Metal-Centered Photoredox Catalysis using d6 Transition Metal-based Chromophores.

Controlling excited-state reactivity in transition metal chromophores is central to light-driven chemistry, with applications spanning from solar energy conversion to photoredox catalysis. Ru(II) and Ir(III) chromophores currently dominate applications within this field due to their long-lived charge-transfer (CT) excited states, which support bimolecular electron transfer chemistry. However, their elemental scarcity and relative lack of selectivity for differentiating oxidative versus reductive pathways has in part motivated the exploration of alternative platforms that could, in principle, offer distinct mechanistic opportunities. Earth-abundant, valence-isoelectronic 3d6 complexes of Fe(II) and Co(III) have attracted significant attention due to their electronic similarity to Ru(II) and Ir(III), suggesting the potential for complementary photophysical behavior. In contrast to their heavier congeners, however, the weaker ligand fields of first-row transition metals promote rapid relaxation from initially populated CT states to low-lying metal-centered (MC) excited states, typically of 5T2 or 3T1 character. Because these MC states involve redistribution of electron density within MC d-orbitals rather than charge separation between metal and ligand orbitals, they were historically considered poor candidates for use in photochemical transformations. In this Account, we summarize our and others' recent work that challenges this paradigm by establishing MC excited states in Fe(II) and Co(III) complexes as distinct and mechanistically powerful platforms for productive photochemistry. We begin by outlining how the nature of the reactive MC excited state can be identified and how its spin-state character impacts photochemical reactivity. This is important because MC excited states are defined by complex potential energy landscapes in which multiple competing pathways are present and can be profoundly different with regard to both spin state and equilibrium geometry. For Fe(II) complexes, 5T2 excited states are unreactive in photoredox processes due to substantial reorganization energy and limited access to low-spin Fe(III) products. In contrast, 3T1 excited states of Fe(II) and Co(III) enable productive electron-transfer pathways by minimizing spin and structural reorganization, providing access to Fe(III) and Co(II) products under favorable conditions. These insights establish key design principles for tuning reactivity through ligand field strength and excited-state electronic structure. Together, these findings establish MC excited states as a fundamentally distinct platform for photoredox catalysis with advantages - including enhanced selectivity - that can be leveraged in ways difficult, if not impossible, to realize in conventional CT-based systems. More broadly, this work has allowed for the development of design rules for exploiting MC excited states of earth-abundant, first-row transition-metal complexes, presenting new opportunities in photoredox catalysis by controlling spin state, ligand field strength, and structural reorganization associated with excited-state electron transfer reactions.