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 rejuvenated 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 micro-symmetry of
{CrIIIN6} 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 temperature
for the [Cr(L)3]3+ series. Additionally, the photoluminescent quantum yield climbs to 0.8% for (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 (c.a. 14100 cm-1) and the ¹Sg+¬³Sg- 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 photo-oxidant behavior. The pH-sensitivity
of (2) and (3) luminescence is discussed along with the reactivity of their β-diketiminate derivatives.