Synthesis And Properties Of A New Type Of Asymmetric Liquid Crystal Dimer Containing Cholesterol Groups

Liquid crystals are an important type of soft materials with self-assembled structures.Its special stimuli responsiveness makes it a great potential in chemical sensors,information storage media,full color display,and thermo-optical devices.The introduction of chiral molecules in liquid crystal structures is an interesting modification of liquid crystal materials,of which cholesterol is one of the most widely used chiral entities.In this paper,a number of experiments were carried out to the structure design of the small fraction liquid crystal in different directions.For example,the chiral terminal group(cholesteryl group)was introduced,the linkage was changed,the length of the inner spacer flexible chain was varied,and a special rigid core acted as a chromophore(with a fluorescent effect)and a mesogenic moiety was introduced.The molecular structures were confirmed by ~1H-NMR,FT-IR and elemental analyses.The mesomorphic behavior of the liquid crystals was recorded by DSC and POM.Detailed phase structure information was supported by X-ray diffraction(SAXS and WAXD).UV and PL spectra were used to record the UV absorption and fluorescence emission of the samples,respectively.In Chapter 2,ester-linked liquid crystal dimers with polyphenyl rings and cholesterol were synthesized and characterized.C6OAR showed an enantiotropic Sm C*and N*phase with wide phase ranges and well thermal stability.SAXS gave more detailed structure information about Sm C*and N*phase.In addition,C6OAR also exhibits unique optical properties that can characteristically reflect white light.In Chapter 3,a series of asymmetric cholesteryl twin liquid crystals with thioester linkages were successfully synthesized.The clearing points and entropy changes(?S/R)of the dimer series show a significant parity effect.All of the target products exhibited a enantiotropic mesophase behavior with a Sm C*phase(focal conic texture)and a typical N*phase(oily streaks texture).In comparison with analogs containing a carboxylate linkage as a linker,it was found that the COS-containing dimer has a more stable mesophase,a higher phase transition temperature and an enthalpy value.It is worth noting that in the heating and cooling cycles,the desired dimers could undergo Bragg reflection,and multiple colors were observed.In Chapter 4,Cyanostilbene is a strong candidate for chromophores and mesogens.Therefore,a series of twin liquid crystals based on cyanostilbene and cholesterol were synthesized and characterized.The length and parity of the inner flexible segment cause a large difference in the mesomorphic properties.The clearing points(/enthalpy values)show strict odd-even effects common to liquid crystal dimers.The cooperation of molecular configuration simulation and variable-temperature XRD further provides the structural information of the Sm C*,Sm A and N*phases of this series LCs.Unsurprisingly,the molecules of Ch M-CN show a distinct AIEE effect.