Efficient non-doped bluish-green organic light emitting devices based on N-1 functionalized star-shaped phenanthroimidazole fluorophores

In this paper, two star-shaped fluorescent phenanthroimidazole fluorophores, tris(4-(1-(3-(trifluoromethyl)pheny1)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PIMCFTPA) and tris(4-(1-(4-(trifluoromethyl)pheny1)-1H-phenanthro[9,10-d]imidazol-2-yl)phenyl)amine (PIPCFTPA) with a D-pi-A structure were designed and synthesized by attaching a hole-transporting triphenylamine and an electron transporting phenanthroimidazole moiety. A detailed photophysical, thermal, electrochemical and related properties were systematically studied. Furthermore, theoretical investigations (DFT) were performed to get a better understanding of the electronic structures. In particular, PIMCFTPA shows blue shifted emission due to the most twisted conformation and reduced intermolecular interaction as compared with PIPCFTPA. Both the fluorophores exhibit high glass transition temperatures and high thermal stabilities with decomposition temperatures up to 377 degrees C. The excellent stability renders them promising materials in electroluminescent devices. Non-doped organic light-emitting devices (OLEDs) using these fluorophores as emissive materials display bluish green emissions with high efficiency (6.58 cdA(-1), 5.911mW(-1), 3.62% at 100 cdm(-2)), low turn-on voltages (2.83 V) (PIMCFTPA) and excellent spectral stability. Our results suggest that the molecular design strategy of integrating TPA with phenthroimdazole play an important role in the device performance. (C) 2017 Elsevier B.V. All rights reserved.