Preparation And Properties Of Benzoheterocyclic Fluorine-containing Liquid Crystal Compounds

As one of the calamitic mesogens containing heterocyclic core,benzoxazole derivatives have attracted growing interests in recent years for fundamental research and potential application due to their optical and photochemical properties.It is found that by modifying the molecular structure,introducing a lateral or terminal fluorine atom into the mesogen can effectively improve the molecular mesomorphic properties.In order to prepare more benzoheterocyclic liquid crystal compounds,and also to further investigate the effect of fluorine substitution on mesophase type and stability,benzoxazole derivatives bearing difluoro-substituted atoms and benzimidazole derivatives bearing a lateral monofluorine atom on the inter-ring of the biphenyl unit are synthesized and their propertie are investigated.(1)A series of lateral disfluoride nematic heterocyclic compounds such as 2-(2,3'-difluoro-4'-alkoxyciphenyl)benzoxazole(nPF(2)PF(3)Bx)(nPF(2)PF(3)BH,nPF(2)PF(3)BM and nPF(2)PF(3)BN).The preparation of the compounds was carried out by using four-step organic reactions of 3-fluoro-4-bromophenol as starting materials by Williamson etherification,Suzuki coupling,nucleophilic reaction and dehydrogenation reaction.A total of 24 target products were prepared.The rate was 8.7 to 51.5%and GC/HPLC was 98%or more.And the structures of the intermediates and the target products were confirmed by 'H-NMR,MS,IR and EA.And then through the DSC,POM its liquid crystal performance test,after analysis can be drawn,They mainly display enantiotropic nematic mesophases with wider mesophase ranges of 12.4-107.4?(heating process)and 22.1-134.1?(cooling process)than the corresponding analogs.The enhanced nematic mesophase stability is achieved via slightly increasing inter-ring twist angle with inter-ring lateral fluorine substitute in biphenyl unit,as well as through improving the molecular polarity with multifluorine substitutes.Meanwhile,two inter-ring lateral fluorine atoms lead to a decrease in melting/clearing points and a wide nematic mesophase range,which makes it possible for heterocyclic mesogens nPF(2)PF(3)Bx to use in nematic liquid crystal display mixtures.(2)Series of compounds,2-(3,3'-difluoride-4'-alkoxy-1,1'-biphenyl)-benzoxazole derivatives(nPF(3)PF(3)Bx)bearing different substituents((H,CH3,NO2,coded as nPF(3)PF(3)BH?nPF(3)PF(3)BM and nPF(3)PF(3)BN)at 5-position are prepared.The preparation of the compounds was carried out by using four-step organic reactions of 3-fluoro-4-bromophenol as starting materials by Williamson etherification,Suzuki coupling,nucleophilic reaction and dehydrogenation reaction.The structures of interme-diates and the desired products are confirmed by 1H-NMR,MS,IR and EA.The yield is from 15.9%to 72.1%,and the purity of the desired products are over 98%which we re tested by GC or HPLC.According to the results from differential scanning calorimetr y(DSC)and polarizing optical microscopy(POM),all of the compounds exhibit enanti otropic smectic mesophases and nematic mesophase,for which the mesophase ranges ar e 25.4-39.6? and 17.3-76.8? on heating and cooling for nPF(3)PF(3)BH,42.5-93.2? and 53.6-123.1? for nPF(3)PF(3)BM,and 60.4-81.8? and 73.8-152.1? for nPF(3)PF(3)BN,respectively.It display a wide mesophase range,which may be caused by the lateral flurine,locating in the outer of the biphenyl crystal unit.(3)Series of laterally monofluorinated compounds,bearing different substituents(H,CH3,NO2,coded as nPPF(3)MH,nPPF(3)MM and nPPF(3)MN,respectively)at 5-position,were prepared and their structures were characterized.A series of 2-(3'?fluoro-4'-alkoxylbiphenyl)benzimidazole liquid crystal compounds(nPPF(3)Mx)were synthesized by four-step organic reactions via Williamson etherification,Suzuki coupling,nucleophilic reaction and aldolamine condensation).According to the results from differential scanning calorimetry(DSC)and polarizing optical microscopy(POM),the present compounds nPPF(3)Mx exhibit enantiotropic smectic mesophases,for which the mesophase ranges are 12.7-66.7? and 46.9-111.4? on heating and cooling for nPPF(3)MH,84.0-111.7? and 126.3-154.4? for nPPF(3)MM,and 23.1-102.3? and 48.9-117.4? for nPPF(3)MN,respectively.Compared to non-fluorinated analogs,monofluorinated nPPF(3)Mx have low melting/clearing points and display enhanced mesophase range both in heating and cooling,which are attributed to the disruption of the side-to-side intermolecular packing caused by the ortho lateral fluoro substituents,and the increased dipole-dipole interaction between the polar fluoro substituted molecules,respectively.It is noted that nPPF(3)MM and nPPF(3)MN show a much wider mesophase range than nPPF(3)MH,which suggest that the substituent at benzimidazole moiety can improve the mesophase stability.