Cationic Ir(III) Complexes Featuring Phenylimidazole-type Cyclometalated Ligands: Fluorine-Free Blue Phosphorescent Emitters for Light-Emitting Devices

Abstract

The development of blue emissive cationic Ir(III) complexes with no fluorine substitutions but with sufficient blue color purity and high phosphorescence efficiency has remained challenging. Here, fluorine-free cyan to deep blue emissive cationic Ir(III) complexes with phenylimidazole-type cyclometalated ligands (C∧N) are reported, which are [Ir(dphim)2(dmapzpy)]PF6 (1), [Ir(ipr-dphim)2(dmapzpy)]PF6 (2), [Ir(ipr-dphim)2(bipz)]PF6 (3), and [Ir(ipr-dphim)2(bicb)]PF6 (4). 1,2-Diphenyl-1H-imidazole (dphim) and 1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole (ipr-dphim) are the phenylimidazole-type C∧N ligands, and 4-dimethylamino-2-(1H-pyrazol-1-yl)pyridine (dmapzpy), di(1H-pyrazol-1-yl)methane (bipz), and 3,3′-methylenebis(1-methyl-1H-imidazol-3-ium-2-ide) (bicb) are the neutral ancillary ligands (A∧A). In both solution and diluted films, complex 1 shows a cyan emission with the emission maxima at ∼472 and 495 nm, and complexes 2–4 provide a deep blue emission with the emission maxima at ∼460 and 480 nm. While the complexes exhibit low to moderate phosphorescence efficiencies (0.05–0.35) in a degassed CH3CN solution, they exhibit high phosphorescence efficiencies (up to 0.82) in diluted films. Theoretical calculations revealed that the mixed 3π–π* (C∧N-centered)/3MLCT (Ir → C∧N) states are responsible for the emission afforded by complexes 1–4, which undergo nonradiative deactivations induced by different types of metal-centered states. Organic light-emitting diodes with complexes 1–4 as phosphorescent dopants are fabricated by a solution process, which affords a blue-green to blue emission with the emission maxima at ∼460 and 490 nm for the blue devices and a high current efficiency at 28.1 cd A–1 for the blue-green device. Solid-state light-emitting electrochemical cells are also fabricated with complexes 1–2 as phosphorescent dopants, which provide green-blue to blue emission with a high luminance (up to 840 cd m–2) and current-efficiency (up to 16.8 cd A–1) under a constant-current driving. The work reveals that, by using phenylimidazole-type C∧N ligands and optimized A∧A ligands, blue emissive cationic Ir(III) complexes with no fluorine substitutions but with sufficient blue-color purity and a high phosphorescence efficiency can be developed.