Synthetic Control over Ligand-Field Photophysics and Photooxidation Potentials in Cr(III) Chromophores

The ambiguous design rules for controlling light absorption, photoluminescence (PL) emission, and excited-state decay in pseudo-octahedral Cr(III) chromophores pose key challenges for their implementation as photosensitizers in photocatalysis and optoelectronic applications. Tailored metal-ligand interactions ultimately controlling the excited-state decay in these open-shell coordination complexes remain underdeveloped. Here, we evaluate the influence of 1,10-phenanthroline (phen) ligand substitution on the static and dynamic photophysical properties of six distinct homoleptic [Cr(phen)3]3+ chromophores in CH3CN solutions. Installing electron-donating methyl or methoxy groups tunes the excited-state landscape, thereby modulating the photoluminescence lifetime and ultrafast transient absorption dynamics. For example, [Cr(tmp)3]3+ (tmp = 3,4,7,8-tetramethyl-1,10-phenanthroline) exhibits a ligand-field excited doublet state lifetime of 695 μs, which represents a 3-fold increase over [Cr(phen)3]3+, 199 μs in CH3CN. The photoluminescence quantum yield (ΦPL) is also enhanced 5-fold in the former, increasing from 0.66% to 3.4%. These facile changes in ligand substituents markedly influence excited-state reduction potentials ranging from 1.00 to 1.44 V vs SCE across the series. Long-lived 2E/2T1 excited states drive efficient singlet oxygen sensitization (ΦΔ = 0.57 to 0.95) that match and surpass the reactivity of [Ru(bpy)3]2+ under identical conditions. These results suggest that diimine ligand functionalization represents a simple design strategy for tuning the photophysical and photoredox properties of earth-abundant Cr(III) LF chromophores.