CAS: Toward Molecular Control of Cage Escape Yields in Bimolecular Photochemistry

University Of North Carolina At Chapel Hill, Chapel Hill NC

Investigators

Abstract

With support from the Chemical Structure, Dynamics & Mechanisms B Program of the Chemistry Division, Professor Gerald Meyer of the Department of Chemistry at the University of North Carolina at Chapel Hill and his team are studying chemical reactions that are initiated with visible light with the aim of improving the efficiency of such transformations. The goal in most cases is to optimize the reactivity such that each absorbed photon produces one desired product. Downstream applications of the findings from this research may include solar energy conversion, as well as the photosynthesis of high-value organic compounds. The project lies at the interface of photochemistry and inorganic chemistry, with a focus on electron transfer reactivity that is well suited to the education of scientists at all levels. The Meyer research team endeavors to support the training of students underrepresented in science. Outreach activities involving K-12 students in the Research Triangle and rural North Carolina are being planned as part of the project. This project seeks to obtain mechanistic insight into photo-initiated electron transfer between a photosensitizer and a quencher that will allow optimization and a means to predict quantum yield. Focus will be placed on excited state electron transfer and charge recombination within the ‘encounter complex’ inherent to bimolecular redox reactions. Charge recombination within the encounter complex is known to significantly lower the yield of most photochemical reactions to values far below one, yet the origin(s) of such behavior remain largely unknown. Professor Gerald Meyer and his team propose systematic electron transfer studies that will exploit the charge-transfer excited states of vintage second- and third-row transition metal photosensitizers, as well as emerging earth abundant photosensitizers-based on cobalt, iron, zirconium, and copper. Electron donor and acceptor quenchers with tailored charge, size, and reduction potentials are expected to provide the insight into encounter complex structure that is necessary to understand the factors that control escape of desired products, i.e., the cage escape yield. A fundamental question to be addressed is whether the photosensitizer and/or the quencher structure can be tuned at the molecular level to impact the encounter complex and thereby the cage escape yield. Supramolecular assembly will be used to quantify the impact of non-covalent interactions within the encounter complex on electron transfer and cage escape. Variable temperature kinetic measurements are proposed to quantify the electronic coupling within the encounter complex and to determine the adiabaticity of the excited state electron transfer reaction. The proposed research is fundamental in nature yet are expected to impact approaches used in solar energy conversion and organic photoredox catalysis. In the long term, insights gained from these studies have the potential to provide a rational means for the molecular-level design of photosensitizer encounter complexes capable of efficiently harvesting solar photons, driving electron transfer reactions, and releasing the sought-after products. 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.