Electron-transfer sensitization of H₂ oxidation and CO ₂ 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 + H₂ → CO + H₂O), where the reducing equivalents are provided by renewable H₂. The system consists of the chromophore zinc tetraphenylporphyrin (ZnTPP), H₂ oxidation catalysts of the form [Cp^RCr(CO)₃]^-, and CO₂ reduction catalysts of the type Re(bpy-4,4′-R₂)(CO) ₃Cl. 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 T₁ state of ZnTPP. Under the experimental conditions, the catalytically competent state decays with a second-order half-life of ∼15 μs, which is of the right magnitude for substrate trapping of sensitized catalyst intermediates.