Regioselective Syntheses of Imidazo[4,5-b]pyrazin-2-ylidene-Based Chelates and Blue Emissive Iridium(III) Phosphors for Solution-Processed OLEDs

Six homoleptic Ir(III) complexes bearing imidazo-[4,5-b]pyrazin-2-ylidene chelates were successfully designed and synthesized. Narrowband blue emission (λ_max = 466-485 nm) and broadened green emission (λ_max = 518-532 nm) in degassed toluene solution with high photoluminescent quantum yields in the range of 75-81 and 45-48% were observed for ƒ-timpz, t2impz, and t2empz as well as m-timpz, t2impz, and t2empz, respectively. In addition, the tert-butylphenyl cyclometalate is more electron donating than N-phenyl cyclometalate and, hence, all tert-butylphenyl-substituted derivatives, that is, m- and ƒ-t2impz and m- and ƒ-t2empz, give more red-shifted emission in comparison to that of m- and ƒ-timpz. Moreover, solution-processed OLED with ƒ-t2empz (20 wt %) as the dopant gave electrophosphorescence at 474 nm with maximum external quantum efficiency (max. EQE) of 5.1%, while hyper-OLED with assistant sensitizer ƒ-t2empz (10 wt %) and the multi-resonance thermally activated delayed fluorescence emitter BCzBN (0.5 wt %) afforded narrowband emission centered at 485 nm and max. EQE up to 17.4%, confirming the high potential of this class of Ir(III) metal phosphors.