Decoupling redox potentials and excited state energies in substituted chromium(III) chromophores

Abstract

Redox and excited state properties of classical organic and precious metal charge-transfer (CT) photocatalysts can be tuned by substituent effects—however, not independently, as redox and chromophore units are strongly entangled. In contrast, substituent effects should selectively address electrochemical properties of photoactive complexes based on metal-centered spin-flip (SF) excited states lacking CT character without compromising excited state properties. Yet, detailed structure-activity relationships for SF chromophores are lacking. We demonstrate that the redox potentials of polypyridine chromium(III) complexes, that feature purely metal-centered SF states, can be selectively tuned by incremental substituent effects, while the excited state energies and lifetimes remain unaffected. This delivers a unique series of chromophores with identical optical properties, but redox potentials tuned by increments. We believe, that such precisely designed chromophore series provide a unique tool for the systematic investigation of photoredox reactions and recombination processes relevant to photoredox catalytic cycles.