Asymmetric tris-heteroleptic iridium(iii) complexes containing three different 2-phenylpyridine-type ligands: A new strategy for improving the electroluminescence ability of phosphorescent emitters

A series of asymmetric tris-heteroleptic Ir(iii) phosphorescent complexes adopting both IrLL′L′′ (Ir3-1 and Ir3-2) and IrLL′(acac) (Ir2-1 and Ir2-2) chemical constitution have been successfully prepared, where L, L′ and L′′ represent different ppy-type (2-phenylpyridine) ligands. The IrLL′L′′ asymmetric tris-heteroleptic Ir(iii) phosphorescent complexes Ir3-1 and Ir3-2 bearing three different ppy-type ligands can show better thermal stability, higher Φ_P and improved ability of trapping both holes and electrons than IrLL′(acac) asymmetric analogs Ir2-1 and Ir2-2. Thanks to these advantages, Ir3-1 and Ir3-2, especially Ir3-2, can show phenomenal EL performance with a maximum external quantum efficiency (η_ext) of 26.2%, a maximum current efficiency (η_L) of 88.7 cd A^-1 and a maximum power efficiency (η_P) of 75.3 lm W^-1, much higher than the data achieved by not only IrLL′(acac) asymmetric analogs Ir2-1 and Ir2-2, but also the traditional Ir(L)₃ symmetric counterparts. To the best of our knowledge, these data represent the highest EL efficiencies ever achieved by the asymmetric ppy-type Ir(iii) phosphorescent complexes reported in the literature. All these encouraging results not only indicate the great potential of the unique asymmetric IrLL′L′′ structures bearing three different ppy-type ligands in improving the EL ability of ppy-type Ir(iii) phosphorescent emitters, but also represent important progress in the design and synthesis of new asymmetric ppy-type Ir(iii) phosphorescent complexes with high EL ability.