Hexaphenylphenylene dendronised pyrenylamines for efficient organic light-emitting diodes

A series of blue-emitting triarylamines containing pyrene and hexaphenylbenzene dendrons were successfully synthesized by employing the Diels-Alder and palladium catalyzed C-N coupling reactions. They display high glass transition temperatures (>140 degrees C) in differential scanning calorimetry and facile reversible oxidation couple in cyclic voltammetry. They are also thermally stable exhibiting decomposition temperature above 385 degrees C. Efficient blue-emitting electroluminescent devices were fabricated using these novel amines as the hole transporting layer and 1,3,5-tris(N-phenylbenzimidazol-2-yl) benzene (TPBI) as the electron transporting layer. Colour mixing or green emission was observed when tris(8-hydroxyquinoline) aluminium (Alq(3)) was used as the electron transporting layer. However insertion of a thin layer of TPBI or 1,3,5-tris(4-tert-butylphenyl-1,3,4-oxadiazolyl) benzene (TPOB) in between the HTL and Alq(3) layers led to pure blue emission owing to the confinement of recombination inside the HTL layer containing the compounds.