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

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 ƒ-ct5c, ƒ-ct5d, and ƒ-ct11, bearing imidazo[4,5-b]pyrazin-2-ylidene cyclometalates are prepared and characterized in detail. Both ƒ-ct5c and ƒ-ct5d (also their mixture ƒ-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, ƒ-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 ƒ-ct11 delivered a maximum external quantum efficiency (EQE_max) of 6.5% with serious efficiency roll-offs. In contrast, ƒ-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 ƒ-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 ƒ-ct5mix to serve as both the emitter and sensitizer in OLEDs. © 2024 Wiley-VCH GmbH.