Structural Control of Metal-Centered Excited States in Cobalt(III) Complexes via Bite Angle and π-π Interactions.

Co(III) complexes have recently become an important focus in photophysics and photoredox catalysis due to metal-centered excited states with strong oxidizing properties. Optimizing chelate ligand bite angles is a widely used strategy to strengthen metal-ligand interactions in coordination complexes, with the resulting enhanced ligand fields often contributing to extended excited-state lifetimes that are advantageous for photochemical applications. We demonstrate that bite-angle optimization exerts the opposite effect on Co(III) polypyridines compared to previously studied transition metal complexes, as polypyridine ligands function as π-donors to Co(III) rather than π-acceptors. Our findings reveal two counterintuitive paradigms: while bite-angle optimization weakens the ligand field in Co(III) complexes, the resulting lower-energy metal-centered excited states can be accompanied by extended excited-state lifetimes, driven by increased rigidification through intramolecular π-π interactions. These insights, along with additional experiments investigating the possibility of photoreactions from higher excited states, advance the current understanding of the photophysics and photochemistry of first-row transition metal complexes and highlight key distinctions from the more extensively studied photoactive complexes of second- and third-row transition metals.