A narrowband red-emitting asymmetric iridium(iii) complex featuring B- and N-embedded pi-conjugation units: structure, photophysics and OLED application

We have designed and synthesized a new B- and N-embedded asymmetric heteroleptic phosphorescent iridium(iii) complex IrBNacac with ambipolar characteristics. The X-ray diffraction technique shows that the two B- and N-embedded pi-conjugation units in the complex exhibit good planarity and induce rich pi-pi interactions in the crystal state. More importantly, it exhibits an effective red emission (623 nm) in the thin film state with a very narrow full width at half maximum (FWHM = 50 nm, 0.158 eV), even narrower than that of model complexes (53 nm, 0.162 eV for IrBBacac and 62 nm, 0.224 eV for IrNNacac). DFT calculation shows that the lowest excited triplet state (T-1) of the complex IrBNacac exhibits richer excited state characteristics than those of the model complexes, including a charge transfer transition from the N-embedded ligand to the B-embedded ligand ((LLCT)-L-3) and a metal-to-ligand charge transfer transition ((MLCT)-M-3), along with an intraligand charge transfer ((ILCT)-I-3). Considering the good photophysical properties and solubility, the red-emitting complex IrBNacac was used as the emitting layer of a solution-processed OLED device and showed good maximum external quantum efficiency (EQE) (4.9%) peaking at 625 nm with the CIE coordinates (0.64, 0.35), accompanied by a low turn-on voltage. This research provides an important strategy for the design of narrowband red-emitting phosphorescent iridium complexes and their optoelectronic applications.