Can Remote N-Heterocyclic Carbenes Be Used for Designing Efficient Blue Triplet Emitters? An Answer from Quantum Chemical Investigation

摘要

For the first time, we have tried to show the effect of remote carbene ligands on the emission and quantum efficiency of Ir-based triplet emitters useable for organic light-emitting diodes. It has already been shown that N-heterocyclic carbenes (NHCs) are comparatively stronger than mesoionic carbene (MICs) and hence provide a blue emissive color, but the latter ones are superior in terms of quantum efficiency. Similar arguments prevail for remote NHCs (rNHCs), which are supposed to be even less stronger than MICs. Therefore, any triplet emitter designed with rNHC might experience a bathochromic shift and might not therefore achieve the blue emission. However, we demonstrate that the efficiency of rNHC-based triplet emitters is much higher than that of NHCs and MICs. Hence, our target is toward designing efficient blue emitters having rNHCs. We have used both pyrazolium-pyridine- and phenyl-pyridine-based rNHCs, and we find that the combination of NHC as a cyclometallating ligand and phenyl-pyridine-based rNHC as an ancillary ligand is the best for designing efficient blue emitters. Density functional theory (DFT) and time-dependent DFT have been employed to elucidate the effect of different substitutions on the emission color tuning and quantum efficiency.