Exploring the Role of the Nephelauxetic Effect in Circularly Polarized Luminescence of Chiral Chromium(III) Complexes

A novel chiral chromium(III) molecular ruby [Cr(qpp)₂]³⁺ (qpp = N-methyl-N-(pyridin-2-yl)-6-(quinolin-8-yl)pyridin-2-amine) has been synthesized, enantiomerically resolved, and fully characterized. The circularly polarized luminescence (CPL) spectra revealed two emission bands of opposite polarization in the near-infrared region (700-800 nm), corresponding to the metal-centered transitions Cr(²T(1) → ⁴A₂) and Cr(²E(1) → ⁴A₂). Notably, the dissymmetry factor g_lum reached 0.11 for the former transition, which is among the highest reported for chromium(III) systems. Comparison with structurally related homo- and heteroleptic chromium(III) complexes underscores the important role of the nephelauxetic effect in tuning CPL properties. Increased metal-ligand covalency, indicative of a stronger nephelauxetic effect, enhances orbital mixing and modifies the electronic character of the emissive states. These changes influence both electric and magnetic transition dipole moments, leading to noticeable variations in dissymmetry factor g_lum. Altogether, these observations highlight the potential of fine-tuning metal-ligand covalency as a rational strategy for optimizing the chiroptical properties of chromium(III) complexes, with promising implications for bioimaging, molecular probes, and circularly polarized optoelectronic devices.