Photochemistry is the branch of science concerned with light and its ability to cause chemical changes. Excitation by light can enable chemicals to react in ways they cannot react under non-photochemical conditions. This is because light changes the electronic structures of chemicals by taking them from their normal (“resting” or “ground”) states to higher-energy states. The way a chemical reacts is dictated by its electronic structure; modifying electronic structure means rethinking the types of changes a molecule will undergo. Photochemistry is studied in many disciplines, from photosynthesis studied in the life sciences to fluorescent dyes developed by materials chemists for television.

Synthetic organic chemists – chemists who create specific carbon-based molecules like the active agents in medications – can use light to accomplish steps on the way to target molecules. Since light excitation dramatically changes the way a molecule reacts, using photochemical transformations can dramatically cut down the number of steps required to synthesize a molecule. Photochemistry is also appealing as a potentially green methodology because it is safe and it produces little to no waste.

There has recently been a surge in interest among synthetic chemists surrounding a branch of photochemistry called photoredox catalysis: that is, chemistry that allows the transfer of single electrons to proceed using light, as mediated by a substance that is not consumed in the reaction. In 2008 and 2009, David MacMillan at Princeton, Tehshik Yoon at the University of Wisconsin-Madison, and Corey Stephenson at the University of Michigan independently published novel synthetic methodology papers wherein low-energy visible light was used to catalyze photoredox reactions.¹ The field proliferated in the decade that followed. For example, one notable innovation that followed was the pairing of photoredox catalysis with catalysis using nickel, which allows certain uncooperative carbon centers to form new bonds; Gary Molander at Penn, Abigail Doyle (in collaboration with David MacMillan) at UCLA, and Dan Weix at the University of Wisconsin-Madison have all published in this area.

One of the greatest challenges to this field of chemistry is the expense and availability of photoredox catalysts. Typically, these catalysts rely on the use of precious metals, especially iridium and ruthenium. Catalyst production by chemical suppliers has increased as research in the field has raised demand for them, so they have become commercially available. They remain expensive, however, because precious metals can be difficult to procure and are a limited resource. This is a barrier both to adoption by industry and to experimentation by new researchers. In addition to continuing to improve the cost-effectiveness of generating iridium and ruthenium catalysts, there are also efforts to employ photocatalysts that are not based on precious metals. Photochemistry utilizing organocatalysts (nonmetallic catalysts based on carbon) is has gained attention among researchers in recent years as a possible route to diminish costs and improve sustainability.²

An additional challenge that emerges when photochemical research is implemented in industry is the scaling-up of reactions for mass chemical production. Light cannot penetrate uniformly through large flasks. The use of photochemical flow reactors (where a reaction mixture is exposed to light as it travels through a tube) instead of batch reactors (where a reaction mixture is exposed to light as it stirs in a vessel) is an open area of research that provides a solution to this issue.³

While photochemistry for organic synthesis is not a new field, many branches of photocatalysis have emerged only over the last decade. As scientific knowledge about this methodology continues to expand and its current issues are ameliorated, the field will surely encounter new obstacles. Just as surely, progress in this promising area will continue to meet new heights, and the search for solutions will continue to spark new discoveries.

REFERENCES

1. The Brilliant History of Photoredox Catalysis. Nature 2019. https://www.nature.com/articles/d42473-019-00033-7

2. Duvadie, R.; Pomberger, A.; Mo, Y.; Altinoglu, E. I.; Hsieh, H.-W.; Nandiwale, K. Y.; Schultz, V. L.; Jensen, K. F.; Robinson, R. I. Photoredox Iridium–Nickel Dual Catalyzed Cross-Electrophile Coupling: From a Batch to a Continuous Stirred-Tank Reactor via an Automated Segmented Flow Reactor. Org. Process Res. Dev. 2021, 25 (10), 2323–2330. https://doi.org/10.1021/acs.oprd.1c00251.

3. Yao, W.; Bazan-Bergamino, E. A.; Ngai, M.-Y. Asymmetric Photocatalysis Enabled by Chiral Organocatalysts. ChemCatChem 2022, 14 (1), e202101292. https://doi.org/10.1002/cctc.202101292.