A novel homologous series of thermotropic mesogens of ethylene derivatives: α-4-[4′-n-Alkoxy Benzoyloxy] phenyl β-4″ methoxy benzoyl ethylenes

Article abstract of DOI:10.1080/15421406.2012.703916

A novel homologous series consists of 12 compounds. The shorter chain homologues do not show liquid crystal phases, but from six carbons onwards, the nematic phase is exhibited enantiotropically, with some members also showing a smectic phase. The phase behavior of the transition curves of the series shows a usual trend. The texture of the nematic phase is the threaded or Schlieren type and that of smectic phase is a smectic A. An odd-even effect is observed for smectic-nematic and nematic-isotropic transition curves. Analytical data confirm the molecular structure. The transition temperatures were determined by an optical polarizing microscopy. The mesogenic properties of the novel series are compared with a structurally similar homologous series. The average smectic-nematic and nematic-isotropic thermal stabilities are 133.4°C and 159.8°C, respectively, with a mesogenic temperature range of 10°C to 62°C.

Full text of DOI:10.1080/15421406.2012.703916

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A Novel Homologous Series of
Thermotropic Mesogens of Ethylene
Derivatives: α-4-[4′-n-Alkoxy
Benzoyloxy] Phenyl β-4″Methoxy
Benzoyl Ethylenes
H. N. Chauhan a b & A. V. Doshi c
a Shri JJT University, Jhunjhunu, Rajasthan, India
b K. K. Shah Jarodwala Maninagar Science College, Ahmedabad,
Gujarat, India
c Shri Jagdish Prasad Jhabarmal Tibrewala University, Jhunjhunu,
Rajasthan, India
Published online: 22 Jan 2013.
To cite this article: H. N. Chauhan & A. V. Doshi (2013) A Novel Homologous Series of Thermotropic
Mesogens of Ethylene Derivatives: α-4-[4′-n-Alkoxy Benzoyloxy] Phenyl β-4″Methoxy Benzoyl
Ethylenes, Molecular Crystals and Liquid Crystals, 570:1, 12-19, DOI: 10.1080/15421406.2012.703916
To link to this article: http://dx.doi.org/10.1080/15421406.2012.703916
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Mol. Cryst. Liq. Cryst., Vol. 570: pp. 12–19, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2012.703916
A Novel Homologous Series of Thermotropic
Mesogens of Ethylene Derivatives: α-4-[4ꢀ-n-Alkoxy
Benzoyloxy] Phenyl β-4ꢀꢀMethoxy Benzoyl Ethylenes
H. N. CHAUHAN1,2,∗ AND A. V. DOSHI3
1Shri JJT University, Jhunjhunu, Rajasthan, India
2K. K. Shah Jarodwala Maninagar Science College, Ahmedabad, Gujarat, India
3Shri Jagdish Prasad Jhabarmal Tibrewala University, Jhunjhunu, Rajasthan,
India
A novel homologous series consists of 12 compounds. The shorter chain homologues
do not show liquid crystal phases, but from six carbons onwards, the nematic phase
is exhibited enantiotropically, with some members also showing a smectic phase. The
phase behavior of the transition curves of the series shows a usual trend. The texture of
the nematic phase is the threaded or Schlieren type and that of smectic phase is a smectic
A. An odd–even effect is observed for smectic–nematic and nematic–isotropic transition
curves. Analytical data confirm the molecular structure. The transition temperatures
were determined by an optical polarizing microscopy. The mesogenic properties of the
novel series are compared with a structurally similar homologous series. The average
smectic–nematic and nematic–isotropic thermal stabilities are 133.4◦C and 159.8◦C,
respectively, with a mesogenic temperature range of 10◦C to 62◦C.
Keywords Liquid crystal; mesogen; mesogenic; nematic; smectic
Introduction
The molecular geometry, shape and size, of a molecular structure has a direct relation
with molecular rigidity and flexibility, which in turn influences the mesomorphic properties
[1–3]. The rigid core of a structure is made up of ring structures (aromatic or heterocyclic)
bridged through two central groups, and the flexible portion is made up of lateral and/or
terminal end groups or chains. The novel homologous series consists of three phenyl rings
linked through two central bridges –COO– and –CH CH–CO–, which act as a rigid core.
The left and right terminal end groups of the molecule are n-alkoxy and –OCH3 respectively,
and these act as flexible units of the molecule without any lateral substitution. Thus, the
induced mesomorphism can be assessed and discussed in terms of molecular rigidity
and flexibility and their correlation with molecular structure and suitable magnitudes of
anisotropic intermolecular forces of attraction.
∗Address correspondence to H. N. Chauhan, K. K. Shah Jarodwala Maninagar Science College,
Ahmedabad, Gujarat, India. E-mail: shreeyashailee@yahoo.in
12

Novel LC Methoxy Benzoyl Ethylenes
13
Experimental
Synthesis
4-Hydroxybenzoic acid is alkylated by suitable alkylating agents and the resulting n-
alkoxy benzoic acids were converted to the corresponding n-alkoxybenzoyl chloride (A)
using thionyl chloride by the method of Dave and Vora [4]. α-4-Hydroxy phenyl β-4ꢀ
methoxy benzoyl ethylene (B) was prepared by reacting 4-hydroxy benzaldehyde with p-
methoxyacetophenone by the usual established method [5]. Components (A) and (B) were
condensed in dry cold pyridine by a known method [6–8]. The final products were de-
composed, filtered, washed, dried, and recrystallized until constant transition temperatures
were obtained. The transition temperatures were determined by the hot stage polarizing
microscopy. 4-Hydroxybenzoic acid, alkylating agents (R-X), thionyl chloride, pyridine,
KOH, methanol, ethanol, 4-hydroxybenzaldehyde, and 4-methoxyacetophenone were used
as received. The synthetic route to the series is outlined in Scheme 1.
Scheme 1. Synthetic route to the novel series.
Characterization
Selected members of the novel homologous series were characterized by elemental analysis
1
(Table 1), infra red (IR) spectroscopy, and HNMR spectroscopy techniques. Microanal-
ysis was performed on a Perkin-Elmer PE2400 CHN analyzer. IR spectra were recorded

14
H. N. Chauhan and A. V. Doshi
Table 1. Elemental analysis for the ethyloxy, butyloxy, heptyloxy, and tetradecyloxy
derivatives
Elements % found (% calculated)
Sr. No.
Molecular formula
C
H
1.
2.
3.
4.
C25H22O5
C27H26O5
C30H32O5
C37H46O5
74.63 (74.60)
75.35 (75.32)
76.27 (76.30)
77.89 (77.86)
5.47 (5.42)
6.05 (6.01)
6.78 (6.81)
8.07 (8.03)
1
on Perkin-Elmer spectrum GX, and HNMR spectra were recorded on a Bruker spec-
trometer using CDCl3 as solvent. The mesogenic properties of the members of the series
were recorded by polarizing microscopy. The textures of nematic and smectic mesophases
were determined by the miscibility method. Thermodynamic properties enthalpy (ꢀH) and
entropy (ꢀS) are discussed qualitatively.
Analytical Data
Spectral Data
NMR in ppm for octyloxy derivative
0.983 (–CH3 of –OC8H17), 1.422 (–(CH2)n– of –OC8H17), 3.675 (–OCH2 of
–OC8H17), 6.565 and 6.582 (–CH CH–CO–), 7.404, 7.093, and 7.636 (phenyl
ring).
NMR in ppm for dodecyloxy derivative
0.945 (−CH3 of OC12H23), 1.465 (–(CH2)n– of –OC12H23 group), 3.689 (–OCH2 of
–OC12H23), 6.575 and 6.669 (–CH CH–CO–), 6.691, 7.610, and 7.632 (phenyl
ring).
IR in cm−1 for hexyloxy derivative
720 (–(CH2)n– of –OC6H13), 815 (phenyl ring), 1175 (C–O of –OC6H13), 1215, 1260,
and 1640 (–COO–), 930 (–CH CH–).
IR in cm−1 for decyloxy derivative
760 (–(CH2)n– of –OC10H21), 830 (phenyl ring), 1150 (C–O of –OC10H21), 1250,
1610, and 1680 (–COO–), 945 (–CH CH–).
Texture Determination
• Tetradecyl – Nematic – Threaded
• Decyl – Smectic – Smectic-A Type
• Hexyl – Smectic – Smectic-A Type
• Octyl – Nematic – Schlieren

Novel LC Methoxy Benzoyl Ethylenes
15
Figure 1. The phase behavior of the novel series.
Results and Discussion
4-n-Alkoxybenzoic acids are dimeric, where two molecules are joined through hydrogen
bonding. Dimerisation disappears on breaking of hydrogen bonding by esterification. The
methyl to pentyl derivatives of the novel series are non-mesogenic, but all others are meso-
genic. The pentyl to dodecyl derivatives are enantiotropically smectogenic and nematogenic
in character. The tetradecyl and hexadecyl derivatives of the series are only enantiotrop-
ically nematogenic without the exhibition of a smectic phase. The phase behavior of the
novel series is shown in the phase diagram (Fig. 1) with the number of carbon atoms present
in n-alkyl chain versus the transition temperatures (also shown in Table 2) of the members
of the series as determined by the hot stage polarizing microscopy. The phase diagram
indicates the following features:

16
H. N. Chauhan and A. V. Doshi
Table 2. Transition temperatures of the novel series
Transition temperatures (◦C)
Compound
No.
n-alkyl group
–CnH2n+1 (n)
Smectic
Nematic
Isotropic
1.
2.
3.
4.
5.
6.
7.
8.
9.
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
125.0
113.0
112.0
110.0
106.0
—
—
—
—
—
—
133.0
135.0
139.0
131.5
129.0
126.0
128.0
164.0
151.0
169.0
148.0
152.0
171.0
172.0
174.0
164.0
157.0
145.0
138.0
10.
11.
12.
—
• The solid-isotopic or mesomorphic transition curve rises and falls as the series is
ascended with an overall falling nature.
• The smectic–nematic transition curve initial rises, passes through maxima, and then
falls in usual manner up to the decyloxy homologue, but it rises by 1◦ in dodecyl
derivative. The curve is extrapolated [8] to the pentyl, butyl, and tetradecyl derivatives
to predict their latent transition temperatures (LTT) for the smectic phase, which are
exhibited monotropically as 111◦C, 118◦C, and 126◦C. The curve shows very narrow
range of odd–even effect.
• The nematic–isotropic transition curve initially rises and then adopts a descending
tendency as the series is ascended and behaves in the usual manner with an odd–even
effect. Both curves for the odd and even members are extrapolated [8] to predict the
LTT for nematic–isotropic transition of the butyl and pentyl members of the series,
which coincide with the melting points of the respective homologues.
The smectic and nematic mesophases commence from the fifth member of the series.
The non-mesogenic behavior of the first five homologues is attributed to their high crystal-
lizing tendency arising from their amount of energy released to stabilize in crystalline solid
state at room temperature. These non-mesogenic molecules are unable to resist thermal
vibration exposed upon them and are disaligned on the plane of the surface without any
2D order, i.e., molecules are oriented randomly in all possible directions with high order of
disorder or entropy (ꢀS = ꢀH/T). Thus, the non-mesogenic members of the series sharply
transform into the isotropic state at their melting point without passing through a meso-
genic state. The exhibition of the nematogenic mesophase with or without smectic phase
is attributed to the disalignment of molecules at an angle less than 90◦ under the influence
of suitable magnitudes of anisotropic intermolecular forces of attraction. Under such situ-
ations, the sample molecules (thermodynamic system) are able to resist thermal vibrations
exposed upon them to float on the surface with their molecular arrangement statistically
parallel in two dimensions controlled by entropy (ꢀS) at the transition temperature. The
observed smectogenic property exhibited by the hexyloxy to dodecyloxy homologues are

Novel LC Methoxy Benzoyl Ethylenes
17
Table 3. Average thermal stability of three homologous series in ◦C
Series
1
S1
S2
Smectic–Nematic or isotropic
Commencement of smectic phase
Nematic–isotropic
133.4 (C6–C12)
C6
159.8 (C6–C16) 166.6 (C5–C14) 119.8 (C8–C16)
C6 C5 C8
106.6 (C10–C16)
C10
—
Commencement of nematic phase
attributed to lamellar packing of molecules in their crystal lattices, causing a layered ar-
rangement of molecules. Such layers of molecules tend to slide out of plane under the
influence of applied heat, resisting thermal vibrations, maintaining a 2D array of molecules
in a floating condition. The smectic–nematic mesophase range is from 8◦C to 23◦C for the
hexyloxy and dodecyloxy homologues respectively. Similarly, the nematic–isotropic phase
range varies from 10◦C to 42◦C. Overall, the mesogenic (smectic plus nematic) phase
range varies from a minimum of 10◦C at the hexadecyloxy homologue to a maximum of
62◦C at the octyloxy homologue. Thus, the novel series is predominantly nematogenic and
partly smectogenic. Variations in the degree of mesomorphism and an odd–even effect from
homologue to homologue are due to sequential addition of methylene units in the n-alkyl
chain. The odd–even effect diminishes for higher homologues because the longer n-alkyl
chain may coil, bend or flex, or couple to lie in the major axis of the core that hinders the
odd–even effect [9]. The mesogenic properties of the series, including the average thermal
stability and the commencement of mesophases (Table 3) of the novel series, are compared
with two structurally similar homologous series as shown in Fig. 2.
The homologous series 1 and S1 [10] are identical in all respects except for the linking
groups of –COO– and –CH CH–COO– respectively. Therefore, the length to breadth
ratio and the ratio of molecular polarity to polarizability, molecular rigidity and flexibility,
the magnitude of anisotropic intermolecular forces of attraction differs appreciably [9].
Homologous series 1 and S2 [5] are identically similar in all respects except for the terminal
units of –OCH3 and –H respectively. Therefore, the observed differences of mesogenic
properties and the degree of mesomorphism are directly related to the structural differences
among series 1, S1, and S2. The average thermal stabilities and the commencement of
smectic and nematic phases are shown in Table 3.
Figure 2. The molecular structure of the series under comparison.

18
H. N. Chauhan and A. V. Doshi
Table 3 indicates that all the three structurally similar homologous series, 1, S1 and S2,
under comparison are partly smectogenic and partly nematogenic. The smectic–nematic
thermal stability of series 1 is 133.4◦C, and that of S2 is 106.6◦C, but S1 does not show
the smectic phase. The nematic–isotropic thermal stability of series 1 is lower than that
of series S1 and higher than that of series S2. The smectic and nematic mesophases com-
mence from sixth and fifth members of series 1 and S1 respectively; while the smectic and
nematic phases commence relatively later from the decyloxy and octyloxy derivative of the
series S2. Homologous series 1 and S1 differ in respect of their central bridges (–COO–
and –CH CH–COO–) linking the first and middle phenyl rings. Therefore, the length to
breadth ratio and the ratio of molecular polarity to polarizability appreciably differs and
the magnitudes of end-to-end anisotropic intermolecular forces of attractions differ as a
consequence of differing molecular rigidity and flexibility [1–3]. Molecules of series 1
enable lamellar packing, while it is absent in the case of molecules of series S1. Both the
central bridges are comparable, although the vinyl carboxylate –CH CH–COO– has a
greater length, which causes more non-coplanarity due to a twist generated as the oxygen
atoms of the vinyl carboxy group affect the adjacent hydrogen atoms of aromatic rings in
case of –COO– central bridge; the similar effect causes considerable strain on the molecule.
Consequently, a twist around C–O bond will occur forcing the phenyl ring out of the plane
of the molecule. Thus, the coplanarity of the molecule is reduced to a lesser extent than
the –CH CH–COO– group, making phenyl ring broader. On account of these differences,
the smectic–nematic thermal stability of series S1 is lower than series 1. The enhanced
length of the vinyl carboxy central group by the –CH CH– unit increases conjugation,
molecular rigidity, and lateral attractions [1,2,3,5]. Hence, the intermolecular attractions
due to –CH CH– unit increases, while in the case of –COO– group the multiple bond is
absent. Consequently, the stereochemistry of the molecule containing –COO– does pre-
serve the linearity of the molecule, but results in a less thermally stable nematogenic phase
due to the relatively poor end-to-end attractions. Moreover, early or late commencement of
mesophase depends upon the extent of non-coplanarity caused by the molecule as discussed
above. On comparing the thermal stabilities of series 1 and S2, they differ in respect of right
terminal end group, which causes difference in end-to-end attractions due to the polarity
difference of –OCH3 and –H terminal units. Thus, the smectic phase commences from
the hexyloxy homologue in series 1, while it commences from the decyloxy homologue
in series S2. Thus, variations in mesogenic properties from series to series for the same
homologue varies with different polarity of a right terminal end group.
Conclusions
1. The present series is predominantly nematogenic and partly smectogenic.
2. The central group bearing a multiple bond induces conjugation and increases molec-
ular rigidity to emerge more thermally stable mesophase or mesophases.
3. The group efficiency order derived on the basis of thermal stability for smectic and
nematic mesophases are as follows.
Smectic: –OCH3 with –COO– > –H with –COO– > –OCH3 with –CH CH–COO–
Nematic: –OCH3 with –CH CH–COO– > –OCH3 with –COO– > –H with –COO–
4. The variation of molecular rigidity and flexibility are the basis of inducing meso-
genic properties in a substance.

Novel LC Methoxy Benzoyl Ethylenes
19
Acknowledgments
The authors give thanks to the principal, Dr. R. R. Shah, and Dr. R. B. Patel, head of
chemistry department of K. K. Shah Jarodwala Maninagar Science College, Ahmedabad,
for their valuable cooperation, and also to the Vaibhav Laboratory for analytical services.
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