The effect of solute-solvent interaction on the radiationless relaxation rate of the ^3MLCT states of tris(alpha, alpha'-diimine)metal(II) complexes

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The radiationless decay rates of Ru(bpy)_3^<2+>, Ru(phen)_3^<2+> Os(bpy)_3^<2+> and Os(phen)_3^<2+> were measured in water-alcohol and -dioxane mixted solvents. In the Ru(II) complexes, we divided the radiationless decay rate into two non-radiative processes from the results of the measurements for the temperature dependence : one is the path to the ground state directly (IC path) and another is the path to the metal center state above the ^3MLCT state (MC path). On the contrary, the IC path is predominant in Os(II) complexes. By using the deuterated solvent, it has been revealed that no isotope effect is observed in the MC path. For the IC path, a large isotope effect on the rate was observed in water while a slight isotope effect was observed in alcohol. On adding alcohol to water, the rate constant decreased nonlinearly With increase of mole fraction of alcohol. In order to explain the observed solvent effect on the rates, we proposed a new model for the radiationless transition to … More the ground state as below instead of the mechanism of charge-transfer-to-solvent : the water molecule included between ligands of the complex may accept the excitation energy followed by the energy dissipation toward the cluster of outer sorrounding solvents with which the included water combines in advance. The interaction between the included solvent and the ligands varies accompanying with the change in the interaction between the included solvent and the outer solvent cluster. Therefore, the transmission coefficient, kappa, of the energy tranefer from the metal complex to the included water molecule is change with the microscopic structure of solvent, since the interaction between the included solvent and the outer solvent cluster is changed by the strength of the hydrogen bond of the outer cluster. In alcohol, since the included alcohol molecule have only one OH,the molecule must reorientate to dissipate the excitation energy to the outer solvent cluster. Thus the kappa should be small. Less