Synthesis And Properties Of Novel Multi-arm Azobenzene Liquid Crystal Macromolecules Based On Cholic Acid

With the development of liquid crystal chemistry in recent years,the design and synthesis of novel liquid crystal compounds and liquid crystal polymers has become an important research direction in the field.The synthesis of multi-arm liquid crystal compounds composed of central atoms or groups and several mesogenic units has broken through the restriction of traditional long-chain rod-shape liquid crystal in theory.They belong to unconventional liquid crystal compounds with the differences from ordinary liquid crystal dimers,trimers,tetramers,oligomers and traditional rod-shape liquid crystal compounds.This kind of liquid crystals have excellent mesogenic properties,and their applications involve many fields,such as supramolecular chemistry,nanomaterials,biomedical carriers and optical transmission.Centerd as cholic acid multi-arm azobenzene liquid crystals are novel unconventional liquid crystal compounds.With the different terminal substituents and the different flexible spacer lengths of mesogenic arms will cause excellent and stable mesophase performance,and the mesogenic ranges will change regularly.Because of the existence of the azo group,when heated or irradiated by different wavelengths,these compounds will show optical activity,resulting in photoisomerization.Twenty azobenzene mesogenic arms in five homologous groups with different termial substituents,each with different flexible chain lengths,were designed and synthesized on the basis of consulting relevant literatures at home and abroad.Two groups of mesogenic arms with excellent properties were screened out through contrasting and analyzing their properties.Eight three-arm azobenzene liquid crystal macromolecules and eight four-arm azobenzene liquid crystal macromolecules were prepared through that selected mesogenic arms were linked to cholic acid by esterification reaction.Eight azobenzene liquid crystal polymers were synthesized by the reaction of two selected homologous groups mesogenic arms with polyvinyl alcohol.This paper includes the following three major parts:In the first part,five azobenzene monophenols with different terminal substituents were designed and synthesized,named as 4-(methoxy phenylazo)phenol(AP1),4-(methyl phenylazo)phenol(AP2),4-phenylazo phenol(AP3),4-(chloro phenylazo)phenol(AP4),4(nitro phenylazo)phenol(AP5).They were reacted with adipic acid,suberic acid,sebacic acid and dodecanedioic acid to prepare mesogenic arms with different terminal substituents and different flexible chain lengths,named as 4-(p-methoxyphenyl azophenoxy)formyl acid(LCA-O-n),4-(p-methylphenyl azophenoxy)formyl acid(LCAM-n),4-(phenyl azophenoxy)formyl acid(LCA-H-n),4-(p-chlorophenyl azophenoxy)formyl acid(LCA-C-n),4-(p-nitrophenyl phenoxy)formyl pentanoic acid(LCA-N-n),n=4,6,8,10,respectively.The structures of the compounds were analyzed by FT-IR,1H-NMR and elemental analysis(EA).The results showed that the synthesized compounds conformed to the molecular design.The properties of the compounds were measured by differential scanning calorimetry(DSC),polarized optical microscopy(POM)and ultraviolet-visible spectrophotometer(UV-Vis).It indicated that homologous series of compounds(LCA-O-n,LCA-N-n)with methoxy and nitro as terminal substituents belonged to photosensitive thermotropic enantiotropic nematic liquid crystals.Among the compounds with methyl terminal substituents,LCA-M-4 belonged to thermotropic enantiotropic nematic liquid crystals.The other homologous series of compounds did not belong to liquid crystal compounds.For the homologous series of compounds with different terminal substituents,their melting transitions(Tm),mesophase-isotropic transitions(Ti)on heating and isotropic-mesophase/crystalline transition on cooling decreased with the increase of flexible chain length.In the second part,we designed and synthesized multi-arm liquid crystal macromolecules based on cholic acid.Three-arm liquid crystal macromolecules(TACAO-n,n=4,6,8,10)with methoxy terminal substituent and those with nitro(TACA-N-n)were synthesized by esterification reaction.Four-arm liquid crystal macromolecules(FACAE-O-n,FACAE-N-n)were synthesized by further reaction of homologous three-arm liquid crystal macromolecules with azophenol.Through structural analysis,it showed that the synthesized liquid crystal compounds conformed to the molecular design.The properties of three-arm and four-arm liquid crystal compounds were tested by DSC,POM and UV-Vis.The results showed that FACAE-O-n and FACAE-N-n all belonged to photosensitive thermotropic enantiotropic nematic liquid crystal compounds.Their melting transitions(Tm),liquid crystal-isotropic transitions(Ti)on heating and cooling all decreased with the carbon numbers of flexible chain in mesogenic arms increasing.Under ultraviolet irradiation,the homologous series of compounds did not shift red or blue with the increase of carbon numbers of flexible chain.After photo-induced,the melting transitions(Tm)and LC-isotropic transitions(Ti)on heating and cooling of the four-arm liquid crystal compounds decreased,and the mesogenic ranges became shorter.Among them,the mesogenic ranges of FACAE-O-n on heating and cooling decreased with the increase of carbon numbers in the flexible chain,and the effect of photo-induced decreased with the increase of the flexible chain length.The mesophase performance of FACAE-O-4 was the most excellent with the mesogenic range 100.2℃ on heating while 129.0℃ on cooling.For the nitro-terminated homologous series FACAE-N-n,the mesogenic ranges decreased with the increase of carbon numbers of flexible chain only on heating.The cooling mesogenic ranges showed a trend of rising firstly and then falling,reaching the maximum 103.1℃ when carbon numbers of flexible chain were ten(FACAE-N-8).The effect of photo-induced increased with the length of the flexible chain grew longer.In the third part,the azobenzene side chain liquid crystal polymer with PVA as the main chain was designed and synthesized.The mesogenic arms designed and synthesized in the first part were used as monomers to be grafted onto polyvinyl alcohol(PVA)by esterification forming side chain polymer series PVA-O-n and PVA-N-n.The lengths of side chain flexible interval of each series were different.The structure analysis showed that the synthesized liquid crystal polymers conformed to the molecular design.DSC and POM were used to test the properties of them.The results showed that the two series of polymers belonged to crystalline thermotropic enantiotropic nematic liquid crystal.The melting transition temperatures(Tm)of PVA-O-n were lower than those of the monomers while the side chain was the series of methoxybenzazobenzene monomers.With the increase of the length of the side chain flexible interval,the melting point of the series showed a decreasing trend.At the same time,with the increase of carbon number of side chain flexible spacer,liquid crystal-isotropic transitions(Ti)decreased both on heating and cooling,and the mesogenic ranges became shorter.PVA-O-4 behaved the excellent mesogenic performance.The melting transition temperatures(Tm)of PVA-N-n decreased significantly when the side chains were nitrobenzene azobenzene monomers comparing with the monomers.With the increase of the length of the side chain flexible interval,the melting point of the series decreased.With the increase of carbon number of side chain flexible spacer,liquid crystal-isotropic transitions(Ti)increased firstly and then decreased on heating and cooling.And the mesogenic ranges also showed the trend of increasing firstly and then decreasing on heating and cooling,reaching the maximum at n=6.PVAN-6 behaved an excellent mesogenic performance.