Browsing by Author "Mahmood, Wan Ahmad Kamil"

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Alkyl chain self ordering, induction and suppression of mesophase by Cu(II) containing [1,2,3]-triazole-based bidentate salicylaldimine ligands: synthesis, characterisation and X-ray diffraction studies
(2014-12-02)
Heng, Boon-Teck
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Yeap, Guan-Yeow
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Mahmood, Wan Ahmad Kamil
;
Han, Chun-Chieh
;
Lin, Hong-Cheu
Some new Cu(II) complexes containing [1,2,3]-triazole-based bidentate salicylaldimine and its analogues with terminal substituent (F, Cl, Br and I) have been synthesieed. All the target complexes and their uncoordinated ligands were elucidated by elemental analysis and spectroscopic techniques (UV-visible, FT-IR, 1D, 2D H-1 and C-13-NMR). The polarising optical microscope and differential scanning calorimetry (DSC) have disclosed all complexes and ligands are mesomorphic except the complex without any terminal substituent. The fluoro-substituted complexes with even parity C14H29 and C16H33 exhibit new enantiotropic nematic phase which was absent in their corresponding ligands, whereas the suppression of SmC phase occurred for all complexes with longer C16H33 and C18H37. X-ray diffraction confirmed the existence of SmA, SmC and N phases for complexes and ligands. The other notable feature is that the self-ordering of terminal alkyl chain occurred in SmA and SmC phases of complexes with even terminal alkyl chain ranging from C14H29 to C18H33. Their corresponding ligands exhibit intercalated structure of SmA and SmC phases. The thermal behaviour studies show that the fluoro-substituted triazole-based complexes possess lowest phase transition temperature and more stable as compared to other substituent which decomposed during the isotropisation.
Liquid crystal dimers containing Cholesteryl and Triazole-containing mesogenic units
(1970-01-01)
Al-shargabi, Arwa
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Yeap, Guan-Yeow
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Mahmood, Wan Ahmad Kamil
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Han, Chun-Chieh
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Lin, Hong-Cheu
New liquid crystals categorised as cholesteryl dimers have been successfully synthesised through the reaction between cholesteryl 4-(prop-2-ynyloxy)benzoate moieties with n-azido(cholesteryloxy-carbonyl)alkane. All the dimers display enantiotropic mesophases. Whilst the odd-numbered dimers exhibit chiral nematic (N*), twisted grain boundary (TGB) and chiral smectic C (SmC*) phases, the even-numbered members from the same series show chiral smectic A and C. A detailed inspection on mesophase reveals that the chiral centres and the bent conformation of the odd-numbered members are essential for the induction of TGB phase. However, upon decreasing the temperature, the ratio of the transition temperatures (TSmC*-SmA*/TSmA*-I) is found to be 0.95, which indicate the second order transition according to the McMillan's molecular theory. In addition, the X-ray diffraction study supports the presence of the smectic A phase on the even members rather than the N* by the appearance of the Bragg diffraction peaks at 190 degrees C. A comparison study with the other analogues in which the cholesterol entity is substituted by azobenzene or biphenyl tails has been carried out to assess the relationship between the molecular structure and mesomorphic behaviour. [GRAPHICS] .
Non-symmetrical liquid crystal dimers armed with azobenzene and 1,2,3-triazole-cholesterol
(2015-09-02)
Alshargabi, Arwa
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Yeap, Guan-Yeow
;
Mahmood, Wan Ahmad Kamil
;
Han, Chun-Chieh
;
Lin, Hon-Cheu
New non-symmetrical dimers which contain respective side units of azobenzene and 1,2,3-triazole-choloesteral have been synthesised from the reaction between 6-azido-[4-(4-acetylphenylazo)phenoxy]alkane and cholesteryl-4-(prop-2-ynyloxy)benzoate. The results show that the derivatives of 6-bromo[4-(4-acetylphenylazo)phenoxy]alkanes which possess the odd alkyl spacers exhibit dimesomorphism of N and SmA phases whilst the members with even numbers are non-mesogenic. However, the 6-azido-[4-(4-acetylphenylazo)phenoxy]-alkanes show N and SmA phases in a wide temperature range. On heating and cooling the target compounds cholesteryl 4-[(1-(6-(4-(4\'-acetylphenylazo)phenoxy)alkyl)-1H-1,2,3-triazol-4-yl)methoxy]benzoate show chiral nematic (oily streak and fan-shaped texture) and focal conic texture characteristics of smectic A phases with high thermal stability. Present investigation also reveals the effect of the spacer length upon the phase transition temperature. Intercalated arrangement is favoured by the molecules in the smectic phase. The observation is well established by XRD measurement wherein the actual molecular length is smaller than the theoretical value.
Synthesis, characterization and molecular organization for induced smectic phase of triazole ring in non- symmetric liquid crystalline dimer
(2015-06-10)
Yeap, Guan-Yeow
;
Alshargabi, Arwa
;
Mahmood, Wan Ahmad Kamil
;
Han, Chun-Chieh
;
Lin, Hong-Cheu
New non-symmetric liquid crystal dimers containing cholesterol, triazole and biphenylene segments have been synthesized via click reaction. Increase in the flexible spacers length affects the mesophase formation. The homologues with short and medium alkyl spacer exhibit SmA* and SmC* phases whilst the analogue with the longest spacer favors the formation of enantiotropic SmA and N* phases. The XRD studies reveal the emergence of the SmA* and SmC* phases that can be associated with the monolayer ordering of these dimers. Further investigation through a comparison study shows that the triazole ring affects the mesophase formation wherein apart from the molecular dipole the subtle electrostatic interaction and van der Waal\'s forces enhance the SmC* phase. (C) 2015 Elsevier Ltd. All rights reserved.