Bis-oxazoline derivatives as ancillary ligands for bis-cyclometalated iridium complexes

Organometallic iridium complexes with two cyclometalated ligands (ĈN) and one bis-oxazoline derived ancillary ligand (L^X), i.e. (ĈN)₂Ir(L^X), are reported. The ĈN ligands are 1-phenylpyrazoline (ppz), 2-(4,6-difluorophenyl)pyridine (F₂ppy), 2-phenylpyridine (ppy), 1-phenylisoquinoline (piq). The box ligand is (4S)-(+)-phenyl-α-[(4S)-phenyloxazolidin-2-ylidene]-2-oxazoline-2-acetonitrile. The emission of these complexes span across the visible and into the near-ultraviolet region of the electromagnetic spectrum with moderate to high photoluminescence quantum yields (Φ_PL = 0.45-1.0). These complexes were found to emit from a metal-ligand to ligand charge transfer (ML'LCT) state and have lifetimes (1.3-2.1 μs), radiative rates (10⁵ s^-1), and nonradiative rates (10⁴-10⁵ s^-1) comparable to state-of-the-art iridium emitters. The (ppy)₂Ir(BOX-CN) complexes were resolved into the Δ- and Λ- diastereomers using differences in their solubility and additionally characterized by x-ray crystallography, stability, and chiroptic studies. The high Φ_PL of these isomers results in the best to date brightness for circularly polarized luminescence (CPL) from iridium complexes (7.0 M^-1 cm^-1), with dissymmetry factors of -0.57 × 10^-3 and +1.9 × 10^-3 for 3 and 3, respectively. The significant difference in CPL magnitude between 3 and 3 likely arises from interligand interactions (edge-to-face arrangement versus strong π-π interaction) for the pendant phenyl ring of the BOX-CN ligand which differ for the two isomers.