However, in acetonitrile, the ΔG dependence of k_q in the Ru(bpy)₂(CN)₂ system differed from that in the Ru(phen)₂(CN)₂ system. This difference is explained in terms of solvation in the excited state; Ru(bpy)₂(CN)₂ is more solvated and stabilized in the excited state in polar solvents than is Ru(phen)₂(CN)₂. The activation enthalpy (ΔH^‡) and the activation entropy (ΔS^‡) of electron transfer were determined from the temperature dependence of k_q. In acetonitrile, ΔH^‡ was constant and nearly equal to zero in the region of ΔG convered in this work, while ΔS^‡ was negative and its absolute value rose as ΔG increased in the normal region. On the other hand, in 1,2-dichloroethane, ΔH^‡ decreased and |ΔS^‡| greatly increased in the normal region. while in the exothermic region ΔH^‡ and ΔS^‡ showed the same tendency as in acetonitrile. These results indicate that in acetonitrile the activation is induced by the change of entropy caused by the rearrangement of solvent molecules coordinating around the reactants. However, in 1,2-dichloroethane, activation seems to be induced not only by the rearrangement of solvent molecules coordinating around the reactants, but also by the deformation of the reactant complexes and/or by the solvation of free solvent molecules.