Rethinking the Excited-State Redox Properties of Iron(III) Complexes for LMCT Photoredox Catalysis

Abstract

The reduction potentials of electronically excited states are crucial input values for photoredox reaction design. Since they are not directly measurable, they are typically estimated from the corresponding ground-state potentials and excited-state energies. Here, we demonstrate that this commonly applied approach breaks down for low-spin d⁵ complexes of iron(III) with photoactive ligand-to-metal charge transfer (LMCT) excited states. Stern–Volmer luminescence quenching, photocatalytic experiments, and detailed thermodynamic analyses demonstrate that the true potentials for the oxidation of excited-state iron(III) complexes are up to 0.7 V lower than anticipated, resulting in a nearly 70 kJ/mol change in the driving forces of photoinduced electron transfer reactions. Our analysis further indicates that other complexes with LMCT-excited states and partially filled d-orbitals are likely to exhibit the same behavior, because LMCT-excited-state quenching removes the highest-energy electron from the t_2g orbital but results in a formally ligand-centered oxidation, whereas the first ground-state oxidation is typically metal-centered. These findings have significant implications for the use of the emerging class of complexes with photoactive LMCT-excited states as well as for the broader field of LMCT photoredox catalysis in synthetic chemistry.