Exploring the Mesomorphic Potential of 2,4-disubstituted Thiophenes: A Structure-property Study
The synthesis and mesomorphic properties of 25 novel esters derived from 4-substituted thiophene-2-carboxylic acids and either (S)-1-methylheptyl 4-hydroxybenzoate or (S)-1-methylheptyl 4-hydroxybiphenyl-4-carboxylate are described. A structure–property relationship has been deduced, and mesogenic...
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| Main Authors: | , , , , |
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| Format: | Article |
| Published: |
Taylor & Francis
2011
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| Subjects: | |
| Online Access: | http://dx.doi.org/10.1080/02678292.2010.539786 http://dx.doi.org/10.1080/02678292.2010.539786 http://umpir.ump.edu.my/5462/1/Liquid_crystals.pdf |
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| Summary: | The synthesis and mesomorphic properties of 25 novel esters derived from 4-substituted thiophene-2-carboxylic
acids and either (S)-1-methylheptyl 4-hydroxybenzoate or (S)-1-methylheptyl 4-hydroxybiphenyl-4-carboxylate
are described. A structure–property relationship has been deduced, and mesogenic properties are found to be
dependent on the total number of rings in the molecular core and the nature of the substituent in the 4-position of
the thiophene ring. Four-ring systems have greater thermal stability and are more likely to form mesophases than
their three-ring counterparts. A 1,4-phenylene substituent in the 4-position of the thiophene ring (Series I and II)
gives better thermal properties than does a 2,5-thienyl substituent (Series III and IV). A rationale is suggested in
terms of conjugative effects that may be important in mesophase thermal stability. A preliminary molecular modelling
study (B3LYP/6–31G(d)) reveals that 2,4-disubstituted systems offer a slight preference for a folded geometry
structure compared with their 2,5-counterparts. Homologues (n = 7–10,12) in Series II show an interesting chiral
phase behaviour between the SmA and SmC∗ ferrielectric phase, tentatively assigned SmX∗, which then gives a
SmC∗ antiferroelectric phase. Textures have been characterised by thermal polarising microscopy using a conventional
glass slide cover-slip and free-standing film. Although the SmX∗ phase has still to be fully characterised, this
is the first reported occurrence of such a phase in 2,4-thiophene systems. |
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