Direct Evidence for Excited Ligand Field State-based Oxidative Photoredox Chemistry of a Cobalt(III) Polypyridyl Photosensitizer

• DOI: 10.1021/jacs.3c09374
• Publication Date: 2023-09-13T20:02:55Z
• Authors: Micheal M. Alowak...; Atanu Ghosh; James K. McCusker
• Keywords: Chromophore; Reactivity; Photoredox catalysis

Increasing interest in sustainable chemistry coupled with the quest to explore new reactivity has spurred research on first-row transition metal complexes for potential applications a variety of settings. One more active areas is photoredox catalysis, where synthetically tunable nature their electronic structures provides rich palette options tailoring desired chemical transformation. Understanding mechanism excited-state critical informed development next-generation catalysts, which turn requires information concerning propensity excited states engage electron or energy transfer processes. Herein we provide direct evidence highly oxidizing lowest-energy ligand-field (LF) state d6-low-spin Co(III) photosensitizer [Co(4,4′-Br2bpy)3]3+ (where 4,4′-Br2bpy 4,4′-dibromo-2,2′-bipyridine). The redox associated LF complex was bracketed by performing bimolecular quenching studies using series simple organic donors. Time-resolved absorption spectroscopy confirmed dynamic process attributed reductive chromophore. Analysis Stern–Volmer plots each chromophore-quencher pair revealed limiting value Ered* ∼ 1.25 V vs Fc/Fc+ metal-centered state, significantly stronger than that commonly employed metal-based agents such as [Ru(bpy)3]2+ (Ered* = 0.32 Fc/Fc+) and [Ir(ppy)2(bpy)]+ 0.27 Fc/Fc+). These results suggest this class chromophores could find utility requiring activation oxidatively resistant substrates catalysis.