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 subst ituents 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-emi tting diode (OLED). The 4,4 ' -bis[N-(1-naphthyl)-N-phenylamino]biphenyl (a lpha -NPB) and 4,4 ' -dicarbozylyl-1,1 ' -biphenyl (CBP) were used as hole- transporting materials, whereas electron-transporting 2,2 ' ,2 "-(1,3,5-phe nylene)-tris[1-phenyl-1H-benzimidazole] (TPBI) was used as a host for the P AQ-X dopant. The device with a general configuration of ITO/NPB/CBP/TPBI:PA Q-X/TPBI/Mg:Ag, where the PAQ-X dye concentration was kept around 2 wt %, w as fabricated, and emits a bright blue light for all PAQ dyes except diethy lamino-substituted PAQ, which emits blue-green light. With the same structu re details, the electroluminescence, turn-on voltage, and external quantum efficiency show a dependence on the substitution. In particular, the extern al quantum efficiency and power efficiency are higher for devices doped wit h electron-rich PAQs and lower for electron-deficient PAQs.