Study of mesomorphism and its relation to molecular structure with special reference to central bridges viz. -COO- and -CH=CH-COO- of the homologous series

Article abstract of DOI:10.1080/15421406.2015.1073564

Two ester homologous series of mesogens, viz., Methyl-p-(p′-n-alkoxy benzoyloxy) cinnamates (X) and Methyl-p-(p′-n-alkoxy cinnamoyloxy) cinnamates (Y) being structurally similar are discussed. Both (X) and (Y) differ in respect of central bridges linking two phenyl rings. Only enantiotropic nematogenic character is observed without exhibition of any smectic character by all members of series (X) and (Y). Thermal stability of series (X) is relatively low as compared to (Y), but nematogenic mesophase lengths are of reverse order. Solid-nematic/solid-isotropic and nematic-isotropic transition curves in the phase diagrams behave in normal manner.

Full text of DOI:10.1080/15421406.2015.1073564

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Study of mesomorphism and its relation to
molecular structure with special reference to
central bridges viz. -COO- and -CH=CH-COO- of the
homologous series
V. S. Sharma, R. B. Solanki, P. K. Patel & R. B. Patel
To cite this article: V. S. Sharma, R. B. Solanki, P. K. Patel & R. B. Patel (2016) Study of
mesomorphism and its relation to molecular structure with special reference to central bridges
viz. -COO- and -CH=CH-COO- of the homologous series, Molecular Crystals and Liquid Crystals,
625:1, 137-145, DOI: 10.1080/15421406.2015.1073564
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1073564
Published online: 19 Feb 2016.
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Date: 05 March 2016, At: 22:33

MOL. CRYST. LIQ. CRYST.
, VOL. , –
http://dx.doi.org/./..
Study of mesomorphism and its relation to molecular structure
–
–
with special reference to central bridges viz. COO and
–
=
–
–
CH CH COO of the homologous series
V. S. Sharma, R. B. Solanki, P. K. Patel, and R. B. Patel
Chemistry Department, K. K. Shah Jarodwala Maninagar Science College, Gujarat University, Ahmedabad, India
ABSTRACT
KEYWORDS
Mesogen; mesophase;
nematic; smectic
Two ester homologous series of mesogens, viz., Methyl-p-(pꢀ-n-alkoxy
benzoyloxy) cinnamates (X) and Methyl-p-(pꢀ-n-alkoxy cinnamoyloxy)
cinnamates (Y) being structurally similar are discussed. Both (X) and (Y)
differ in respect of central bridges linking two phenyl rings. Only enan-
tiotropic nematogenic character is observed without exhibition of any
smectic character by all members of series (X) and (Y). Thermal stabil-
ity of series (X) is relatively low as compared to (Y), but nematogenic
mesophase lengths are of reverse order. Solid-nematic/solid-isotropic
and nematic-isotropic transition curves in the phase diagrams behave
in normal manner.
GRAPHICAL ABSTRACT
Introduction
The relation between mesomorphism and molecular structure has been established with ref-
erence to varying terminally and/or laterally substituted functional groups of molecules but
less attention has been given to central bridges linking phenyl rings. Therefore, the present
CONTACT V. S. Sharma
vinaysharma@gmail.com
College, Gujarat University, Ahmedabad , Gujarat, India.
Chemistry Department, K. K. Shah Jarodwala Maninagar Science
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

138
V. S. SHARMA ET AL.
investigation is planned with a view establishing the effect of central bridge on mesomorphic
behavior of molecules. The novel liquid crystal compounds consist of rigid part as two or
three phenyl rings bridged through central bridge and flexible part as varying left polar ter-
minal, alkyl or alkoxy end group keeping right terminal end group intact of fix polarity with
or without lateral substitution.
Experimental
(i) n-Alkoxy (benzoic and cinnamic) acids: p-n-alkoxy benzoic acids and p-n-alkoxy cin-
namic acids were prepared by usual established methods[1–5] by reaction of p-hydroxy
acids and the corresponding alkyl halide (Scheme 1).
(ii) p-n-Alkoxy acids were converted to the acid chlorides by refluxing with freshly distilled
thionyl chloride until the evolution of SO2 ceases and then the excess of thionyl chloride
was distilled off [6].
(iii) Acid chlorides were carefully treated with a pre-cooled solution of p-hydroxy methyl
cinnamate dissolved in pyridine. Final products were obtained and subsequently dried,
purified, and crystalized from alcohol.
(iv) The methyl ester of cinnamic acid was prepared by a usual established method [7–10]
m.p. 117.0 ºC, yield is 69.6%.
The transition temperatures of the homologs were determined by hot stage polarizing
microscope. Infra-red (IR) and nuclear magnetic resonance (NMR) spectra of representative
homologs of the series under comparison confirm the structure of the molecules. Analytical
data support the structure.
Scheme . Preparation of series X and series Y (R = CnHn+).

MOL. CRYST. LIQ. CRYST.
139
Spectral Data for Series-X: NMR in ppm
Octyl.
–
–
–
0.890—CH3 of C8H17, 1.308—CH2 of C8H17, 1.772—OCH2 CH2 of C8H17, 3.816—O CH3,
–
–
=
–
4.021—OCH2 of O C8H17, 6.380 and 6.460—CH CH , 6.905 and 8.026—P-Sub. Phenyl
ring, 7.260—another P-Sub. Phenyl ring.
NMR confirms the structure.
Decyl.
–
=
–
0.880—CH3 of C10H21, 3.82—OCH3, 4.31—OCH2 of O C10H21, 5.344— CH Ar, 6.380—
=
–
–
CH CO , 6.90 and 8.04—P-Sub. Phenyl ring with the difference groups, 7.26—P-Sub.
Phenyl ring with the two same like groups.
NMR confirms the structure.
IR: in cm−1
Octyl.
−1
2854.5 cm−1 confirms alkyl group, 1685.7 cm−1 and 1168.8 cm confirms COO group and
=
ether group, 844.8 cm−1 confirms p-sub. Phenyl ring, 721.3 cm−1 confirms polymethylene of
C8H17, 698.2 cm−1 confirms cis CH CH group.
IR confirms the structure.
–
=
–
Dodecyl.
−1
2960.0 cm confirms alkyl group, 1257.5 cm−1 and 1685.7 cm−1 confirms –COO group,
–
1168.8 cm−1 confirms ether group, 941.2 cm−1 confirms trans CH CH group, 844.8 cm
confirms p-sub. Phenyl ring, 721.3 cm−1 confirms polymethylene of C11H22.
IR confirms above structure.
−1
–
=
–
For Series-Y
NMR:
Propyl.
–
–
–
–
–
–
–
1.84—CH2 CH2 CH3, 3.806—O CH3, 4.00—O CH2 of O CH2 CH2 CH3, 4.44 and
=
–
4.02—CH CH , 6.43 and 6.94, 6.97 and 7.20—P-sub. phenyl two p-sub. benzene, 7.23 and
7.55, 8.09 and 8.12—P-sub. phenyl two p-sub. benzene.
NMR confirms the structure.
Octyl.
–
–
–
–
–
–
–
–
–
1.81—O CH2 CH2 (CH2)5 CH3, 3.8—O CH3, 4.034—O CH2 of (O CH2 (CH2)6 CH3),
=
–
4.41—CH CH , 6.38 and 6.43, 7.23 and 7.24—two p-sub. phenyl ring, 7.55 and 7.58, 8.09
and 8.12—two p-sub. phenyl ring.
NMR confirms the structure.
IR: in cm−1
Pentyl.
720.0 cm−1 Poly –CH2 (long methyl chain), 1050.0 cm−1 confirms ether group of ester(-
COO), 820.0 cm−1 confirms p-sub. benzene ring, 1250.0 cm−1 confirms >C=O bend,

140
V. S. SHARMA ET AL.
−1
698.2 cm confirms cis CH CH group, 1750.0 cm−1 confirms > C=O of ester,
–
=
–
3000.0 cm−1 confirms >C-H of aromatic group.
IR confirms the structure.
Tetradecyl.
1050.0 cm−1 confirms >C-O of ester group, 650.0 cm−1 confirms cis CH CH group
1250.0 cm−1 confirms >C=O of ester group, 840.0 cm−1 confirms p-sub. benzene ring
750.0 cm−1 confirms poly –CH2- of n-octyl, 1750.0 cm−1 confirms > C=O of ester group
1250 cm−1 confirms >C=O of ester group, 3000.0 cm−1 confirms =C-H of aromatic group.
IR confirms the structure.
–
=
–
Results and Discussion
n-Alkoxy benzoic acids and n-alkoxy cinnamic acids are dimeric through hydrogen bond-
ing. Dimerization disappears as a result of breaking of hydrogen bonding on esterification to
give p-hydroxy methyl cinnamate. Thus, transition temperatures of homologs of series (X)
and (Y) are relatively lower than the corresponding n-alkoxy acids from which final prod-
ucts are obtained. p-Hydroxy methyl cinnamate is nonmesomorphic but on linking with n-
alkoxy phenyl ring through central bridge mesomorphic characteristics are exhibited, except
for the first and/or second homologs for the series under comparison. Homologs of the series
(X) and (Y) are entirely nematogenic without exhibition of any smectic character even in the
monotropic condition except first and/or first and second homologs, respectively.
Transition temperatures of homologous series from Table 1 are plotted versus the number
of carbon atoms in n-alkyl chain of left n-alkoxy terminal, smooth curves are drawn through
the like or related points, the phase diagrams obtained are shown separately in Figs. 1 and 2,
respectively. From the phase diagrams of Figs. 1 and 2, the solid-isotropic or solid-nematic
transition curves partly follow zigzag path of rising and falling and behave in normal manner.
Nematic isotropic transition curves also behave in normal manner.
Nonmesomorphic behavior of first one or two homologs is attributed to their high crystal-
lizing tendency arising out from their strong intermolecular attractions due to the presence
of left shorter methyl and ethyl end groups linked through with phenyl ring. Enantiotropic
nematogenic mesophase exhibited by the ethyl or propyl to hexadecyl derivatives of series (X)
and (Y), respectively, is due to the maintenance of statistically parallel orientational order of
Table . Homologous series (X) and (Y): Transition temperatures
Transition temperatures in ºC
Smectic
series
Nematic
series
Isotropic
series
Sr. no.
n-alkyl group
Methyl
Ethyl
Propyl
Butyl
Pentyl
Hexyl
Heptyl
Octyl
(x)
(y)
(x)
(y)
(x)
(y)
.
.
.
.
.
.
.
.
.
.
.
.
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
–
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
–
.
.
.
.
.
Decyl
Dodecyl
Tetradecyl
Hexadecyl
.

MOL. CRYST. LIQ. CRYST.
141
Figure . Methyl p-(pꢀ-n-alkoxy benzoyloxy) cinnamates (series X).
two-dimensional array of molecules in floating condition. Thus, methyl and/or ethyl deriva-
tives are unable to resist the thermal vibrations and passed directly into isotropic liquid with-
out passing through any mesomorphic state whereas the rest of the homologs, i.e., ethyl (series
X) or propyl to hexadecyl derivatives are capable to resist the thermal vibrations and conse-
quently exhibit nematic mesophase without exhibition of any smectic mesophase.
The structurally similar homologous series (x) and (y) are as under:
Scheme . Series X and series Y.

142
V. S. SHARMA ET AL.
Figure . Homologous series: methyl p-(p’-n-alkoxy cinnamoyloxy) cinnamates (series Y).
In Figs. 4 and 5, we represents the three-dimensional (3D) diagram of series X and series
Y having different central linkage group and exhibit different mesomorphic properties. The
mesomorphic properties of substances depend upon polarity, polarizability, aromaticity, lin-
earity, length to breadth ratio, geometrical shape and size, pai electron density, and many
other factors of molecules, which contribute to the net intermolecular forces of attractions
and hence to the mesomorphic properties and the degree of mesomorphism.
Figure . D diagram of series X (R = side chain).

MOL. CRYST. LIQ. CRYST.
143
Figure . D diagram of series Y (R = side chain).
Figure . Space filling diagram of series (X) and series (Y).
Figure . D arrangement of the atoms in series (Z) (R = CnHn+).
Suspicious observation of the general structure of molecules is shown in Figs. 3 and 4.
In Fig. 6 of series (X) and (Y), it is clear that, geometrical shape of molecules is linear rod
like without any lateral substitution. Both molecules consist of left n-alkoxy terminal, two
–
=
–
phenyl rings, and CH CH COOCH3 right terminal. They differ only with respect to cen-
tral bridges for the same homologs from series to series. Molecules of (X) homologous series
–
=
–
–
link two phenyl rings through COO (ester) central bridge while, that of series (Y) link
–
two phenyl rings through CH CH COO- central bridge (Scheme 2). Thus, aromaticity
–
=
–
–
due to two phenyl rings of molecules, left n-alkoxy terminals, right CH CH COOCH3 ter-
=
–
minal, and linearity remains unchanged while length of molecules due to CH CH part

144
V. S. SHARMA ET AL.
–
–
of central bridge and its unsaturation for series (Y), i.e., central bridges, viz., COO and
–
=
–
CH CH COO- differs. Therefore, length to breadth ratio, arrangement of atoms in space,
net polarity, and polarizability cause differences of inter molecular forces of attractions and
hence the difference in mesogenic characteristics of series (X) to series (Y).
In comparison transition temperatures of series (X) are relatively lower than transition
temperatures of series (Y) for the same homolog from series to series. This difference of tran-
sition temperatures can be directly linked to the difference in intermolecular forces of attrac-
–
=
–
tions caused by the difference in length by CH CH and length to breadth ratio resulting
into difference in polarity as well as polarizability of molecules. In spite of lowering of tran-
sition temperatures of homologs of series (X), it is observed that the overall phase length or
degree of mesomorphism (only Nematic) exhibited by series (X) is more than (maximum 52.0
ºC for series (X) and 46.0 ºC for series (Y)) the corresponding homologues of series (Y). Thus,
two-dimensional array of the molecules even in the floating condition maintains more statis-
tically parallel orientational order, resisting thermal vibrations in case of series (X) than series
(Y). Average thermal stability for nematic–mesophase for homologous series (Y) is more than
homologous series (X).
Generally, ester homologous series are smectogenic, however present two homologous
series are nematogenic, i.e., smectic mesophase is totally absent. The exhibition of smec-
tic mesophase, the degree of smectogenic character, and early or late commencement
of smectic mesophase depend [11–15] upon the extent of noncoplanarity caused by the
molecules. The terminal end group in the molecules of both the series under discussion bear
–
=
–
CH CH COO- functional group with a double bond. The presence of double bond can
cause cis–trans isomerism and arrangement of atoms in space may operate and create such
an extent of noncoplanarity of molecules that smectic mesophase formation, i.e., arrangement
of sliding layer formation of molecules during course of heating the sample, is not maintained
or say, fully eliminated till the last homolog and only statistically parallel orientational order
of molecules is maintained in floating condition. Thus, nematic mesophase is occurred at the
cost of smectc mesophase [16–21].
In comparison the type of mesomorphic phase exhibited by (X) and (Y) homologous series
with a series (Z) and Fig. 6, i.e., methyl-p-(pꢀ-n-alkoxy benzoyloxy) benzoates with its struc-
ture is as follows:
Series Z is entirely smectogenic without exhibition of any nematogenic characteristic.
Thus a series (Z) is smectogenic while series (Y) and (X) are nematogenic. Hence, same
–
–
central bridge and terminal groups bearing COO (series Z) is smectogenic and a same
–
=
–
central and terminal group bearing CH CH COO- (series Y) is nematogenic. Thus,
–
–
–
–
=
–
–
COO group can induce smectic character more favorably and CH CH COO group can
induce nematogenic character more favorably. Replacement of COOCH3 terminal group

MOL. CRYST. LIQ. CRYST.
145
–
=
–
–
by CH CH COO CH3 terminal (series Y) wonderfully causes emergence of nematic
–
=
–
–
mesophase from smectic mesophase (series Z). Hence CH CH COO is strong enough
to emerge nematic character from smectic character and vice versa. Thus, group efficiency
order derived from induction of smectic and or nematic characters are as under.
–
–
–
–
–
=
–
–
–
–
Smectic character group efficiency order: COO > CH CH COO
Nematic character group efficiency order: COO > CH CH COO
–
=
Conclusion
In summary, ester homologous series are not necessarily the smectogenic but it may be entirely
smectogenic or entirely nematogenic or partly smectogenic and partly nematogenic. We were
able to compare mesomorphism property of series (X) and series (Y) with series (Z), which
contain on both the side only ester linkage group and exhibit smectogenic behavior without
any kind of nematogenic property.
Acknowledgments
The authors thank Dr. Rutesh Shah the Principal of K.K. Shah Jarodwala Maninagar Science College and
supporting staff of Chemistry department for valuable cooperation in the work. The authors are also
thankful to the authority of Uma laboratory, Vadodra, Vaibhav laboratory, Ahmedabad, and National
Salt and Marine research laboratory, Bhavnagar for analysis of samples.
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