DFT/TD-DFT investigation on the photoinduced electron transfer of diruthenium and viologen complexes

Abstract Photoinduced electron transfer, chemical stability, and redox property of diruthenium−viologen complexes were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Target systems include a series of diruthenium and viologen complexes consisting of Ru(bpy)3-viologen-Ru(bpy)3; Ru–V–Ru, Ru(bpy)3-viologen-Ru(bpy)(dcbpy)2; Ru–V–RuCOOH, and Ru(bpy)3-viologen-viologen-Ru(bpy)3; Ru–V–V–Ru, where bpy = 2,2′-bipyridyl and dcbpy = 4,4′-dicarboxyl-2,2-bipyridyl. The B3LYP functional was used to investigate the ground state properties for the three diruthenium and viologen complexes and for two parent complexes: ruthenium-bipyridine (Ru(bpy)32+) and methyl viologen (MV2+) as well as their oxidized states. Singlet excited states were examined using the TD-DFT calculations with CAM-B3LYP functional in acetonitrile using conductor polarizable continuum model (CPCM). The TD-DFT calculations reproduced the experimentally observed variation of the absorption spectra due to oxidation states. The correlation between the calculated UV–Vis transition and molecular orbitals for the excited state clearly established that the viologen dication (V2+) changed their reductive product into the radical cation (V•+) and neutral (V0), whereas ruthenium retained its formal Ru(bpy)32+ oxidation state. The metal to ligand charge transfer state of Ru(bpy)32+ was shown to undergo electron transfer quenching by methyl viologen dication (MV2+). These findings are relevant for the development of novel optoelectronic materials owing to a combination of chemical stability, redox activity, and long-lived excited states.