Browsing by Author "Ha, Sie-Tiong"

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Heterocyclic benzothiazole-based liquid crystals: synthesis and mesomorphic properties
(2009)
Ha, Sie-Tiong
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Koh, Teck-Ming
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Lin, Hong-Cheu
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Yeap, Guan-Yeow
;
Win, Yip-Foo
Two homologous series of 2-(4-alkanoyloxybenzylidenamino)benzothiazoles and 2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles were synthesised and characterised. Their molecular structures differed wherein the latter comprised a lateral hydroxyl group, unlike the former. Spectroscopic techniques such as FT-IR, 1H 13C NMR and mass spectrometry together with elemental analysis were employed to elucidate the molecular structures. The transition temperatures and their mesophases were determined by differential scanning calorimetry, optical polarising microscopy and X-ray diffraction techniques. Members with decanoyloxy till hexadecanoloxy chain in the series without the lateral hydroxyl group each exhibited a smectic A phase, while those in the series with the lateral hydroxyl group were non-mesogenic. The mesomorphic properties of the present series were compared with other structurally related series to establish the chemical structure-mesomorphic properties relationship.
Heterocyclic pyridine-based liquid crystals: synthesis and mesomorphic properties
(2018-01-01)
Ong, Lay-Khoon
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Ha, Sie-Tiong
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
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材料科學與工程學系
A series of new calamitic liquid crystals, 4-{[(pyridin-4-yl)methylidene]amino}phenyl 4-alkoxybenzoates comprising a heterocyclic (pyridine) and two phenyl rings core system, terminal alkoxy chain, imine and ester linkers were synthesised and characterised. This series consists of nine members wherein the members differ by the length of alkoxy chain (CnH2n+1O-, where n=2, 4, 6, 8, 10, 12, 14, 16, 18). Spectral analysis results were in accordance with the expected structure. Their thermotropic behaviours were studied by using differential scanning calorimetry, optical polarising microscopy and powder X-ray diffraction techniques. A single mesophase (nematic) was observed for the first three members of the series (n=2, 4 and 6). As the alkoxy chain increased to n=8 and n=10, the nematic phase appeared together with an additional smectic A (SmA) phase. When moving from n=12 until the highest members (n=18), the nematic phase disappeared and these compounds only exhibited a single mesophase (SmA). [GRAPHICS] .
Mesogenic Schiff base esters with benzothiazole core: synthesis and phase transition studies
(2010)
Ha, Sie-Tiong
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Koh, Teck-Ming
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
;
Lee, Siew-Ling
A series of new rod-shaped mesomorphic compounds, 6-ethoxy-2(4-alkanoyloxybenzylidenamino)benzothiazoles, consisting of a 2,6-disubstituted benzothiazole core and a Schiff base central linkage, were synthesized and their structures were ascertained via elemental analysis and spectroscopic techniques. Their mesomorphic properties were studied using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD) analysis. All compounds showed enantiotropic mesomorphism. Whilst the lower members of the series, hexanoyloxy and octanoyloxy derivatives exhibited nematic phase, the higher members (decanoyloxy, dodecanoyloxy, tetradecanoyloxy, hexadecanoyloxy and octadecanoyloxy derivatives) exhibited nematic and smectic C phases.
Mesogenic Schiff base esters with terminal chloro group: Synthesis, thermotropic properties and X-ray diffraction studies
(2010-05-01)
Ha, Sie-Tiong
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Ong, Lay-Khoon
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Sivasothy, Yasodha
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
A new homologous series of Schiff base esters comprising a terminal chloro substituent was studied. The chloro substituent contributes to the molecular polarizability, thus, affecting intermolecular interactions, hence, resulting in smectic polymorphism. The mesomorphic properties were studied using differential scanning calorimetry, polarizing optical microscopy and temperature-dependent X-ray diffractometry. Whilst the n-pentanoyloxy and n-heptanoyloxy derivatives exhibited enantiotropic smectogenic A and B phases, monotropic smectogenic A and B phases were observed in the n-butanoyloxy and n-hexanoyloxy derivatives. Enantiotropic smectogenic A and monotropic smectogenic B phases were observed in the n-octanoyloxy to n-hexadecanoyloxy derivatives. The highest member of this series, the n-octadecanoyloxy derivative, exhibited an enantiotropic smectogen A phase. The homologous members were compared with structurally related series to establish their chemical structure-mesomorphic behavior relationships.
Mesogenic Schiff Base with Bromo End Group: Synthesis and Thermotropic Properties
(2012-08-01)
Ha, Sie-Tiong
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Lee, Teck-Leong
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Lin, Hong-Cheu
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Ito, Masato M.
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Abe, Kazuma
A new homologous series of Schiff base esters comprising a terminal bromo substituent was studied. The bromo substituent contributes to the molecular polarizability, thus, affecting intermolecular interactions, hence, resulting in smectic polymorphism. The mesomorphic properties were studied using differential scanning calorimeter and polarizing optical microscope attached to a Linkam hotstage. Whilst the lower members, n-ethanoyloxy and n-butanoyloxy derivatives were non-mesogens, n-hexanoyloxy member exhibited monotropic (less stable) smectic A and smectic B phases. Further lengthening of the alkanoyloxy chain from C8 to C12 (medium members), led to formation of enantiotropic (more stable) smectic A and smectic B mesophases based on the observation of both phases during heating and cooling cycles. The high members of the series, C14, C16 and C18 exhibited enantiotropic smectic A phase and monotropic smectic B phase. The homologous members were compared with structurally related series to establish their chemical structure-mesomorphic behaviour relationships.
Mesogenic Schiff's base ether with dimethylamino end group
(2009)
Ha, Sie-Tiong
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Ong, Lay-Khoon
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Wong, Joanna Pik-Wan
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
A series of Schiff base ethers comprising a dimethylamino group at one terminal position and an even number of carbons at the other along the molecular axis was studied. The dimethylamino unit, acting as a charge-transfer donor in the formation of conjugative interactions, contributes to the molecule's polarizability and in turn affecting its intermolecular interactions, leading to the formation of less-ordered mesophase. The mesomorphic properties were studied using polarizing optical microscopy and differential scanning calorimetry. Whilst the n-butyloxy to n-hexadecyloxy derivatives exhibited nematic phase, the n-ethyloxy, n-propyloxy and n-octadecyloxy derivatives were not mesogenic. Their phase transitions during heating and cooling run were presented. The title compounds were compared with other structurally related series to establish their chemical structure-mesomorphic property relationships.
New mesomorphic benzothiazol derivatives: Synthesis and characterization
(2013-10-01)
Koh, Teck-Ming
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Ha, Sie-Tiong
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
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材料科學與工程學系
In this paper, the synthesis of new mesomorphic benzothiazolyl derivatives, 6-ethoxy-2-[4-(4-alkyloxybenzoyloxy)benzylidenamino]benzothiazoles, is presented. The structures of the title compounds were elucidated using spectroscopic techniques, such as FT-IR, NMR (H-1 and C-13), elemental analysis and EI-MS. The mesomorphic behaviours of these compounds were determined by differential scanning calorimetric and polarizing optical microscopic techniques. Compounds exhibited nematic and tilted smectic phases upon heating from crystal phase. An obvious odd-even effect was observed in this homologous series. (C) 2013 Sie-Tiong Ha. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Synthesis and mesomorphic behaviours of a homologous series of 2-(4-alkyloxybenzylidenamino)benzothiazoles
(2014-11-02)
Foo, Kok-Leei
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Ha, Sie-Tiong
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Win, Yip-Foo
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
The synthesis and characterisation of a series of new rod-shaped liquid crystal (LC), 2-(4-alkyloxybenzylidenamino)benzothiazoles possessing even number of carbon atoms at the alkyloxy chain (CnH2n + 1O-, n=2, 4, 6, 8, 10, 12, 14, 16, 18) are reported. The molecular structures of title compounds were elucidated using FTIR and nuclear magnetic resonance spectroscopic techniques. The phase behaviour of these new compounds was characterised and studied by differential scanning calorimetry and polarising optical microscopy. Lower members (n=2, 4 and 6) did not exhibit mesophase. Octyloxy derivative was monotropic LC. Enantiotropic smectic A phase was observed from n-decyloxy derivatives onward to the last homologous synthesised.
Synthesis and Mesomorphic Properties of 2-(4-Alkyloxyphenyl)benzothiazoles
(2009)
Ha, Sie-Tiong
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Koh, Teck-Ming
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
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Boey, Peng-Lim
A homologous series of 2-(4-alkyloxyphenyl)benzothiazoles with an even number of carbons in the terminal alkoxyl chain was synthesized and characterized. The structure of the compounds was postulated using elemental analyses, FT-IR, NMR, and mass spectrometry techniques. Their liquid crystalline properties have been investigated by optical polarizing microscopy, DSC, and X-ray diffraction (XRD) techniques. Whilst the n-octyloxy to n-hexadecyloxy derivatives exhibited a monotropic smectic A phase, the n-ethoxy to n-hexyloxy and the n-octadecyloxy derivatives were not mesogenic. The mesomorphic properties of the present series were compared with other structurally related series to establish the chemical structure-mesomorphic properties relationship.
Synthesis and Mesomorphic Properties of 4-(Methylthio)benzylidene-4 '-n-alkanoyloxyanilines
(2011-10-01)
Ha, Sie-Tiong
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Lee, Teck-Leong
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
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Ito, Masato M.
A homologous series of Schiff base esters, 4-(methylthio)benzylidene-4'-n-alkanoyloxyanilines possessing even number of carbon atoms at the alkanoyloxy chain (C(n-1)H(2n-1)COO, n = 2 to 18) was synthesized. The structure of the 4-(methylthio)benzylidene-4'-n-alkanoyloxyanilines was confirmed by elemental analysis and spectroscopic techniques including FT-IR, (1)H and (13)C NMR and EI-MS. Mesomorphic properties were evaluated by differential scanning calorimetry and polarizing optical microscopy techniques. Whilst n-butanoyloxy to n-octanoyloxy derivatives exhibited monotropic nematic phase, n-ethanoyloxy and n-decanoyloxy to n-octadecanoyloxy derivatives were non-mesogenic compounds. The mesomorphic properties of the present series were compared with other structurally related series to establish the chemical structure-mesomorphic property relationship.
Synthesis and Mesomorphic Properties of 6-Methoxy- and 6-Ethoxy-2-(2-Hydroxy-4-Alkanoyloxybenzylidenamino)Benzothiazoles
(2010)
Ha, Sie-Tiong
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Koh, Teck-Ming
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
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Lee, Siew-Ling
Two new series of Schiff base thermotropic liquid crystals, 6-methoxy-2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles and 6-ethoxy-2-(2-hydroxy-4-alkanoyloxybenzylidenamino)benzothiazoles, comprising different terminal groups, a methoxyl and ethoxyl group, respectively, were synthesized. Structural elucidation was carried out using elemental analysis and spectroscopic techniques such as Fourier transform infrared (FTIR), 1H and 13C nuclear magnetic resources (NMR), and mass spectrometry. The mesomorphic properties and thermal stabilities of the title compounds were studied by using differential scanning calorimetry, optical polarizing microscopy, and thermogravimetric analysis. No liquid-crystal phases were observed for the short-chain members (n=2 and 3) in both series, and the remaining members all exhibited nematic phase with Schlieren or marble-like textures. Effects of the lateral hydroxyl group, terminal group, and the length of the terminal alkanoyloxy chain on the mesomorphic properties are discussed. Structure-property relationships were established upon comparison with other structurally related compounds.
Synthesis and phase transition behaviours of laterally substituted liquid crystals containing methylhydroquinone: emerging of smectic C phase for higher homologues
(2017-01-01)
Seou, Chi-Kien
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Ha, Sie-Tiong
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Lee, Siew-Ling
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Yeap, Guan-Yeow
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Lin, Chien-Min
A series of laterally substituted low-molar-mass liquid crystals with molecular geometry were constructed with three phenyl rings bridged through ester central groups as the rigid core and a lateral methyl group as the flexible part of a molecule, with a view to understanding and establishing the effect of molecular structure on liquid crystal behaviour. Low-molar-mass mesogens known as 1,4-bis[(4-n-alkyloxybenzoyl)oxy]toluene with different number of carbon (n) at the alkyl chain have been prepared. Their molecular structures were proposed via physical measurements and spectroscopic techniques. Mesomorphic properties were studied by using differential scanning calorimetry, optical polarizing microscopy and powder X-ray diffraction techniques. The results showed that the melting points as well as the clearing temperatures decreased upon lengthening of the terminal alkyloxy chain lengths. Members with the shorter chain (n = 2-10) exhibited nematic phase. As for higher homologues, members with n = 12, 14, 16 and 18 showed polymorphism, whereby these compounds displayed both smectic and nematic properties.
Synthesis of new schiff base ester liquid crystals with a benzothiazole core
(2010)
Ha, Sie-Tiong
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Koh, Teck-Ming
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Lee, Siew-Ling
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Yeap, Guan-Yeow
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Lin, Hong-Cheu
A series of new calamitic liquid crystals, 6-methoxy-2-(4-alkanoyloxybenzylidenamino)benzothiazoles, comprising a benzothiazole core, terminal methoxy group and a Schiff base linkage were synthesised and characterised. This series comprises 12 members wherein members differ by the length of the alkanoyloxy chain (Cn-1H2n-1COO-, where n = 2-8, 10, 12, 14, 16, 18). Their mesomorphic properties were studied by using differential scanning calorimetry, optical polarising microscopy and powder X-ray diffraction techniques. The short chain derivatives (n = 2 and 3) were non-mesogenic compounds, while an enantiotropic nematic phase was present throughout the remaining members of the series. The smectic C phase emerged from the decanoyloxy derivative onwards.