The Rehm-Weller Approach to Experimentally Determine the Ground-State Geometry Triplet Energy of a Co(III)...

As an isoelectronic metal center to ruthenium(II) and iron(II), cobalt(III) has recently gained some attention as a possible alternative to form first-row transition metal complexes with nanosecond to microsecond excited-state lifetimes. In the recently reported [Co^III(phtmeimb)₂]⁺, where phtmeimb stands for phenyl[tris(3-methylimidazolin-2-ylidene)]borate, the lowest excited state is a metal-centered triplet (³MC) and the Laporte-forbidden transition together with the absence of charge-transfer absorption band result in weak visible-light absorption. Herein, we experimentally determine the previously computationally estimated energy of the ³MC state at the ground-state geometry and provide clear indications that its population is potentially possible in a sensitized manner. To allow for more quantitative insights, a Rehm-Weller-type analysis was performed via Stern-Volmer quenching studies using 25 organic and inorganic photosensitizers. Photophysical characterization of all photosensitizers by cyclic voltammetry, steady-state and time-resolved absorption, and emission spectroscopy allowed us to clearly showcase the cobalt as potential energy acceptor. The triplet energy was estimated at 2.5 eV, indicating that visible-light-mediated photosensitization is feasible. This energy is significantly below the previously calculated energy of 3.0 eV. Additionally, for highly oxidizing photocatalysts, the cobalt complex was identified as an essentially colorless quencher for reductive electron transfer.