Red and near-infrared emissive palladium(II) complexes with tetradentate coordination framework and their application in OLEDs

Palladium(II) complexes based organic light-emitting diodes (OLEDs) have made great progress in emission regions from blue to yellow. However, the red emission OLEDs based on palladium(II) complexes have not been reported, and the related near-infrared (NIR) emission OLEDs are extremely rare. Herein, two novel tetradentate cyclometalated palladium(II) complexes of DPA-Cz-bpy-Pd (Pd-1) and 2Cz-bpy-Pd (Pd-2) were designed and synthesized as emissive dopants for red and NIR phosphorescent OLEDs. These complexes exhibited a short-lived (0.5–1 μs) metal-to-ligand charge transfer (MLCT) excited state in neat and doped films. NIR photoluminescence peaks at 700 nm and 722 nm were observed in their dichloromethane solution for Pd-1 and Pd-2, respectively. Optimized OLED devices using Pd-1 exhibited peak external quantum efficiency (EQE) of 2.6% with emission peak at 643 nm. This work presents the first example of palladium(II) complexes as red electroluminescent emitters, and sheds light on the potential of palladium(II) complexes for long-wavelength molecular emitters and electroluminescence.