Preparation And Properties Of Lateral Difluoro Substituted Bibenzoxazole Liquid Crystals

Due to the introduction of unsaturated heteroatoms(O,N,S)and/or others,the properties of heterocyclic liquid crystals,such as,mesophase behavior,phase transition temperature and dielectric anisotropy are strongly affected.As a kind of heterocyclic structure,benzoxazole has been extensively studied depended on its strong intermolecular force and π-electron conjugation efficiency in nonlinear optical materials and polymer materials.However,the application of benzoxazole derivatives is restricted due to the layered structure of the smectic mesophase.So we tried to eliminate the smectic mesophase and broaden the nematic mesophase by introducing different terminal substituents and adjusting positions and/or numbers of lateral fluorine substituents.This paper designed and synthesised a series of 2,3’-difluoro-1,1’-biphenylberunzoxazole derivatives and 2,2’-difluoro 1,1’-biphenylbenzoxazole derivatives.The research concentrated on the effects of terminal substituents,alkoxy chains and lateral fluorine atoms on mesophase type and stability.The details are as follows:(1)A total of 39 benzoxazole compounds(nPF(3)PF(2)Bx,x = H,CH3,NO2)were synthesised via four step reactions,including Williamson etherification,palladium-catalyzed Suzuki-Miyaura cross-coupling,nucleophilic addition and intramolecular cyclization in a yield of 68-99%,and the purity by high performance liquid chromatography(HPLC)was greater than 98%.The structure was confirmed by a nuclear magnetic resonance spectrometer(NMR)and a gas chromatography-mass spectrometer(GC-MS),etc.The compounds nPF(3)PF(2)BH present a monotropic nematic mesophase,nPF(3)PF(2)BM and nPF(3)PF(2)BN both present an enantiotropic nematic mesophase.The nematic phase stability is extremely high for nPF(3)PF(2)BH and nPF(3)PF(2)BM,and compounds with methyl terminal substituent have the widest mesophase range of about 33.8-109.1 ℃.(2)A total of 39 benzoxazole compounds(nPF(2)PF(2)Bx,x = H,CH3,NO2)were prepared from 4-bromo-3-fluorophenol via the same reaction above.The yield was 77-98%and the purity of HPLC was more than 98%.The structure was confirmed by 1H-NMR,GC-MS,etc.The dihedral angle increased above 37.8° by introducing two fluoro atoms at the 2-and 2’-positon of 1,1’-biphenyl unit.nPF(2)PF(2)BM and nPF(2)PF(2)BM both exhibited a stable nematic phase for 35.7-85.8 ℃ and 50.9-92.3℃,respectively.(3)The smectic phase of terminal chloro-substituted benzoxazole derivatives was inhibited by regulation of lateral fluorine substituent and terminal electron-donating/-withdrawing substituents.Ten of nematic phase chlorine-substituted benzoxazole derivatives(nPF(2)PF(2)BC)were obtained.All of these compounds are enantiotropic liquid crystals,and the nematic phase range is 12.5-53.8 ℃ and 8.1-83.4 ℃ for heating and cooling processes,respectively.(4)The optical anisotropy and dielectric anisotropy of 2,2’-difluoro-1,1’-biphenylbenzoxazole derivatives were tested,and the results indicated that benzoxazole derivatives have large optical anisotropy,and it could improve the clearing point and birefringence properties by doping them into the liquid crystal mixtures.