| Fluorinated liquid crystals are widely used in liquid crystal displays due to their low viscosities and melting points, as well as improved dielectric properties. Benzoxazole-liquid crystals are among the mesogenic heterocyclic compounds and have been widely studied in recent years, including several kinds of benzoxazole derivatives bearing lateral fluorine atoms in the mesogenic core. However, most of them exhibit a smectic mesophase, which restrict their further possible application in liquid crystal display mixtures because of their poor nematic mesophase.To develop new nematic heterocyclic liquid crystals, and also to further investigate the effect of the monofluorine substituent on mesophase type and stability, benzoxazole derivatives bearing a lateral monofluorine atom on the inter-ring of the biphenyl unit are synthesized and their propertie s investigated. This thesis includes the following aspects:1. A series of nematic heterocyclic liquid crystals,2-(2’-fluoro-4’-alkoxy-1,1-biphenyl-4-yl)-benzoxazole derivatives (coded as nPF(2)PBx) bearing hydrogen (nPF(2)PBH), methyl (nPF(2)PBM), nitro (nPF(2)PBN) and chloro (nPF(2)PBC) terminal groups, respectively, are developped and investigated. Thirty two benzoxazole derivatives nPF(2)PBx are prepared via four-step reactions, including nucleophilic substitution, palladium catalyzed coupling reaction, nucleophilic addition, and subsequent intramolecular cyclisation with overall yields 14-64% and high purity greater than 98% (HPLC or GC). The results show that the nematic mesophase stability is obviously improved via slightly increasing inter-ring twist angle between the planes of two phenyl rings in biphenyl unit, where the modification of the twist angle can be achieved by adjusting the position of the lateral monofluorine substituent from outer-ring to inter-ring of biphenyl mesogenic core unit. Meanwhile, the inter-ring lateral fluorine substituent results in a decrease in both melting and clearing points but only nematic mesophase with an acceptable wide mesophase range for nPF(2)PBx (except nPF(2)PBC), and thus a potential application in liquid crystal display mixtures is expected, especially for the compounds nPF(2)PBM and nPF(2)PBN. The reduced π-π interaction/conjugation caused by the increased twist angle is considered as one of the possible reasons for the formation of nematic mesophase. These results suggest that modification of the twist angle for mesogenic unit is an effective method to improve the mesophase type and stability of the heterocyclic liquid crystals. The present compounds nPF(2)PBx have potential for use in liquid crystal display mixtures.2. Fluorine is widely used as a lateral substituent to modify the physical properties of liquid crystals. Here, Thirty two laterally monofluorinated compounds,2-(4’-alkoxy-2-fluoro biphenyl-4-yl)-benzoxazole derivatives (nPPF(2)Bx) bearing different substituents (H, CH3, NO2, Cl, coded as nPPF(2)BH, nPPF(2)BM, nPPF(2)BN and nPPF(2)BC, respectively) at 5-position, are synthesized via four-step reactions, including nucleophilic substitution, palladium catalyzed coupling reaction, nucleophilic addition, and subsequent intramolecular cyclisation with overall yields 15-53% and high purity greater than 98% (high performance liquid chromatography (HPLC) or gas chromatography (GC)). The structures of nPPF(2)Bx and the corresponding Schiff base precursors are confirmed by infrared (IR), proton nuclear magnetic resonance (’H-NMR), gas chromatography with electron impact-mass spectrometry (GC/EI-MS), and elemental analysis (EA). It is interesting to note that these only display enantiotropic nematic mesophases with mesophase ranges of 12-28℃ and 13-45℃ on heating and cooling for nPPF(2)BH,46-97℃ and 62-120℃ for nPPF(2)BM and 82-108℃ and 87-113℃ for nPPF(2)BN, which are very different from the corresponding monofluorine-substituted analogue (lateral monofluorine substituent at outer-ring of biphenyl) with enantiotropic smectic or smectic/nematic mesophases. The enhanced nematic mesophase is attributed to the reduced π-π interaction/conjugation resulting from the twisted structure of the molecule caused by the introduction of a fluorine atom into the inter-ring of the biphenyl unit. These results suggest that modification of the monofluorine substituent position is an effective method to improve the nematic mesophase in benzoxazole-liquid crystals. Meanwhile, the inter-ring lateral fluorine substituent also plays an important role in photophysical properties due to the decrease in π-conjugation resulting from the increased twist angle. The compounds nPPF(2)Bx have potential for use in liquid crystal display mixtures because of their acceptably wide nematic mesophase ranges.3. Eutectic mixtures of 4-cyano-4’-pentylbiphenyl (5CB) and laterally monofluorinated benzoxazole derivatives (nPF(2)PBx and nPPF(2)Bx) are prepared by mixing. Their anisotropy properties are investigated, where the benzoxazole derivatives display both high birefringence (An) and large dielectric anisotropy (△ε).4. It is noted that nitro terminated Schiff base intermediates nPF(2)PSN and nPPF(2)SN exhibit no mesophase on both heating and cooling, while other Schiff base intermediates bearing a short terminal alkoxy chain exhibit poor mesophase stability with a narrow smectic mesophase. The lower mesophase stabilities with narrower mesophase ranges for nPF(2)PSx and nPPF(2)Sx maybe related to their twisted molecular shapes resulted from the inter-ring fluorine substituent on biphenyl unit. |
