Synthesis and evaluation of a novel liquid crystalline homologous series: α-4-[4′-n-alkoxy cinnamoyloxy] benzoyl-β-3′,4′- dimethoxy phenyl ethylenes

Article abstract of DOI:10.1080/15421406.2013.785812

The synthesis and evaluation of a novel series of 12 homologues is presented. Enantiotropic nematogenic mesomorphism is exhibited from the pentyl to hexadecyl homologues. The other homologues are not liquid crystalline. Smectic mesomorphism is absent in all homologues. Transition temperatures were determined using an optical polarizing microscope equipped with a heating stage. The transition curves of a phase diagram behave in a normal manner. An odd-even effect is observed for the N-I transition curve. The textures of the nematic phase are of the threaded or Schlieren type. Mesomorphic-isotropic transition temperatures range from 152°C to 178°C. The nematogenic phase length varies between 24°C and 51°C. The nematic thermal stability is 163.1°C. Thus, the novel homologous series is entirely nematogenic without the exhibition of smectogenic character, and is of a middle ordered melting type. Analytical and spectral data support the molecular structures. Liquid crystal (LC) properties of the novel series are compared with structurally similar homologous series.

Full text of DOI:10.1080/15421406.2013.785812

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Synthesis and Evaluation of a Novel
Liquid Crystalline Homologous Series:
α-4-[4′-n-Alkoxy Cinnamoyloxy]
Benzoyl-β-3″,4″-Dimethoxy Phenyl
Ethylenes
D. M. Suthar a , A. A. Doshi b & A. V. Doshi c
a Chemistry Department , K. K. Shah Jarodwala Maninagar Science
College , Ahmedabad , 380008 , GujaratIndia/J.J.T. University,
Rajasthan, India
b Department of Biochemistry , The Maharaja Sayajirao University of
Baroda , Vadodara , Gujarat
c Chemistry Department , Shri Jagdish Prasad Jhabarmal Tibrewala
University , Jhunjhunu , Rajasthan , India
Published online: 09 Jul 2013.
To cite this article: D. M. Suthar , A. A. Doshi & A. V. Doshi (2013) Synthesis and Evaluation of
a Novel Liquid Crystalline Homologous Series: α-4-[4′-n-Alkoxy Cinnamoyloxy] Benzoyl-β-3″,4″-
Dimethoxy Phenyl Ethylenes, Molecular Crystals and Liquid Crystals, 577:1, 51-58, DOI:
10.1080/15421406.2013.785812
To link to this article: http://dx.doi.org/10.1080/15421406.2013.785812
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Mol. Cryst. Liq. Cryst., Vol. 577: pp. 51–58, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.785812
Synthesis and Evaluation of a Novel Liquid
Crystalline Homologous Series:
α-4-[4ꢀ-n-Alkoxy Cinnamoyloxy]
Benzoyl-β-3ꢀꢀ,4ꢀꢀ-Dimethoxy Phenyl Ethylenes
D. M. SUTHAR,1,∗ A. A. DOSHI,2 AND A. V. DOSHI3
1Chemistry Department, K. K. Shah Jarodwala Maninagar Science College,
Ahmedabad (380008), Gujarat, India / J.J.T. University, Rajasthan, India
2Department of Biochemistry, The Maharaja Sayajirao University of Baroda,
Vadodara, Gujarat
3Chemistry Department, Shri Jagdish Prasad Jhabarmal Tibrewala University,
Jhunjhunu, Rajasthan, India
The synthesis and evaluation of a novel series of 12 homologues is presented. Enan-
tiotropic nematogenic mesomorphism is exhibited from the pentyl to hexadecyl homo-
logues. The other homologues are not liquid crystalline. Smectic mesomorphism is
absent in all homologues. Transition temperatures were determined using an optical
polarizing microscope equipped with a heating stage. The transition curves of a phase
diagram behave in a normal manner. An odd–even effect is observed for the N-I tran-
sition curve. The textures of the nematic phase are of the threaded or Schlieren type.
Mesomorphic–isotropic transition temperatures range from 152◦C to 178◦C. The ne-
matogenic phase length varies between 24◦C and 51◦C. The nematic thermal stability
is 163.1◦C. Thus, the novel homologous series is entirely nematogenic without the ex-
hibition of smectogenic character, and is of a middle ordered melting type. Analytical
and spectral data support the molecular structures. Liquid crystal (LC) properties of
the novel series are compared with structurally similar homologous series.
Keywords Liquid crystal; mesomorphism; nematic; smectic; thermotropic
Introduction
The exhibition of liquid crystal properties by any substance is related directly to the
molecular structure. Molecular structure [1] includes aspects of geometrical shape, size,
number of phenyl rings, molecular aromaticity, polarities of the central, lateral and terminal
end groups, and the molecular width. Molecular polarity, polarizability, steric hindrance,
and electronic interactions are also significant factors. Overall favorable molecular rigidity
and flexibility [2–4] enable suitable magnitudes of anisotropic intermolecular forces of
attractions; which maintain ordered molecular arrangement [1] facilitating mesomorphism.
The molecular geometry in this novel series consists of three phenyl rings bridged through
∗Address correspondence to D. M. Suthar, Chemistry Department, K. K. Shah Jarodwala Mani-
nagar Science College, Ahmedabad 380008, Gujarat, India. E-mail: dipakmsuthar@gmail.com
51

52
D. M. Suthar et al.
–CH = CH–COO– and –CO–CH = CH– central groups, with –OR and –OCH3 terminal
end groups, and a –OCH3 lateral group positioned at the ortho position to the right handed
terminal end group on the third phenyl ring. Thus, the effect of molecular structure on
liquid crystal properties in terms of broadening the molecule with a highly polar –OCH3
group is discussed.
Experimental
Synthesis
4-Hydroxycinnamic acid was alkylated with suitable alkylating agents (R-X) and cis-4-n-
alkoxycinnamic acids formed were converted to the corresponding 4-n-alkoxycinnamoyl
chlorides (A) using freshly distilled thionyl chloride by the modified method of Dave and
Vora [5]. α-4-Hydroxy benzoyl β-3ꢀ,4ꢀ-dimethoxy phenyl ethylene (B) was prepared by the
usual established method [6]. Components (A) and (B) were condensed in dry cold pyridine.
Final products were individually decomposed, filtered, washed, dried, and purified until
they gave constant transition temperatures. The p-hydroxybenzaldehyde, malonic acid,
pyridine, alkyl halides, MeOH, KOH, ethanol, thionyl chloride, p-hydroxy acetophenon,
and 3,4-dimethoxy benzaldehyde required for synthesis were used as received except for
solvents, which were dried and purified prior to use. The synthetic route to the series is
shown below in Scheme 1.
Characterization
Some selected homologues of the novel series were characterized by elemental analysis,
Infra red, and 1HNMR spectroscopy techniques. Micro analysis of the compounds was per-
formed on a Perkin–Elmer PE 2400 CHN analyzer with results shown in Table 1. Nuclear
magnetic resonance (NMR) spectra were recorded using CDCl3 as solvent and IR spectra
Table 1. Transition temperatures in ◦C of series-1
Compound
no.
n-Alkyl group
R = CnH2n+1 (n)
Sm
Sm
N
N
Isotropic
Isotropic
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
10
12
14
16
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
200.0
206.0
190.0
201.0
170.0
160.0
174.0
169.0
178.0
166.0
152.0
156.0
142.0
124.0
123.0
110.0
132.0
120.0
125.0
132.0
10
11
12
Note: Sm = Smectic; N = Nematic.

A Novel Liquid Crystalline Homologous Series
53
Scheme 1. Synthetic route to the series.
were recorded using a Perkin–Elmer GX spectrometer. The liquid crystal properties, transi-
tion temperatures, and melting temperatures were investigated by using optical polarizing
microscopy.
The optical texture of the nematic mesophase were determined by the miscibility
method. Pentyloxy homologue shows athreded type nematic texture and tetradecyloxy
homologue ahows a schlieren type texture.
Analytical Data
NMR in ppm for Hexyloxy Derivative. 0.88 (–CH3 of –OC6H13 group), 1.28 (–CH2–)n
Polymethylene group of –OC6H13, 4.08 (–OCH2–CH2– of –OC6H13), 3.88 (–OCH3), 4.74
broad (–CH = CH– group), 6.90 & 6.92 (–CO–CH = CH–group), 7.90, 7.92, & 8.08
(substituted phenyl ring). The NMR data supports the molecular structure.
NMR in ppm for Decyloxy Derivative. 0.87 (–CH3 of –OC10H21group), 1.25 (–CH2–)n
Polymethylene group of –OC10H21, 4.00 (–OCH2–CH2– of –OC10H21), 3.86 (–OCH3),

54
D. M. Suthar et al.
Table 2. Elemental analysis for propoxy, butoxy, octyloxy, and dodecyloxy derivatives
Elements % found (% calculated)
Sr. no.
Molecular formula
C
H
73.78 (73.73)
74.19 (74.07)
75.38 (75.28)
76.40 (76.25)
5.83 (5.93)
6.35 (6.17)
6.92 (7.01)
7.78 (7.69)
2
3
4
C30H30O6
C34H38O6
C38H46O6
4.696 broad (–CH = CH– group), 7.04 & 7.22 (–CO–CH = CH–group), 6.90, 7.88, & 7.90
(substituted phenyl ring). The NMR data supports the molecular structure.
IR in cm−1 for Ethyloxy Derivative. 690 (polymethylene – (CH2)n– group of –OC2H5),
1170 (>C = O group), 1260, 1605 (–COO ester group), 990 (–CH = CH– group), 770
(1,2,4-trisubstituted phenyl ring). The IR data supports the molecular structure.
IR in cm−1 for Dodecyloxy Derivative. 720 (polymethylene –(CH2)n– group of –OC12H25),
765 (p-substituted phenyl ring), 1165 (>C = O group),1260, 1600 (–COO ester group),
960 (–CH = CH– group), 845 (1,2,4-trisubstituted phenyl ring). The IR data supports the
molecular structure (Table 2).
Results and Discussion
Cis-4-n-Alkoxycinnamic acids and α-4-hydroxy benzoyl β-3ꢀ,4ꢀ-dimethoxy phenyl ethy-
lene (B) are not liquid crystals. However, liquid crystalline properties are induced on linking
ethylene derivative (B) to the corresponding acid chlorides of 4-n-alkoxycinnamic acids.
The methyl to butyl homologues are not liquid crystals, while the pentyl to hexadecyl
homologues of the series are enantiotropic liquid crystals with nematogenic character only,
without the exhibition of smectogenic properties even in the monotropic condition. Thus,
8 out of 12 homologues of the series are liquid crystalline. A plot of a phase diagram
drawn for transition temperatures versus the number of carbon atoms present in the left n-
alkoxy terminal end group consists of two transition curves. The solid-isotropic or nematic
transition curve adopts a zigzag path of rising and falling values as the series is ascended
with an overall descending tendency and behaves in normal manner. The nematic–isotropic
transition curve initially rises up to the decyl homologue and then descends as the series
is ascended with exhibition of an odd–even effect and a negligible rise of few degrees at
the hexadecyl homologue. Thus, the N-I transition curve behaves in the usual established
manner. The N-I transition curves for the odd and even members of the series merge into
each other at the nonyl homologue of the series. The odd–even effect diminishes as series
is ascended for higher homologues because the longer n-alkyl chains may coil, bend, flex,
or couple to lie with the major axis of the core. The texture of the nematic mesophase is
threaded or Schlieren. The maximum nematogenic mesophase length is 59◦C at the octyl
homologue and the minimum mesophase length is 24◦C for the hexadecyl homologue. The
thermal stability for nematic mesophase is 163.1◦C. Thus, the presently investigated novel
homologous series is entirely nematogenic without exhibition of smectogenic character
and is of a middle ordered melting type. The mesomorphic (LC) properties vary from
homologue to homologue in the same series.

A Novel Liquid Crystalline Homologous Series
55
240
220
200
180
160
140
120
100
80
SOLID-ISOTROPIC OR MESOMORPHIC
NEMATIC - ISOTROPIC
ISOTROPIC
NEMATIC
SOLID
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
No. of carbon atoms in n-alkyl chain- R
Figure 1. Phase behavior of series.
The occurrence of a liquid crystalline state from and beyond the pentyl homologue is
attributed to the occurrence of suitable magnitudes of anisotropic intermolecular forces of
end-to-end attractions as a consequence of favorable molecular rigidity and flexibility [1–4].
The molecules forming the nematogenic mesophase resist thermal vibrations imposed upon
them and set themselves at an angle of less than 90◦ on the plane of a surface at a particular
temperature t1 and continue up to temperature t2; between t1 and t2 the molecules float
with a statistically parallel orientation ordered. The temperature difference, t2 – t1, is
called phase length or range of liquid crystallinity (nematic). The temperature above t2
converts the sample into the isotropic state, at which point the molecules are randomly

56
D. M. Suthar et al.
oriented in all possible directions in disordered manner. Thus, suitable magnitudes of
anisotropic intermolecular forces of adhesion [1] as a consequence of favorable rigidity
and flexibility [2–4] maintain ordered molecular arrangement in a floating condition. The
nonliquid crystalline behavior of the methyl to butyl homologues is attributed to their high
crystallizing tendency, which arises from their unsuitable magnitudes of intermolecular
forces of attraction. Therefore, the molecules concerned transform sharply into the isotropic
state from the solid state without passing through a liquid crystalline state. The absence of
smectogenic character indicates absence of lamellar packing of molecules in their crystal
lattices. The shape of either the nematic–isotropic transition curve or solid-nematic or
isotropic curve adopts the path of their propagation in a phase diagram according to the
resultant molecular rigidity and/or flexibility. The odd–even effect and the variations in
mesomorphic properties from homologue to homologue in the same series is due to the
sequentially added methylene unit of the left n-alkoxy terminal end group [1–4]. The liquid
crystal properties of the presently investigated novel homologous series 1 are compared
with the other structurally similar homologous series X [7] and Y [8] as shown in Fig. 2.
Homologous series X and Y chosen for comparison and the novel homologous series 1
as shown in Fig. 2 are similar in respect of three phenyl rings bridged through two identical
central groups –CH = CH–COO– and –CO–CH = CH– as well as commonly substituted
left n-alkoxy (–OR) terminal end group. However they differ in respect of the groups in
the terminal and/or lateral positions in the right phenyl ring, as can clearly be seen in
Fig. 2. Therefore, the observed variations in their mesomorphic properties and the degree
of mesomorphism can be attributed to the differing features of series 1, X and Y [1–4].
The length to breadth ratio, intermolecular end-to-end and lateral attractions, molecu-
lar polarizability due to molecular width by laterally substituted –OCH3 and –NO2 groups
contributing to the magnitudes of anisotropic intermolecular forces of adhesion consider-
ably differ, based on molecular rigidity and flexibility [1–4]. Similarly, series 1 and Y differ
in respect of only one of the –OCH3 groups substituted at the lateral position (3ꢀꢀ), i.e., two
–OCH3 groups present on 3ꢀꢀ–4ꢀꢀ position of series 1 and only one –OCH3 group present on
4ꢀꢀ position of series Y; or 3ꢀꢀ –OCH3 group is missing in series Y, which results in a linear
shape of series Y and a nonlinear shape of series 1. Thus, the magnitudes of anisotropic
intermolecular forces of adhesion differ by molecular width or intermolecular closeness,
molecular polarity and polarizability as a consequence of the resultant rigidity and flexibil-
ity. Table 3 shows the thermal stability of mesophases, commencement of mesophase, type
of mesophase exhibition, as shown below.
Figure 2. Structurally similar series.

A Novel Liquid Crystalline Homologous Series
57
Table 3. Average thermal stability in ◦C
Series
1
X
Y
Smectic–nematic or
smectic–isotropic
commencement of
smectic phase
—
141.0
(C5–C10)
C5
159.7
(C5–C8)
C5
Nematic–isotropic
commencement of
nematic phase
163.1
(C5–C16)
C5
156.0
(C5–C16)
C5
174.1
(C5–C14)
C5
Table 3 shows that homologous series X and Y, selected for comparison, are smecto-
genic in addition to being nematogenic, but the novel series 1 does not exhibit smectogenic
character at all. The intermolecular closeness due to the varying molecular width plays an
important role to the net magnitudes of intermolecular anisotropic forces of attractions.
The laterally substituted –OCH3 and –NO2 groups increase molecular width as compared
to series Y, chosen for comparison. When molecular width increases, the intermolecular
forces of attraction depend upon two opposing forces operating at a time. (1) As molecu-
lar width increases, the intermolecular distance increases or the intermolecular closeness
reduces, consequently intermolecular attractions diminish. (2) Increased molecular width
raises molecular polarizability, which results in an increase of intermolecular attractions
[1]. The net resultant effect of intermolecular attractions depends upon the predominating
effect out of two opposing effects (1) and (2) operating at the time. Thus, the (1) effect
predominates for the nonlinear series 1 while; the (2) effect predominates for the non-
linear series X. The intermolecular distance is closest for a linear shaped series Y. Thus,
suitable magnitudes of anisotropic intermolecular forces of attractions are favorable and
sufficient to facilitate lamellar packing of molecules of series X and Y in their crystal
lattices but it is insufficient, unfavorable, and unsuitable to facilitate lamellar packing of
molecules in the crystal lattices of series 1. Hence, the statistically parallel orientation
order of molecules is maintained by only end-to-end attractions, but the sliding layered
arrangement of molecule is missing in the floating condition for the presently investigated
series 1. Homologous series Y being linear induces the highest intermolecular closeness
to exhibit smectic and nematic mesophase formation. The highest value of smectic and
nematic thermal stability of series Y among the series under comparison is obvious from
the point of view of intermolecular closeness and end group polarity. The nematic–isotropic
thermal stability of series 1 is higher than the series X, because series 1 contains two highly
polar groups –OCH3, while series X contains only one polar group –NO2. Thus only ne-
matic thermal stability has occurred at the cost of the missing smectogenic mesophase.
The ematic mesophase commences from the fifth homologue of all the series 1, X and Y,
but the smectic phase does not occur or commence until the last hexadecyloxy homologue
of series. However, smectic mesophase formation commences from the fifth member of
the series X and Y. This difference of behavior is attributed to the different magnitudes of
noncoplanarity caused [9] by the molecules of series 1, X, and Y, under comparison. Thus,
the variations in mesomorphic properties for the same homologue from series to series is
attributed to the unchanging lateral and/or terminal end group throughout the same series.
The group efficiency order derived on the basis of thermal stability for smectic and nematic
is mentioned in the conclusions below.

58
D. M. Suthar et al.
Conclusions
1. Group efficiency order for the smectic phase:
2. Group efficiency order for the nematic phase:
3. Molecular width plays an important role in inducing smectic and/or nematic
mesophase.
4. Molecular width variation by lateral substitution regulates the desired type
mesophase formation.
5. Molecular rigidity and/or flexibility is an important criteria to predict mesophase
formation based on molecular shape, size, and the polarity of a substituted functional
group.
Acknowledgments
Authors thank the president and all members of management of Bai Jivkor Lalubhai Trust
as well as the principal Dr. R. R. Shah and the head chemistry department Dr. R. B.
Patel of K. K. Shah Jarodwala Maninagar Science College, Ahmedabad, for their valuable
cooperation into this work. Also, thanks are due to the Vaibhav Laboratory and head
chemistry department of S. P. T. Science College, Godhra, for their valuable cooperation to
facilitate microscopic observations and analytical services.
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