Chenhe Yao,Jiaxing Tang,Ran Chen,Zihao Mao,Xinbing Chen,Pei Chen,Zhongwei An
A series of liquid crystals (LCs) containing bis-tolane, lateral fluorine substituents and 2,2-difluorovinyloxyl group were designed and synthesized. Their LC phase behavior, birefringence, dielectric anisotropy and viscosity properties were discussed by comparing with previously reported LCs with high birefringence and large electrical anisotropy. Meanwhile, the effects of lateral fluorine numbers on the melting point, birefringence and dielectric anisotropy of these LCs were explored, and density functional theory (DFT) calculations of molecular conformation and polarizability were used to correlate the experimental results. Research results indicated that the 2,2-difluorovinyloxyl terminal group decreased the melting points of these LCs, and these LCs showed relatively low melting point (<113.7 ℃), broad nematic-phase temperature interval, high birefringence (>0.4), and large dielectric anisotropy (35.87). Furthermore, performance in high birefringence LC mixture demonstrated that they could effectively improve the birefringence and dielectric anisotropy, and maintain low viscosity, which is expected to possess widespread applications in fast response LC optical devices.
Single-crystal X-ray diffraction is a powerful characterization technique that enables the determination of atomic arrangements in crystalline materials. Growing or retaining large single crystals amenable to it has, however, remained challenging with covalent organic frameworks (COFs), especially suffering from post-synthetic modifications. Here we show the synthesis of a flexible COF with interpenetrated qtz topology by polymerization of tetra(phenyl)bimesityl-based tetraaldehyde and tetraamine building blocks. The material is shown to be flexible through its large, anisotropic positive thermal expansion along the c axis ( α c = +491 × 10 –6 K –1 ), as well as through a structural transformation on the removal of solvent molecules from its pores. The as-synthesized and desolvated materials undergo single-crystal-to-single-crystal transformation by reduction and oxidation of its imine linkages to amine and amide ones, respectively. These redox-induced linkage conversions endow the resulting COFs with improved stability towards strong acid; loading of phosphoric acid leads to anhydrous proton conductivity up to ca. 6.0 × 10 −2 S cm −1 .
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