Blue phosphorescent platinum(II) complexes based on tetradentate ligand with rigid and highly distorted linkage for efficient organic light-emitting diodes

Abstract

Blue phosphorescent Pt(II) complexes comprising two C^N chelates, namely, 2,3′-bipyridine that are connected by a rigid linker, were designed and synthesized. In order to examine the effects of substitution of C^N ligands on the intermolecular interactions in solid-state, optical properties and electroluminescence, the electron withdrawing atom (F) or electron donor group (methoxy unit) as the substituents were introduced into the C-coordinated pyridine ring. The molecular structures of both complexes were confirmed by single crystal X-ray diffraction analysis. The coordination sphere of the Pt(II) cation in both complexes displayed a highly distorted square-planar geometry. The fluorine substituted Pt(II) compound (1) exhibited bright blue emission with λmax = 458 nm, whereas the red-shifted bluish-green emissions with λmax = 482 nm are observed in the methoxy-functionalized Pt(II) compound (2). The time-dependent density-functional theory (TD-DFT) calculations suggest that the electronic transition for both complexes arise from intraligand charge transfer (ILCT) (πbpy−π*bpy) mixed with metal-to-ligand charge transfer (MLCT) (Ptd−π*bpy) in the absence of contribution from the phenyl linker. Thus, multi-layered blue phosphorescent organic light-emitting diodes (PHOLEDs) were successfully fabricated using both Pt(II) compounds as the dopants. The PHOLEDs at 10% doping level displayed bright sky-blue emission with an external quantum efficiency of approximately 7.8–8.0% at 100 cd/m2.