Artificial Photosynthesis: Progress Toward Molecular Systems for Photoconversion

A great deal of progress has been made in identifying the criteria necessary to design molecular systems for the efficient photochemical separation of charge over extended periods of time. Our own research in this area has focused on preparing supramolecular assemblies of electron donors and acceptors which fulfill the biomimetic criteria dictated by photosynthetic charge separation in Nature. First, it is important that the electron donors and acceptors occupy structurally well-defined spatial relationships relative to one another. A key consequence of studying well-defined structures is the simplification of mechanistic arguments that are used to describe electron transfer events in these systems. Second, the electronic coupling between the donors and acceptors must be controlled by the structure to insure that strong coupling results in rapid charge separation from the initial donor-acceptor state, and that weak coupling in the product ion pair state slows charge recombination. Considerations of electronic structure and orbital symmetry are necessary to achieve this goal. Third, the surrounding medium plays a critical role in determining the rates of electron transfer reactions. This is especially true when charges are created or annihilated in electron transfer reactions. Once again, Nature provides a high standard to be achieved. Photosynthetic reaction centers are fully competent in the solid state, even at very low temperatures, undergoing charge separation with unity quantum yield at temperatures < 4.2K. In order for our artificial photosynthetic systems to have general applicability as photocatalysts, it is highly desirable to have systems that are fully functional in the solid state.