Photo-driven Molecular Systems for CO2 Reduction

Photosynthesis uses assemblies of organized chromophores and catalysts within proteins to optimize the environment necessary to carry out solar energy conversion. Artificial photosynthesis for solar fuels production on a practical scale must capture light energy, carry out charge separation, and furnish these charges to catalytic sites where multi-electron redox reactions occur. Many aspects of this complex problem have received wide attention, yet researchers have not developed a complete system having fully-integrated components to accomplish this goal. The design and synthesis of complex, covalent molecular systems comprising chromophores, electron donors, and electron acceptors, which mimic both the light-harvesting and the charge separation functions of photosynthetic proteins, has been demonstrated. These synthetic systems have been used to study electron transfer rate constant dependence on donor-acceptor distance and geometry, electronic interactions, and the free energies of reaction. Controlling both the distance and orientation between the electron donors and acceptors provides the most useful approach to obtaining this information. Bio-inspired systems for photochemical solar energy conversion must be robust, easy to synthesize, and capable of absorbing the entire solar spectrum transmitted through the Earth’s atmosphere. Multi-component donor-acceptor arrays, a strategy used in photosynthetic reaction center proteins, are an excellent approach to producing long-lived charge-separated states. In order to determine these mechanisms, it is important to be able to identify in an unambiguous fashion both the short- and long-lived intermediates produced by these photo-initiated electron transfer events. Time-resolved optical and electron paramagnetic resonance spectroscopy are the principal techniques that can yield this information.