Synthesis and characterization of blue-to-green electrophosphorescence emitter based on pyrazole iridium complexes

•A series of (ppz)2Ir(NˆN) phosphorescent iridium complexes with larger steric hindrance were synthesized.•The molecular structures of complexes (ppz)2Ir(NˆN) were defined by X-ray single diffraction.•The emission colors had been tuned from blue to green by modification the ancillary ligands.•The device using (ppz)2Ir(tfmpptz) as dopant had almost no roll-off efficiency.

To overcome the roll-off efficiency of electrophosphorescent organic light-emitting devices, Ir(III) complexes of (ppz)2Ir(NˆN) were synthesized and named for (ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz), (ppz)2Ir(tfmptz) and (ppz)2Ir(pybi) (ppz: 1-phenylpyrazolato; NˆN: N, N′-heteroaromatic). Their molecular structures, photophysical properties and cyclic voltammetric data were reported. The crystals of (ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz) are monoclinic, space group P2(1)/n, and (ppz)2Ir(tfmptz) is triclinic, space group P-1. The emission maximum peaks of (ppz)2Ir(tfmpptz), (ppz)2Ir(fpptz), (ppz)2Ir(tfmptz) and (ppz)2Ir(pybi) at room temperature appear at 486, 497, 473 and 530 nm, respectively. The devices with architecture of ITO/NPB (30 nm)/CBP: (ppz)2Ir(NˆN) (6%, 30 nm)/BAlq(10 nm)/Alq3(30 nm)/LiF(1 nm)/Al (100 nm) were fabricated by thermal evaporation. The results indicate that the larger steric hindrance of ancillary ligands efficiently suppressed the roll-off of efficiency at high current density.

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