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

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.