Novel Cinnamate Esters - Synthesis and Mesomorphic Properties in Relation to Molecular Structure

Article abstract of DOI:10.1080/15421406.2014.884392

A novel homologous series 4-[4′-n-alkoxy cinnamoyloxy] benzyl cinnamates consisting of 11 homologs was synthesized and studied with a view to understanding and establishing the relation between molecular structure and mesomorphic behavior of a substance. Mesomorphic behavior of the series commences from the third homolog and then continues until the last hexadecyloxy homolog. The first and second members of a series are non-mesomorphic. Nematogenic mesophase formation is observed from the propoxy homolog to the hexadecyloxy homolog, but the smectogenic mesophase formation is observed from the hexyloxy homolog to the tetradecyloxy homolog. All mesomorphic transitions are enantiotropic in nature. Transition temperatures were determined by an optical polarizing microscopy equipped with a heating stage. The textures of the nematic mesophase are threaded or Schlieren in type and that of the smectic mesophases are focal conic fan shaped of the smectic A or smectic C type. A phase diagram of the series shows a normal behavior of the transition curves with a minor abnormality of the last three homologs for the nematic-isotropic transition curve. The average thermal stability for smectic and nematic mesophases is 176.0°C and 219.3°C, respectively. Analytical and spectral data agree with the molecular structures of homologs. Mesomorphic phase length varies between 22°C and 76°C. Smectic phase length varies from 6°C to 33°C and nematic phase length varies from 22°C to 54°C. Thus, the novel present series is predominantly nematogenic and partly smectogenic. Mesomorphic properties of the novel series are compared with structurally similar other known homologous series.

Full text of DOI:10.1080/15421406.2014.884392

Mol. Cryst. Liq. Cryst., Vol. 605: pp. 42–51, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.884392
Novel Cinnamate Esters—Synthesis
and Mesomorphic Properties in Relation
to Molecular Structure
BRIJESH H. PATEL1 AND A. V. DOSHI2,∗
1Huntsman, Baroda Textile Effects Pvt. Ltd., Umraya Village, Taluka Padra,
Vadodara, Gujarat, India
2Applied Science, Dr K. N. Modi University, Dist. Tonk, Rajasthan, India
A novel homologous series 4-[4ꢀ-n-alkoxy cinnamoyloxy] benzyl cinnamates consisting
of 11 homologs was synthesized and studied with a view to understanding and establish-
ing the relation between molecular structure and mesomorphic behavior of a substance.
Mesomorphic behavior of the series commences from the third homolog and then con-
tinues until the last hexadecyloxy homolog. The first and second members of a series are
non-mesomorphic. Nematogenic mesophase formation is observed from the propoxy
homolog to the hexadecyloxy homolog, but the smectogenic mesophase formation is
observed from the hexyloxy homolog to the tetradecyloxy homolog. All mesomorphic
transitions are enantiotropic in nature. Transition temperatures were determined by an
optical polarizing microscopy equipped with a heating stage. The textures of the nematic
mesophase are threaded or Schlieren in type and that of the smectic mesophases are
focal conic fan shaped of the smectic A or smectic C type. A phase diagram of the series
shows a normal behavior of the transition curves with a minor abnormality of the last
three homologs for the nematic-isotropic transition curve. The average thermal stability
for smectic and nematic mesophases is 176.0◦C and 219.3◦C, respectively. Analytical
and spectral data agree with the molecular structures of homologs. Mesomorphic phase
length varies between 22◦C and 76◦C. Smectic phase length varies from 6◦C to 33◦C
and nematic phase length varies from 22◦C to 54◦C. Thus, the novel present series
is predominantly nematogenic and partly smectogenic. Mesomorphic properties of the
novel series are compared with structurally similar other known homologous series.
Keywords Enantiotropic; liquid crystals; mesomorphic; mesophase; nematic
Introduction
Benzoic and cinnamic acid derivatives exhibiting mesomorphic or non-mesomorphic prop-
erties are known for their microbial activity or as biologically active molecules. Their
biological activity and/or mesomorphic behaviors are related to the molecular structure
of the substance concerned as a consequence of rigid and flexible parts of the molecule.
Thus, molecular rigidity and/or flexibility [1–3] play an important role to cause variations
of properties and varying usefulness of all different substances. Thus, varying combined
effects of molecular rigidity and flexibility can yield varying magnitudes of anisotropic
or isotropic forces of intermolecular attractions [4], out of which suitable magnitudes of
∗Address correspondence to Brijesh H. Patel, A-21 Saket Bunglows Opp Yash Complex, Gotri,
Baroda 390021, Gujarat, India. Tel: 919898345428. E-mail: brijpatel79@gmail.com
42

Novel Cinnamate Ester Liquid Crystals
43
anisotropic forces of intermolecular attractions inducing a specific property, of some sub-
stances (not all) as mesomorphic property, which flows like a liquid on the surface and
possess optical properties like a solid crystal. This special state of matter has a number
of applications, particularly displays. The present investigation involves the synthesis of
novel molecules that consist of three phenyl rings bridged through CH CH COO and
CH CH COO CH2 groups, which act as the rigid core and the terminal chain (–OR),
which acts as a flexible moiety. Thus, the effect of molecular structure on mesomorphic
properties can be studied by the proposed investigations.
Experimental
Synthesis
4-Hydroxy cinnamic acid was prepared by the reaction between 4-hydroxy benzaldehyde
and malonic acid in pyridine by a usual established method [5] (m.p. 213◦C, yield 84%).
4-Hydroxy cinnamic acid was alkylated by suitable alkylating agents (R-X) by a modi-
fied method of Dave and Vora [6] to form 4-n-alkoxy cinnamic acids. 4-Hydroxy benzyl
cinnamate was prepared by a usual established method [7] (m.p. 150◦C, yield 80%) and
then condensed with the appropriate 4-n-alkoxy cinnamic acids through the corresponding
acid chlorides [8] to form final cinnamoyloxy cinnamate ester derivatives. The final prod-
ucts [9] were individually decomposed, filtered, washed, dried, and purified until constant
transition temperatures were obtained. 4-Hydroxy benzaldehyde, malonic acid, pyridine,
aniline, toluene, alkyl halides, MeOH, EtOH, thionyl chloride, 4-hydroxy cinnamic acid,
benzyl alcohol, H2SO4, HCl, and KOH were used as received, except solvents that were
dried and purified prior to use. The synthetic route to the series is outlined in Scheme 1.
Characterization
Some selected members of the present novel series were characterized by elemental anal-
1
ysis, infrared spectra, H NMR spectra, and mass spectra. Microanalysis was performed
using a Perkin-Elmer PE 2400 CHN analyser (Waltham, Massachusetts, U.S.A.), 1H NMR
spectra were recorded on a Bruker spectrometer (Billerica, Massachusetts, U.S.A.) using
DMSO solvent. Infrared spectra were recorded on a Perkin Elmer spectrum GX spectrome-
ter (Shelton, Connecticut, U.S.A.). Transition temperatures and the textures of smectic and
nematic mesophases were determined by a miscibility method, and were characterized by
an optical polarizing microscope equipped with a heating stage. Thermodynamic properties
enthalpy change (ꢀH) and entropy change (ꢀS = ꢀH/T) are qualitatively discussed.
Analytical Data (Table 1)
Spectral Data
1H NMR in ppm for the Decyloxy Derivative. 1.24 (long alkyl chain), 1.7 ( CH2 CH2
O
of C10H21), 3.3 ( OCH2 of –OC10H21 group), 3.9 ( COO CH2 C6H5), 6.3 and 6.9
(p-substituted phenyl ring), 7.5 (mono-substituted phenyl ring), 4.00 (trans CH CH
group).
1H NMR in ppm for the Tetradecyloxy Derivative. 1.3 (long alkyl chain), 1.7 ( CH2 CH2
O
of C14H29), 3.3 ( OCH2 of OC14H29 group), 3.9 ( COO CH2 C6H5), 6.2–6.3

44
B. H. Patel and A. V. Doshi
Scheme 1. Synthetic route to the series.
and 6.8–6.9 (p-substituted phenyl ring), 7.5 (mono-substituted phenyl ring), 4.1 (trans
CH CH group).
IR in cm−1 for the Octyloxy Derivative. 696 (mono-substituted phenyl ring), 831 (p-
substituted phenyl ring), 1217 (ether linkage of -C8H17O-C6H4), 1171, 1285, 1676
( COO ester group), 982 (trans CH CH group).
Table 1. Elemental analysis for methoxy, ethoxy, and propoxy derivatives
Elements % found (% calculated)
Sr. no.
Molecular formula
C
H
1
2
3
C26H22O5
C27H24O5
C28H26O5
75.06 (75.36)
75.58 (75.70)
75.98 (76.01)
5.14 (5.31)
5.62 (5.60)
5.71 (5.88)

Novel Cinnamate Ester Liquid Crystals
45
Table 2. Transition temperatures of series in ◦C
Transition temperature in ◦C
Compound no.
n-alkyl group (–CnH2n+1) (n)
Sm
Nm
Isotropic
1
2
3
4
5
6
7
8
1
2
3
4
5
6
8
10
12
14
16
—
—
—
—
—
160.0
158.0
161.0
159.0
170.0
—
—
—
198.0
199.0
248.0
242.0
221.0
220.0
200.0
202.0
216.0
215.0
210.0
202.0
166.0
162.0
166.0
164.0
180.0
192.0
182.0
188.0
9
10
11
Note: Sm: smectic, Nm: nematic.
IR in cm−1 for the Dodecyloxy Derivative. 694 (mono-substituted phenyl ring), 828
(p-substituted phenyl ring), 981 (trans CH CH group), 1217 (ether linkage of
C12H25O C6H4), 1173, 1288, 1677 ( COO- ester group), 1217 (ether linkage of -
C12H25O-C6H4 group).
Mass Spectra
Mass spectra for the butoxy derivative:
Molecular formula is C29H28O5, calculated molecular weight is 456. Practical mass
observed by mass spectroscopy is 456.
Texture of Nematic Phase by Miscibility Method
Pentyloxy derivative —————–→Threaded nematic
Hexadecyloxy derivative ———–→Schlieren nematic
Hexyloxy derivate ——————–→Threaded nematic and smectic A
Results and Discussion
4-Hydroxy benzyl cinnamate (m.p. 150◦C) and 4-n-alkoxy cinnamic acids are non-
mesomorphic, but on linking them through their corresponding acid chlorides, they yielded
mesomorphic homologs from the third member of the novel homologous series of 4-
(4ꢀ-n-alkoxy cinnamoyloxy) benzyl cinnamates. The methoxy and ethoxy homologs are
non-mesomorphic. The propoxy to hexadecyloxy homologs are nematogenic only or are
nematogenic with smectogenic behavior. The hexyloxy to tetradecyloxy homologs exhibit
a smectogenic mesophase prior to nematogenic mesophase formation. Transition tempera-
tures (Table 2) of the homologs are plotted versus the number of carbon atoms present in

46
B. H. Patel and A. V. Doshi
CH=CH-COO-CH2
CH=CH-COO
RO
260
SOLID-ISOTROPIC OR
SOLID-MESOGENIC
SMECTIC - NEMATIC
240
220
200
180
160
140
120
NEMATIC - ISOTROPIC
ISOTROPIC
NEMATIC
SMECTIC
SOLID
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
X axis
No.of carbon atoms in n-alkyl chain-R
Figure 1. Phase behavior of series.
n-alkyl chain of the left n-alkoxy terminal end group. A phase diagram (Fig. 1) showing
phase behavior is obtained by joining like or related points. The solid-isotropic or meso-
morphic transition curve adopts a zigzag path of rising and falling values with a drastic rise
from and beyond the dodecyloxy homolog to the hexadecyloxy homolog. Thus, it behaves
in a normal manner. The smectic-nematic transition curve commences from the hexyloxy
homolog and initially rises, passes through maxima, and then descends as series is ascended
up to the tetradecyloxy homolog. The Sm-Nm transition curve is smoothly extrapolated
[10] to predict the latent transition temperature (LTT) following the trend of the curve to

Novel Cinnamate Ester Liquid Crystals
47
determine the probable latent ability to form the smectic mesophase, which indicates the
LTT to be 154◦C below (monotropic) the solid-nematic transition temperature. However,
LTT is not realizable due to the high crystallization tendency of the hexadecyloxy homolog.
Thus, the smectic-nematic transition curve perfectly behaves in the usual established man-
ner without the exhibition of an odd–even effect. The nematic-isotropic transition curve
commences from propoxy homolog to the hexadecyloxy homolog. Initially, it descends
from the propoxy to decyloxy homolog and then it descends for the tetradecyloxy and
the hexadecyloxy homologs after a 14◦C rise at the dodecyloxy homolog. Thus, it gen-
erally behaves in a normal manner except for the slight abnormality from and beyond
the decyloxy homolog derivative. An odd–even effect is observed for the Nm-I transition
curve. Both curves, for the odd and even merge into each other at the nonyloxy homolog
and then the effect diminishes and disappears. The Sm-Nm mesophase length varies be-
tween 6◦C and 33◦C, Nm-I mesophase length varies between 22◦C and 54◦C, and the total
mesophase length varies between 22◦C and 76◦C. Thus, the present novel series bears rich
mesomorphism at relatively high temperatures. Hence, the present series is a high melt-
ing series whose melting temperatures are varied between minimum of 198◦C at the first
homolog to maximum of 242◦C at the third homolog, from which mesomorphism com-
mences enantiotropically. The average thermal stabilities for smectic and nematic phases
are 176◦C and 219.3◦C, respectively. The novel series under discussion is predominantly
nematogenic and partly smectogenic whose mesomorphic behavior varies from homolog
to homolog, with changing molecular length by the methylene unit at the left n-alkoxy ter-
minal end group. The mesomorphic behavior is induced on linking two non-mesomorphic
components, viz., 4-n-alkoxy cinnamic acid and 4-hydroxy benzyl cinnamate, because the
molecular rigidity is strengthened by increasing the number of phenyl rings to the acid
molecule. Hence, the molecular aromaticity length to breadth ratio, molecular polarity is
enhanced to create suitable magnitudes of anisotropic forces of intermolecular attractions,
which can resist exposed thermal vibrations to cause mesomorphism from third member
(propoxy homolog) of the series under discussion. The methoxy and ethoxy homologs are
non-mesomorphic because of their high crystallizing tendency arising from imbalanced
intermolecular attractions as combined effect of suitable magnitudes of molecular rigidity
and flexibility, which induces an inability of molecules concerned to resist or withstand
exposed thermal vibrations from thermodynamic surroundings. Therefore, molecules of the
methoxy and ethoxy homologs are transformed into the isotropic liquid state directly from
the crystalline solid state without passing through mesomorphic states. Molecules of the
homologs from and beyond the propoxy homolog resist enough externally exposed thermal
vibrations and are disaligned to take on a floating surface at an angle of less than 90◦ within
a definite range of temperature. The molecules of the methoxy and ethoxy derivatives
due to relatively shorter n-alkyl chain length in left n-alkoxy terminal end group show an
inability to disalign to set on the surface at an angle of less than 90◦ against exposed ther-
mal vibrations. Therefore, they neither acquire statistically parallel orientational ordered
arrangement of molecules nor ordered sliding layered arrangement of molecules in floating
condition to exhibit either nematic or smectic mesophase or mesophases. The propoxy to
hexadecyloxy homologs under the influence of exposed thermal vibrations acquire suit-
able magnitudes of anisotropic forces of intermolecular end to end attractions to maintain
statistically parallel orientational order of molecules with or without prior sliding layered
arrangement of molecules to cause nematic and/or smectic mesophase formation. Thus, the
propoxy, butoxy, pentyloxy, and hexadecyloxy homologs are only nematogenic without ex-
hibition of any smectogenic character, but the hexyloxy to tetradecyloxy homologs exhibit

48
B. H. Patel and A. V. Doshi
Figure 2. Structurally similar homologous series.
nematogenic mesophase formation in addition to smectogenic mesophase formation. Smec-
togenic mesophase formation occurs due to the lamellar packing of molecules in their crystal
lattices and the resistivity of exposed thermal vibration is high enough to maintain a sliding
layered arrangement of molecules in a floating condition within definite temperature range
and then, on further heating, the sample substance loses intermolecular attractions, but
at this stage intermolecular end to end attractions are still strong enough to maintain a
relatively less ordered statically parallel orientational order of molecules to enable nematic
mesophase formation. An odd–even effect of alternation in transition temperatures and
changing mesomorphic behavior from homolog to homolog is attributed to the sequentially
and progressively added methylene group to the n-alkyl terminal end group, which causes
variations in molecular flexibility with stronger molecular rigidity as a result of three phenyl
rings and two central bridges bearing the linking group CH CH . The disappearance
of the odd–even effect from and beyond nonyloxy homolog in Nm-I transition curve is
attributed to the coiling or bending or flexing or coupling of the longer n-alkyl chain with
the major axis of the core of left n-alkoxy terminal end group. High thermal stabilities for
mesophases and a high degree of mesomorphism are attributed to the stronger intermolec-
ular attractions as a consequence of relatively higher molecular rigidity and flexibility
induced by longer molecules with CH CH COO and CH CH COO CH2
central bridges. The mesomorphic behavior of present novel series-1 is compared
with structurally similar other known series X [11] and Y [7], as shown in
Fig. 2.
As can be seen from Fig. 2, homologous series 1, X, and Y (Fig. 2) are similar in
structure with respect to three phenyl rings and left n-alkoxy terminal end group, but they
differ with respect to their central bridges. The three phenyl rings and the differing linking
bridge groups contribute to the total molecular rigidity, and changing n-alkyl chain of left
n-alkoxy terminal end group contributes to the total molecular flexibility of each varying
homologs of the series. However, the combined effect of molecular rigidity and flexibility
controls the suitable magnitudes of anisotropic forces of intermolecular attractions through
which mesomorphic behavior of a substance controlled. Varying suitable magnitudes of
molecular rigidity and flexibility of present series-1 and the series X and Y chosen for

Novel Cinnamate Ester Liquid Crystals
49
Table 3. Average thermal stability in ◦C
Series
Series 1
Series X
—
Series Y
Smectic-isotropic or 176.0 (C6–C14)
smectic-nematic
106.6 (C10–C16)
Commencement of
smectic phase
C6
C10
Nematic-isotropic
Commencement of
nematic phase
219.3 (C3–C16)
C3
144.5 (C6–C16)
C6
119.8 (C8–C16)
C8
comparison cause variations in mesomorphic behavior and the degree of mesomorphism
shown by homologous series-1 to X and Y.
Table 3 represents thermal stabilities for smectic and/or nematic, commencement of
smectic and/or nematic mesophase formation, etc.
The thermal stability for nematic mesophase decreases from series-1 to X to Y (Table 3).
The smectogenic thermal stability of series-1 is also greater than a series Y, whereas series
X is non-smectogenic.
The commencement of nematic mesophase formation takes place earlier from the third
member of a present series-1, but it commences from the sixth and eighth member of the
series X and Y, respectively.
The molecular length of series-1 is longer than series X by the O CH2 unit
of the second central bridge, CH CH COO CH2 , of series-1. Therefore, length to
breadth ratio and the ratio of the molecular polarity to polarizability as well as anisotropic
intermolecular attractions differ between series-1 and X. This situation reflects the molec-
ular rigidity of series-1 and the thermodynamic quantity, enthalpy change, and entropy
change ꢀS = ꢀH/T of the system, i.e., the resistivity to exposed thermal vibrations of
the molecule is enhanced enabling two-dimensional order in a floating condition. The
reasons for early or late commencement of the nematic mesophase formation is also re-
lated to the factors such as suitable magnitudes of anisotropic forces of intermolecular end
to end attractions, as a consequence of more or less favorable molecular rigidity and/or
flexibility [1–4]. Early or late commencement (including absence) of smectic mesophase
appearance, i.e., lamellar packing of molecules in crystal lattices, depends upon favorable
magnitudes of intermolecular attractions and the extent of non-coplanarity caused by the
molecule.
The observed difference of mesomorphic behavior between a series-1 and Y is at-
tributed to the above discussed reasons of molecular rigidity versus flexibility, and also
to the differing central bridges CH CH COO and COO , joining first and mid-
dle phenyl rings. Both these central bridges are comparable, though the vinyl carboxylate
unit, CH CH COO , has a relatively greater length and causes more molecular non-
coplanarity. On account of this difference, the smectic-isotropic or smectic-nematic thermal
stability of series Y is relatively lower than the corresponding smectic thermal stability of a
present investigated novel homologous series-1. Thus, variations in mesomorphic behavior
for the same homolog from series to series are attributed to the differing central bridge or
bridges.

50
B. H. Patel and A. V. Doshi
Conclusions
1. Group efficiency order derived for smectic and nematic on the basis of thermal
stabilities are as under.
Smectic : − CH=CH−COO− >
with
−COO−
with
> −CH=CH−COO−
with
−CH=CH−COO − CH2−
−CH=CH−CO−
−CH=CH−CO−
Nematic : − CH=CH−COO−CH2− > −CH=CH−CO− > −CH=CH−CO−
with
with
with
−CH=CH−COO−
−CH=CH−COO−
−COO−
2. Group efficiency order derived on the basis of early commencement of smectic
and nematic phase are as under.
Smectic : − CH=CH−COO−CH2− > −CH=CH−CO− > −CH=CH−CO−
with
with
−CH=CH−COO−
−COO−
Nematic : − CH=CH−COO − CH2− > −CH=CH−CO− > −CH=CH−CO−
with
with
−CH=CH−COO
−COO−
3. Molecular rigidity and flexibility of favorable or suitable magnitudes can only
induce mesomorphic property in a substance.
4. The -COO- group is generally smectogenic only, however, nematogenic character
can be incorporated by introducing second central group like CH CH CO .
5. The CH CH COO group is generally nematogenic but smectic and nematic
mesophases can be induced by high degree of mesomorphism and high melting
type mesomorphism by introducing CH CH COO CH2− as second central
group with CH CH COO central group.
6. Mesomorphic behavior of a substance is very sensitive to molecular structural
variation of a substance.
7. Mesomorphic behavior from homolog to homolog in the same series varies with
the sequential addition of methylene unit or units.
8. Mesomorphic behavior of a same homolog from series to series varies with the
varying molecular structure of the mesomorphic substances.
9. Mesomorphic behavior of any homologous series (including novel series-1) de-
pends upon the varying positional substitution of same functional group or groups
whose individual self-group polarity is same; especially in the case of isomeric
series.
10. Presently investigated novel series is predominantly nematogenic and partly smec-
togenic, with exhibition of good degree of mesomorphism and high melting type.
Acknowledgments
Authors acknowledge thanks to the Green Circle Inc. Lab for research facilities’ services
as and when needed. Authors also thank Dr. N. N. Vyas, Dr. Vipul Patel, and Dr. M.

Novel Cinnamate Ester Liquid Crystals
51
L. Chauhan for their valuable helping hand and microscopic facility. Thanks are due to
the Sophisticated Analytical Instrumentation Facility, Punjab University, Chandigarh for
analytical services.
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