Utilization of an Aromatic [4.4.4]Propellane as Thermally Stable Hole-Transport Materials in OLEDs

In order to develop the thermally stable hole-transport materials (HTMs) in organic light-emitting diodes (OLEDs), hexabenzo-annulated [4.4.4]propellane that has three-biphenyl substructures around the C-C hub has been exploited as the core substance. Introduction of bis-, tetrakis-, or hexakis(diphenylamino) substituents at the 4 and 4'-positions of each biphenyl moiety leads to monomeric, dimeric, and trimeric derivatives of N,N,N',N'-tetraphenylbenzidine; N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) has excellent electroluminescent (EL) characteristics but low thermal stability. The key step in the preparation of these derivatives (2S-, 4S-, and 6S-P) was the coupling of appropriate fluorenes with requisite ortho-bromo biphenyl compounds. All of these had not only excellent EL characteristics, but also glass transition temperatures (T g ) of over 120°C which was higher than that of TPD (62°C) representative of the HTM. This result is consistent with that the parent hydrocarbon is unusually stable even at 300 °C. Structural effects of the present monomeric to trimeric TPD analogues on the thermal stability and EL characteristics are discussed.