The Rehm–Weller Approach to Experimentally Determine the Ground-State Geometry Triplet Energy of a Co(III) Photosensitizer
(2026) Inorganic Chemistry — Vol. 65, n° 16, p. 9091-9106 (2026)
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- 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 (3 MC) 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 3 MC 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. ■ INTRODUCTION The development of photosensitizers based on earth-abundant elements has become a priority to enable technologies for solar energy conversion, chemical synthesis, and environmental remediation. 1−6 For decades, photosensitizers based on ruthenium(II) and iridium(III) complexes have dominated the field, 7−11 owing to their well-defined photophysical properties, long-lived excited states, high photostability, and tunable redox potentials. However, both Ru II and Ir III are classified as critical raw materials, characterized by extremely low natural abundance, limited geographic availability, and energy-intensive mining processes that contribute significantly to environmental degradation. Their scarcity not only drives high material costs but also poses a major barrier to large-scale deployment of light-driven technologies, such as solar fuel production or photoredox catalysis. The shift to earth-abundant elements requires overcoming intrinsic photo-physical limitations of lighter transition metals, such as faster nonradiative decay pathways, shorter excited-state lifetimes, and reduced ligand-field stabilization compared to heavier congeners. 2,12 Nevertheless, advances in ligand-field engineering , computational photochemistry, and time-resolved spec-troscopy are enabling the rational design of photosensitizers based on abundant 3d transition metals that display photo-physical properties previously thought to be exclusive to heavier elements. 2,13−17 Among these earth-abundant candidates, iron, 4,12,18−20 manganese, 21−25 and chromium have been extensively studied, 26,27 but cobalt has recently emerged as another alternative due to its favorable electronic structure, redox versatility, and relatively high crustal abundance. 28−39 Unfortunately , cobalt mining is still heavily concentrated in politically unstable regions, raising concerns over supply chain resilience and ethical sourcing that parallel those associated with precious metals. From a scientific viewpoint, cobalt's d 6 configuration makes it isoelectronic with Ru II , Ir III , and Fe II (Figure 1a), positioning it as a strong candidate for the development of sustainable photosensitizers. Cobalt-based photosensitizers have significantly evolved since the late 1970s when the octahedral complex [Co III (CN) 6 ] 3− was first spectroscopically characterized. 40−43 Early studies revealed radiative decay originating from a triplet metal-centered (3 MC) state, 40 establishing cobalt as a unique platform for exploring long-lived 3 MC excited states. 31,44 In recent years, similar to other earth-abundant metals, cobalt has received
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Thunissen, T., Troian-Gautier, L., & Glaser, F. (2026). The Rehm–Weller Approach to Experimentally Determine the Ground-State Geometry Triplet Energy of a Co(III) Photosensitizer. Inorganic Chemistry, 65(16), 9091-9106. https://doi.org/10.1021/acs.inorgchem.6c00585 (Original work published 2026)