Solvent Dependence of Absorption and Emission Spectra of Ru(bpy)2(CN)2: Experiment and Explanation Based on Electronic Structure Theory

Abstract

Measurements in acidic media and time-dependent density functional theory and DeltaSCF calculations were performed for Ru(bpy)2(CN)2 in 11 solvents of varying polarity to determine the solvent's influence on the absorption and emission spectra of the complex. The solvent effect caused by both the polarizable continuum nature of the solvent (characterized by the polarizable conductor model), and by the coordination of the cyano groups of the complex by solvent molecules were investigated. Both the absorption and emission maxima show a strong blue shift as the solute-solvent interaction increases, the magnitude of which is in good linear correlation with Gutmann's acceptor number of the solvent. The calculations reproduce the location, shape, and shift of the experimental metal-to-ligand charge transfer bands. The solvent shift is shown to be in good correlation with the charge difference between the Ru atom and the bpy ligand, which in turn is closely related to the HOMO energy. The coordination of the solvent molecule to the cyano group causes a smaller blue shift than the polarizable continuum solvent. The specific solute-solvent interaction becomes dominant, however, when the pH in a protic solvent is small and the complex is protonated.