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 (CN)-N-boolean AND chelating ligands have been designed and synthesized. To investigate the effect of the substitution of an electron-donating/withdrawing unit on the solidstate structure and photo- and electroluminescence, either fluorine or methoxy substituents were incorporated into the Ccoordinating 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)center dot center dot center dot Pt(II) intermolecular interactions in crystal packing. Pt(II) complexes bearing fluorine substituents emit bright blue phosphorescence with lambda(max) 460 nm, while methoxy-functionalized Pt(II) complexes display red-shifted and bluish green emissions with lambda(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 electrondonating counterpart. However, the substituents on the linker do not affect luminescence properties. All of the complexes possess high thermal stabilities (decomposition temperature >300 degrees 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).