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Abstract
The cage escape yield (Φ CE), i.e., the separation of the geminate radical pair formed upon bimolecular excited-state electron transfer, is a key parameter that governs the efficiency of photoredox transformations. For Fe(III) photosensitizers featuring doublet ligand-to-metal charge transfer excited states, Φ CE are notoriously small in polar solvents such as acetonitrile yet increase markedly in dichloromethane, an empirical observation that has long remained unexplained. Herein, excited-state quenching and Φ CE were quantified for the reaction between the prototypical [Fe(phtmeimb) 2 ] + photosensitizer and a curated series of 17 nitrogen-based aromatic electron donors whose one-electron oxidation potentials span a potential window of ∼0.8 eV. We show that Φ CE is dictated by the position of the geminate charge recombination on the Marcus parabola: as the driving force for charge recombination becomes more negative, Φ CE increases from below 5% to 63% in acetonitrile. Because dichloromethane exhibits a smaller reorganization energy than acetonitrile, its inverted region is reached at less negative driving forces, which quantitatively accounts for the systematically larger Φ CE measured in this solvent, even reaching unity. The complete data sets are reproduced by a single nonadiabatic Marcus model, providing a predictive framework for cage escape based solely on driving force and reorganization energy. These results establish that cage escape in open-shell Fe(III) photosensitizers obeys Marcus theory and deliver rational design guidelines for earth-abundant photoredox catalysis.
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Lempereur, S., Glaser, F., Olivier, Y., Elias, B., & Troian-Gautier, L. (2026). Resolving Anomalous Cage Escape in Fe(III) Photosensitizers: A Marcus Theory Framework. Journal of the American Chemical Society, 148(40), 43847-43853. https://doi.org/10.1021/jacs.6c16361 (Original work published 2026)