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Chiral (R)- and (S)-3-alkoxy-2-methylpropionic acids were
synthesized as building blocks for the preparation of
chiral liquid crystalline materials. The investigation about
the effect of molecular structures on the mesophases and
mesomorphic properties was divided into three parts. The first
part was to synthesize a homologous series of (R)-6-(3-
ethoxy-2-methylpropionyloxy)-2- naphthyl 4- alkoxybenzoates ,
(R)EMPNmB (m=6~13) , in order to understand the effect of
extending the achiral peripheral alkyl chain length on the
mesomorphic properties. The second part was to synthesize
(R)-6-(3-ethoxy-2-methylpropionyl- oxy)-2-naphthyl 4?-
decyloxybiphenyl -4-carboxylate ,(R)EMPNBC , in order to
understand the effect of extending the rigid core structure by
an additional phenyl ring on the formation of the TGB
phases . The third part was to synthesize a homologues
series of (R)-6-(3-alkoxy-2-methylpropionyloxy)-2- naphthyl
4-decyloxybenzoates , (R)nMPNB (n=2~5) , in order to understand
the effect of alkyl chain length attached to the chiral
tail on the effect of TGB phases. Furthermore, the
influence of chiral moiety between oxygen- and sulfur-
containing liquid crystallines on the mesomorphic
properties was also compared and discussed.
The results of the first homologous (R)EMPNmB reveal that stable
frustrated phases exhibit in this series . The stability of
the TGBA* phase decreases with the increasing peripheral
alkyl chain length, m, whereas the temperature range of the
TGBc* phase tents to ascend as m increases.
The investigation of compound (R)EMPNBC shows that, when the
rigid core structure is extended by an additional phenyl ring
, the thermal stability of the mesophases increases , the
temperature range of BPI phase depresses, the TGBc* phase
disappeares, and a relatively wide temperature range of the
Sc* phase appears.
The third homologous compounds (R)nMPNB , show that all
mesophases are enantiotropic. When extending the alkyl chain
length at the chiral tail group, the clearing points exist an
odd-even effect, the TGBA* phase disappears and a relatively
wide temperature range of the TGBc* phase appears . The
results demonstrate that the elongation of the alkyl chain
length has no significant effect on the thermal stability of
BP, N* and TGBc* phases. When the
sulfur atom is replaced by oxygen in the chiral tail group , the
clearing points for oxygen-containing materials are higher than
that for sulfur containing materials . It is suggested that
the oxygen contributes more dipole than sulfur. From binary
mixture studies,it is found that the handedness of the helical
twist sense for oxygen-containing materials, (R)EMPNmB, is
opposite to sulfur-containing materials, (R)EMMPNmB.
Furthermore, study in the electro-optical properties shows that
the spontane- ous polarizations for oxygen-containing materials
are approximately 20 nC/cm2. The magnitude of the Ps reduces
as the rigid core structure is extended by an additional
phenyl ring.
In conclusion , the results demonstrate that oxygen-
containing materials exhibit wide temperature range of
frustrated phases. It is suggested that these materials possess
high degree of "chirality". Our systematic studies also reveal
that the spatial configuration of the molecules contributes a
significant influ- ence on the formation of frustrated phases.
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