Green and yellow electroluminescent dipolar carbazole derivatives: Features and benefits of electron-withdrawing segments

New multiply substituted carbazole derivatives containing fluorene or phenylene conjugated oxadiazole segments and quinoxaline units were obtained by palladium-catalyzed C-N coupling reactions. They are amorphous with the glass transition temperature (T-g) in the range 104-176 degreesC. The emission color of the materials varies from blue to yellow and is dependent on the nature of the electron-withdrawing segments and solvents. Two reversible one-electron oxidations were observed for these molecules in cyclic voltammograms, which originate from the peripheral 3,6-diarylamino units in the 3,6,9-trisubstituted derivatives and diarylamine and carbazole segments in the 3,9-disubstituted compounds. Reductions originating from quinoxaline segments were also located for the molecules incorporating quinoxaline moieties. The double-layer organic light-emitting diodes fabricated using these compounds as hole-transporting/emitting layers and TPBI or Alq(3) as an electron-transporting layer emit bluish green to yellow colors. The recombination zone is restricted in the HTL layer for the quinoxaline-containing molecules irrespective of the electron-transporting layer used and emission occurs from them. However, for the oxadiazole derivatives emission in the Alq(3)-based devices is either red-shifted or resembles that of Alq(3). Cyclic voltammetric and spectroscopic data support more pronounced electron affinity for the quinoxaline-incorporated carbazole derivatives than for the oxadiazole-tethered carbazole materials.