Blue-emitting iridium(<sc>iii</sc>) phosphors with functional imidazo[4,5-<i>b</i>]pyridin-2-ylidene cyclometalates: designs aimed at greater steric hindrance

Abstract

Ir(iii) carbene complexes are a class of promising phosphors used in constructing high-performance 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. These Ir(iii) carbene complexes possessed bulky substituents (both 2-xylenyl and tert-butyl group) at both cyclometalating and non-coordinated aryl sites and a dislocated 6- or 5-cyano substituent on the imidazo[4,5-b]pyridin-2-ylidene fragment, achieving blue phosphorescence. These Ir(iii) emitters showed excellent photophysical properties in degassed toluene solution with high PLQYs (72-98%) and short radiative lifetimes (0.91-3.08 mu s), making them excellent candidates for the fabrication of blue PhOLEDs. As a result, the blue PhOLEDs based on f-ct9ax and f-ct9bx with a 5-cyano substituent exhibited maximum EQEs of 24.2% and 23.3%, respectively, which were higher than those of f-ct3ax and f-ct3bx with a 6-cyano substituent (EQEmax = 22.1% and 20.0%, respectively). Moreover, the hyper-OLEDs featuring the f-ct9ax sensitizer and v-DABNA terminal emitter delivered a narrowband blue emission with a peak maximum at 468 nm and CIEx,y coordinates of (0.123, 0.095). This champion device also achieved a highest maximum EQE of 31.9% and an EQE of 20.9% at a practical luminance of 1000 cd m-2. These results demonstrate the potential of these Ir(iii) carbene complexes in constructing blue OLEDs with high efficiency and color purity for future applications.