title: Electron-transfer sensitization of H2 oxidation and CO2 reduction catalysts using a single chromophore.
abstract: 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(R)Cr(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.
authors: Department of Chemistry, The University of Chicago, Chicago, IL 60637.
authors: Nathan T
authors: NT
authors: La Porte
authors: Department of Chemistry, The University of Chicago, Chicago, IL 60637.
authors: Davis B
authors: DB
authors: Moravec
authors: Department of Chemistry, The University of Chicago, Chicago, IL 60637 mhopkins@uchicago.edu.
authors: Michael D
authors: MD
authors: Hopkins
mesh_terms: D002245:Carbon Dioxide / Q000737:chemistry*; D002384:Catalysis; D004583:Electrons; D006859:Hydrogen / Q000737:chemistry*; D008665:Metalloporphyrins / Q000737:chemistry*; D010084:Oxidation-Reduction; D010777:Photochemistry; D013816:Thermodynamics
chemical_list: D008665:Metalloporphyrins; D002245:Carbon Dioxide; C048955:zinc tetraphenylporphyrin; D006859:Hydrogen
keywords: artificial photosynthesis; catalysis; dual catalysis; photocatalysis; photoredox