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.