Synthesis and characterization of some calamitic liquid crystals consisting of cholesterol, rod-like imine and biphenyl-4-carboxylate components
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Date
2006
Authors
Ha, Sie Tiong
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Abstract
Seven series of calamitic liquid crystals were successfully synthesized
and characterized. These liquid crystals are categorized into three different
types according to their core structure; either a cholesterol, rod-like aromatic
imine or biphenyl-4-carboxylate component. The synthesis of these compounds
basically involved alkylation, esterification and condensation reactions. The
structures of the synthesized compounds were established by CHN
microanalysis along with several spectroscopic techniques such as FTIR, 1Dand
2D-NMR. Whilst the phase transitions and liquid crystal textures were
observed by using polarized optical microscope, the respective transition
temperatures and associated enthalpies were determined by using DSC
analysis. The first and second series which consist of seven homologues each,
include cholesteryl 4-n-alkoxybenzoates (nOACh) and cholesteryl 4-(4-nalkoxyphenyl)
benzoates (nOABCh). Various alkoxy groups consisting of even
numbered carbons ranging from six to eighteen have been adopted throughout
this study. The various NMR techniques which were employed revealed that
the O=C-O bridging the phenyl group and the cholesteryl fragment was bent
and led to the cholesteryl fragment to fold up. This unique conformation has
been observed for the first time in cholesteryl esters. Both of the series showed
that the lower members have a higher tendency to exhibit chiral nematic (N*)
phase whereas the higher members showed a greater tendency to display the
smectic A (SmA) phase. The third, fourth, fifth and sixth series each consists of
a rod-like aromatic imine component. The third series is further divided into
three sub-series; 2-hydroxy-4-methoxybenzylidene-4’-n-alkanoyloxyanilines
(nSBA), 2-hydroxy-3-methoxybenzylidene-4’-n-alkanoy-loxyanilines (nSBB)
and 3-methoxy-4-n-alkanoyloxybenzylidene-4’-n-alkanoy-loxyanilines (nSBC).
Each of them comprised four members which differed in the alkanoyloxy chain
length. Whilst the nSBA homologues exhibited the enantiotropic nematic or
SmA phase, the nSBC homologues displayed the monotropic smectic C (SmC)
phase. However, the nSBB homologues did not exhibit any mesophase. The
fourth, fifth and sixth series encompassed 2-hydroxy-4-nhexadecanoyloxybenzylidene-
4’-substituted-anilines (16OHA-R), N-[4-(4-nhexadecanoyloxybenzoyloxy)
benzylidene]-4-substituted-anilines (16AB-R) and
N-[4-(4-n-hexadecanoyloxybenzoyloxy)-2-hydroxybenzylidene]-4-substitutedanilines
(16OHAB-R), in which R is H, F, Cl, Br, OCH3, CH3, C2H5, CN, OH, SH
or NO2. While compounds 16OHA-R with the substituents R = H, F, Cl, Br,
OCH3, CN, OH and NO2 exhibited SmA phase, those with the substituents R =
CH3 and C2H5 showed monotropic SmC phase and finally the compound with R
= SH displayed monotropic nematic phase. Compounds 16AB-R and 16OHABR
(where R = H, OCH3, CH3, C2H5, OH and NO2) showed evidence of nematic
phase while compounds 16AB-R and 16OHAB-R (where R = F, Cl, Br and CN)
exhibited SmA phase. Compound 16OHAB-SH exhibited nematic phase
whereas compound 16AB-SH did not show any mesophase. The 16AB-R and
16OHAB-R molecules have a higher mesophase stability compared to the
16OHA-R molecules owing to their higher molecular length. The mesomorphic
properties of these compounds were also influenced by the intermolecular and
intramolecular interactions owing to the presence of the ortho-hydroxyl group.
The seventh series with a biphenyl-4-carboxylate core consists of four (S)-2-
methylbutyl 4’-(4”-n-alkanoyloxybenzoyloxy)biphenyl-4-carboxylate (S-MBOOCn)
homologues. All the S-MB-OOCn homologues exhibited the chiral
smectic C (SmC*) and SmA phases.
Description
PhD
Keywords
Chemical science , Calamitic liquid crystals , Rod-like imine , Biphenyl-4-carboxylate