Synthesis, Characterization And Mesomorphic Properties Of Heterocyclic Liquid Crystal Oligomers Containing Thiazole And 1,2,3-Triazole Moieties
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Date
2015-06
Authors
AL-SHARGABI, ARWA ABDULWAHAB MANSOOR
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Abstract
This dissertation comprises synthesis of six series of new heterocyclic oligomers based on thiazole and 1,2,3-triazole moieties. The molecular structure of these oligomers were elucidated by different spectroscopic techniques. The anisotropic properties were investigated by using polarizing optical microscopy, differential scanning calorimetry and X-ray diffraction. The first series of oligomers possess U-shaped molecular structure. It comprise of two identical mesogenic units (4-((6-ethoxybenzothiazol-2-yl)azenyl)alkanoxyphenyl) connected by different aromatic core (unsubstituted and Br substituted benzene). Most of the homologues in this series show nematic and smectic A in which the lateral Br enhanced the thermal stability of the smectic A phase upon cooling to room temperature. The XRD experiment suggested a hair barrette arrangement for the smectic phase and the lateral Br caused disordering in the molecular arrangement in which these homologues having short d-spacing as compared with the members having lateral H. In the second series, the effect of the different chemical composition (catechol, dibromocatechol, resorcinol, hydroquinone and 4-4`-dihydroxybiphenyl) incorporated with (4-((6-ethoxybenzothiazol-2-yl)azenyl)decanoxyphenyl) has been investigated. The position of the mesogen affected the mesomorphic properties. The U-shaped oligomers in this series exhibited nematic and smectic phases whereas the rod-like homologues show predominantly enantiotropic nematic phase. Derivative of bent core structure is found to exhabit monotropic nematic phase. The third series is
derived from the reaction of bis(4-hydroxybenzylidene)1,4-phenylenediamine with 4-((6-ethoxybenzothiazol-2-yl)azenyl)alkanoxyphenyl. All the derivatives in this series exhibited enantiotropic nematic phase and the XRD experiment confirms the absence of smectogenic properties in this series. However, the other three series are non-symmetrical dimers. The dimers in the fourth series are cholesteryl ester and triazole in one arm connecting to diazophenyl mesogen as a peripheral unit via different alkyl spacers. All the homologues in this series exhibited enantiotropic nematic and smectic phases with high thermal stability. The XRD analysis has substantiated the presence of smectic phase and suggested the intercalated molecular arrangement for these homologues. In the fifth series in which the other peripheral arm made up by cholesteryl ester shows various mesophases (chiral nematic, twisted grain boundary A and chiral smectic C). The study on the phase transition temperature shows that the even-numbered dimers show higher transition temperatures as compared with odd-numbered dimers. The sixth series belong to the aforementioned series wherein the other peripheral mesogenic group is 2-methylbutyl-4-(4-alkoxyphenyl)benzoate. The members with short and medium alkyl spacers are purely chiral smectogenic whereas the member with long spacer exhibited chiral nematic and smectic A phases. The influence of the peripheral group on the thermal stability as well as on the mesomorphic properties for the dimers is evaluated. The study revealed that the dimers with more rigidity and bulky moieties show higher melting and clearing temperatures. Furthermore, the incorporation of highly chiral segment in the dimers led to chiral and frustrated mesophases such as SmC*, SmA* and TGBA*.
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Synthesis, Characterization And Mesomorphic Properties Of Heterocyclic Liquid Crystal Oligomers , Containing Thiazole And 1,2,3-Triazole Moieties