Synthesis And Mesophase Structure Of Several Types Of Non-conventional Liquid Crystals

Liquid crystals,which combine order and mobility on a molecular level,are materials with anisotropic physical properties,switching-able under the influence of external stimuli(electric or magnetic field).Based on this unique feature,liquid crystals found practical applications in the fields of display,light shutter,sensor, and laser etc..In this dissertation,after a short review on recent progress in the sub-fields of liquid crystal dimers,polycatenar mesogens,and banana liquid crystals,the most attention was moved to our own research work,which mainly consists of three projects.1) Synthesis and mesomorphic properties of symmetric liquid crystal dimers containing hydrazide groupRecently,due to their novel molecular architecture(linear,H-shaped,and T-shaped et.al.),unique phase behaviours(especially,odd-even effect),and mesophase polymorphism(for example,biaxial nematic,intercalated smectic,and columnar B₁ phase),liquid crystal dimers have attracted much attention. In this project,a series of symmetric liquid crystal dimers containing hydrazide group were synthesized.Different terminal groups,such as nitro(α,ω-bis[N-(4-nitrobenzoyl)-N'-(benzoyl-4'-oxy) hydrazine]alkane,denoted as Nn, n indicates the number of the carbon atoms in the spacer),alkoxyl(α,ω-bis[N-(4-hexadecyloxybenzoyl)-N'-(benzoyl-4'-oxy) hydrazine]alkane,C16.n.C16), phenyl(1,6-bis[N-(4-biphenyl carbonyl)-N'-(benzoyl-4'-oxy)hydrazine]hexane, B6) and methyl(1,6-bis[N-(4-methylbenzoyl)-N'-(benzoyl-4'-oxy) hydrazine] hexane,M6) were taken to see their effect on the thermotropic phase behaviors and mesophase structures.It was found that intercalated smectic phase was observed in Nn series,and monolayer smectic phase was observed in C16.n.C16 series,while only nematic phase was formed by B6,and no mesophase was formed by M6.Temperature-dependent IR spectroscopic analysis on these two kind of dimers suggests that intermolecular hydrogen:bonding,as well as the dipole-dipole interaction arising from the strong polar substituents(-NO₂) synergistically drives the intercalated structure of Nn;although hydrogen bonding still exists in the monolayer smectic phase of C 16.n.C16,microphase segregation is in favor of the monolayer smectic structure,preventing the formation of the intercalated structure.2) Synthesis and mesomorphie properties of polyeatenar 1,3,4-oxadiazole derivativesLiquid crystalline organic semiconductors,which exhibit high carrier conduction in their mesophases,without the need to consider the boundary effects. were recognized as a new type of quality organic semiconductors.In this project,a series of polycatenar 1,3,4-oxadizole derivatives,i.e.1,4-bis[(3,4, 5-trialkoxyphenyl) - 1,3,4- oxadiazolyl]benzene(P-P-oxd-n,n indicates the number of carbons in the terminal chains) were synthesized.The liquid crystalline properties were investigated by differential scanning calorimetry,polarized optical microscopy and X-ray diffraction.It was found that P-P-oxd-n,exhibit columnar mesophases,and the symmetry of the mesophase changed from rectangular to hexagonal by increasing the length of the alkoxy chains.A detailed x-ray diffraction study suggests that each columnar slice should consist of two hexacatenar oxadiazole molecules with a tilt angle of 50°in Colt and 46°in the Colh phase with.respect to the columnar axis.The driving force for the columnar phase was considered to be micro-phase segregation between the rigid-rod core and the flexible chains,theÏ€-Ï€stacking between theÏ€-conjugated central aromatically substituted oxadiazole groups,as well as efficient filling of the space.3)Phase behaviors of binary mixtures of bent-core molecules and hexadecanePolarity and chirality are the most attractive two aspects of banana liquid crystals.In this project,banana molecules(Nobow) were doped with non-polar flexible carbohydrate chains,hexadecane;and the phase behavior of these binary mixtures was studied by polarized optical microscope,X-ray diffraction and Freeze fracture TEM.It was found that in these mixtures the B2 phase was depressed;the B3 phase disappeared;while the B4 phase seemed immune to the doped hexadecane.In the B2 phase of these mixtures,it was thought that a number of hexadecane molecules went in between the bent-core molecules,or the domains, while the rest are completely phase separated.Similarly,in the B4 phase,besides of completely phase-separated hexadecane molecules,a lot of hexadecane molecules went in between the banana molecules,the filaments of the twist smectic layers,or the domains;and the mixed hexadecane molecules restrained the nucleation of the B4 phase,so it is much easier to get lager chiral domains by mixing Nobow with hexadecane.Moreover,based on the microscopy observations, it could be concluded that the handness of the B4 chiral domains is determined during the nucleating,regardless of the layer chirality of the B2 phase.