Collaborative Research: RUI: Coupled Proton/Electron Transfer Reactions of Excited States

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In this project, funded by the Chemical Structure, Dynamics and Mechanisms-B Program, Professors Russell Schmehl of the Department of Chemistry at Tulane University and Jared Paul of the Villanova University Chemistry Department are developing new transition metal complexes that serve as light absorbers capable of transferring both protons and electrons to other molecules. The metal complexes under study are important as model systems which may help improve the efficiency of solar energy converters (solar cells). The project is of direct significance to the design of solar fuels such as water oxidation to oxygen, water reduction to hydrogen and carbon dioxide reduction to hydrogenated products. The investigators are committed to educating the public on issues relating energy and the environment. As a part of this work, Dr. Paul is establishing a Villanova center for understanding and communicating issues relating to energy and the environment and has developed an undergraduate course on sustainability. Tulane has regular science cafes on topics related to energy. Both investigators are committed to bringing energy-related science topics to children, particularly middle school aged students. The research will involve long-term projects at the Ph.D. level at Tulane as well as undergraduate and master's level research at both universities. In a collaborative venture between Villanova University and Tulane University, chemists plan to examine optically-triggered proton transfer, electron transfer, and proton-coupled electron transfer (PCET) between the excited states of a series of transition metal complexes of copper(I), ruthenium(II) and osmium(II) having one hydroxylated bipyridine (or phenanthroline) ligand and a variety of electron and/or proton acceptors. The ability to independently examine the free energy dependence of the kinetics of each of these reactions provides motivation for the research. Since the reactions are initiated by pulsed laser excitation, it is possible to track the temporal evolution of the processes and distinguish true PCET from reactions in which, perhaps, electron transfer precedes proton transfer. Preliminary work has illustrated that the optical spectra differ for the complex, the singly deprotonated complex and the oxidized and deprotonated species in a series of ruthenium(II) hydroxybipyridine complexes. Nanosecond pulsed laser transient absorption spectroscopy can be used to clearly distinguish systems that exhibit solely proton or solely electron transfer from others exhibiting electron transfer followed by proton transfer and those that react by coupled proton and electron transfer.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.