Blue Phosphorescent Platinum Complexes Based on Tetradentate Bipyridine Ligands and Their Application to Organic Light-Emitting Diodes (OLEDs)

Blue phosphorescent Pt(II) complexes with 2,3′-bipyridine-based tetradentate C^N chelating ligands have been designed and synthesized. To investigate the effect of the substitution of an electron-donating/withdrawing unit on the solid-state structure and photo- and electroluminescence, either fluorine or methoxy substituents were incorporated into the C-coordinating pyridine ring. The molecular structures of all the complexes were fully characterized by different spectroscopic methods, including X-ray diffraction analysis. There are no Pt(II)···Pt(II) intermolecular interactions in crystal packing. Pt(II) complexes bearing fluorine substituents emit bright blue phosphorescence with λ _max 460 nm, while methoxy-functionalized Pt(II) complexes display red-shifted and bluish green emissions with λ _max 490 nm. The electron-withdrawing group at the C-coordinating pyridine ring produced a deeper HOMO (highest occupied molecular orbital) level in comparison to its electron-donating counterpart. However, the substituents on the linker do not affect luminescence properties. All of the complexes possess high thermal stabilities (decomposition temperature >300 °C) and electrochemical stabilities. Blue phosphorescent organic light-emitting diodes (PHOLEDs) using four Pt(II) dopants have been successfully fabricated with an external quantum efficiency of 17.6% (maximum) and 17.3% (at 100 cd/m ² ). Moreover, a fluorine-substituted Pt(II) complex with a doping level of 50 wt % in nonoptimized white organic light-emitting diodes (WOLEDs) exhibits an external quantum efficiency of 8.5% and pure white emission with Commission Internationale de L'Eclairage (1931) coordinates (x, y) of (0.32, 0.34).