Iridium(III) Blue Phosphors with Heteroleptic Carbene Cyclometalates: Isomerization, Emission Tuning, and OLED Fabrications

Ir(III) complexes are particularly noted for their excellent photophysical properties in giving blue OLED phosphors. In this study, two distinctive carbene pro-chelates LAH2+ and LBH2+ (or LCH2+) were employed in preparation of heteroleptic Ir(III) complexes, to which LAH2+ bears a cyano substituted benzoimidazolium along with N-mesityl appendage, while LBH2+ (or LCH2+) carries the symmetrical benzoimidazolium entity. Notably, the reversible equilibration at high temperature was observed for m, f-ct14 and m, f-ct15 with a single LA chelate. In contrast, only the mer-substituted m-ct16 was obtained upon employing two LA chelates. All Ir(III) complexes exhibited blue photoluminescence (ΦPL ≥ 78%) with short radiative lifetimes (τrad ≤ 1.05 µs) in solution. The Ph OLED device with m-ct16 afforded an external quantum efficiency (EQE) of 22.8% at 5000 cd m‒2. Moreover, the hyper-OLED based on m-ct16 and v-DABNA exhibited EQE1000 of 32.1% (EQE recorded at 1000 cd·m-2) and J90 of 15.0 mA cm‒2 (current density at 90% of max. EQE). Its suppressed efficiency roll-off (EQE of 32.1% and 27.7% at 1000 cd·m‒2 and 10000 cd·m‒2) demonstrated a milestone in fabrication of blue OLED devices.