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

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 in a variety of settings. One of the more active areas of research is photoredox catalysis, where the synthetically tunable nature of their electronic structures provides a rich palette of options for tailoring their reactivity to a desired chemical transformation. Understanding the mechanism of excited-state reactivity is critical for the informed development of next-generation catalysts, which in turn requires information concerning the propensity of their electronic excited states to engage in the desired electron or energy transfer processes. Herein we provide direct evidence of the highly oxidizing nature of the lowest-energy ligand-field (LF) excited state of a first-row d(6)-low-spin Co(III) photosensitizer [Co(4,4'-Br(2)bpy)(3)](3+) (where 4,4'-Br(2)bpy is 4,4'-dibromo-2,2'-bipyridine). The redox potential associated with the LF excited state of the Co(III) complex was bracketed by performing bimolecular quenching studies by using a series of simple organic electron donors. Time-resolved absorption spectroscopy confirmed a dynamic quenching process attributed to reductive quenching of the lowest-energy ligand-field excited state of the Co(III) chromophore. Analysis of the Stern-Volmer plots for each chromophore-quencher pair revealed a limiting value of E-red * similar to 1.25 V vs Fc/Fc(+) for the metal-centered excited state, which is significantly stronger than that of more commonly employed transition metal-based photoredox agents such as [Ru(bpy)(3)](2+) (E-red * = 0.32 V vs Fc/Fc(+)) and [Ir(ppy)(2)(bpy)](+) (E-red * = 0.27 V vs Fc/Fc(+)). These results suggest that this class of chromophores could find utility in applications requiring the activation of oxidatively resistant organic substrates for photoredox catalysis.

Automated summary

This study provides direct evidence of the highly oxidizing nature of the lowest-energy ligand-field excited state of a first-row d(6)-low-spin Co(III) photosensitizer. The redox potential associated with the excited state was determined through quenching studies and time-resolved absorption spectroscopy. The results indicate that this class of chromophores has stronger excited-state properties compared to commonly used transition metal-based photoredox agents, making them suitable for activating oxidatively resistant organic substrates in photoredox catalysis.