Syntheses And Properties Of Cholesterol Ester Liquid Crystal Compounds With Solid-to-Solid Phase Transition

In recent decades,a solid-to-solid phase transition has played an important role in dielectric switching,ferroelectricity,piezoelectric,nonlinear optics,and other materials,which refers to the phase transition between multiple crystal states of the same material under the influence of different physical factors.Compared with rigid inorganic materials,molecular-based phase transition materials have the advantages of rich variety,environmental friendliness,and tunable structure,which provides a promising platform for the design and optimization of various functional materials.Liquid crystal is a mesocrystalline state between a solid state and a liquid state,which combines the anisotropy of the crystalline state with the fluidity of a liquid.Different liquid crystal phases are exhibited due to non-covalent and orientation-dependent interactions between condensed phase molecules.Cholesteric liquid crystal has a selforganized periodic spiral structure,which can realize the selective reflection of incident light,and has great development value in the fields of optics and information storage,new display devices and sensors.Liquid crystal materials with solid-to-solid phase transition have the advantages of both solid phase transition materials and liquid crystal materials,which provides a new possibility for the development of new multifunctional intelligent materials.However,there are few researches on liquid crystal materials with reversible solid-to-solid phase transition in single-component compounds.So the development and research of such liquid crystal materials have vital theoretical and application value.In this paper,a series of cholesteryl ester liquid crystal compounds were synthesized and their single crystal structure,phase transition behavior,and liquid crystal texture properties were characterized.The details are as follows:1.Three cases of cholesterol ester liquid crystal compounds were synthesized from 4-Picolinic acid,4-Pyridineacrylic acid,cinnamic acid,and cholesterol:Cholesteryl 4-Picolinate(4-PACS),Cholesteryl 4-Pyridineacrylate(4-PDACS),Cholesteryl Cinnamate(CACS).Among them,4-PACS is a unidirectional cholestericphase liquid crystal,and 4-PDACS and CACS are cross-variant cholesteric-phase liquid crystals.By comparison,it was found that the presence of double bonds in the compound 4-PDACS leads to a lower melting temperature and broader temperature range of liquid crystal phase.More importantly,thermal analysis and dielectric analysis results show that 4-PACS has reversible solid-to-solid phase transition before melting point,making it a potential application prospect in biosensors,optoelectronic devices,switches,and other fields.2.Three cases of cholesterol ester liquid crystal compounds were synthesized from 4-Picolinic acid,4-Pyridineacrylic acid,cinnamic acid,and dihydrocholesterol:Dihydrocholesteryl Cinnamate(CAHCS),Dihydrocholesteryl 4-Picolinate(4-PAHCS),Dihydrocholesteryl 4-Pyridineacrylate(4-PDAHCS).The experimental results show that all three compounds belong to the invariant cholesteric phase liquid crystal,and the compounds containing the benzene ring side chain have a lower melting point and broader temperature range of liquid crystal phase than the compounds containing nitrogen heterocyclic ring side chain.In addition,CAHCS undergoes a reversible solid-to-solid phase transition due to the order-disorder movement of carbon atoms at the tail of the molecule.3.Four cases of esters were synthesized from 4-Picolinic acid,4-Pyridineacrylic acid,cinnamic acid,4-bromocinnamic acid,and β-sitosterol: β-Sitosteryl 4-Picolinate(4-PASS),β-Sitosteryl 4-Pyridineacrylate(4-PDASS),β-sitosteryl Cinnamate(CASS),β-sitosteryl 4-bromocinnamate(4Br-CASS).The results show that both 4-PASS and4-PDASS have a reversible solid-to-solid phase transition,but neither is a liquid crystal.Both CASS and 4Br-CASS are invariant cholesteric phase liquid crystals.Compared with CASS,the introduction of bromine atoms makes 4Br-CASS have a lower melting temperature,higher clear point,and wider temperature range of liquid crystal phase.It is worth noting that the reversible phase transition of 4Br-CASS in the solid crystal phase was induced by the introduction of halogen bromine atoms.