Design of Metal-to-Metal Charge-Transfer Chromophores for Visible-Light Activation of Oxygen-Evolving Mn Oxide Catalysts in a Polymer Film

Abstract

To construct photoresponsive unidirectional charge transfer units for the activation of oxygen-evolving manganese oxide (MnOx) catalyst, metal-oxide nanoclusters consisting of cerium (CeIII) or cobalt (CoII) ions and Keggin-type polyoxotungstate (PW12O403–) were synthesized in a polymer matrix as visible-light-absorbing chromophores. The utilization of the polymer matrix enabled the molecularly dispersed PW12O403– states and was advantageous to achieve product-separable energy conversion systems. The reaction of PW12O403– with Ce(NO3)3 or CoCl2 in the polymer matrix generated the new broad absorption tails extending from the UV to visible region, assignable to metal-to-metal charge transfer (MMCT) transitions of oxo-bridged binuclear WVI–O–CeIII and WVI–O–CoII units. Cyclic voltammetry analysis of the oxo-bridged binuclear units in the polymer membrane revealed that the CoIII/CoII couple had a 300 mV more positive redox potential than that of CeIV/CeIII and was capable of extracting electrons from MnOx ca...