The synthesis and characterisation of chiral thiophene-based liquid crystals

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Grover, C., 2000. The synthesis and characterisation of chiral thiophene-based liquid crystals. PhD, Nottingham Trent University.

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Abstract

To meet a global demand for compounds which exhibit ferro-, ferri- and antiferro-electricity and to gain an insight into sbucture-property relationships of non-linear liquid crystals, the synthesis and mesomorphic properties of a variety of novel thiophene-based chiral esters [Series I-XIV] is reported.

[Series I] (S)-4-(l-methylheptyloxycarbonyl)phenyl 5-(4-n-alkoxyphenyl)thiophene-2-carboxylates.
[Series II] (S)-4'-(l-methylheptyloxycarbonyl)biphenyl-biphenyl-4-yl 5-(4-n-alkoxyphenyl)thiophene-2-carboxylates.
[Series III] (S)-4-(l-methylheptyloxycarbonyl)phenyl 5'-n-alkyl-2',2-bithienyl-5-carboxylates.
[Series IV] (S)-4'-(l-methylheptyloxycarbonyl)biphenyl-4-yl 5'-n-alkyl-2',2-bithienyl-5-carboxylates.
[Series V] (S)-4-(l-methylheptyloxycarbonyl)phenyl 4-(4-n-alkoxyphenyl)thiophene-2-carboxylates.
[Series VI] (S)-4'-(l-methylheptyloxycarbonyl)biphenyl-4-yl 4-(4-n-alkoxyphenyl)thiophene-2-carboxylates.
[Series VII] (S)-4-(l-methylheptyloxycarbonyl)phenyl 5'-n-alkyl-2',4-bithienyl-2-carboxylates.
[Series VIII] (S)-4'-(l-methylheptyloxycarbonyl)biphenyl-4yl 5'-n-alkyl-2',4-bithienyl-2-carboxylates.
[Series IX] (S)-4-(l-methylheptyloxycarbonyl)phenyl 5"-n-alkyl-2",2':5',2-terthienyl-5-carboxylates.
[Series X] (S)-4'-(l-methylheptyloxycarbonyl)biphenyl-4-yl 5"-n-alkyl-2",2':5',2-terthienyl-5-carboxylates.
[Series XI] (S)-4-(l-methylheptyloxycarbonyl)phenyl 5'-4(4-n-decyloxyphenyl)-2',2-bithienyl-5-carboxylates.
[Series XII] (S)-4-(l-methylheptyloxycarbonyl)phenyl 5-(4'-n-decyloxybiphenyl-4-yl)thiophene-2-carboxylate.
[Series XIII] (S)-4-(l-methylheptyloxycarbonyl)phenyl 5-(4'-n-decyloxy-3'-fluorobiphenyl-4-yl)thiphene-2-carboxylate.
[Series XIV] (S)-4-(l-methylheptyloxycarbonyl)phenyl 4-(4'-n-decyloxybiphenyl-4-yl))thiophene-2-carboxylates.

Thiophene-based esters [Series I-XTV] were prepared from an appropriate thiophene-containing carboxylic acid and either (S)-l-methylheptyl 4-hydroxybenzoate or (S)-1-methylheptyl 4'- hydroxybiphenyl-4-carboxylate, as source of 'chiral phenol'. The latter were employed to ensure a strong possibility for the occurrence of the Sc*ferro-, ferri- and antiferro-electric phase types and, to this effect, certain members of Series I-IV, VI and IX-XV exhibit the aforementioned phase types. Results for the n=12 homologue of Series IV have been published (Mol. Cryst. Liq. Cryst., 1999, 332, 303) whereas results for the n=10 homologue of Series II have been accepted for publication (J. Mater. Chem., 2000).

An in depth structure-property relationship which impinges on the aspects of molecular length and molecular geometry is discussed. Increasing the overall molecular length by increasing the number of aromatic rings, enhances mesophase thermal stability and mesophase type, e.g.: Series I (three-ring), SA, SC*ferro-, ferri- and antiferro-electric; Series II (four-ring), SA, SC*ferro-, ferri-, antiferro-electric and S1*. Four-ring compounds containing three-rings in the molecular core are thermally more stable than their isomeric counterparts containing two rings in the core and two rings in the terminal chiral moiety. (S)-4-(1-Methylheptyloxycarbonyl)phenyl 4"-n-decyloxy-1", 1' :4',1-terphenyl-4-carboxylate [Series XV] was prepared to serve as a model compound for comparison with Series IX, XI and XII.

Either one-, two- or three-thiophene rings can be readily accommodated by the central molecular core but their disposition dictates liquid crystallinity. Unlike 2,5-disubstituted thiophene, 2,4-disubstitued thiophene is extremely poor at promoting liquid crystallinity, e.g., members of Series VII are non- mesomorphic whereas Series III are mesomorphic.

A short study of the influence of lateral fluorination is also reported. As expected, the inclusion of a lateral fluoro-substituent lowers the mesophase thermal stability and eliminates high order smectic phase types when compared with its non-fluorinated counterpart.