Photosubstitution Reaction of cis-[Ru(bpy)(CHCN)] and cis-[Ru(bpy)(NH)] in Aqueous Solution via Monoaqua Intermediate.

The photoinduced ligand exchange reaction of Ru(II) complexes in aqueous solution was studied using density functional theory (DFT). The optimized structures of the lowest triplet state of cis-[Ru(bpy)(CHCN)] (bpy = bipyridine), cis-[Ru(bpy)(NH)], and their monoaqua complexes were analyzed. The metal-centered (MC) structure was lower than the metal-to-ligand charge transfer (MLCT) structure for cis-[Ru(bpy)(CHCN)], whereas the MLCT structure was lower than the MC structure for cis-[Ru(bpy)(NH)]. Such a difference would correlate with the higher quantum yield of the former complex. For the monoaqua complexes, the most stable local minimum structure was the MC structure, in which the Ru-O and Ru-N ( trans to the oxygen) bonds were elongated. Therefore, the dissociation of the HO ligand would be preferred to that of the CHCN (or NH) ligand from the monoaqua intermediate, which might result in the reformation of the monoaqua intermediate, and thus, the formation of the bis-aqua product would take a longer time than that of the monoaqua intermediate.