Blue Electrophosphorescence from Iridium(III) Phosphors Bearing Asymmetric Di-N-aryl 6-(trifluoromethyl)-2H-imidazo[4,5-b]pyridin-2-ylidene Chelates.

Efficient blue phosphors remain a formidable challenge for organic light-emitting diodes (OLEDs). To circumvent this obstacle, a series of Ir(III)-based carbene complexes bearing asymmetric di-N-aryl 6-(trifluoromethyl)-2H-imidazo[4,5-b]pyridin-2-ylidene chelates, namely, f-ct6a‒c, are synthesized, and their structures and photophysical properties are comprehensively investigated. Moreover, these emitters can undergo interconversion in refluxing 1,2,4-trichlorobenzene, catalyzed by a mixture of sodium acetate (NaOAc) and p-toluenesulfonic acid monohydrate (TsOH·H2O) without decomposition. All Ir(III) complexes present good photoluminescence quantum yield (ΦPL = 83-88%) with peak maximum (max.) at 443-452 nm and narrowed full width at half maximum (FWHM = 66-73 nm). Among all the fabricated OLED devices, f-ct6b delivers a max. external quantum efficiency (EQE) of 23.4% and Commission Internationale de L'Eclairage CIEx , y coordinates of (0.14, 0.12), whereas the hyper-OLED device based on f-ct6a and 5H,9H,11H,15H-[1,4] benzazaborino [2,3,4-kl][1,4]benzazaborino[4',3',2':4,5][1,4]benzazaborino[3,2-b]phenazaborine-7,13-diamine, N7,N7,N13,N13,5,9,11,15-octaphenyl (ν-DABNA) exhibits max. EQE of 26.2% and CIEx , y of (0.12, 0.13). Finally, the corresponding tandem OLED with f-ct6b as dopant gives a max. luminance of over 10 000 cd m-2 and max. EQE of 42.1%, confirming their candidacies for making true-blue OLEDs.