Using Substituted Cyclometalated Quinoxaline Ligands To Finely Tune the Luminescence Properties of Iridium(III) Complexes

The syntheses of five new heteroleptic iridium complexes [Ir­(L1–4)2(Diobpy)]­PF6 (where Diobpy = 4,4′-dioctylamido-2,2′-bipyridine)
and [Ir­(L3)2(bpy)]­PF6 (where L = para-substituted 2,3-diphenylquinoxaline
cyclometalating ligands; bpy = 2,2′-bipyridine) are described.
The structures of [Ir­(L3)2(Diobpy)]­PF6 and [Ir­(L3)2(bpy)]­PF6 show that the complexes each adopt a distorted octahedral
geometry with the expected trans-N, cis-C arrangement of the cyclometalated ligands. Electrochemical studies
confirmed subtle perturbation of the IrIII/IV redox couple
as a function of ligand variation. Luminescence studies showed the
significant contribution of 3MLCT to the phosphorescent
character with predictable and modestly tunable emission wavelengths
between 618 and 636 nm. DFT studies provided approximate qualitative
descriptions of the HOMO {located over the Ir­(5d) center (11–42%)
and the phenylquinoxaline ligand (54–87%)} and LUMO {located
over the ancillary bipyridine ligands (ca. 93%)} energy levels of
the five complexes, confirming significant MLCT character. TD-DFT
calculations indicate that UV–vis absorption and subsequent
emission has substantial MLCT character, mixed with LLCT. Predicted
absorption and emission wavelengths are in good general agreement
with the UV–vis and luminescence experiments.