Breaking Kasha’s Rule to Enable Higher Reactivity in Photoredox Catalysis
Björn Pfund, Oliver S. Wenger
Abstract
High Resolution Image Download MS PowerPoint Slide Nearly all photochemical transformations known to date follow Kasha’s rule, implying that reactions occur only from the lowest electronically excited state of a given spin multiplicity due to the fast relaxation of higher-energy states. We challenge this foundational principle by demonstrating with time-resolved laser spectroscopy that the 4,4″-dicyano- p -terphenyl radical anion can undergo photoinduced electron transfer directly from a higher-energy excited state, enabling reactivity inaccessible to the lowest excited state with the same spin multiplicity. Preassociation with the substrate and driving-force optimization are critical for overcoming the kinetic barrier to subnanosecond electron transfer, enabling bimolecular anti-Kasha reactivity. This advance establishes a general and broadly applicable framework for bypassing one of the most fundamental principles of photophysics and photochemistry, Kasha’s rule, and opens new possibilities in photoredox catalysis and solar energy conversion by rethinking the energetic and kinetic landscape.