Cationic Iridium Complexes with 5-Phenyl-1H-1,2,4-triazole Type Cyclometalating Ligands: Toward Blue-Shifted Emission

Four cationic iridium complexes with 5-phenyl-1H-1,2,4-triazole (phtz) type cyclometalating ligands (C^N) and different ancillary ligands (N^N), namely, [Ir(dphtz)₂(bpy)]PF₆ (1), [Ir(dphtz)₂(pzpy)]PF₆ (2), [Ir(Mephtz)₂(pzpy)]PF₆ (3), and [Ir(Mephtz)₂(dma-pzpy)]PF₆ (4), have been designed, synthesized, and fully characterized (dphtz = 1-(2,6-dimethylphenyl)-3-methyl-5-phenyl-1H-1,2,4-triazole, Mephtz = 1,3-dimethyl-5-phenyl-1H-1,2,4-triazole; bpy = 2,2'-bipyridine, pzpy = 2-(1H-pyrazol-1-yl)pyridine, dma-pzpy = 4-dimethylamino-2-(1H-pyrazol-1-yl) pyridine). In solution, complex 1 emits efficient yellow light (λ_max = 547 nm), which is blue-shifted by nearly 40 nm (or by 1187 cm^-1) compared with that from the archetypal complex [Ir(ppy)₂(bpy)]PF₆ (Hppy = 2-phenylpyridine), owing to the stabilization of the highest occupied molecular orbital by the phtz-type C^N ligand. In the lightly doped rigid films, complex 1 emits green light with a high luminescent efficiency of 0.89. Although complexes 2-4 with electron-rich N^N ligands are weakly emissive or nearly nonemissive in the solution, they emit relatively strong deep-blue light peaked around 435 and 461 nm in the lightly doped films, which is among the bluest reported for cationic iridium complexes. Theoretical calculations reveal that for complex 1, the emission always comes from the charge-transfer (CT) (Ir/C^N → N^N) state; for complexes 2 and 3, the ³CT and C^N-centered ³π-π* states lie close in energy and the emission could originate from either or both of them; for complex 4, the emission comes predominantly from the C^N-centered ³π-π* state. For blue-emitting complexes 2-4, metal-centered states play an active role in the nonradiative deactivation of the emitting triplet states. Solid-state light-emitting electrochemical cells (LECs) based on complexes 1-3 show yellow-green, blue, and blue-green electroluminescence, respectively, with the yellow-green LEC affording a peak current efficiency of 21.5 cd A^-1.