Synthesis and photophysical studies on cyclometalated iridium complexes containing various monodentate phenyl-azole ancillary ligands

In this study, five cyclometalated iridium complexes containing phenyl-azole ancillary ligands were synthesized and characterized by X-ray crystallography, and their photophysical properties were studied. The reaction of [Ir(ppy)2(μ-Cl)]2 (ppy: 2-phenylpyridine) with pim (4-phenylimidazole), ptr4h (4-phenyl-4H-1,2,4-triazole), and ptr1h (1-phenyl-1H-1,2,4-triazole) ligands furnished mononuclear [Ir(ppy)2ClL] complexes (1–3; L = pim, ptr4h, ptr1h) featuring monodentate neutral N → Ir coordination. In contrast, the corresponding reaction with ptz (5-phenyl-1H-tetrazole) ligand yielded a hetero-bridged dimeric iridium complex, [Ir(ppy)2(μ-Cl)(μ-ptz)Ir(ppy)2] (4). Notably, adding dimethylsulfoxide (DMSO) solvent to complex 4 resulted in its division to [Ir(ppy)2(ptz)(DMSO)] (5) and [Ir(ppy)2Cl(DMSO)]. Complexes 1–4 exhibited blue-green phosphorescence in solution with marginal shifts in the emission wavelengths. Complex 1, having a pim ligand, exhibited a significantly higher phosphorescence quantum efficiency (ΦPL = 31%) than those of complexes 2–4 (ΦPL = 4–9%), which was attributed to the relatively small non-radiative decay rate constant in 1. Density functional theory calculations confirmed that the phosphorescence has originated from both the metal–ligand charge transfer (3MLppyCT) and the ligand-centered (3LCppy) states. An increased LC character was observed in the phosphorescence of the dinuclear complex 4.