Rational design and characterization of heteroleptic phosphorescent iridium(iii) complexes for highly efficient deep-blue OLEDs

Abstract

Two new deep-blue iridium(III) complexes, (dfpypy)₂IrFptz (Ir1) and (Medfpypy)₂IrFptz (Ir2), comprising difluoro-bipyridyl (dfpypy) derivatives as cyclometaling ligands and a chelated pyridyl-triazole (Fptz) ancillary ligand are reported. The bipyridyl ligands lead to a significantly increased HOMO–LUMO gap and a hypsochromic shift of the phosphorescence compared to phenylpyridyl analogs. Density function theory (DFT) calculations and electrochemical measurements for Ir1 and Ir2 support their genuine blue phosphorescent emission. The combination of ancillary and cyclometalating ligands in Ir1 and Ir2 significantly influences the molecular orbitals of both complexes, leading to clearly distinct electron density distributions of the HOMO and LUMO compared with other blue-emitting Ir(III) derivatives. Both complexes Ir1 and Ir2 show deep-blue emission with λ_max values in the region of 435–465 nm with high PLQYs and short excited-state lifetimes. The phosphorescent organic light emitting diodes (PhOLEDs) based on Ir1 and Ir2 achieve remarkably high EL performance with low efficiency roll-off at high luminance. The bluest color (CIE_x,y 0.14, 0.11) and the highest EL efficiency were achieved in the device based on Ir2 (Device 2), where the peak EQE/PE of 13.0%/11.2 lm W⁻¹ together with the corresponding values of 12.6%/8.8 lm W⁻¹ and 10.1%/5.0 lm W⁻¹ at the practical luminances of 100 and 1000 cd m⁻² respectively, strongly compete with those of any deep-blue fluorescent and/or phosphorescent OLEDs with similar CIE coordinates previously reported.