Dipyrazolopyridine derivatives as bright blue electroluminescent materials

Abstract

Very bright blue organic light emitting diodes were fabricated using highly fluorescent dipyrazolopyridine derivatives, 4-(4-substituted phenyl)-1,7-diphenyl-3,5-dimethyl-1,7dihydrodipyrazolo[3,4-b,4^′,3^′-e]pyridine (PAP–X, $\mathrm{X=CN},$ Ph, and OMe), as emitter by doping the dye in an electron-transporting host, 2,2^′,2^″-(1,3,5-benzenetriyl)tris-[1-phenyl-1H-benzimidazole] (TPBI). Two hole-transporting layers, 4,4^′-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl (NPB) and 4,4^′-dicarbazolyl-1,1^′-biphenyl (CBP) were used to achieve the emission from PAP–X. The devices with a general configuration of indium tin oxide/NPB/CBP/TPBI:PAP(2%)/Mg:Ag showed a bright blue emission. The PAP–CN-based device is exceptionally good, with a brightness of 11 200 cd/m² at 14.2 V and the peak external quantum efficiency of 3.2%. The efficiency is the highest for the blue emission.