Novel Oligomers of 9,9-Dialkylated Fluorenes for Thermally Stable Hole-Transport Materials in OLEDs

Using 9,9-dialkyl-functionalized fluorenes as monomers, novel fluorene-based oligomers were prepared in order to improve the thermal stability of a typical hole-transport material (HTM), N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD), in organic light emitting diodes (OLEDs). 2,7-Bis(diphenylamino)fluorene was utilized as a TPD unit throughout the present synthetic work. 9,9-Bis(2-methylallyl)fluorene and its TPD congener underwent a new cyclopolymerization reaction affording six-membered oligomers under acid catalysis with p-toluenesulfonic acid. Intermolecular Suzuki coupling of a 9,9-bis(p-bromobenzyl)fluorene analogue with the corresponding bis-boronate afforded a new oligomer containing the TPD substructure. These oligomers (3 and 6) for use as HTMs have high glass-transition temperatures (Tg’s) (147 and 167 °C, respectively); the values are much higher than the Tg for TPD (63 °C). These materials are soluble in organic solvents to allow easy processing for preparing layered devices, though their electroluminescent performances were lower than that of the single molecule TPD.