Ambipolar conductive 2,7-carbazole derivatives for electroluminescent devices

A series of 2,7-disubstituted carbazole (2,7-carb) derivatives incorporating arylamines at the 2 and 7 positions are synthesized via palladium-catalyzed C-N or C-C bond formation. These compounds possess glass transition temperatures ranging from 87 to 217 °C and exhibit good thermal stabilities, with thermal decomposition temperatures ranging from 388 to 480 °C. They are fluorescent and emit in the purple-blue to orange region. Two types of organic light emitting diodes (OLEDs) were constructed from these compounds: (I) indium tin oxide (ITO)/2,7-carb (40 nm)/1,3,5-tris(N- phenylbenzimidazol-2-yl)benzene (TPBI, 40 nm)/Mg:Ag; and (II) ITO/2,7-carb (40 nm)/tris(8-hydroxyquinoline) aluminum (Alq₃,40 nm)/Mg:Ag. In type I devices, the 2,7-disubstituted carbazoles function as both hole-transporting and emitting material. In type II devices, light is emitted from either the 2,7-disubstituted carbazole layer or Alq₃. The devices appear to have a better performance compared to devices fabricated with their 3,6-disubstituted carbazole congeners. Some of the new compounds exhibit ambipolar conductive behavior, with hole and electron mobilities up to 10 ^-4 cm² V^-1 s^-1.