AIE-active Pt(II) complexes based on a three-ligand molecular framework for high performance solution-processed OLEDs

Abstract

Cyclometallated Pt(II) complexes are one of the most competitive light-emitting materials for fabricating high-performance OLEDs. The properties of cyclometallated Pt(II) complexes can be conveniently manipulated by altering the ligand structures. However, because of the typical design fashion which leads to Pt(II) complexes with one or two ligands, the conventional cyclometallated Pt(II) complexes always have a rigid planar structure and are prone to form close packing, resulting in a change in the emission spectrum and a decrease in the color purity. Besides, it is also relatively difficult or cumbersome to simultaneously introduce electron-withdrawing and electron-donating groups into one molecule to improve the balance of electron and hole injection/transport process. In this work, we propose a new molecular framework which consists of one bidentate ligand and two independent monodentate ligands. Based on this three-ligand molecular framework, nineteen Pt(II) complexes are synthesized to investigate the influence of each ligand on the emission as well as the charge injection/transport properties. Some of these three-ligand Pt(II) complexes display impressive aggregation-induced emission properties, high photoluminescent quantum yields and improved balance of hole and electron injection/transport ability. Consequently, solution-processed phosphorescent OLEDs exhibit very pure yellow emissions with a remarkably high external quantum efficiency (EQE) reaching up to 24.2%, which is the highest EQE reported for pure yellow OLEDs fabricated with the solution-processed method. This study proposes a new molecular framework, demonstrates its promising potential for developing high-performance electroluminescent materials and provides clear clues on how to engineer molecular structure for desired properties.