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-Crafts copolycondensation reaction. The oligomers, PF3-TPA and PF3-TPA(3), consist of three fluorene pentamer arms that are connected non-conjugately through a triphenylamine (TPA) and 1,3,5-tris(triphenylamino)benzene core (TPA(3)), respectively, at the 9-position of the central fluorene 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 (phi(pl-film) = 67 % for PF3-TPA(3) and 86% for PF3-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/PF3-TPA(3) and PF3-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(2) at 7.5 V in the PF3-TPA(3) device and 1055 cd/m(2) at 8 V in the PF3-TPA device. The EL efficiencies at this luminance were 1.63 and 1.57 cd/A, respectively.