Efficient tetradentate palladium(II) emitters for sky-blue OLEDs through tuning triplet excited states

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Compared with Pt(II) and Ir(III) complexes, phosphorescent Pd(II) complexes typically suffer from low quantum efficiencies and long luminescent lifetimes due to a significantly weaker spin–orbit coupling (SOC) effect, thereby limiting their application in blue organic light-emitting diodes (OLEDs). In this work, a series of new tetradentate Pd(II) complexes with 1,8-dimethyl-9-(pyridin-4-yl)-9 H -carbazole (PydMCz)-containing ligands were designed and synthesized. Their triplet excited state properties can be efficiently tuned by simple structural modification. Introducing two methyl groups to the ortho positions of the Cz group stabilizes the LUMO orbitals, and significantly increases metal-to-ligand charge transfer ( 3 MLCT) character in the T 1 state for 8 wt% Pd(II) complexes doped in hosts (65 wt% SiCzCz:27 wt% SiTrzCz2) film by vacuum deposition, resulting in enhanced quantum efficiencies up to 99 % and short luminescent lifetimes ( τ 1 = 2.1–4.0 μs, τ 2 = 6.9–8.2 μs) at room temperature. Particularly, employing these Pd(II) emitters as dopants, sky-blue OLEDs with small efficiency roll-offs achieved high maximum external quantum efficiency (EQE max ) of 17.6 % and a high maximum luminance ( L max ) of 25688 cd/m 2 , respectively. This study demonstrates that the Pd(II) complexes can act as good phosphorescent emitters through rational molecular design.