Functionalized phenylimidazole-based facial-homoleptic iridium(iii) complexes and their excellent performance in blue phosphorescent organic light-emitting diodes

Abstract

Herein, we present the preparation of two functionalized phenylimidazole-based facial-homoleptic iridium complexes, FIr (fac-tris(1-(2,6-diisopropylphenyl)-2-(4-fluorophenyl)-1H-imidazolyl)iridium(III)) and CNIr (fac-tris(1-(2,6-diisopropylphenyl)-2-(3-cyanophenyl)-1H-imidazolyl)iridium(III)), and their full characterisation by multinuclear NMR spectroscopy and elemental analysis. Investigation of the molecular structures of the two iridium complexes by single-crystal X-ray diffraction revealed the facial-isomerism. Both complexes showed a definite blue-emission band (λ_em = 454 nm for FIr and 462 nm for CNIr) in solution and film states at ambient temperature. Moreover, the complexes exhibited significantly high phosphorescent quantum efficiencies in a mCP (1,3-bis(N-carbazolyl)benzene) film doped with 5 wt% system (0.87 for FIr and 0.99 for CNIr). Thermogravimetric analysis of the complexes displayed their high thermal stabilities as T_d5 values (5 wt% loss) above 390 °C. The frontier energy levels of both complexes calculated from the oxidation and reduction potential were measured to be from approximately −6.1 eV (HOMO) to approximately −2.8 eV (LUMO), which indicated their suitability as novel phosphorescent emitters for blue OLEDs. In addition, multi-layer phosphorescent organic light-emitting diodes using FIr and CNIr as emitters and mCP as a host have been fabricated. The devices exhibited low turn-on voltages and high external quantum efficiencies (18.9% for FIr and 22.5% for CNIr) with blue phosphorescent emission under CIEy = 0.3 (y coordinate of Commission Internationale de l’Eclairage). Moreover, the CNIr based device showed stable lifetimes greater than 550 h at 200 cd m⁻², which is much longer than that of the common phenylpyridine-type blue triplet emitter based device.