Molecular Orbital Engineering of Mixed-Addenda Polyoxometalates Boosts Light-Driven Hydrogen Evolution Activity

Inspired by the principle of molecular orbital engineering, two structurally well-defined polyoxometalate (POM)-based hydrogen-evolving catalysts, namely, [H2N(CH3)(2)](9.24)Na3H4[Cu2.06W1.94O2(P2W16O60)(2)]<middle dot>40H(2)O (POM-1) and [H2N(CH3)(2)](12.6)Na2H3[Cu2.4Mo6.48W3.12O26(P2W12O48)(2)]<middle dot>27H(2)O (POM-2), have been successfully synthesized and systematically characterized. Both POM compounds exhibited similar twin-Dawson-type polyoxoanion structures in which the monomer was connected through two mu(2)-O atoms bonded to the disordered Cu centers, as revealed by single-crystal X-ray diffraction analyses. Electronic structure analyses confirmed that the introduction of mix-addenda Mo atoms could readily adjust the lowest unoccupied molecular orbital (LUMO) energy level of POM-2, leading to a more negative lowest unoccupied molecular orbital (LUMO) position compared with that of POM-1. Various spectroscopic and theoretical studies confirmed that the molecular orbital engineering modulation of POM-2 could provide a higher driving force for thermodynamically favorable and efficient electron transfer from the photosensitizer to POM-2. In a three-component photocatalytic system, POM-2 exhibited the most efficient photocatalytic hydrogen evolution activity compared to all reported similar catalytic systems, achieving a catalytic turnover number (TON) of 5362 after 6 h of photocatalysis under visible-light irradiation.