Organic Light-Emitting Diodes Based on Variously Substituted Pyrazoloquinolines as Emitting Material

1-Methyl-3-phenyl-1 H -pyrazolo[3,4- b ]quinolines (PAQ) carrying various substituents at the 6- or 7-position (PAQ-X, X = OMe, t -Bu, H, F, CN, CF 3, NEt 2 ) were synthesized and studied as emitting materials in an organic light-emitting diode (OLED). The 4,4‘-bis[ N -(1-naphthyl)- N -phenylamino]biphenyl (α-NPB) and 4,4‘-dicarbozolyl-1,1‘-biphenyl (CBP) were used as hole-transporting materials, whereas electron-transporting 2,2‘,2‘ ‘-(1,3,5-phenylene)tris[1-phenyl-1 H -benzimidazole] (TPBI) was used as a host for the PAQ-X dopant. The device with a general configuration of ITO/NPB/CBP/TPBI:PAQ-X/TPBI/Mg:Ag, where the PAQ-X dye concentration was kept around 2 wt %, was fabricated, and emits a bright blue light for all PAQ dyes except diethylamino-substituted PAQ, which emits blue-green light. With the same structure details, the electroluminescence, turn-on voltage, and external quantum efficiency show a dependence on the substitution. In particular, the external quantum efficiency and power efficiency are higher for devices doped with electron-rich PAQs and lower for electron-deficient PAQs.