Decoupling Redox Potentials and Excited State Energies in Substituted Chromium(III) Chromophores

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.