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

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)(3)(2+)) 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 center dot+) and neutral (V-0), whereas ruthenium retained its formal Ru(bpy)(3)(2)(+) oxidation state. The metal to ligand charge transfer state of Ru(bpy)(3)(2+) 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.