Phosphorescent Dinuclear Pd-Pd Emitters in OLED Applications

Four phosphorescent dinuclear Pd-Pd complexes were synthesized by using the clamping ligand N,N'-diphenylformamidine and primary ligands of C∧N, which both have high ligand field strength to enhance the luminescence of the complexes. The single-crystal data confirmed their clamshell structures and revealed Pd-Pd distances between 2.829 and 2.864 Å. In addition, time-dependent density functional theory calculations revealed the phosphorescent nature of the metal-metal-to-ligand charge transfer and ligand-to-ligand charge transfer in these complexes. Moreover, complexes 3 and 4 were used to fabricate an OLED by a vapor phase deposition technique. At an optimal doping concentration of 5%, the 3-based device showed a maximum external quantum efficiency (EQE_max) of 8.79% for CIE coordinates of (0.49, 0.48) and the 4-based device showed an EQE_max of 13.10% for CIE coordinates of (0.61, 0.37).