Rigid Oxygen-Bridged Boron NHC-Based Homoleptic Phosphorescent Iridium Complexes: Structures, Photophysics and OLED Application
Two novel N-heterocyclic carbene (NHC)-based ligands featuring rigid boron-oxygen (BO) fused-ring units, named Bpmi and Bpmb, and the two corresponding homoleptic meridianal iridium complexes, namely mer-Ir(Bpmi)3 and mer-Ir(Bpmb)3, were designed and synthesized. Single-crystal structures revealed a meridional coordination geometry for both complexes. Shorter Ir–C carbene bond lengths and rigid planar BO-fused ring units contribute to enhanced stability. Both complexes exhibit efficient green phosphorescence (λem = 536/521 nm in toluene, ΦPL > 78%) with short lifetimes (τ = 846/1083 ns), leading to high radiative rate constants (Kr = 10.04 × 10^5 and 7.29 × 10^5 s−1, respectively). Theoretical calculations indicate significantly increased metal-to-ligand charge transfer (MLCT) character (21.69% for mer-Ir(Bpmi)3; 17.30% for mer-Ir(Bpmb)3) compared to reference complexes (13.01% for mer-Ir(pmi)3; 15.99% for mer-Ir(pmb)3). Both complexes exhibit exceptional thermal stability with decomposition temperatures of 491°C (mer-Ir(Bpmi)3) and 540°C (mer-Ir(Bpmb)3). OLED devices using mer-Ir(Bpmb)3 and mer-Ir(Bpmi)3 as emitters demonstrate maximum external quantum efficiencies of 20.0% and 15.6%, respectively. This research pioneers boron-fused ring-containing NHCs and their phosphorescent iridium(III) complexes, establishing a novel design strategy for high-performance NHC-based OLED phosphorescent emitters.