Resolving Anomalous Cage Escape in Fe(III) Photosensitizers: A Marcus Theory Framework

Abstract

The cage escape yield (ΦCE), 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, ΦCE 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 ΦCE 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 ΦCE is dictated by the position of the geminate charge recombination on the Marcus parabola: as the driving force for charge recombination becomes more negative, ΦCE 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 ΦCE 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.