Mechanism for the light-induced O(2) evolution from H(2)O promoted by Ru(II) PNN complex: A DFT study.

The density functional theory (DFT) method was used to explore the light-induced O(2) formation from H(2)O promoted by Ru(II) PNN complex in the present work. The elimination of H(2)O(2) was found to be highly endothermic, which is not in competition with the H(2)O elimination and hydrogen transfer. The calculated results reported here do not support the mechanism proposed in a recent experiment, where H(2)O(2) was suggested as an important intermediate for formation of O(2). We proposed a new mechanism for formation of the triplet O(2) molecule, which contains the two steps of the concerted hydrogen transfer and dehydration. The light-induced O(2) evolution from water promoted by the Ru(II) complex was found to be a nonadiabatic process. The O-O bond is formed along the T(1) pathway as a result of the efficient S(1) → T(1) intersystem crossing. All experimental findings on the light-induced O(2) evolution can be explained by the mechanism proposed in the present work.