Additional Insights into the Design of Cr(III) Phosphorescent Emitters Using 6‑Membered Chelate Ring Bis(imidazolyl) Didentate Ligands

The interest in Cr­(III) complexes has been renewed over the past decades for building practical guidelines in the design of efficient earth-abundant phosphorescent near-infrared emitters. In that context, we report the first family of homoleptic tri­(didentate) Cr­(III) complexes [CrL₃]³⁺ based on polyaromatic ligands inducing 6-membered chelate rings, namely, the bis­(1-methylimidazol-2-yl)­ketone (L = bik), bis­(1-methylimidazol-2-yl)­methane (L = bim), and bis­(1-methylimidazol-2-yl)­ethane (L = bie) ligands. The programmed close-to-perfect octahedral microsymmetry of {Cr^IIIN₆} chromophores found in [Cr­(bik)₃]­(OTf)₃ (1), [Cr­(bim)₃]­(OTf)₃ (2), and [Cr­(bie)₃]­(BF₄)₃ (3) ensures a ligand-field strength large enough to induce intense and long-lived Cr-based phosphorescence. Impressive excited-state lifetimes (5.0–8.2 ms) were obtained at low temperatures for the [Cr­(L)₃]³⁺ series. Additionally, the photoluminescent quantum yield climbs to 0.8% for compound 1 in deaerated solutions. Moreover, the photophysical features of the three homoleptic complexes are barely influenced by the presence of dioxygen presumably because of the poor overlap between the Cr-based phosphorescence spectra (ca. 14100 cm^–1) and the ¹Σ_g⁺ ← ³Σ_g^– transition in the absorption spectrum of dioxygen (13100 cm^–1). The multiredox electrochemical pattern of 1 is evidenced by cyclic voltammetry as well as its strong photooxidant behavior. The pH sensitivity of 2 and 3 luminescence is discussed, along with the reactivity of their β-diketiminate derivatives.