/ Ronny Hardegger
Breaking Rules in Photochemistry: Unlocking Hidden Light Energy
When molecules absorb light, they usually “jump” to a higher energy level only to quickly “climb” down to the lowest one before reacting. This behavior is known as Kasha’s Rule. Researchers from Basel now show how reactions can take place directly from these higher, short-lived levels, making better use of light energy and opening new possibilities for photochemistry. (Illustration: University of Basel, D. Gejsnæs-Schaad)
Traditional synthetic chemistry often relies on multi-step procedures and thermal activation, making it both time consuming and energy intensive. Photochemistry offers a compelling alternative: by using light as a direct energy source, reactions can proceed under milder conditions and with fewer steps. However, a long-standing assumption in the field is that photochemical reactions occur primarily from the lowest excited states, as higher-energy states are believed to deactivate too fast to react. This view has significantly limited the potential of light-driven synthesis, until now. Dr. Björn Pfund from the research group of Prof. Dr. Oliver Wenger at the University of Basel, has shown that even short-lived, higher-energy excited states can be harnessed for chemical reactions. In a new open-access article published in the Journal of the American Chemical Society, the team shows that these so-called “anti-Kasha” pathways can, in fact, lead to productive photochemical transformations. This work challenges a foundational assumption in photochemistry and opens new avenues for designing more efficient and selective light-driven reactions.
For decades, one of the cornerstones of photochemistry, Kasha’s Rule, has shaped our understanding of light-driven reactions. It states that reactions and light emission typically occur exclusively from the lowest electronically excited state of a given spin multiplicity, as higher-energy states lose their excess energy often within just a few femtoseconds (quadrillionths of a second). In solution, this was thought to make reactions impossible from these higher states: molecules simply can’t find each other fast enough, since they move randomly and slowly compared to the ultrafast energy loss. The new study demonstrates that this obstacle can be overcome. In certain cases, the reacting molecules weakly associate in solution before absorbing light, a phenomenon known as preassociation. In simple terms, the molecules “shake hands” and stay near to each other ready to react. Because they are already in position, the electron transfer can occur almost instantly after light absorption, fast enough to compete with the rapid energy loss that normally enforces Kasha’s Rule.
Proving Higher-Energy States Can React
Using advanced time-resolved spectroscopic methods, the study compared two excited states of the molecule 4,4″-dicyano-p-terphenyl radical anion (DCT•−). In a pump–pump–probe experiment, a first laser pulse at 355 nm (UV light) generated the active photocatalyst, which then associated with the substrate through weak intermolecular interactions. A second pulse selectively excited the photocatalyst (with substrate still in close proximity) either to its lower or higher excited state. A probe pulse monitored the subsequent spectroscopic changes in real time. The results showed that electron transfer to molecules such as chlorobenzene occurred only when the higher excited state was populated, while no reaction was detected from the lowest excited state. These findings demonstrate that even short-lived higher-energy states can drive chemical reactions in solution under the right conditions.
Why It Matters
Being able to drive reactions from higher-energy excited states expands the possibilities of modern photochemistry. Harnessing these extreme short-lived states could lead to more efficient and selective photochemical processes with applications ranging from solar fuel production to greener synthetic methods or pollutant degradation. The implications are wide-ranging, as this work demonstrates the potential of bypassing one of the most fundamental principles of photophysics and photochemistry – Kasha’s Rule.
Original publication
Björn Pfund and Oliver S. Wenger
Breaking Kasha’s Rule to Enable Higher Reactivity in Photoredox Catalysis
J. Am. Chem. Soc.2025, DOI: https://doi.org/10.1021/jacs.5c06115
Further Information
Website Wenger research group
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