Oxygen-Bridged Boron NHC-Based Red-Emitting Iridium Complexes Exhibiting Tri-Charge (0, −1, −2) Ligands

Phosphorescent iridium complexes bearing N -heterocyclic carbenes (NHCs) have attracted significant attention for violet and blue emission, yet red-emitting variants remain challenging. This study introduces two rigid oxygen-bridged boron NHC ligands (Bpmi and Bpmb) as monoanionic (−1) ligands into an iridium system with tri-charge (0, −1, −2) ligands, with biphenyl (bp) as the dianionic (−2) ligand and bipyridine (bpy) or phenanthroline (phen) as the neutral (0) ligand, affording four neutral complexes: Ir(Bpmi)bpy, Ir(Bpmi)phen, Ir(Bpmb)bpy, and Ir(Bpmb)phen. Single-crystal structures reveal that the carbon atom of the oxygen-bridged boron unit exhibits a trans -C–C arrangement with that of the biphenyl ligand, while the NHC carbon atom shows a trans -C–N arrangement with the nitrogen atom of the neutral (0) ligand, with π–π, C–H···π, and C–H···O intermolecular interactions present. All complexes exhibit efficient red luminescence (637–650 nm), with phen and Bpmb ligands enhancing efficiency through greater rigidity. As OLED dopants, Ir(Bpmi)phen, Ir(Bpmb)bpy, and Ir(Bpmb)phen achieved maximum EQEs of 9.8%, 4.3%, and 10.3% with emission peaks at 632, 629, and 624 nm (CIE: 0.65,0.34; 0.63,0.36; 0.62,0.37), respectively. This research provides significant insights for the design and optoelectronic applications of red phosphorescent iridium complexes based on oxygen-bridged boron fused-ring NHC ligands.