Syntheses And Properties Of Molecular Ferroelectrics With Biferroelectricity

Ferroelectrics,whose discovery can be traced back to 1920,refer to a class of electro-active materials with spontaneous polarization and reorientable spontaneous polarization under the external electric field.Due to their excellent electrical,mechanical,and optical switching characteristics,they are widely studied and applied to various types of memory elements,capacitors,transducers,actuators,and sensors.Compared with traditional inorganic ferroelectrics,molecular ferroelectrics have the advantages of environmental friendliness,tunable structure,easy synthesis,and low acoustic impedance,which are expected to become the next generation of high-performance functional devices.Liquid crystal is a mesocrystalline state intermediate between solid and liquid,which retains the anisotropy of the crystalline state while simultaneously maintaining the fluidity of the liquid.Its discovery has completely changed the display industry.The connection between ferroelectricity and liquid crystals（LCs）started in 1975 with the discovery of the first ferroelectric liquid crystal（FLC）p-decyloxybenzylidene p′-amino 2-methyl butyl cinnamate（DOBAMBC）.Ferroelectric LCs have unique application advantages in display and optoelectronics due to their exceptionally fast electro-optical response and the special photoelectric effect.Whether in ferroelectrics,liquid crystals,or even liquids,their physical properties are always constrained by symmetry,which is like a bridge to unify them.Therefore,the study of the ferroelectric change and symmetry relationship during continuous phase transitions in a single component is of great scientific significance for the design of high-performance electronic devices and theoretical research.In light of the above,this thesis regards liquid crystal compounds as the research object,and through reasonable chemical design,successfully designed multiple biferroelectric liquid crystal compounds with polarization switching characteristics in both solid and liquid crystal phases.In Chapter 2,three kinds of cholesterol-ester liquid crystal compounds were synthesized:4I-CACS,4I-CAHCS,and 4I-CASS.Among them,4I-CASS exhibits unique ferroelectricity,which shows polarization-switching behaviors in both SC and LC phases.Single chirality not only promotes 4I-CASS to crystallize in the polar point group C₂ in the solid phase,achieving a polarized and orderly arrangement of molecules but also ensures that it can form a layered spiral structure in the cholesteric liquid crystal phase,allowing 4I-CASS to exhibit dual ferroelectricity in both solid and liquid crystal phases.In Chapter 3,referring to the H/F substitution strategy,using hydrogenated cholesterol benzoate as a prototype,a ferroelectric material 4-F-BDC with ferroelectric properties in both solid and cholesteric liquid crystal phases was prepared by chemically modifying the benzene ring portion of the molecule,that is,introducing fluorine atoms and H/D substitution at different sites.The fluorinated 4-F-BDC equips higher solid-to-liquid crystal phase transition temperature,broader temperature window of liquid crystal phase,larger dipole moment for enhancing polarization,and greater piezoelectric response.More importantly,H/F substitution induces a new solid-solid phase transition in 4-F-BDC.In Chapter 4,three molecular ferroelectrics 4Cl-CB,4Br-CB,and 4I-CB have been successfully designed and synthesized through the substitution of halogen,which show interesting biferroelectric properties in both solid and liquid crystal phases.The perfect polarization-voltage hysteresis confirms the dual ferroelectricity of 4X-CB in both solid and liquid phases.In addition,4X-CB exhibits obvious polarization switching behavior in the solid phase and sensitive electro-optical response in the liquid crystal phase.As a pioneering study,these findings provide an effective approach for designing ferroelectrics with biferroelectric in solid and liquid crystal phases and will trigger more research efforts to further explore biphasic ferroelectrics with great application prospects.