Study of novel thermotropic liquid crystals with lateral nitro substituent

Article abstract of DOI:10.1080/15421406.2016.1147325

In this article, we report synthesis and characterization of a novel thermotropic liquid crystals, namely, 4-bromo-[3′-nitro-4′(4″-n-alkoxybenzoyloxy)] phenyl azobenzenes. The first member of a homologous series is nonmesogenic in nature. The n-ethyl to n-decyl homologues exhibit enantiotropic nematic mesophases, while the n-dodecylton -hexadecyl homologues exhibit only smectic mesophases. The synthesized compounds were characterized by a combination of elemental analysis and standard spectroscopic methods. The mesogenic behavior of present series is explained by comparing each homologue of the related mesogenic series. The impact of the lateral nitro group on mesomorphism is also discussed.

Full text of DOI:10.1080/15421406.2016.1147325

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Study of novel thermotropic liquid crystals with
lateral nitro substituent
S. Dixit & K. Intwala
To cite this article: S. Dixit & K. Intwala (2016) Study of novel thermotropic liquid crystals
with lateral nitro substituent, Molecular Crystals and Liquid Crystals, 631:1, 1-8, DOI:
10.1080/15421406.2016.1147325
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Date: 14 July 2016, At: 07:07

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Study of novel thermotropic liquid crystals with lateral
nitro substituent
S. Dixit and K. Intwala
Applied Chemistry Department, Faculty of Technology and Engineering, Kalabhavan, The M.S. University of
Baroda, Vadodara, Gujarat, India
ABSTRACT
KEYWORDS
Liquid crystals; mesogenic;
nematic; smectic;
enantiotropic
In this article, we report synthesis and characterization of a novel
thermotropic liquid crystals, namely, 4-bromo-[3ꢀ-nitro-4ꢀ(4ꢀꢀ-n-
alkoxybenzoyloxy)] phenyl azobenzenes. The first member of a
homologous series is nonmesogenic in nature. The n-ethyl to n-decyl
homologues exhibit enantiotropic nematic mesophases, while the n-
dodecylton -hexadecyl homologues exhibit only smectic mesophases.
The synthesized compounds were characterized by a combination
of elemental analysis and standard spectroscopic methods. The
mesogenic behavior of present series is explained by comparing each
homologue of the related mesogenic series. The impact of the lateral
nitro group on mesomorphism is also discussed.
Introduction
Liquid crystals had become quintessential advanced materials in our daily life today for more
than a century of studies. Its applications were developed in various areas such as advanced
technological devices, prototypical self-organizing molecular materials, smart biological and
pharmacological uses. Liquid crystalline materials are of great interest for material sciences
as well as for life sciences. The design of novel thermotropic liquid crystals as advanced func-
tional materials involves the suitable selection of core fragment, linking group, and terminal
functionality. The mesomorphic behavior of mesogens depends upon the molecular struc-
ture of molecules and molecular forces arising, therefore, a slight change in the molecular
geometry brings about considerable change in its mesomorphic properties [1–10]. Lateral
substitution makes the molecule broad and hence plays an effective role in the mesogenic
properties of the materials which can be used advantageously for applications. Studies on the
effect of lateral substitution have been carried out by several researchers. Recently, we have also
reported work on lateral methyl and aryl substituents and observed that lateral substitution
in the mesogenic core of the molecules causes reduction in the mesophase thermal stabili-
ties [11–12]. In this continuation, we have synthesized a novel homologous series containing
three rings in the main core linked through ester and azo central linkages and substituted by
a lateral nitro group on the central benzene ring, and investigated the influence of the lateral
nitro group on mesomorphism.
CONTACT S. Dixit
snddxt_@yahoo.com
Kalabhavan, The M.S. University of Baroda, Vadodara , Gujarat, India.
Applied Chemistry Department, Faculty of Technology and Engineering,
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

2
S. DIXIT AND K. INTWALA
Materials and methods
4-Hydroxybenzoic acid, n-alkyl halides, thionyl chloride, pyridine, ethanol, potassium
hydroxide, 2-nitrophenol, 4-bromoaniline, and sodium nitrite were chemically pure of lab
grade. Solvents were dried and distilled prior to use. The microanalysis of the compound
was performed on a Coleman carbon, hydrogen analyzer. Fourier transform infrared spec-
troscopy (FTIR) spectra were determined on FTIR-8400S (Shimadzu) instrument. 1H NMR
spectra was performed on Bruker AV III HD 300 MHz spectrometer using tetramethylsilane
(TMS) as internal references substance and CDCl3 as solvent and the chemical shift values
recorded as (ppm units). Liquid crystalline properties were determined on a Leitz Laborlux
12 POL microscope equipped with a heating stage. Thermodynamic quantities, enthalpies
(ꢀH) and entropies (ꢀS = ꢀH\T) of transition, were determined using differential scanning
calorimeter (DSC) via a Mettler star SW 9.30 system and reported in J g−1 at a scanning rate
of 5°C min−1.
Synthesis of compounds
4-n-Alkoxy benzoic acids and 4-n-alkoxy benzoyl chlorides were synthesized by the modified
method of Dave and Vora [13]. 4-Bromo-3ꢀ-nitro-4-hydroxyazobenzene (DA-1) was synthe-
sized by known method [14].
The series, namely, 4-bromo-[3ꢀ-nitro-4ꢀ(4ꢀꢀ-n-alkoxybenzoyloxy)] phenyl azoben-
zenes was synthesized by adding dropwise one mole solution of 4-bromo-3ꢀ-nitro-4-
hydroxyazobenzene (DA-1) in dry pyridine to the respective one mole solution of 4-n-
alkoxybenzoyl chloride at 0°C–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 representative compounds was found to be satisfactory and recorded in Table 2. The route
of synthesis of series I compounds is illustrated in Scheme 1.
Table . Transition temperatures (°C) of series I.
n-Alkyl, CnHn+, group
Transition temperatures (°C)
(n) =
Series I
SmC
N
I












–
–
–
–
–
–
–
–
–
–








–













–
–


Note. Sm – Smectic, N – Nematic, and I –Isotropic.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
3
Table . Elemental analysis for ethyloxy, butyloxy, octyloxy, and decyloxy derivative.
Element % found (compared with % calculated)
H
Molecular formula
C
N
CHONBr
CHONBr
CHONBr
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
. (.)
C
HONBr
. (.)
Spectral data
FTIR (KBr) spectra (cm−)
–
–
–
n-C6H13 homologue: 3000 (C H str., aromatic), 2900 (C H str., aliphatic), 1700 ( C=O str.,
–
–
ester), 1603 ( N=N str., azo), 1340, 1270, 1165 (aryl ether), 1050, 850, and 750. n-C10H21
–
–
–
homologue: 3100 (C H str., aromatic), 2925–2850 (C H str., aliphatic), 1734 ( C=O str.,
–
–
ester), 1603 ( N=N str., azo), 1380, 1285, 1155 (aryl ether), 1050, 880, and 765.
H NMR (CDCl,  MHz, δ, ppm, standard TMS)
n-C5H11 homologue: δ = 0.9 (t, 3H, CH3), 1.29–1.71 (m, 6H, -(CH2)3-), 3.94 (t, 2H, -
Ar(OCH2)-), and 6.91–8.93 (m, 11H, aromatic H). n-C16H33 homologue: δ = 0.96 (t, 3H,
RBr, KOH
EtOH
SOCl2
HO
COOH
RO
COOH
RO
COCl
(A)
NaNO2, HCl
0-5oC
Br
NH2
Br
N2Cl
OH
NO2
NaOH
0-5oC
Br
N
N
OH
DA1 (B)
NO2
DA1
(B)
RO
COCl
+
(A)
Dry Pyridine
5-10oC, HCl
RO
C
O
O
N
N
Br
O2N
R = CnH2n+1 : n = 1,2,4 to 8, 10, 12, 14 & 16
Scheme . The route of synthesis of series I compounds.

4
S. DIXIT AND K. INTWALA
1.0
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
Wavelength
Figure . UV-visible spectra of C homologue in chloroform.
CH3), 1.29–1.71 (m, 28H, -(CH2)14-), 3.94 (t, 2H, -Ar(OCH2)-), and 6.92–8.93 (m, 11H, aro-
matic H).
UV study of azo compounds
The presence of an azobenzene group leads to the possibilities of photoisomerization and pho-
tochromic behavior. Therefore, permeability studies of photophysical properties were carried
out in chloroform solution of concentration C = 2.5 ×10−3 g dm−3 for n-decyl homologue
and shown in Fig. 1. The compounds displayed primarily two absorptions with maximum
absorbance at about 250 and 342 nm.
The azo compound in the trans form shows a strong band in ultraviolet region 342 nm
which is attributed to the trans-to-cis (π-π∗) transition. The trans form is much more stable
than the cis form but each isomer can be converted into the other by light irradiation of the
appropriate wavelength.
Calorimetry
Calorimetry is valuable method for the detection of phase transitions. Conclusion may
bedrawn concerning the nature of the phases that participate in the transition with help
ofcalorimetric studies. In the present study, enthalpies of decyl homologue is determined by
DSC and data are recorded in Table 3.
Table . DSC data.
Series
I
Member
Decyl
Transition
Peak temperature (microscopic reading)
ꢀ(H/J g−
)
ꢀ(S/J g− K−
)
Cr-N
N-Iso
 ()
 ()
.
.
.
.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
5
Figure . The phase behavior of series.
Results and discussion
In the present study, 12 homologues of the series 4-bromo-[3ꢀnitro-4ꢀ(4ꢀꢀ-n-alkoxy benzoy-
loxy)] phenyl azobenzenes were synthesized and their mesomorphic properties were studied.
All members except the first member of the series exhibit mesomorphism. The n-ethyloxy
to n-decyloxy homologues exhibit enantiotropic nematic mesophases. The n-dodecyloxy to
n-hexadecyloxy members exhibit only smectic mesophases. The transition temperatures are
plotted versus number of carbon atoms in alkoxy chain and shown in Table 1. The phase
diagram showing phase behavior is depicted in Fig. 2. Both nematic-isotropic transition and
smectic-isotropic transition curves show falling tendency as series is ascended. The nematic
phase length vary from 20°C to 66°C and smectic phase length vary from 27°C to 31°C.
There is close relationship between mesomorphism and molecular constitution of organic
compounds. Hence, thermal stability, a measure of mesomorphism, can be correlated with
the molecular constitution of the compounds. Figure 3 shows the transition temperatures and
molecular structures of the n-decyloxy derivative of the present series I and the structurally
related compounds A [9] and B [15] reported in literature.
The comparison of n-decyloxy derivative (compound 1) of the present series with the struc-
turally related compound A shows that compound 1 exhibits only nematic mesophase with
phase length of 20°C, whereas compound A exhibits both smectic and nematic mesophases
with phase lengths of 21°C and 32°C, respectively. The absence of smectic phase and lowering
of nematic phase length by 12°C in compound 1 can be explained on the basis of difference in
the molecular geometries possessed by these compounds. The compound 1 have three phenyl
rings joined through ester and azo central linkages with an alkoxy and bromine as terminal
groups and a lateral nitro group at the second phenyl ring. The compound A differs, firstly
in the type of one terminal group which is fluorine in place of bromine, and secondly, it do
not possess any lateral group. The presence of lateral nitro group in compound 1, forces apart
the molecular axes of the molecules, causes breadth increasing effect and changes molecular
conformation which results in depression of transition temperatures and thermal stabilities
and also to the absence of smectic mesophase as compared to compound A.
Comparing compound 1 with compound B, it is clear that compound 1 has nematic phase
length of 20°C whereas compound B has nematic phase length of 36°C. The lowering of
nematic phase length by 16°C in compound 1 as compared to compound B can be explained
on the basis of their structures. The molecular structure of compound 1 differs from the

6
S. DIXIT AND K. INTWALA
Figure . Comparative molecular geometries of compounds , A, and B.
molecule of compound B in terminal as well as in the central linkage. The molecule of com-
–
–
–
pound 1 has -Br group at one terminal and N=N central linkage, whereas molecules of
–
–
compound B do not have any terminal group and CH=CH CO central linkage. Lateral
–
NO2 group is common feature in both the compounds. The compound B have longer length
–
–
–
–
–
due to presence of longer CH=CH CO linkage as compared to short N=N linkage in
compound 1. Increased molecular length of compound B interact with one another and sta-
bilizes the molecular packing required for nematic phase. Gray [1] has also explained that
increased length and polarizability enhances both mesogenic properties and thermal stabili-
ties. Hence, nematic thermal stability of compound B should be higher than that of compound
1, which is the case.
In the present series, SmC (Schlieren texture) and nematic (marble structure) are inferred
from the textures observed under a polarizing microscope and are shown in Figs. 4 and 5,
respectively.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
7
Figure . Optical micrograph Schlieren texture of smectic C mesophase of C homologue on cooling at
°C.
Figure . Optical micrograph of threaded texture of nematic mesophase of C homologueobtained on cool-
ing at °C.
5. Conclusion
In this article, we have presented the synthesis and characterization of a homologous series
with central ester and azo linkages and a lateral nitro substituent. Compounds of the
present series exhibited nematic mesophases along with the smectic C mesophases. The
study revealed that introduction of lateral nitro group increases the breadth of molecule and
decreases thermal stabilities of nematic mesophase.
Acknowledgment
The authors thank the Head of the Applied Chemistry Department, Faculty of Technology and Engi-
neering, The M.S. University of Baroda, Vadodara for providing research facilities.

8
S. DIXIT AND K. INTWALA
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