Binuclear Mn oxo complex as a self-contained photocatalyst in water-splitting cycle: Role of additional Mn oxides as a buffer of electrons and protons.

We theoretically propose a photoinduced water-splitting cycle catalyzed by a binuclear Mn oxo complex. In our "bottom-up approach" to this problem, we once proposed a working minimal model of water-splitting cycle in terms of a mononuclear Mn oxo complex as a catalyst along with water clusters [K. Yamamoto and K. Takatsuka, Phys. Chem. Chem. Phys. 20, 6708 (2018)]. However, this catalyst is not self-contained in that the cycle additionally needs buffering molecules for electrons and protons in order to reload the Mn complex with electrons and protons, which are lost by photoinduced charge separation processes. We here show that a binuclear Mn oxo complex works as a self-contained photocatalyst without further assistant of additional reagents and propose another catalytic cycle in terms of this photocatalyst. Besides charge separation and proton relay transfer, the proposed cycle consists of other fundamental chemical dynamics including electron-proton reloading, radical relay-transfer, and Mn reduction. The feasibility of the present water-splitting cycle is examined by means of full dimensional nonadiabatic electron-wavepacket dynamics based on multireference electronic wavefunctions and energy profiles estimated with rather accurate quantum chemical methods for all the metastable states appearing in the cycle.