Synthesis and characterization of carbazole-based dendrimers as bipolar host materials for green phosphorescent organic light emitting diodes

A group of novel, carbazole-based dendrimers comprised of the electron-accepting dibenzothiophene core and the electron-donating oligo-carbazole dendrons, namely G1SF and G2SF, are synthesized utilizing the Ullmann C-N coupling reaction. The dendrimers are designed in such a way to show good solubility in common organic solvents, excellent thermochemical stability with decomposition temperatures (T-d) up to 430 degrees C, and high HOMO levels in a range from -5.45 eV to -5.37 eV. Results of density functional theory calculations (DFT) indicate G2SF has an almost complete separation of HOMO and LUMO levels at the hole and electron-transporting moieties; while G1SF exhibits only partial separation of the HOMO and LUMO levels possibly due to intramolecular charge transfer. Green phosphorescent OLEDs were fabricated by the spin coating method with the dendrimers as hosts and traditional green iridium phosphor as doped emitter. Under ambient conditions, a maximum luminance efficiency (eta(L)) of 19.83 cd A(-1) and a maximum external quantum efficiency of 5.85% are achieved for G1SF, and 15.50 cd A(-1) and 4.57% for G2SF. (C) 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.