| Liquid crystal displays (LCD) are predominant information terminals in the human society. In-plane switch (IPS) mode has been rapidly expanded by the promotion of huge market demand for mobile displays, such as smartphones and tablet computers. To date, the most of IPS LCD use liquid crystals with positive dielectric anisotropy. Additionally, it is found that the IPS mode with negative dielectric anisotropy liquid crystals has higher transmittance than that of the positive liquid crystals and a fast response time. The large birefringence is favorable and crucial factor for LCs as base material for switchable terahertz (THz) devices. Hence, the development of the liquid crystal compounds with negative dielectric anisotropy and large birefringence become an urgent and important topic for the liquid crystal chemistry. Based on above backgrounds, in this paper a series of 2,3-difluoro tolane liquid crystals were designed and synthesized and their thermal performance, optical performance and dielectric anisotropy were investigated. The results exhibit that liquid crystals have large negative dielectric anisotropy and high birefringence, which are potential important for the fast response displays and liquid crystal photonic devices. The detailed contents include the following aspects:(1) The series of lateral difluoro substituted tolane allyloxy-based liquid crystals, which composed of the tolane unit, lateral fluoro substituents, alkoxy and allyloxy terminal groups, were synthesized with 4-bromo-2,3-difluorophenol as the starting materials via nucleophilic substitution, Sonogashira reaction, deprotection of hydroxyl group and Williamson reaction. These compounds with carbon atoms in the alkoxy chain from 2 to 5. All of the products were obtained with purities higher than 99% (high performance liquid chromatography (HPLC)) and overall yields of 25%. Their structures were confirmed by infrared spectroscopy (IR), mass spectra (MS) and nuclear magnetic resonance (NMR). The mesomorphic properties were investigated by differential scanning calorimetry (DSC), polarising optical microscopy (POM). The dielectric measurement was performed with an independence analyzer EC-1 and birefringence was determined by Abbe refractrometer. The results show that the nT-1 and nT-2 exhibit moderate nematic phase with low melting points (Tm< 70 ?) and high birefringence (?n> 0.3). The region of the 2,3-difluorophenylene has no significant effect on the phase behavior and birefringence, but the dielectric anisotropy is quite different, in which the nT-1 result in higher dielectric anisotropy than that of nT-2.(2) The series of lateral tetrafluoro substituted tolane allyloxy-based liquid crystals were synthesized with 4-bromo-2,3-difluorophenol as the starting materials via five steps reaction. All of the products were obtained with purities higher than 99% (HPLC) and overall yields of 30%. The structures of the products were characterised by IR, MS and NMR. Their properties were investigated by DSC, POM and Abbe refractrometer. The results show that the nT-3 with double 2,3-difluorophenylene units delivers a high birefringence and highest dielectric anisotropy but brings about the higher melting points and vanishing of mesophase.(3) The series of lateral difluoro substituted tolane acrylic ester-based liquid crystals were synthesized with 4-bromo-2,3-difluorophenol as the starting materials via six steps reaction. The products are obtained with yields of 20-25% and purity above 99% (HPLC). The structures of the products and the intermediates have been characterized via 1H-NMR, IR, and MS measurements. Compared with lateral difluoro substituted tolane allyloxy-based liquid crystals nT-1, the acrylic ester-based liquid crystals nT-4 exhibits lower birefringence, higher melting points and narrower nematic phase range. |
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