Photoactivation of Stable Bonds for Chemical Conversions

Harvard University, Cambridge MA

Investigators

Abstract

With the support of the Chemical Catalysis Program in the Division of Chemistry, Professor Daniel G. Nocera of Harvard University is studying the generation of highly energetic species that are both critical to storing energy in the form of fuels and relevant to the development of new synthetic methods for drug discovery. The light-driven rearrangement of bonds with low energy content to ones of high energy content lies at the heart of converting renewable energy to fuels and of efficiently harnessing it to drive catalytic processes. Accordingly, the funded research focuses on facilitating bond rearrangements at the metal center of a molecular complex with solar light or solar electricity as an energy input. A variety of modern analytical techniques will be deployed to understand the fundamental principles underpinning how the metal complexes of interest mediate light- and electro-driven bond rearrangements. The findings from this work are anticipated to provide the tools and knowledge necessary for the discovery of new sustainable chemical processes for the manufacture of fuels and other materials of relevance to society. Beyond the benefits of the basic science being pursued, which are expected to contribute to the economic competitiveness of the US, the broader impacts of the award are augmented by educational and outreach activities performed by Professor Nocera and his team of coworkers. The research program introduces a talented and diverse pool of students at Harvard to forefront problems in chemistry and provides a mechanism to educate these young people with the broad technical skill set needed for them to contribute to critical problems in their future independent scientific careers. In the realm of outreach, Professor Nocera continues to raise public awareness of the role that chemistry, and science in general, can play in addressing challenges in energy and catalysis through his many contributions across a variety of platforms including national news, TV, radio programs, movies, and other broadcast media. The Nocera laboratory has a successful track record of broadening the participation of individuals belonging to groups underrepresented in science and the plan is to continue this effort to build excellence through diversity in the team. Through an interplay of spectroscopic and synthetic studies, the funded research focuses on the creation of new reagents, reactions, and processes for the generation of high energy species via the activation of stable bonds by transition metal complexes in excited states. Three lines of inquiry are being pursued: (1) photoactivation of metal-halide bonds within sculpted secondary coordination spheres with the goal of enhancing the selectivity of C-H bond activation by halogen radicals; (2) generation of super-reducing/-oxidizing closed-shell photoreagents from radical intermediates [such closed-shell species derived from radicals can greatly expand the redox window of photoredox reagents, thus allowing for the development of new methods based on the generation of highly reactive intermediates], and; (3) expansion of the types of super-reducing/-oxidizing reagents that are accessible by exploiting the oxidizing holes (in the valence band) and the reducing electrons (of the conduction band) of semiconductor photocatalysts. Among the impactful transformations that are anticipated to emerge from this work are perfluoromethylations and perfluoroalkylations, as well as efficient new processes for the decomposition of environmentally harmful perfluorocarboxylic acids. In all photoconversion reactions (1)-(3), an intriguing possibility also of interest is to introduce photoelectrochemical methods to allow a closed catalytic cycle to be achieved as well as the use of solar light. The photoelectrochemical approach opens a potential pathway to sustainable chemical manufacturing using photoredox methods which are otherwise limited in their range of applications. 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.