SUPRAMOLECULAR STRUCTURES FOR PHOTOCHEMICAL ENERGY CONVERSION

SUPRAMOLECULAR STRUCTURES FOR PHOTOCHEMICAL ENERGY CONVERSION Supramolecular Structures for Photochemical Energy Conversion Supramolecular Structures for Photochemical Energy Conversion A sustainable society requires an abundant, inexpensive, environmentally clean, geographically widely distributed, and renewable source of energy. The only such source with the capacity to meet all of societys needs is solar energy. However, to be useful light must be converted to other forms of energy such as electricity and fuels. The most successful, largestscale, and most-tested solar conversion technology is natural photosynthesis. The goal of this research is to discover the fundamental knowledge that will lead to the realization of artificial photosynthesis: the use of the basic chemical and physical principles of natural photosynthesis for the design of synthetic solar energy harvesting systems. In previous years, this project has focused on the design and synthesis of organic molecular species that mimic light gathering and energy transfer in photosynthetic antenna systems, transfer of the resulting molecular excitation energy to artificial photosynthetic reaction centers, and highly efficient conversion of the excitation energy to electrochemical potential energy in the form of long-lived charge separation. This research, coupled with results from many other investigators around the world, has uncovered the design principles for construction of highly efficient, molecule-based artificial antenna-reaction center constructs. In the current proposal, the focus is on the conversion of the electrochemical potential produced by these systems into the types of energy useful to society: electricity and fuel. The project will investigate the synthesis, optical, electrical and photochemical properties of a new class of conducting polymers. The repeating units consist of porphyrins, or porphyrinfullerene dyad and triad artificial reaction centers. They are conveniently formed on transparent conducting glass substrates by electropolymerization. This work may lead to new materials for the construction of organic photovoltaic cells for solar energy conversion. A second thrust is the coupling of artificial reaction centers to catalysts for solar water splitting. If successful, the resulting systems will use sunlight to oxidize water to molecular oxygen, producing electrons with sufficient electrochemical potential to generate solar fuels by reducing protons to hydrogen gas. Three types of water oxidation catalysts will be examined. Two of these are based on inorganic nanomaterials: iridium oxide colloids and cobalt salts, which collaborators on the project have demonstrated to be viable water oxidation catalysts. The third, plastoquinol terminal oxidase (PTOX), is a little-studied iron-containing enzyme, found in some photosynthetic organisms, that catalyzes the O2/H2O half reaction. This enzyme will be studied with a goal of using it, in natural or modified form, as a catalyst for solar water oxidation or an oxygen reduction catalyst in fuel cells. The final aspect of the research is the investigation of the incorporation of photosynthetic carotenoid photoprotective function into artificial photosynthetic systems. Photosynthetic organisms have several systems that protect them from damage by high levels of solar radiation. In this project, we will investigate two of them, non-photochemical quenching (NPQ) and carotenoid quenching of chlorophyll triplet states, by abstracting the basic features into model systems and studying the models with a variety of time-resolved spectroscopic techniques. These three subprojects, taken together, will help establish the basis for solar fuel and electricity production using synthetic systems inspired by natural photosynthesis. Supramolecular Structures for Photochemical Energy Conversion. Supramolecular Structures for Photochemical Energy Conversion The aim of this project is to continue our work to design, prepare and study photochemically active synthetic molecular-level constructs useful in solar-to-fuel conversion processes. This project will both increase our understanding of the fundamental chemistry and physics of solar energy conversion in natural photosynthesis, and provide the essential fundamental knowledge necessary for the design of artificial photosynthetic constructs. Artificial photosynthetic constructs can perform more efficiently than photosynthesis because they will have been rationally designed with one objective to provide energy rich fuel to meet human needs. Their performance will not be compromised by the requirement to support living photosynthetic organisms. The molecules to be prepared for this study consist primarily of covalently linked synthetic chromophores for light absorption and electron/proton/energy donors and acceptors, which will be involved in storing the solar energy in energy rich chemical fuels. In artificial photosynthesis, the chemical linkages control the distances, orientations and electronic interactions among the components that lead to function. In natural photosynthesis, these interactions are mostly enforced by complex, non-covalent interactions involving the protein matrixes and membranes. The project focuses on two synergistic topics: Proton coupled electron transfer (PCET) in artificial photosynthesis. This section deals with the development and study of systems that transfer one or multiple protons as a consequence of an electron transfer reaction. The investigation will include photoinitiated PCET processes and spectroscopic measurements that could answer fundamental questions about the mechanism of this important class of reactions. Interactions involving carotenoids studied in artificial photosynthetic systems. In this section new systems designed to promote strong interactions between carotenoids and between carotenoids and tetrapyrroles will be studied. Detailed understanding of such interactions will provide important insight into the mechanism of photoprotection and energy management in general. Spectroscopic markers for strong interactions between carotenoid pigments and carotenoids and chlorophyll-like pigments will be identified and studied. These markers will be used to guide the design of control systems for routing energy flow in functioning artificial photosynthetic constructs. Supramolecular Structures for Photochemical Energy Conversion The goal of this project is the design, synthesis, and study of photochemically active molecular constructs inspired by nature and potentially useful for artificial photosynthetic solar energy conversion. The proposed research will also advance our unders