Photocatalytic Water Oxidation by a Mixed-Valent (Mn3MnO3)-Mn-III-O-IV Manganese Oxo Core that Mimics the Natural Oxygen-Evolving Center

The functional core of oxygenic photosynthesis is in charge of catalytic water oxidation by a multi-redox MnIII/MnIV manifold that evolves through five electronic states (Si, where i=0-4). The synthetic model system of this catalytic cycle and of its S0→S4 intermediates is the expected turning point for artificial photosynthesis. The tetramanganese-substituted tungstosilicate [MnIII3MnIVO3(CH3COO)3(A-α-SiW9O34)]6- (Mn4POM) offers an unprecedented mimicry of the natural system in its reduced S0 state; it features a hybrid organic-inorganic coordination sphere and is anchored on a polyoxotungstate. Evidence for its photosynthetic properties when combined with [Ru(bpy)3]2+ and S2O82- is obtained by nanosecond laser flash photolysis; its S0→S1 transition within milliseconds and multiple-hole-accumulating properties were studied. Photocatalytic oxygen evolution is achieved in a buffered medium (pH 5) with a quantum efficiency of 1.7 %.