NHC-Based Deep-Red Phosphorescent Iridium Complexes Featuring Three-Charge (0, -1, -2) Ligands

N-heterocyclic carbene (NHC)-based phosphorescent iridium complexes have attracted extensive attention due to their good optical properties and high stability in recent years. However, currently reported NHC-based iridium complexes can easily achieve emission of blue, green, or even ultraviolet light, while emission of red or deep-red light is relatively rare. Here, we report a new family of NHC-based deep-red iridium complexes (Ir1, Ir2, Ir3, and Ir4) featuring three-charge (0, -1, -2) ligands. The single-crystal structures confirm that all complexes exhibit a trans-C-N configuration between the NHC carbon atom of the monoanionic (-1) ligand and the nitrogen atom of the neutral (0) ligand and that there are abundant intermolecular and intramolecular interactions in the crystalline state. Notably, all complexes exhibited an effective deep-red emission (650-664 nm). Moreover, the iridium complexes (Ir2 and Ir4) based on benzimidazol-2-ylidene (pmb) exhibited a higher emission efficiency and longer emission lifetime than the corresponding iridium complexes (Ir1 and Ir2) based on imidazol-2-ylidene (pmi), respectively. Density functional theory calculations demonstrate that the pmb ligand of Ir3 and Ir4 is more involved in the excited state than the pmi ligand of Ir1 and Ir2, which is caused by the stronger electron-donating ability of the pmb ligand. Considering better optical properties, Ir2 and Ir4 were eventually used as dopant emitters of the optical light-emitting diode (OLED) devices to obtain good maximum external quantum efficiency (8.5 and 10.1%) in the deep-red region (628 and 624 nm) with Commission Internationale deL'Eclairage (CIE) coordinates of (0.65, 0.34) and (0.63, 0.36), respectively, with a low turn-on voltage (2.4 V). This research provides an important idea for the design and optoelectronic applications of NHC-based deep-red phosphorescent iridium complexes.