Structural Engineering of Iridium(III) Phosphors with Imidazo[4,5‐b]pyrazin‐2‐ylidene Cyclometalates for Efficient Blue Electroluminescence

Abstract Iridium(III) complexes are particularly noted for their excellent potentials in fabrication of blue organic light‐emitting diodes (OLEDs), but the severe efficiency roll‐off largely hampered their practical applications. To reveal the underlying characteristics, three Ir(III) complexes, namely f ‐ct5c , f ‐ct5d, and f ‐ct11 , bearing imidazo[4,5‐b]pyrazin‐2‐ylidene cyclometalates are prepared and characterized in detail. Both f ‐ct5c and f ‐ct5d (also their mixture f ‐ct5mix ) gave intensive blue emissions peaking at ≈465 nm with short radiative lifetimes of 1.76 and 2.45 µs respectively, in degassed toluene. Alternatively, f ‐ct11 with two 4‐ tert ‐butylphenyl substituents on each imidazo[4,5‐b]pyrazin‐2‐ylidene entity, possessed a bluish‐green emission (508 nm) together with an extended radiative lifetime of 34.3 µs in the dispersed PMMA matrix. Consequently, the resulting solution‐processed OLED with f ‐ct11 delivered a maximum external quantum efficiency (EQE max ) of 6.5% with serious efficiency roll‐offs. In contrast, f ‐ct5mix based device achieved a high EQE max of 27.2% and the EQE maintained at 23.0% of 1000 cd m −2 . Furthermore, the hyper‐OLEDs with f ‐ct5mix as the sensitizer and v‐DABNA as the terminal emitter afford narrowed emission with a considerably high EQE max exceeding 32%, affirming the potential of f ‐ct5mix to serve as both the emitter and sensitizer in OLEDs.