3D monodisperse oligofluorenes with non-conjugated triphenylamine-based cores : Synthesis and optoelectronic properties

Two 3D monodisperse oligofluorenes with non-conjugated triphenylamine-based cores have been synthesized by Friedel\u2013Crafts copolycondensation reaction. The oligomers, PF\u2083-TPA and PF\u2083-TPA\u2083, consist of three fluorene pentamer arms that are connected non-conjugately through a triphenylamine (TPA) and 1,3,5-tris(triphenylamino)benzene core (TPA\u2083), respectively, at the 9-position of the central fluorine of the pentafluorene arms. The coplanar structures of the cores and the linkages at the centre of the pentafluorene arms produced a 3D structure of oligomers. This specific structure efficiently retarded the crystallization tendencies of the pentafluorene arms and gave the materials completely amorphous morphological structures. Both oligomers emit deep-blue fluorescence with high efficiencies in thin films ( 8pl\u2013film = 67% for PF\u2083-TPA\u2083 and 86% for PF\u2083-TPA). The introduction of triphenylamine units into the core promoted the hole-injection ability while not obviously scarificing the electron- injection ability of the oligomers. The multi-layer devices ITO/PEDOT-PSS/PF\u2083-TPA\u2083 and PF\u2083-TPA/TPBI/LiF/Al were fabricated to investigate the electroluminescence (EL) properties of the two oligomers. Both oligomers showed a low turn-on voltage of 4 V. The luminances reached 1946 cd/m\ub2 at 7.5 V in the PF\u2083-TPA\u2083 device and 1055 cd/m\ub2 at 8 V in the PF\u2083-TPA device. The EL efficiencies at this luminance were 1.63 and 1.57 cd/A, respectively.Peer reviewed: YesNRC publication: Ye