Building Electron Transfer Cascades into Amphiphlic Donor-Acceptor Assemblies

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In this project funded by the Macromolecular, Supramolecular and Nanochemistry Program in the Division of Chemistry, Professors Sarah Tolbert, Benjamin Schwartz, and Yves Rubin of the Department of Chemistry at the University of California, Los Angeles are developing complex molecules that will convert light into separated electrical charges that can drive useful chemical reactions. In natural systems, such as photosynthesis, the absorption of light energy can be translated efficiently into separated electrical charges. These charges can then be used to conduct a range of important chemical reactions, and the overall process is called photochemistry. Using modern synthetic chemistry, Professors Tolbert, Schwartz and Rubin are creating molecules that absorb light efficiently and then separate the electrical charges via a series of cascades across the molecules. Then, they are examining ways to use those separated charges to do useful chemistry. In the process, they are training a group of graduate students in interdisciplinary science. Those students are further working to share their knowledge of photochemistry and photochemical technologies with a broad range of middle and high school students in the greater Los Angeles area. Professors Tolbert, Schwartz and Rubin are studying ways to create and exploit artificial electron transfer cascades using strongly absorbing pi-conjugated polymers and molecules. The principal challenges in producing artificial electron transfer cascades are first to create well-defined donor/acceptor pairs and second to place additional acceptors with progressively lower LUMOs at precise positions so that their energy gradient leads to spatial separation of the initially created charge pair. The cascades are being constructed starting with amphiphilic, water-soluble pi-conjugated polymers that assemble into networks of electrically-interconnected micelles. The conjugated polyelectrolytes are then assembled with charged C60 derivatives or other pi-conjugated acceptors. The structure of the assembly and the energetics of both the polymer donor and the pi-conjugated electron acceptor are being tuned. The electron transfer cascades are being characterized using ultrafast transient absorption spectroscopy, and the structures are being correlated with the observed photochemical properties. Optimized assemblies are being adapted for use as photosensitizers in reduction reactions.