High-efficiency orange to near-infrared emissions from bis-cyclometalated iridium complexes with phenyl-benzoquinoline isomers as ligands

Abstract

We report the synthesis and characterization of three novel bis-cyclometalated iridium complexes with phenyl-benzoquinoline (pbq) analogs as ligands, namely, Ir(pbq-f)₂acac, Ir(dpbq-f)₂acac, and Ir(pbq-g)₂acac, where pbq-f, dpbq-f and pbq-g representing 3-phenyl-benzo[f]quinoline, 1,3-diphenyl-benzo[f]quinoline, and 2-phenyl-benzo[g]quinoline, respectively. Interesting distinctions were observed in the electronic structures, photophysical and electroluminescent properties of these complexes. Ir(pbq-f)₂acac and Ir(dpbq-f)₂acac are orange-red emissive phosphors with strong metal–ligand charge transfer (³MLCT) emission bands centered at 577 and 604nm, respectively, while Ir(pbq-g)₂acac shows the largest red-shift to near-infrared (NIR) region with a peak emission at 708nm and a shoulder around 780nm in solution. All the phosphors exhibit strong electrophosphorescence with negligible triplet-triplet annihilation due to quite short phosphorescent lifetimes (∼0.5µs) and high emission quantum yields. Orange-red emissive Ir(dpbq-f)₂acac gives a maximum current efficiency of 17.4 cd/A and external quantum efficiency (η_ext) of 10.5%. NIR emissive Ir(pbq-g)₂acac shows a promising emission centered at 720nm with a shoulder above 780nm. Forward light output is 4.6 mW/cm² at 13V and the maximum η_ext is nearly 1.1%. Our study demonstrates that the constitutional isomers of cyclometallated ligand distinctly control the electronic structures and emissive properties of the corresponding Ir complexes and the obtained NIR emissive Ir(pbq-g)₂acac implies the potential to realize highly efficient NIR OLEDs based on Ir(III) complexes.