Isoelectronic Effects of Pt(II) Complexes with Nitrogen-Embedded Tetradentate Ligands on Blue Phosphorescence

Pt(II) complexes with tetradentate ligands have emerged as high-performance emitters in phosphorescent organic light-emitting diodes. Herein, a comprehensive study of PtON1 [η⁴-(dimethylpyrazolyl)phenoxy-9-(pyridinyl-N-carbazole)Pt] analogues was done to learn the isoelectronic ligand effect on photo- and electroluminescent properties. By exchanging the phenyl ring in the PtON1 framework for a pyridine ring, complexes of PtON1-o and PtON1-p exhibited better planarity in both ground and excited states and improved blue color with narrow emission spectra. A disproportionate energy variation was disclosed corresponding to the embedded nitrogen site. Compared with the initial prototype of PtON1-dmp, PtON1-o exhibits enlargement in the lowest triplet excite-state energies (T₁), whereas PtON1-p shows an opposite trend. The devices based on the nitrogen-embedded complex showed improved color purity in deep-blue emissions, in which PtON1-p also exhibited the highest external quantum efficiency of 15.5% and the lowest turn-on voltage at 2.7 eV. This investigation envisioned an effective strategy to improve the emission properties via nitrogen-embedded isoelectronic derivatives.