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H. N. Chauhan and A. V. Doshi
terminal or lateral position. Therefore, molecular rigidity and flexibility [5,6] vary from
series to series due to varying n-alkoxy terminal end group and varying positional differ-
ence of NO2 group as well as variation arising from central groups from series to series
except molecular aromaticity. However, molecular aromaticity can also vary with molecu-
lar polarity and polarizability, length to breadth ratio, and hence the suitable or unsuitable
magnitudes of anisotropic forces of intermolecular attractions [8,10] which are directly
related with the tendency of inducing or not inducing liquid crystal mesophase in a sample
substance under examination. As mentioned above, even minor structural variations affect
molecular rigidity and flexibility [5,6] and hence cause variation in mesogenic properties
in terms of the degree of mesomorphism, thermal stability, commencement of mesophase
formation (Table 3), transition temperatures, and mesophase range.
Homologous series-1 and series-Y are structurally identical with respect to n-alkoxy(-
OR) terminal end group, three phenyl rings, and COO central groups, but differ in
their substitutional position of NO2 and the linking of the CO group to the phenyl
ring in series-1 and series-Y. The observed difference of smectic and nematic thermal
stability as well as the commencement of mesophases can be linked with the structural
difference of series-1 and series-Y. The molecules of series-1 are broader and shorter
than those of series-Y. Therefore, the length to breadth ratio and the ratio of molecular
polarity to polarizability cause suitable magnitudes of anisotropic intermolecular forces
of adhesion. Broader molecules of series-1 with relatively higher molecular width than
the molecular width of series-Y exerts two opposing effects of intermolecular attractions
at a time, as (i) higher molecular width decreases intermolecular closeness and reduces
intermolecular adhesion on the one hand, and (ii) increases molecular polarizability and
enhances intermolecular adhesion on the other hand. The resultant effect depends upon
the predominating effect of the two opposing effects, i.e., (i) and (ii) [10]. Thus, effect
(ii) due to molecular polarizability is a predominating effect for series-1 of the present
investigation for the smectic and nematic average thermal stabilities, which are higher
than series-Y. This indicates that though intermolecular closeness of the linearly shaped
molecules of series-Y whose intermolecular adhesion are expected to be more than series-
1, but, molecular polarizability of series-1 exceeds the magnitudes of anisotropic forces
of intermolecular attractions. On comparing series-1 with series-X, both differ in their
one central group, i.e., COO and CH CH COO respectively, but the rest of the
structure is identical. However, series-X is only nematogenic, while series-1 is smectogenic
in addition to nemtogenic mesophase, with average nematic isotropic thermal stabilities
of 171.5◦C (series-X) and 163.8◦C (series-1) respectively. Therefore, the difference in
thermal stability can be explained on the basis of the effect due to carboxy COO and
vinylcarboxylate CH CH COO central groups. Thus, in all other respects, except
central bridges, which are COO and CH CH COO , series-1 and series-X under
discussion are closely similar to each other. Both these central bridges are comparable,
although the vinylcarboxylate CH CH COO has greater length, by the CH CH
unit, increased conjugation by the CH CH , which increases the molecular rigidity
[5,6] and lateral attractions and hence the intermolecular adhesion increases, while in the
case of series-Y, the COO central bridge is shorter. Consequently, the stereochemistry
of molecule does, however, preserve the linearity, but results in less thermally stable
nematogenic mesophase. Thus, thermal stability for nematic of series-1 (163.8◦C) is less
than that of series-X (171.5◦C). Moreover, the vinylcarboxylate being longer than COO
causes more non-coplanarity due to a twist caused by interference of the vinylcarboxylate
group and the adjacent hydrogen atoms of aromatic rings. On account of the difference
caused in the extent of non-coplanarity [4,8], the commencement of smectic phase takes
place from the third member in series-1, it commences from the sixth member of series-Y,