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 [Cr L ₃ ] 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 ^III N ₆ } chromophores found in [Cr(bik) ₃ ](OTf) ₃ ( 1 ), [Cr(bim) ₃ ](OTf) ₃ ( 2 ), and [Cr(bie) ₃ ](BF ₄ ) 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 ¹ Σ _g ⁺ ← ³ Σ _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.