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

The syntheses of five new heteroleptic iridium complexes [Ir-(LI-4)(2)(Diobpy)]PF6 (where Diobpy = 4,4'-dioctylamido-2,2'-bipyridine) and [Ir(L-3)(2)(bpy)]PF6 (where L = para-substituted 2,3-diphenylquinoxaline cyclo-metalating ligands; bpy = 2,2'-bipyridine) are described. The structures of [Ir(L-3)(2)(Diobpy)]PF6 and [Ir(L-3)(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 Ir-III/IV redox couple as a function of ligand variation. Luminescence studies showed the significant contribution of (MLCT)-M-3 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.