Fluorene And Benzimidazole Based Blue Light-Emitting Copolymers: Synthesis, Photophysical Properties, And PLED Applications

Organic/Polymeric light-emitting diodes have received increasing attention due to their potential applications in the next generation flat-panel displays. The benefits of PLEDs include thin, low-cost displays with a low driving voltage, wide viewing angle, high contrast, color gamut and flexible. Among the three prime colors, many good red and green emitters have been developed to satisfy the practical requirements for OLEDs. In contrast, the development of blue emitters with high electroluminescence (EL) efficiencies remains a challenge that has attracted significant research effort.Polyfluorenes and fluorine-containing copolymers have emerged as very promising materials for blue emission due to their unique combination of high thermal stability, chemical stability, high photoluminescence (PL) quantum yields, good film-forming and hole-transporting properties. To improve the electron transport properties and achieve balanced charge carrier injection/transport in polyfluorenes, a common strategy is to incorporate n-type (electron acceptor) comonomers into the predominantly p-type polyfluorenes backbone.Imidazole ring is a non-center symmetrical structure with electron deficient characteristic. Benzimidazole based molecules, such as 1,3,5-tris(1-phenyl-1H-benzimidazole-2-yl)benzene (TPBI) are reported to show large bandgap and effective electron-transport property. In the present study, we designed and synthesized 2,2'-(1,4-phenylene)-bis(benzimidazole) (monomer 5), and copolymerized with 9,9-dioctylfluorene-2,7-diboronic acid (monomer 9) though Suzuki coupling reaction to obtain copolymer PFBI0. But PFBI0 shows poor solubility in common solvents because of intense intra- and intermolecular interactions. For this reason, we synthesized N-octyl substituted 5 (monomer 7) to avoid the strong hydrogen bond of benzimidazole. And as expected, copolymer of 7 and 9 (PFBI8) exhibits excellent solubility due to the diminished interactions. PFBI8 also enjoys good thermal stability, whose decomposition temperature and glass transition temperature are as high as 428 and 103 oC, respectively. It emits blue light in solvents with a quantum yield up to 99% in CHCl3.The electroluminescence (EL) peak of PFBI8 in the device with the configuration of ITO/PEDOT:PSS/ PFBI8/TPBI/LiF/Al is 448 nm, which is comparable to the film photoluminescence maximum of 450 nm. Such polymer light-emitting diode (PLED) exhibits a maximum luminance of 1534 cd/m~2 with the current efficiency and power efficiency of 0.67 cd/A and 0.20 lm/W, respectively.The pure blue emission and good EL performance make PFBI8 promising for blue light-emitting PLED applications.