Redox State Sensitive Spectroscopy of the Model Compound [(H-dcbpy)₂Ru^II(NCS)₂]^2– (dcbpy = 2,2′-Bipyridine-4,4′-dicarboxylato)

The charge transfer reaction mechanism in a ruthenium polypyridine model complex with isothiocyanato ligands, i.e., [(H-dcbpy)₂Ru­(NCS)₂]^2– 2Bu₄N⁺ (Ru2H) (dcbpy = 2,2′-bipyridine-4,4′-dicarboxylato), has been investigated by combining UV–vis absorption, resonance Raman spectroscopy, and electrochemical methods. Understanding the photophysics of light-harvesting complexes of this class is an indispensable prerequisite to improve the efficiency and durability of photocatalysts for hydrogen production, a possible basis of future energy sources. In this study, light-induced excitation of Ru2H has been mimicked by electrochemical reduction, and the intermediate redox state has been extensively characterized by UV–vis absorption and resonance Raman spectroscopy. The reduction occurs at the dcbpy ligand, and the spectra of reduced Ru2H resemble the excited state spectra. The reduced Ru2H exhibits a new absorption band at 440 nm localized at the bipyridine framework. This assignment is consistent with the observed enhancement of the bipyridine bands with respect to nonreduced Ru2H in the resonance Raman spectra of the reduced Ru2H excited at 458 nm. The detected shifts in Raman band positions and changes in the relative band intensities between nonreduced and reduced Ru2H are rationalized on an orbital level by quantum chemical calculations. The results indicate that the reduction is accompanied by deprotonation, i.e., an intramolecular electron-transfer-induced cleavage of the O–H bond splitting off hydrogen, of the carboxylic acid groups of the dcbpy ligand.