Organic light-emitting diodes based on variously substituted pyrazoloquinolines as emitting material

1-Methyl-3-phenyl-1H-pyrazolo[3,4-b]quinolines (PAQ) carrying various substituents at the 6- or 7-position (PAQ-X, X = OMe, t-Bu, H, F, CN, CF3, NEt2) were synthesized and studied as emitting materials in an organic light-emitting diode (OLED). The 4,4 ' -bis[N-(1-naphthyl)-N-phenylamino]biphenyl (alpha -NPB) and 4,4 ' -dicarbozylyl-1,1 ' -biphenyl (CBP) were used as hole-transporting materials, whereas electron-transporting 2,2 ' ,2 -(1,3,5-phenylene)-tris[1-phenyl-1H-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.