Photocatalysis in a New Light: A Biohybrid Approach for Improved Reactivity with Tunable, Low-Energy Light Excitation

Abstract

Since the advent of photoredox catalysis, much thought has been devoted to the development of exciting reaction modalities and the catalysts which perform these reactions. Less thought has been placed into the specific aspects of light absorption as the key step in photocatalytic mechanisms. Natural photosynthetic systems drive the high-energy reactions of photosynthesis with efficient and broadband energy capture. They provide a blueprint toward optimizing these processes in synthetic systems. In photosynthesis, both light capture and reactivity have been optimized by separation into distinct sites. The dominant process by which absorbed sunlight is transferred between these sites is through resonance energy transfer, which is highly efficient over long distances. This work highlights that light capture and energy transfer are crucial steps for the design of highly efficient photocatalysts in the future. Chapter 1 describes the relevant structures in natural photosynthesis as inspiration for synthetic approaches, the different mechanisms of energy transfer, and examples of photocatalytic systems that harness such excitation transfer processes to improve performance. Chapter 2 reports the synthesis of a biohybrid photocatalyst inspired by the modular architecture of photosynthetic apparatus which conjugated a photosynthetic light harvesting protein to a transition metal photocatalyst. Spectroscopic investigation found that absorbed photoenergy was efficiently funneled from the light harvester to the photocatalyst. The utility of the biohybrid photocatalyst was demonstrated via an increase in yields for two test reactions, including enabled reactivity at red wavelengths where the photocatalyst alone does not absorb. Chapter 3 establishes the power of incorporating nature’s design into non-natural photoenzymatic catalysis, generalizing the approach to other systems and methodologies. Photoenzymes require high-intensity light to function because of the poor absorption properties of their photoactive intermediate. A conjugate composed of a covalently linked photoenzyme and light antennae separates light capture from catalysis. Spectroscopic characterization of the conjugate showed the presence of efficient energy transfer from the light-harvesting components to the photoenzyme. In the presence of energy transfer, a maximum ~4-fold increase in product yields was observed as well as enabled reactivity. Chapter 4 highlights spectroscopic exploration into emerging molecular catalyst species. Finally, Chapter 5 provides an outlook to the future possibilities of the topics presented herein.