Synthesis And Property Study Of Liquid Crystalline Bi-oxadiazole Derivatives

Since break through evolving OLED made by C.W.Tang et al in 1987 and then the discovery of electroluminescence (EL) in Poly (1,4-phenylene vinylene) (PPV) by Burroughes et al in 1990,Organic and polymeric light–emitting diodes (LEDs) have gained considerable attention because of their potential application and commercial value in lighting and display.Since most of the EL polymer materials are predominantly hole transporting, it is in most of cases easier to inject and transport holes rather than electrons, and consequently, the devices have relatively low efficiency. In order to create more efficient and high performance device, new materials with high electron transporting capability are demanded. The oxadiazole moieties had been demonstrated to possess high potential for electron transport. In this work, we designed and synthesized a series of twin-tapered bi-1,3,4-oxadiazole derivatives ,i.e. BOXD-Tn (n=3,4,5,6,7,8,10,14) . Their molecular stuctures ,liquid crystalline behaviors and fluorescence characterastic were investigated by 1H NMR, FT-IR spectroscopy, UV-vis absorption spectroscopy, photoluminescence spectroscopy, elemental analysis, , DSC, XRD, POM. The results were outlined as follows:1) Liquid Crystalline BehaviorsBOXD-Tn(n = 5, 6, 7, 8, 10,14) exhibited columnar mesophases depending on the length of the terminal chains, but no liquid crystalline phase was detected for BOXD-T3 and BOXD-T4, which had short terminal chains. BOXD-T5 and BOXD-T6 exhibited COlr phase first and then transfered into ColL in the heating run, and the others exhibited ColL phase. No crystallization of BOXD-Tn (n = 5, 6, 7, 8, 10) were observed even at low temperature (-20℃) during cooling at 10℃/min. A room temperature Colho phase was obtained for BOXD-T14.Crystallization of BOXD-T5 was observed during heating its LC phase and its crystallization temperature decreased with the decrease of heating rate. BOXD-T14 developed a Colho phase during cooling.BOXD-Tn(n=5,6,7,8,10,14) exhibited liquid crystalline characteristic at room temperature. No crystallization was observed for BOXD-T14 even after weeks, and crystallization were observed for the others.2) Fluorescence behavior of BOXD-TnBOXD-Tn derivatives exhibited strong fluorescence either in solid states or in non-polarity solution. The annealed BOXD-Tn derivatives showed strong emission at 426 nm for BOXD-T5, 438 nm for BOXD-T6, 448 nm for BOXD-T7, 436 nm for BOXD-T8, 434 nm for BOXD-T10, and 426 nm for BOXD-T14. BOXD-Tn showed monomeric feature and intramolecular charge transition at concentration lower than 1×10-5mol/L . And the maximum emission of BOXD-Tn red shifted with the increase of polarity of sovent.