Phosphorescent Dinuclear Pd–Pd Emitters in OLED Applications

Four phosphorescent dinuclear Pd-Pd complexes were synthesized by using the clamping ligand <i>N</i>,<i>N</i>'-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 <b>3</b> and <b>4</b> were used to fabricate an OLED by a vapor phase deposition technique. At an optimal doping concentration of 5%, the <b>3</b>-based device showed a maximum external quantum efficiency (EQE_max) of 8.79% for CIE coordinates of (0.49, 0.48) and the <b>4</b>-based device showed an EQE_max of 13.10% for CIE coordinates of (0.61, 0.37).