Pure blue phosphorescence by new N-heterocyclic carbene-based Ir(III) complexes for organic light-emitting diode application

Pure blue phosphorescence with high quantum yield is crucial, yet highly challenging to achieve, for the successful application of phosphorescent organic light-emitting diodes (PhOLEDs) in display and lighting technologies. This study presents the design of three meridional tris-cyclometalated Ir(III) complexes (Ir1, Ir2 and Ir3) by introducing diphenylphosphine oxide (Ph2P = O) and/or fluorine (-F) group(s) on N-heterocyclic carbene (NHC) ligands (3-phenyl-3H-imidazo[4,5-b]pyridine or 1-phenyl-1H-imidazole) with the aim of achieving pure blue phosphorescence. Particularly, the introduction of -F on the NHC ligand greatly enhances the triplet (T-1) energy and photoluminescence (PL) intensity of the Ir(III) complex (Ir3). Consequently, Ir3 exhibits 'pure' blue phosphorescence with high quantum yield (95%) and short triplet lifetimes (tau), the latter being beneficial for high radiative decay rates. Moreover, the Ir3 emitter shows pure blue Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.08), especially the 'pure' blue CIE y coordinate, in the PhOLEDs. However, Ir1 shows a maximum external quantum efficiency (EQE) of 8.6% at higher dopant concentration (20 wt%) with CIE coordinates of (0.16, 0.12).