Electron-transfer sensitization of H2 oxidation and CO2 reduction catalysts using a single chromophore

Energy-storing artificial-photosynthetic systems for CO2 reduction must derive the reducing equivalents from a renewable source rather than from sacrificial donors. To this end, a homogeneous, integrated chromophore/two-catalyst system is described that is thermodynamically capable of photochemically driving the energy-storing reverse water-gas shift reaction (CO2 + H-2 -> CO + H2O), where the reducing equivalents are provided by renewable H-2. The system consists of the chromophore zinc tetraphenylporphyrin (ZnTPP), H-2 oxidation catalysts of the form [(CpCr)-Cr-R(CO)(3)](-), and CO2 reduction catalysts of the type Re(bpy-4,4'-R-2)(CO)(3)Cl. Using time-resolved spectroscopic methods, a comprehensive mechanistic and kinetic picture of the photoinitiated reactions of mixtures of these compounds has been developed. It has been found that absorption of a single photon by broadly absorbing ZnTPP sensitizes intercatalyst electron transfer to produce the substrate-active forms of each. The initial photochemical step is the heretofore unobserved reductive quenching of the low-energy T-1 state of ZnTPP. Under the experimental conditions, the catalytically competent state decays with a second-order half-life of similar to 15 mu s, which is of the right magnitude for substrate trapping of sensitized catalyst intermediates.