Synthesis and mesogenic behavior of novel liquid crystals with a-CH=CH-CO-central bridge

Article abstract of DOI:10.1080/15421406.2013.768884

A novel homologous series α-3-[-4′-n-alkoxy benzoyloxy] phenyl-β-4″-methoxy benzoyl ethylene, which consists of 11 members, has been synthesised and the mesomorphic behavior studied in relation to the molecular structure. The pentyl to hexadecyl homologues are mesomorphic and the remaining homologues are non-mesomorphic. The pentyl, tetradecyl, and hexadecyl derivatives are only enantiotropic nematic, while the hexyl, octyl, decyl, and dodecyl homologue derivatives exhibit smectic mesomorphism in addition to the enantiotropic nematic phase. The transition and melting temperatures were determined by an optical polarizing microscope with a heating stage. Analytical and spectral data confirm the molecular structures. The texture of the nematic phase is of the threaded or Schlieren type and that of the smectic mesophase is a typical smectic A phase. The transition curves of the phase diagram behave in normal manner without showing an odd-even effect. The smectic and nematic thermal stabilities are 129.5°C and 150.7°C, respectively. The novel series is partly smectogenic and predominantly nematogenic with a middle-ordered melting type whose mesomorphic phase length varies from 7°C to 46°C. The mesomorphic properties are compared with other structurally similar homologous series.

Full text of DOI:10.1080/15421406.2013.768884

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Synthesis and Mesogenic Behavior of
Novel Liquid Crystals With a –CH˭CH–CO–
Central Bridge
R. P. Chaudhari a , M. L. Chauhan b & A. V. Doshi c
a Department of Chemistry , K. K. Shah Jarodwala Maninagar
Science College , Ahmedabad , Gujarat , India
b Department of Chemistry , S. P. T. Arts and Science College ,
Godhra , Gujarat , India
c Shri Jagdishprasad Jhabarmal Tibrewala University (JJTU) ,
Jhunjhunu , Rajasthan , India
Published online: 22 Apr 2013.
To cite this article: R. P. Chaudhari , M. L. Chauhan & A. V. Doshi (2013) Synthesis and Mesogenic
Behavior of Novel Liquid Crystals With a –CH˭CH–CO– Central Bridge, Molecular Crystals and Liquid
Crystals, 575:1, 88-95, DOI: 10.1080/15421406.2013.768884
To link to this article: http://dx.doi.org/10.1080/15421406.2013.768884
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Mol. Cryst. Liq. Cryst., Vol. 575: pp. 88–95, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.768884
Synthesis and Mesogenic Behavior of Novel Liquid
Crystals With a –CH CH–CO– Central Bridge
R. P. CHAUDHARI,1,∗ M. L. CHAUHAN,2 AND A. V. DOSHI3
1Department of Chemistry, K. K. Shah Jarodwala Maninagar Science College,
Ahmedabad, Gujarat, India
2Department of Chemistry, S. P. T. Arts and Science College, Godhra,
Gujarat, India
3Shri Jagdishprasad Jhabarmal Tibrewala University (JJTU), Jhunjhunu,
Rajasthan, India
A novel homologous series α-3-[-4ꢀ-n-alkoxy benzoyloxy] phenyl-β-4ꢀꢀ-methoxy benzoyl
ethylene, which consists of 11 members, has been synthesized and the mesomorphic
behavior studied in relation to the molecular structure. The pentyl to hexadecyl ho-
mologues are mesomorphic and the remaining homologues are non-mesomorphic. The
pentyl, tetradecyl, and hexadecyl derivatives are only enantiotropic nematic, while the
hexyl, octyl, decyl, and dodecyl homologue derivatives exhibit smectic mesomorphism
in addition to the enantiotropic nematic phase. The transition and melting temperatures
were determined by an optical polarizing microscope with a heating stage. Analytical
and spectral data confirm the molecular structures. The texture of the nematic phase is
of the threaded or Schlieren type and that of the smectic mesophase is a typical smectic
A phase. The transition curves of the phase diagram behave in normal manner without
showing an odd–even effect. The smectic and nematic thermal stabilities are 129.5◦C
and 150.7◦C, respectively. The novel series is partly smectogenic and predominantly ne-
matogenic with a middle-ordered melting type whose mesomorphic phase length varies
from 7◦C to 46◦C. The mesomorphic properties are compared with other structurally
similar homologous series.
Keywords Liquid crystals; mesogen; mesomorphic; nematic; smectic
Introduction
Liquid crystalline materials are useful in electronic devices (LCDs), medical thermographic
instruments, pharmaceutical preparations, and the textile industry. The exhibition of the liq-
uid crystalline state by a substance depends upon its molecular rigidity and flexibility [1–3],
which in turn directly depends upon its molecular geometrical shape, size, aromaticity, po-
larity, and polarizability of the terminal, lateral, and central groups, which induce suitable
magnitudes of anisotropic intermolecular forces of attractions [4] as a consequence of the
molecular rigidity and flexibility [1–3]. Thus, the present investigation is aimed at under-
standing and establishing the relationship between the liquid crystal (LC) state formation
and molecular structure by synthesizing novel substances with a left n-alkoxy(–OR) and
∗Address correspondence to R. P. Chaudhari, Department of Chemistry, K. K. Shah Jarodwala
Maninagar Science College, Rambaug, Maninagar, Ahmedabad – 380 008, Gujarat, India. E-mail:
rpchaudhari@ymail.com
88

Synthesis and Behavior of Novel Liquid Crystals
89
a right –OCH3 terminal end groups without any lateral substitution with three phenyl
rings bridged through –COO– and –CH CH–CO– central groups in a non-linear shaped
molecule. Hence, the effect of the non-linear shaped molecule inducing different magni-
tudes of polarizability in comparison with a linear molecule is studied. Thus, the liquid
crystalline properties will be discussed and evaluated in comparison with known series.
Experimental
Synthesis
4-Hydroxybenzoic acid is alkylated by suitable akly halides to prepare 4-n-alkoxybenzoic
acids. The dimerized n-alkoxybenzoic acids were converted to corresponding 4-n-
alkoxybenzoyl chlorides (A) using freshly distilled thionyl chloride by a modified method of
Dave and Vora [5]. α-3-Hydroxy phenyl-β-4ꢀ-methoxy benzoyl ethylene (B) was prepared
by reacting 3-hydroxybenzaldehyde with 4-methoxyacetophenone by the usual established
method [6,7]. Components (A) and (B) were condensed in dry cold pyridine by an es-
tablished method [8]. The final products of each homologue were decomposed filtered,
washed, dried, and purified by crystallization until constant transition temperatures were
obtained by an optical polarizing microscope with a heating stage. Synthetic route to the
series is shown in Scheme 1.
Scheme 1. The synthetic route to the novel series.
Characterization
A selected number of representative homologues were characterized by elemental analysis,
Infrared (IR) and 1HNMR spectroscopy. Microanalysis was performed on a Perkin-Elmer
PE 2400 CHN analyzer (Vaibhav Lab. Ahmedabad, Gujarat) as shown in Table 1. IR spectra
were recorded on Perkin-Elmer spectrum GX instrument (Punjab University, Chandigarh).

90
R. P. Chaudhari et al.
Table 1. Elemental analysis for propyloxy, pentyloxy, decyloxy, and hexadecyloxy
derivatives
Elements % found (% Calculated)
Sr. no
Molecular formula
C
H
1
2
3
4
C26H24O5
C28H28O5
C33H38O5
C39H50O5
75.03 (75.00)
75.71 (75.68)
77.02 (77.05)
78.29 (78.26)
5.81 (5.77)
6.33 (6.30)
7.36 (7.39)
8.38 (8.36)
1HNMR spectra were recorded using CDCl3 as the solvent. LC properties were determined
by an optical polarizing microscopy equipped with a heating stage. The textures of the
nematic and the smectic phase were determined by a miscibility method. Thermodynamic
quantities enthalpy (ꢀH) and entropy (ꢀS) are discussed qualitatively instead of using
differential scanning calorimetry.
Analytical Data
Analytical data includes spectral data for dodecyloxy, tetradecyloxy, hexyloxy, and octyloxy
derivatives.
Spectral Data.
NMR in ppm for Dodecyloxy Derivative. 0.870 (–CH3 of –OC12H25 group),
1.4388 (–CH2– of –OC12H25), 4.0038 (–OCH2– of –OC12H25), 6.9024 and 6.9216
(–CH CH–CO– group), 7.8818 and 7.90 (p-substituted phenyl ring). NMR supports the
structure.
NMR in ppm for Tetradecyloxy Derivative. 0.885 (–CH3 of –OC14H29 group), 1.4270
(–CH2)– of –OC14H29 group, 3.49 (–OCH2– of –OC14H29 group), 6.9015, 6.9232, and
6.9397 (–CH CH–CO– group), 7.8850 and 7.9069 (p-substituted phenyl ring). NMR
supports the structure.
IR in cm−1 for Hexyloxy Derivative. 660 –(CH2)n– group of –OC6H13, 850 p-
substituted phenyl ring, 1170 C–O of –OC6H13 group, 1250 and 1600 –COO ester group,
935 –CH CH– group. IR supports the structure.
IR in cm−1 for Octyloxy Derivative. 780 –(CH2)n– group of –OC8H17, 820 p-
substituted phenyl ring, 1160 C–O of –OC8H17 group, 1250 and 1680 –COO ester group,
935 –CH CH– group. IR supports the structure.
Texture
The texture for tetradecyl, octyl, and hextyl are listed as follows.
• Tetradecyl: Threaded nematic
• Octyl: Schlieren nematic
• Hextyl: Smectic-A
Results and Discussion
Dimerzation of 4-n-alkoxybenzoic acids disappears on the esterification process. Compo-
nent B of the ethylene homologue derivative is not liquid crystalline. However, on linking it

Synthesis and Behavior of Novel Liquid Crystals
91
Table 2. Transition temperatures of series in ◦C
Transition temperatures in ◦C
n-alkyl group:
Compound no.
CnH2n+1
Sm
Nm
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
8
10
12
14
16
—
—
—
—
—
126.0
118.0
115.0
113.0
—
—
—
—
—
125.0
136.0
144.0
123.0
115.0
110.0
136.0
210.0
166.0
165.0
172.0
132.0
172.0
163.0
152.0
147.0
146.0
143.0
10
11
—
Note. Sm = Smectic; Nm = Nematic.
with an acid chloride, i.e., with component A, the resultant product exhibits LC properties
from the pentyl to the hexadecyl homologues, while the methyl to butyl homologues do
not exhibit LC properties (Table 2). A plot of the transition temperature versus the number
of carbon atoms present in n-alkyl terminal chain consists of three transition curves, solid-
isotropic or mesomorphic curve, smectic–nematic curve, and nematic–isotropic curve,
as shown in Fig. 1. The solid–isotropic or mesomorphic transition curve rises and falls
as the series is ascended in a zigzag manner with an overall descending tendency. The
smectic–nematic transition curve initially rises from the hexyl homologue and then passes
from a maxima at the octyl derivative and then falls up to the dodecyl derivative of the series.
This curve is extrapolated to the tetradecyl homologue and it merges into a point where the
smectic phase ceases to appear and only the nematic phase appears. Thus, the smectogenic
property is exhibited by the hexyl to the dodecyl homologues of the novel series. The
nematic–isotropic transition curve initially rises and then descends as the series is ascended
without the exhibition of an odd–even effect. Thus, all three transition curves of the phase
diagram (Fig. 1) behave in the usual established manner. The mesomorphic phase length
ranges from 7◦C to 46◦C at the pentyl to the hexadecyl homologues. The series is partly
smectogenic and predominantly nematogenic with a middle-ordered melting type. The me-
somorphic properties vary from homologue to homologue in the same series with different
magnitudes of phase length including smectic and nematic phase lengths. An odd–even
effect is absent in the smectic–nematic and nematic–isotropic transition curves. The smectic
mesophase commences from the hexyl homologue (Table 2). All the mesogenic members
of the series are enantiotropic mesomorphic. The breaking of hydrogen bonding due to
the esterification process gives rise to esters with lower transitions than the corresponding
alkoxy acids. However, the addition of a third phenyl ring bonded through a –CH CH–CO–
central group increases molecular rigidity and flexibility and raises transition temperatures.
Non-mesomorphic behavior of the methyl to the butyl homologue derivatives is attributed to
their high crystallizing tendency due to their relatively short alkoxy terminal chains. Hence,
the corresponding homologue molecules are unable to resist exposed thermal vibrations,

92
R. P. Chaudhari et al.
Figure 1. Phase behavior of series.
which results in the sudden breaking of the crystal structure and converts the substance
sharply into isotropic liquid state from solid state without exhibition of a liquid crystalline
mesophase. Such a situation also arises for a mesomorphic (LC) substance from and beyond
the mesomorphic–isotropic transition temperature. The exhibition of mesogenic properties
from the pentyl to the hexadecyl homologues is attributed to the suitable magnitudes of
anisotropic intermolecular forces [4] of attraction caused by favorable molecular polarity
and polarizability, aromaticity, electronic–electronic interactions, length-to-breadth ratio,
and the ratio of the polarity to polarizability as a consequence of the resultant molecular
rigidity and flexibility [1–3]. The exhibition of only nematogenic character by the pentyl,
tetradecyl, and hexadecyl homologues, or in addition to smectogenic character by the hexyl
to the dodecyl homologues, is attributed to the statistically parallel orientational order of
molecules in a floating condition due to favorable intermolecular end-to-end adhesion in
which molecules appear to lie at an angle less than 90◦ on the plane of a surface, with the
thermal vibration resisting under the influence of applied heat within definite temperature
range. The exhibition of smectogenic character by the hexyl to dodecyl homologues is due
to the presence of lamellar packing of the molecules in their crystal lattices that on heating
gradually yield a sliding layered arrangement of molecules in a floating condition resisting
thermal vibrations within a definite range of temperature. Thus, the presently investigated

Synthesis and Behavior of Novel Liquid Crystals
93
Figure 2. Structurally similar series.
homologous series is predominantly nematogenic and partly smectogenic without showing
an odd–even effect. The absence of an odd–even effect in either of the curves in the phase
diagram (Fig. 1) is due to the absence of mesophase formation by the methyl and propyl
derivatives of the series. The disappearance of the smectogenic character in the tetradecyl
and hexadecyl homologues indicates the absence of lamellar molecular packing due to the
longer terminal chains, which may coil, bend, flex, or couple to lie with the major axis of the
core adversely affecting the molecular rigidity and flexibility [1–3], which hinders lamellar
molecular packing. The variation of mesogenic properties from homologue to homologue
in the same series is due to the sequential addition of methylene unit [4] in the terminal
alkoxy chain, which causes variation in molecular polarity and polarizability affecting the
intermolecular lateral attractions.
The mesomorphic properties of the novel Series 1 is compared with two other struc-
turally similar homologous Series X [8] and Y [9] in Fig. 2.
Figure 2 shows structurally similar homologous series with minor variations in their
geometrical shape and the central bridge linking the first and middle phenyl rings. Such
minor variations cause a difference in the mesomorphic behavior in terms of the com-
mencement of the smectic and/or nematic phase, transition temperatures, magnitudes
of intermolecular anisotropic forces of adhesion, thermal stabilities, etc., as recorded in
Table 3. The homologous Series 1 of the present investigation and Series X differ in their
central bridge, viz., –COO– and –CH CH–COO–, linking the first and middle phenyl
Table 3. Relative average thermal stability in ◦C
Average transition temperatures in ◦C
Series
Series (1)
Series (X)
Series (Y)
Smectic–nematic or
isotropic
129.5 (C6–C12)
138.80 (C10–C14)
133.4 (C6–C12)
Commencement of
smectic phase
C6
C5
C6
Nematic–isotropic
Commencement of
nematic phase
150.7 (C5–C16)
C5
165.7 (C5–C14)
C5
159.8 (C6–C16)
C6

94
R. P. Chaudhari et al.
rings with the remaining molecular units and shape unchanged. The vinyl carboxylate
–CH CH–COO– group is relatively longer than the –COO– and has a conjugated double
bond. The presence of the double bond strengthens the molecular rigidity of Series X,
which reflects in the degree of mesomorphism and the lateral intermolecular attractions.
Homologous Series 1 and Y are isomeric or identical in all respect except their molecular
shape. Homologous Series 1 is broader than a Series Y, while Series Y has a lath like
shape. The polarity due to the three phenyl ring central groups and terminal end group are
the same, but their molecular polarity and polarizability varies with the molecular shape,
which is reflected in the magnitudes and degree of mesomorphism due to intermolecular
distance.
Table 3 indicates that the smectic–nematic thermal stability is lowest of Series 1 in
comparison with Series X and Y chosen for comparison. Series X contains the longer
vinyl carboxylate central group with the –CH CH– additional unit, which increases the
molecular length in comparison with the –COO– central bridge. Hence, the length-to-
breadth ratio and the molecular polarizability of the molecules of Series X are greater than
the molecules of Series 1. Thus, suitable magnitudes of anisotropic intermolecular forces
of adhesion are higher in Series X than in Series 1, and this reflects the smectic–nematic
thermal stability of Series 1 and X. The smectic–nematic thermal stability of Series Y is also
greater than the molecules of Series 1. This is because the molecules of Series Y are lath
like while the molecules of its isomeric Series 1 of present investigation are nonlinear and
broader than the Series Y. Broadening of a molecule raises the intermolecular attractions by
polarizability factor. But at the same time, broadening of a molecule reduces intermolecular
attractions due to increase in the intermolecular distance. Thus, two opposite effects due
to molecular broadening operate at a time [4] in case of Series 1. The net resultant effect
of intermolecular attractions depends upon the predominating effect out of two opposing
effects operating at a time. The second effect, i.e., the reduction in intermolecular attractions
due to broadening of a molecule, predominates in case of Series 1, while closer packing
of molecules of Series Y strengthens the suitable magnitudes of anisotropic intermolecular
forces of attractions in comparison to Series 1 of present investigation. Therefore, the
smectic–nematic thermal stability of Series Y is higher than for Series 1. Similarly, the
nematic–isotropic thermal stabilities of Series X and Y are higher than that of Series 1
of present investigation. The commencement of the smectic mesophase takes place from
fifth or sixth member of the series, which indicates that the flexible identical molecular
part, viz., the –OR and –OCH3 terminal groups, causes an almost equivalent extent of
molecular non-coplanarity of Series 1, X, and Y. Similarly, the magnitudes of the end-
to-end intermolecular attractions result in the formation of the nematogenic mesophase
irrespective of the differing features of the central group or the linearity of a molecule.
Thus, the variation in mesomorphic properties for the same homologue from series to
series is attributed to varying features due to central group linking first and middle phenyl
ring and the molecular shape or linearity.
Conclusions
(1) The group efficiency order derived for smectic and nematic on the basis of thermal
stability is as under:
Smectic: –CH CH–COO– > –COO– > –COO–
(Linear) (Nonlinear)
Nematic: –CH CH–COO– > –COO– > –COO–
(Linear) (Nonlinear)

Synthesis and Behavior of Novel Liquid Crystals
95
(2) Broadening of a molecule causes two opposing effects at a time, viz., enhances
intermolecular attractions related to molecular polarizability and reduces in inter-
molecular attraction due to enhanced intermolecular distance. The resulting net
effect depends upon predominating effect out of two opposing effects operating at
a time.
(3) When terminal end groups and aromaticity of a molecule are identical, then
only the molecular shape predominantly contributes to the suitable magnitudes
of anisotropic forces of intermolecular adhesion.
(4) Central group, or groups, plays an important role in mesophase formation and
molecular rigidity.
Acknowledgments
The authors thank the authorities of K. K. Shah Jarodwala Maninagar Science College,
Ahmedabad, and S. P. T. Arts & Science College, Godhara, for their valuable cooperation
in present work. Authors are also thankful to Vaibhav Laboratory for their analytical
services. Thanks are due to Ankita Doshi, Department of Biochemistry, M. S. University
of Baroda, Vadodara, for valuable support in the work.
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