Blue emitting iridium(III) phosphors with functional imidazo[4,5-b]pyridin-2-ylidene cyclometalates: designs aimed for greater steric encumbrance

Abstract

Ir (III) carbene complexes are considered as one of the most promising phosphors in constructing highly performed blue OLEDs. In this work, two series of efficient blue emitting Ir(III) carbene complexes, namely, f-ct3ax / bx and f-ct9ax / bx, were designed, synthesized and characterized. Accordingly, these Ir(III) carbene complexes possessed bulky substituents (both 2-xylenyl and tert-butyl group) at both cyclometalating and non-coordinated aryl sites, and with dislocated 6- or 5-cyano substituent on imidazo[4,5-b]pyridin-2-ylidene fragment to achieve blue phosphorescence. These as-synthesized Ir(III) emitters showed excellent photophysical properties in degassed toluene solution with high PLQYs (72% – 98%) and short radiative lifetimes (0.91 – 3.08 µs), making them excellent candidates for fabrication of blue PhOLEDs. As a result, the blue PhOLEDs based on f-ct9ax and f-ct9bx exhibited max. EQEs of 24.2% and 23.3%, respectively, which are higher than those of f-ct3ax and f-ct3bx (EQEmax = 22.1% and 20.0%). Moreover, the hyper-OLEDs featuring f-ct9ax sensitizer and v-DABNA terminal emitter delivered a narrowband blue emission with peak max. at 468 nm and CIEx,y coordinates of (0.123, 0.095). This champion device also achieved a high max. EQE of 31.9%, and an EQE of 20.9% at a practical luminance of 1000 cd·m‒2. These results well demonstrate the potential of these Ir(III) carbene complexes in constructing the blue OLEDs with high efficiency and color purity for future applications.