Study On The Synthesis And Properties Of Benzoheterocyclic Liquid Crystal Compounds

The liquid-crystalline state of matter exists between the crystalline solid and amorphous liquid states.Liquid crystals share the anisotropic(direction dependent)properties of crystalline solids and fluid properties of isotropic liquids.As we all know,the properties of liquid crystal depends on the structure of the compounds.The introduction of heteroatoms(N,O,S)can have an effect on the structure and properties of the compound.Many calamitic mesogens containing heterocyclic core,such as benzimidazole,benzoxazole and coumarin,has been developed well in recent years.Because of the large electronegativity of nitrogen atoms,hydrogen bonds are easily formed between the benzimidazole compound,which may make the molecular structure more stable,but this leads to an increase in the melting point of the compound at the same time.And a lower melting points and a wider mesophase would be achieved for fluorinated compounds.In order to investigate the influence of hydrogen bonds which is formed between the imidazole molecules,and the effect on the phase transition temperatures and other properties of the mesogens when introducing allyloxy in the molecule,we designed and synthesized methyl-substituted benzimidazole compounds and benzoxazole compounds with allyloxy groups,and the research content is described in detail.In our research,a series of 1-methyl-1H-benzimidazole-based compounds,2-(4'-alkoxy-1,1'-biphenyl-4-yl)-1-methyl-1H-1,3-benzimidazole derivatives(nPPMx-M)with different terminal moieties were synthesised and their structures were characterized.Using 4-iodophenol as raw material,the target product was obtained through etherification reaction,coupling reaction,nucleophilic reaction,aldol condensation and nucleophilic reaction.The properties were measured by DSC and POM,and the results were displayed as follows:when the terminal group of the compound is hydrogen and methyl,the compound exhibits a wide smectic phase on the heating and cooling.However,when the terminal substituent is nitro and the number of carbon atoms is from 4 to 8,the compounds exhibit a nematic phase on heating and cooling,and for the further carbon atoms,the compound only exhibit a smectic phase.Compared to non-1-methyl substituted analogs,nPPMxM show much lower melting and clearing temperatures,but much wider mesophase range both in heating and cooling process were achieved,which maybe due to the disruption of hydrogen bonding among the molecules caused by methyl substitution at 1-position of benzimidazole moety.Especially,nPPMx-M with a short terminal alkoxy chains(n = 4?5)display relative larger mesophase reanges on heatng and cooling.The compounds give enantiotropic liquid crystalline phases with mesophase ranges of 24-129 ? and 74-207 ? on heating and cooling processes.A series of benzoxazole compounds(nPPF(2)BP)with allyloxy groups were designed and synthesized.Using 4-iodophenol as raw material,the target molecules were prepared via reactions of nucleophilic substitution,Suzuki coupling,nucleophilic reaction,dehydrogenation and methyl bromide and nucleophilic reaction.The propertie of the target molecules is measured by DSC and POM.It was found that the newly designed compounds have both smectic phase and nematic phase in the heating and cooling.And the melting point of the compound has been significantly decreased.This may caused by the introduction of allyloxy at the end of the molecule,which increases the conjugation of the rigid core in the molecule,and the effect leads to a more ordered arrangement among the molecules.The compounds give enantiotropic liquid crystalline phases with mesophase ranges of 24-129 ? and 74-207 ? on heating and cooling processes.The properties of the prepared compounds were measured.The effects of different alkoxy chain lengths and terminal substituents on the properties of the compounds were investigated.With the growth of the alkoxy chain of the compounds,the melting point and clearing point of the compounds showed a decrease trend with the growth of the carbon number,at the same time,the molecular arrangement is more orderly,which is more conducive to the form the smectic phase.The compounds with different terminal substituents have the different properties.The polarity substiuent is helpful to improve the stability of the compounds.The compounds give enantiotropic liquid crystalline phases with mesophase ranges of 71.6-97.3 ? and 87.1-136.2 ? on heating and cooling processes.