Molecular Z-Scheme for H2 Production via Dual Photocatalytic Cycles

Natural photosynthesis uses an elaborate array of molecular structures in a multi-photon Z-scheme for the conversion of light energy into chemical bonds (i.e. solar fuels). Extensive research effort has been dedicated to achieving artificial photosynthesis but a fully molecular artificial Z-scheme for solar fuels production has yet to be realized. Here we show that upon excitation of both a molecular photocatalyst (PC) and an aryl alcohol (ROH) in the presence of a sacrificial electron donor and proton source, we achieve artificial photosynthesis of H2. Chemical, electrochemical, and spectroscopic data support a mechanism consisting of: 1) photoexcitation of PC, 2) reduction of excited state PC, 3) electron transfer from PC- to photoexcited ROH, and 4) generation of H2 from reduced ROH. The system is catalytic with respect to both PC and ROH and operates at a reaction overpotential of ~40 meV. This molecular Z-scheme circumvents the thermodynamic and overpotential constraints associated with reduction of weak acids in their ground-state and offers a new paradigm for the conversion of light energy into H2 bond energy.