Synthesis and Characterization of Novel Thermotropic Liquid Crystals with a Lateral Chloro Group

Article abstract of DOI:10.1080/15421406.2015.1010849

Twelve novel homologues having azolinkage of the series 4-Carbethoxy-[3'chloro-4^' (4^'-n-alkoxy benzoyloxy)] azobenzenes has been synthesized. The methyl to n-hexyl homologues exhibit only a nematic mesophase, while the n-heptyl to n-dodecyl homologues exhibit both smectic and nematic mesophases. The n-tetradecyl and n-hexadecyl homologues exhibit only smectic mesophases. The plot of transition temperatures versus number of carbon atoms in alkoxy chain exhibits an odd-even effect for the nematic-isotropic transition temperatures. The mesogenic behavior of present series is explained by comparing its homologue with other homologue of related mesogenic series. The synthesized compounds were characterized by combination of elemental analysis and standard spectroscopic methods. For the exhibition of mesomorphic property the role of ester and azo linkages has been discussed. The impact of lateral chloro group on mesomorphism is also discussed.

Full text of DOI:10.1080/15421406.2015.1010849

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and Characterization of Novel
Thermotropic Liquid Crystals with a Lateral Chloro
Group
Sandhya Dixit & R. A. Vora
To cite this article: Sandhya Dixit & R. A. Vora (2015) Synthesis and Characterization of Novel
Thermotropic Liquid Crystals with a Lateral Chloro Group, Molecular Crystals and Liquid
Crystals, 623:1, 56-63, DOI: 10.1080/15421406.2015.1010849
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Date: 13 January 2016, At: 09:34

Mol. Cryst. Liq. Cryst., Vol. 623: pp. 56–63, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2015.1010849
Synthesis and Characterization of Novel
Thermotropic Liquid Crystals with a Lateral
Chloro Group
SANDHYA DIXIT∗ AND R. A. VORA
Faculty of Technology and Engineering, Applied Chemistry Department, The
M.S. University of Baroda, Kalabhavan, Vadodara, Gujarat, India
Twelve novel homologues having azolinkage of the series 4-Carbethoxy-[3ꢀchloro-4ꢀ
(4ꢀꢀ-n-alkoxy benzoyloxy)] azobenzenes has been synthesized. The methyl to n-hexyl
homologues exhibit only a nematic mesophase, while the n-heptyl to n-dodecyl homo-
logues exhibit both smectic and nematic mesophases. The n-tetradecyl and n-hexadecyl
homologues exhibit only smectic mesophases. The plot of transition temperatures versus
number of carbon atoms in alkoxy chain exhibits an odd-even effect for the nematic-
isotropic transition temperatures. The mesogenic behavior of present series is explained
by comparing its homologue with other homologue of related mesogenic series. The
synthesized compounds were characterized by combination of elemental analysis and
standard spectroscopic methods. For the exhibition of mesomorphic property the role
of ester and azo linkages has been discussed. The impact of lateral chloro group on
mesomorphism is also discussed.
Keywords Liquid crystals; mesomorphic; monotropic; nematic; smectic
1. Introduction
Liquid crystalline materials are found to have an important role in new and emerging
electro-optical technologies. Thermotropic liquid crystals consists of either linear or disc-
like molecules like organic molecules. The general architecture for linear thermotropic
liquid crystals of linearly linked ring systems and flexible chains, the permanent dipoles
and polarisable moieties also being requirements. The rings may be linked to each other
either directly or through some linking group (e.g. ester,olefinic, acetylenic, azo, azoxy,
dimethylene or oxymethylene) that maintains the linearity of the core [1]. The flexible alkyl
chains may be linked either directly with the ring or through the intervening hetero atoms
such as oxygens, sulphur and nitrogens. These polar groups may also impart dielectric
anisotropy to the molecules [2]. The thermal stability of the mesophase is determined by
the nature of the molecule, steric, and electronic factors. It has also been reported in the
literature [3–5] that the linking units capable of maintaining the rigidity and linearity of the
molecule also promote mesophase stability. The aromatic esters are known for their thermal
stability, easy synthesis and relative resistance to hydrolysis. Azobenzene derivatives are
very effective for controlling liquid crystals by light, because the geometrical change due
∗Address correspondence to Sandhya Dixit, Faculty of Technology and Engineering, Applied
Chemistry Department, The M.S. University of Baroda, Kalabhavan, Vadodara 390001, Gujarat,
India. E-mail: sandhyadixit-appchem@msubaroda.ac.in
56

Synthesis and Characterization of Novel Thermotropic Liquid Crystals
57
to photo-isomerisation gives a concomitant change in chemical and physical properties
not only in the azobenzene residue but also in the environment around it. Such photonic
control has mainly been applied in the nematic phase by means of a transmission, reflection,
and light scattering mode. The photo-responsive properties of azobenzene derivatives due
to photo-isomerisation have also been reported [6–12]. It is reported that the addition of
chromophores such as azo groups are found the lower energy required in optical field [13].
These features have made azo compounds, promising candidates for optical applications.
Lateral substitution at the center of the mesogen increases the molar volume and
hence plays an effective role in the mesogenic properties of the materials. Studies on the
effect of lateral substitution have been carried out by several researchers [14–17]. Recently
we have also reported a homologous series with lateral methyl substituent [18]. In this
continuation, in the present paper, we report the design, synthesis, and characterization
of newly designed homologous series with lateral polar chloro substituent to evaluate the
effect of polar substituent on mesomorphism.
2. Experimental
2.1. Synthesis
4-Hydroxy benzoic acid, n-alkyl halides, thionyl chloride, pyridine, methanol, potassium
hydroxide, ortho chloro phenol, ethyl 4-amino benzoate, and sodium nitrite were chemically
pure of lab grade. Solvents were dried and distilled prior to use.
1. 4-n-Alkoxy benzoic acids and 4-n-alkoxy benzoyl chlorides were synthesized by
the modified method of Dave and Vora [19].
2. 4-Carbethoxy-3ꢀ-chloro-4-hydroxy azobenzene (DA-1) was synthesized by known
method [20].
3. The series namely 4-Carbethoxy-[3ꢀchloro-4ꢀ(4ꢀꢀ-n-alkoxy benzoyloxy)] azoben-
zene was synthesized by adding dropwise the solution of 4-Carbethoxy-3ꢀ-chloro-
4-hydroxy azobenzene (DA-1) in dry pyridine to the respective solution of 4-n-
alkoxy benzoyl chloride at 0–5◦C. The mixture was allowed to stand overnight at
room temperature. It was acidified with cold 1:1 aqueous hydrochloric acid and
the separated solid was washed successively with dilute sodium hydroxide solution
and water. The final product obtained was recrystallized several times from glacial
acetic acid until constant transition temperatures were obtained which are recorded
in Table 1. The elemental analysis of each compound was found to be satisfactory
(Table 2). The route of synthesis of series 1 is illustrated in Scheme 1.
2.2. Characterization
Micro analysis of the compounds was performed on a Coleman carbon, hydrogen analyzer.
FTIR spectra were determined on Nicolet impact 400 FTIR spectrometer using Nujol. 1H
NMR spectra was performed on Perkin-Elmer (90 MHz) spectrometer using tetramethylsi-
lane (TMS) as internal reference and CDCl3 as solvent. Liquid crystalline properties were
determined on a Leitz Laborlux 12 POL microscope equipped with a heating stage. Ther-
modynamic quantities enthalpies (ꢀH) and entropies (ꢀS = ꢀH/T) of transitions were
determined using differential scanning calorimeter (DSC) via a Mettler TA-4000 system

58
S. Dixit and R. A. Vora
Table 1. Transition temperatures (◦C) of Series I
Transition temperatures (◦C)
n-Alkyl CnH2n+1 Group (n)
Series I
Sm C
N
I
1
2
3
4
5
6
7
8
10
12
14
16
—
—
—
—
—
150.0
155.0
145.0
135.0
96.0
107.0
80.0
109.0
118.0
132.0
—
215.0
227.0
195.0
200.0
180.0
177.0
165.0
160.0
154.0
147.0
143.0
137.0
—
(73.0)∗
70.0
80.0
95.0
90.0
94.0
—
()∗monotropy, Sm—Smectic, N-Nematic, I-Isotropic.
and reported in Jg−1 at a scanning rate of 5◦C min−1. The calorimeter was calibrated using
pure indium as standard.
2.3. Analytical Data
FTIR (Nujol) Spectra cm−1. n-Butoxy Derivative : 2900 (C H Str. Aromatic), 2900,
2850 (C H Str.Aliphatic), 1705 (C O Str. Ester), 1600 (N N Str.Azo), 1370, 1270, 1160
(aryl ether), 1060, 960, 890, 760.
n-Hexadecyloxy derivative : 2950 (C H Str. Aromatic), 2900, 2850 (C H Str.
Aliphatic), 1700 (C O Str.Ester), 1600 (N N Str. Azo), 1370, 1250, 1150 (aryl ether),
1050, 860, 750.
Proton NMR spectra in δ PPM (Solvent CDCl3 90 MHz, standard TMS). n-Butoxy
Derivative: δ 1.0 (t, 3H, CH3), 1.4 (t, 3H, CH3), 1.8 (m, 4H, Ar-O-(CH2)2-), 4.0 (t, 2H of
Ar-OCH2), 4.3 (q, 2H of Ar-COOCH2), 6.9 – 8.1 (m, 11H,Ar-H).
n-Hexyloxy Derivative : δ 1.0 (t, 3H, CH3), 1.3 (t, 3H, CH3), 1.7 (m, 8H, Ar-O-(CH2)4-),
4.1 (t, 2H of Ar-OCH2), 4.3 (q, 2H of Ar-COOCH2), 7.0 – 8.3 (m, 11H,Ar-H).
Table 2. Elemental analysis for ethyloxy, pentyloxy, octyloxy, and decyloxy derivative
Element% found (compared with% calculated)
Molecular formula
C
H
N
C24H21O5N2 Cl
C27 H27O5N2 Cl
C30H33O5N2 Cl
C32H37O5N2 Cl
63.42 (63.64)
65.19 (65.52)
67.49 (67.10)
67.82 (68.02)
4.39 (4.64)
5.01 (5.46)
6.05 (6.15)
6.54 (6.55)
6.43 (6.18)
6.07 (5.66)
5.07 (5.21)
5.25 (4.96)

Synthesis and Characterization of Novel Thermotropic Liquid Crystals
59
The synthetic route to the series is mentioned below in Scheme 1.
Scheme 1. Synthetic route to the series I.
2.4. Calorimetry
Calorimetry is valuable method for the detection of phase transitions. Conclusion may be
drawn concerning the nature of the phases that participate in the transition with help of
calorimetric studies. In the present study, enthalpies of hexadecyl homologue of series I is
determined by DSC and data is recorded in Table 3.
3. Results and Discussion
The homologous series 4-Carbethoxy [3ꢀchloro-4ꢀ-(4ꢀꢀ-n-alkoxybenzoyloxy)] azobenzenes
consisting of twelve mesomorphic homologues. The methyl to n-hexyl derivatives exhibit
enantiotropic nematic mesophases. The smectic mesophase commences in the monotropic
form, from the n-heptyl derivative which becomes enantiotropic from the next member i.e.
n-octyl derivative. The n-octyl to n-dodecyl derivatives are enantiotropically smectics and
nematics. The n-tetradecyl and n-hexadecyl derivatives exhibit only enantiotropic smectic
mesophases. The transition temperatures (Table 1) are plotted versus the number of carbon

60
S. Dixit and R. A. Vora
Figure 1. The phase behavior of series 1.
atoms in alkyl chain of n-alkoxy terminal end group. The phase diagram showing phase
behavior is depicted in Figure 1. The smectic–nematic transition curve shows rising ten-
dency. The nematic-isotropic transition curve shows falling tendency as series is ascended
with exhibition of odd–even effect. The nematic phase lengths vary from 15◦ to 84◦C and
the smectic phase lengths vary from 39 ◦ to 53◦C.
There is close relationship between mesomorphism and molecular constitution of or-
ganic compounds. Hence thermal stability, a measure of mesomorphism, can be correlated
with the molecular constitution of the compounds. Figure 2 shows the transition tempera-
tures and molecular structures of the n-dodecyloxy derivative (compound 10) of the present
series I and the structurally related compound A [21], B [22], and C [23] reported in
literature.
Reference to Figure 2 indicates that the compounds 10, A and B consists of three
phenyl rings, two central groups COO− and N N−, one n-alkoxy terminal end group
and laterally substituted chloro Cl group as common identical features. The only one
terminal end groups of compounds 10, A and B viz. COOC2H5, COCH3, and F are
uncommon features. Figure 2 indicates that the compound 10 exhibits smectic and nematic
mesophases whereas compound A exhibits only nematic mesophase. Both the compounds
under comparison possess laterally substituted chloro groups at the middle phenyl rings
Table 3. DSC Data
Peak temperature
(microscopic
reading)
ꢀ (S/Jg−1
Series
I
Member
Transition
ꢀ (H/Jg−1)
K−1)
Hexadecyl
Cr Sm
Sm-Iso
88.3 (94)
132.4 (137)
43.33
0.689
0.119
0.001699

Synthesis and Characterization of Novel Thermotropic Liquid Crystals
61
Figure 2. The Comparative molecular structures of compounds 10, A, B, and C.
which will affect the length to breadth ratio and polarizability to a different extent for com-
pounds 10 and A due to the presence of different end groups COOC2H5 and COCH3.
The molecules of compound 10 are longer due to the presence of longer COOC2H5
terminal group at one end, as compared to shorter COCH3 group in compound A at that
position. The longer terminal COOC2H5 group aids the formation of smectic mesophase
due to enhanced lateral attraction as compared to terminal attraction which facilitates
lamellar packing required for smectic mesophase [26]. Thus compound 10 exhibits smec-
tics mesophase along with nematic phase. The nematic phase length of compound A is
marginally higher (9◦C) than that of compound 10 can be understandable because of the
difference in terminal attraction arising due to different terminal groups. Further both smec-
tic and nematic phase lengths and thermal stabilities of compound 10 are greater than that
of compound B. The molecules of compound 10 and compound B differ only in the right
terminal end groups, other structural features are being the same. The compound 10 posses

62
S. Dixit and R. A. Vora
long and more polarisable COOC2H5 group at one end whereas compound B has short
F group at that end. The presence of long and polarisable COOC2H5 group of com-
pound 10 increases length of the molecules, as a result of its polarizability, increases the
intermolecular cohesive forces responsible for wider mesophase length and higher thermal
stabilities of compound 10 in comparision to compound B with short terminal F group.
The comparison of the compound 10 and compound C shows that the smectic and
nematic thermal stabilities of compound 10 are lower by 17◦C and 72◦C, respectively, than
those of compound C. This can be explained on the basis of their structural variations.
The molecular structure of compound 10 differs from the molecules of compound C in the
terminal as well as in the central part of the benzene ring. The molecules of the compound
10 has COOC2H5 group at the one terminal and Cl lateral group at the central benzene
ring, whereas, molecules of compound C has COOC2H4OCH3 group at the same terminal
position without any lateral substituent. The presence of lateral chloro group increases the
breadth of the molecules of compound 10 and the length is also shorter than the molecules
of compound C because of absence of additional −OCH3 unit. Gray [24] has explained
that increase in breadth of molecules reduces both nematic and smectic thermal stabilities.
Thus, both reduced length and enhanced breadth factors resulted in lower smectic and
nematic thermal stabilities of compound 10 reported in present investigation.
In the present series SmC (schlieren texture) and nematic (marble texture) are inferred
from textures observed under a polarizing microscope.
4. Conclusion
In this paper, we have presented the synthesis and characterization of a homologous series
with central ester and azo linkages and lateral chloro substituent. Compounds of the present
series exhibited nematic mesophases along with the SmC mesophases. The study revealed
that enhanced length of the molecules of the present series due to the presence of long
–COOC2H5 terminal substituent favored formation of smectics mesophase while increased
breadth due to the presence of the lateral chloro group lowered its mesophase thermal
stabilities.
Acknowledgment
The authors thank the Head of the Faculty of Technology and Engineering, Applied Chem-
istry Department, The M.S. University of Baroda, Vadodara for providing research facilities.
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