A New Dial for Controlling Light-Driven Chemistry
RASEI Fellow Niels Damrauer (University of Colorado Boulder) and a multidisciplinary team initiated by Professor Zach Wickens at the University of Wisconsin Madison just found a new way to control which molecules react in light-driven chemistry. The key to unlocking this new pathway? Focus on what happens after the reaction starts, not before.
SET: A powerful reaction for building organic molecules, such as medicines and materials
This type of reaction, called single electron transfer (SET), is one of the most useful tools in modern chemistry. It can be initiated by swapping in light or electricity instead of heat, making it more energy efficient and less prone to generating waste products. But there have always been some limitations with SET. Some molecules just don’t want to accept an electron, particularly when there are other types of molecules nearby that do the reaction faster, meaning that they’ve been locked out of this chemistry, until now. The new study, published in Nature with collaborators Zachary Wickens from the University of Wisconsin-Madison and Robert Paton at Colorado State University, opens the door to reactions that were previously impossible.
How Single Electron Transfer Works
One molecule, commonly after absorbing light, transfers an electron to another, triggering a reaction. Researchers have previously controlled the outcome of the reaction by picking molecules based on how ‘eager’ they are to grab, or accept, that electron. The molecule most ready to accept the electron wins. This approach has produced a wide range of really impressive chemistry, but it leaves out any molecule that isn’t ‘interested’ in the electron to begin with.
Identifying an opportunity
Every SET reaction actually has two steps, not one. First, an electron is transferred from a donor molecule to an acceptor molecule. Then, it has an option, it can either go forward to trigger a reaction, or it can go backward, undoing the first step. It is reversible. Chemists have always focused on controlling the first electron transfer. This team asked what would happen if they found a way to control the second electron transfer instead.
An extreme electron donor
To test this idea the researchers built what they call a “super-potent photoreductant”. It’s a light-activated catalyst that hands out electrons to almost every nearby molecule, good at accepting or not. In this particular research, the reductant is a solvated electron released from a photoexcited catalyst. Damrauer and his student Arindam Sau used time resolved spectroscopies to disentangle mechanism and identify conditions, such as the wavelength for photoexcitation, where catalysts could function. Damrauer notes, “By analyzing time-resolved signals in our experiment, my student Arindam was able to find colors for photoexcitation where the electron, the reductant, would be released deeper into the solvent, thereby affording it time to do its work in reducing the substrate molecules in solution.” But shouldn’t this reducing potency wipe out selectivity entirely? If every molecule gets an electron, which one ‘wins’?
RASEI Fellow Niels Damrauer at the graduation of one of the lead researchers Arindam Sau (Boulder, CO, 2026).
Reversibility is the key
The opportunity comes down to the second electron transfer. If a molecule accepts an electron, but doesn’t react quickly, the electron slips back to the catalyst and molecule floats away unchanged. If the molecule does react quickly, in a non-reversible way, the electron doesn’t go back to the catalyst and the reaction moves forward. Selectivity now depends on how reversible the electron transfer is, not how easy the initial electron transfer was.
Building a model reaction to test the hypothesis
The team tested this using a mixture of two molecules side by side: One that accepts electrons readily, but also gives them back easily, and one that ‘reluctantly’ accepts electrons, but then holds on tight once it does through a structural change. Under the old rules, the ‘eager’ electron acceptor would win. Under the new rules the ‘reluctant’ one wins, moving forward along the reaction pathway, leading to the formation of a new product that would not have been otherwise possible. Through a series of experimental explorative and optimization reactions the team were able to confirm that this selectivity preference held across a broad range of related molecules, not just the one test case.
No molecules left behind
With this works chemists now have a new dial to turn. Molecules that were previously incompatible with this efficient, light-driven chemistry are now available. This opens new pathways for building medicines and advanced materials, using less energy and creating less waste along the way.