λ-shaped to T-shaped azo diester mesogens having methyl (–CH3)/methoxy(–OCH3) terminal substituents with trisubstituted benzene

Article abstract of DOI:10.1016/j.molliq.2021.116863

A two new homologous series of λ-shape to T-shape mesogenic azo diesters were synthesized, and their thermotropic properties were studied by differential scanning calorimetry and a hot-stage polarizing optical microscope. The difference between these two series is in the structure of terminal substituents methyl (–CH₃) for series I and methoxy (–OCH₃) for series II at one terminus. Structure variation from λ-shape to T-shape as we go from lower members to higher members is discussed. In the series I, methoxy to n-pentyloxy derivatives are non-mesogenic. n-hexyloxy derivative exhibits only monotropic nematic mesophase. n-heptyloxy to n-dodecyloxy derivatives exhibit monotropic smectic C mesophase. n-tetradecyloxy derivative exhibits enantiotropic SmA mesophase, whereas n-hexadecyloxy derivatives exhibit monotropic Smectic A mesophase. In series II, methoxy to n-hexyloxy derivatives are non-mesogenic. n-heptyloxy and n-octyloxy derivatives exhibit monotropic smectic C mesophase. Smectic A mesophase commences from the n-decyloxy derivative as a monotropic and persists up to the last member synthesized. The mesomorphic properties of the present series were compared with each other and with the structurally related mesogenic homologous series to evaluate the effect of methyl (–CH₃)/methoxy (–OCH₃) substituents as well as variation in the shape of the molecule by varying the alkoxy chain length of the bulky lateral substituent on mesomorphism.

Full text of DOI:10.1016/j.molliq.2021.116863

Journal of Molecular Liquids 336 (2021) 116863
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
k-shaped to T-shaped azo diester mesogens having methyl (–CH3)/
methoxy(–OCH3) terminal substituents with trisubstituted benzene
Yongfang Yao a,b,c, Rohit R. Koshti d, Akshay Vyas d, Chetan B. Sangani d, Yongtao Duan a,
Rakesh Kumar Ameta d, Umesh P. Tarpada e, H.N. Patel d,
⇑
a Henan Provincial Key Laboratory of Pediatric Hematology, Children’s Hospital Affiliated to Zhengzhou University, Henan Children’s Hospital, Zhengzhou Children’s
Hospital, Zhengzhou 450018, China
b School of Pharmaceutical Science, Zhengzhou University, Zhengzhou, Henan 450001, China
c Key Laboratory of Advanced Drug Preparation Technologies Zhengzhou University), Ministry of Education of China, Zhengzhou 450001, China
d Shri Maneklal M. Patel Institute of Sciences & Research, Kadi Sarvavishwavidyalaya, Gandhinagar, Gujarat, India
e Chemistry Department, Government Science College, Gandhinagar, Gujarat 382015, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A two new homologous series of k-shape to T-shape mesogenic azo diesters were synthesized, and their
thermotropic properties were studied by differential scanning calorimetry and a hot-stage polarizing
optical microscope. The difference between these two series is in the structure of terminal substituents
methyl (–CH3) for series I and methoxy (–OCH3) for series II at one terminus. Structure variation from k-
shape to T-shape as we go from lower members to higher members is discussed. In the series I, methoxy
to n-pentyloxy derivatives are non-mesogenic. n-hexyloxy derivative exhibits only monotropic nematic
mesophase. n-heptyloxy to n-dodecyloxy derivatives exhibit monotropic smectic C mesophase. n-
tetradecyloxy derivative exhibits enantiotropic SmA mesophase, whereas n-hexadecyloxy derivatives
exhibit monotropic Smectic A mesophase. In series II, methoxy to n-hexyloxy derivatives are non-
mesogenic. n-heptyloxy and n-octyloxy derivatives exhibit monotropic smectic C mesophase. Smectic
A mesophase commences from the n-decyloxy derivative as a monotropic and persists up to the last
member synthesized. The mesomorphic properties of the present series were compared with each other
and with the structurally related mesogenic homologous series to evaluate the effect of methyl (–CH3)/
methoxy (–OCH3) substituents as well as variation in the shape of the molecule by varying the alkoxy
chain length of the bulky lateral substituent on mesomorphism.
Received 30 March 2021
Revised 18 June 2021
Accepted 27 June 2021
Available online 30 June 2021
Keywords:
Trisubstituted Benzene
smectic C
k-shape
T-shape
Methyl/methoxy terminal
Monotropic
Ó 2021 Elsevier B.V. All rights reserved.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
4
2.1. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2.1. Preparation of 4-((4-nitrophenyl) diazenyl) benzene-1,3-diol [a] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2. 4-methyl (–CH3)/methoxy (–OCH3) benzoyl chloride [b] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3. 3-hydroxy-4-((4-nitrophenyl) diazenyl) phenyl 4-methyl
4
4
(–CH3)/methoxy (–OCH3) benzoate [c] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4. 4-n-Alkoxybenzoic acids [d]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4
5
2.2.5. 3-((4-alkoxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methyl (–CH3) /methoxy (–OCH3) benzoate (series-I/ Series-
II). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3. Spectroscopic characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5
2.3.1. 3-((4-dodecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methyl benzoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2. 3-((4-tetradecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methyl benzoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
5
⇑
Corresponding author.
E-mail address: hemant71patel@gmail.com (H.N. Patel).
https://doi.org/10.1016/j.molliq.2021.116863
0167-7322/Ó 2021 Elsevier B.V. All rights reserved.

Y. Yao, R.R. Koshti, A. Vyas et al.
Journal of Molecular Liquids 336 (2021) 116863
2.3.3. 3-((4-hexadecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methyl benzoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
5
5
5
6
6
2.3.4. 3-((4-pentyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methoxy benzoate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.5. 3-((4-heptyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methoxy benzoate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.6. 3-((4-dodecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methoxy benzoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.7. 3-((4-hexadecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methoxy benzoate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Effect of molecular constitution on mesomorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Spectroscopic investigation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. The phase behavior and mesomorphic properties of series I and II. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.1. 3-((4-alkoxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methyl benzoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2. 3-((4-alkoxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl 4-methoxy benzoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3. Microscopic observation for series I and II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
9
9
3.4. Calorimetric studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5. Mesogenic properties and molecular constitution:. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
increased in importance since 1977 following the pioneering work
of Chandrasekhar et al. [1].(seeScheme 1.Scheme 2.)
The supramolecular assemblies of disc-shaped molecules lead
to the formation of discotic liquid crystals, discovered in Bangalore
in 1977 by Chandrasekhar et al, for more than 30 years. However,
this is relatively recent compared with the well-developed area of
calamitic liquid crystal research [1-4]. Previously, discotic liquid
crystal (LC) materials have received a great deal of attention due
to their technological importance in low dimensional conductivity
[5], electro optical display devices with optical compensation films
[6,7], holographic data storages [8] and photovoltaic cells [9]. More
research has been focused on the triphenylene structure due to its
relative ease of synthesis [10-12] and its strong tendency to form
columnar mesophases [10]. However, compounds based on a sin-
gle benzene ring as the central core having different substituents
Trisubstituted benzene mesogens molecules have a molecular
assembly having versatility in their synthesis that encompasses
various functional groups and structure the desired physical and
chemical properties [13-16] Molecules with Smectic C, Nematic,
Smectic A and columnar liquid crystalline phases [10,17-19]. The
self-assembly in columnar phases allows the transport of electrons,
protons and ions by building the molecular constitution in one
dimension. The flexibility of the arms in the trisubstituted benzene
mesogens forms aggregates of two or more molecules in a disc or a
half disc, rising to a supramolecular columnar system by self-
assembly resulted by conformational changes. This organization
provides columnar liquid crystals with properties desirable for var-
ious applications like light-emitting diodes, photoconductive semi-
Series
I
X
n=
-CH3
1-8, 10,12,14 and 16.
1-8, 10,12,14 and 16.
II
-OCH3
Scheme 1. Core structure of azo diester mesogens.
2

Y. Yao, R.R. Koshti, A. Vyas et al.
Journal of Molecular Liquids 336 (2021) 116863
Scheme 2. Synthetic route to series I and II compounds.
conductor devices, field-effect transistors, photovoltaic cells, gas
sensors, memory devices and electro-optical and optical fibers
[20,21]. Various molecules incorporating oxadiazole with symmet-
rical and non-symmetrical 1,3,5 tri substituted benzene as a cen-
tral core have been synthesized, and their mesomorphic and
photophysical properties studied [22-24].
A large number of thermotropic liquid crystals have a rigid core
made of two or more aromatic and /or aliphatic rings with one or
more flexible different terminals as alkyl/alkoxy chains [56,57]. In
general, the rings are linked with 1, 2, 4 units, generally producing
a rod-like, lathe-like or disc-like structure. In general, a rigid, lat-
eral substituent perturbs the orderings of liquid crystalline differ-
ent phases [58-60], causing a significant depression in the phase
transition, a reduction of the liquid crystal range, and destabiliza-
tion of ordered smectic A phases. However, the effect of a lateral,
flexible substituent like an alkoxy or alkyl chain is quite different
[61-63]. A methyl or a methoxy substituent causes a strong
decrease in the phase transition, but as the chain length increases,
the effect of the perturbation diminishes. In fact, chains with three
or more carbons perturb the molecular arrangement in the phase
transition to about the same extent, as the lateral alkyl chain is
supposed to be orientated along the long molecular axis. Until
now, only compounds containing three phenyl rings have been
shown to exhibit some different phase transition, if the lateral sub-
stituent is an alkyl or an alkoxy group. Recently molecular consti-
tution [64] has attracted much attention for producing molecules
of different shapes with rich mesogenic behaviour. A recent review
of research work shows that a number of efforts have been going
on for designing molecules of different shapes showing liquid crys-
Aside from the linear combination of two or more mesogenic
entities linked via flexible spacers [25], other modes of interlinked
have been adopted to prepare molecules of a variety of shapes such
as banana-shaped [26-28], S-shaped [29,30], H-shaped [31-33], U-
shaped [34,35], T-shaped [36,37], Y-shaped [38-40] and the k-
shaped mesogens [41-43]. Star-shaped liquid crystals usually have
a core and symmetric or asymmetric mesogens as the side arms,
which is kind of an unconventional liquid crystal. [44-48], which
has led to the growing interest in the synthesis and application.
The simplest star-shaped liquid crystals (LCs) composed of a small
aromatic core unit surrounded by three mesogenic units have been
the most commonly studied liquid crystalline materials [49-51].
The common mesophase of the star-shaped liquid crystals (LCs)
is the columnar phase. In order to obtain cholesteric star-shaped
liquid crystals (LCs), it is undoubtedly effective to use a chiral com-
pound with multiple functional groups as the cores [52-55].
3

Y. Yao, R.R. Koshti, A. Vyas et al.
Journal of Molecular Liquids 336 (2021) 116863
talline behaviour. Reports in the literature have shown that the
majority of the published work on bent shaped, U shaped, banana
or V- shaped molecules based on rigid central 1, 3-phenylene and
2, 7-naphthalene cores self-assemble to yield molecular structures
that show banana liquid crystalline mesomorphism [65]. The swal-
lowed tailed compounds [66] have also been reported with inter-
esting phase transitions. ‘‘Janus-like” supramolecular liquid
crystals have been found with chiral nematic and chiral smectic
A phases [67]. U-shaped molecule having layered structure in the
nematic phase have been reported by Yoshizawa and Yamaguchi
[68]. Berdague et al. [69] reported 1, 2, 4-trisubstituted benzene
derivatives possessing azo-ester and ester linkages having terminal
dodecyloxy substituents.
nance(1H NMR) spectra were recorded on a Bruker Avance Neo-
spectrometer (400 MHz) using Deuterated chloroform (CDCl3) as
a solvent and tetramethyl silane (TMS) as internal reference stan-
dard. The chemical shifts are quoted as d (ppm) downfield from
the reference. The phase appearance and phase transition were
determined by thermal polarizing optical microscopy using a
polarizing microscope equipped with a heating stage. The enthal-
pies of phase transitions, reported in J.gꢀ1, were measured using
differential scanning calorimeter (DSC) Shimadzu DSC-60 plus sys-
tem, with a scanning rate of 10 ℃ minꢀ1. The instrument was cal-
ibrated using pure indium as a standard.
2.2. Synthesis
We [70] have reported the synthesis of two k-shape (Trisubsti-
tuted Benzene Mesogens) mesogenic homologous series in which
one series were having methyl and the other having ethoxy termi-
nal. Mahajan et al. [71] have also reported the synthesis of two k-
shape mesogenic homologous series trisubstituted benzene
derivatives having ester and amide linkages with chloro or meth-
oxy terminal substituents. Vora and Prajapati [72] have reported
a mesogenic homologous series containing three phenyl rings in
the central core and substituted by a lateral acetyl group on the
central benzene nucleus as the effect of lateral acetyloxy group
on mesomorphism has not been studied extensively. The meso-
genic homologous series with lateral acetyl group exhibited
nematic mesomorphism, whereas homologous series with lateral
aromatic branch exhibited smectogenic tendencies. Patel [73] have
reported the synthesis of 1, 2, 4-trisubstituted benzene derivatives
with azo-ester and ester linkages having the terminal nitro group.
Nandedkar [74] has also reported the synthesis of 1,2,4- trisubsti-
tuted benzene derivative possessing azo-ester and ester linkages
without any terminal group. Guan-YeowYeap et al. [75] have also
reported the synthesis of k-Shaped liquid crystal trimers having
dual terminal cholesteryl exhibiting N*, SmA and cholesteric glassy
phases. Mei Tian et al. [76] reported Mesomorphic properties of
non-symmetric three-arm chenodeoxycholic acid-derived liquid
crystals. Irina Carlescu et al. [77] reported star-shaped liquid crys-
tals based on 1,3,5-trihydroxy benzene with pendant alkyloxylated
azobenzene arms.
2.2.1. Preparation of 4-((4-nitrophenyl) diazenyl) benzene-1,3-diol [a]
Dissolve 0.036 Mole of 4-nitroaniline in a warm mixture of
14 ml of concentrated hydrochloric acid and 14 ml of water con-
tained in a 500 ml beaker. Place the beaker in a crushed ice salt
bath and cool to 0–5 °C whilst stirring vigorously; 4-nitroaniline
hydrochloride will separate in a finely divided crystalline form.
Add a cold solution of 0.054 mol of sodium nitrite in 10 ml of water
slowly, stirring to an endpoint with potassium iodide starch paper.
Do not allow the temperature of the solution to rising above 10 °C.
Dissolve 0.035 mol of benzene-1–3-diol in a solution of 7 g of
sodium hydroxide in 30 ml of water, cool in ice, add the diazotized
solution slowly, and stir. Then add cooled concentrated hydrochlo-
ric acid slowly and with vigorous stirring to the cold mixture until
it is strongly acid to congo red paper. The colour will change from
violet to dark red-brown. Filter with gentle suction, wash with
water until free from acid and dry upon filter paper in the air.
The yield is 0.034 Mole (75%). It was crystallized from glacial acetic
acid till constant melting point was obtained. M. P. 198–201 °C
(Reported [73] M. P. 202 °C).
2.2.2. 4-methyl (–CH3)/methoxy (–OCH3) benzoyl chloride [b]
4-methyl (–CH3)/methoxy (–OCH3) benzoyl chloride was
prepared by treating corresponding acids with excess of thionyl
chloride under reflux for 2 h [78].
In this present study, we have synthesized two new mesogenic
homologous series containing tri substituted benzene in the cen-
tral core with azo diester linkage and having nitro substituents,
with the lateral n-alkoxy chain and methyl (–CH3)/methoxy (–
OCH3) terminal groups. The mesomorphic properties of these two
series having the following general structural formula were com-
pared with each other as well as other structurally related com-
pounds to evaluate the effect of different terminal group and
shape of molecule by changing the lateral alkyl chain length.
2.2.3. 3-hydroxy-4-((4-nitrophenyl) diazenyl) phenyl 4-methyl
(–CH3)/methoxy (–OCH3) benzoate [c]
0.01 Mole of 4-((4-nitrophenyl) diazenyl) benzene-1,3-diol [A]
was dissolved in 12 ml of dry pyridine and was added dropwise
with constant shaking to cold 0.01 mol of 4-methyl (–CH3)/
methoxy (–OCH3) benzoyl chloride. The mixture was then warmed
2. Experimental
2.1. Materials
4-Nitroaniline, Benzene-1,3-diol, 4-methylbenzoic acid, 4-
methoxybenzoic acid, 4-Hydroxybenzoic acid, n-bromoalkane, N,
N0-Dicyclohexylcarbodiimide (DCC), 4-Dimethylaminopyridine
(DMAP), Tetrahydrofuran (THF) thionyl chloride (SOCl2), dry pyri-
dine, pottasium hydroxide (KOH), ethyl alcohol were used as
receivd. Distillation and drying of solvent were done prior to using
them. Column chromatography was done on Acme’s silica gel
(100–200 mesh) as a stationary phase. Thin layer chromatography
(TLC) was performed on aluminum sheets pre-coated with silica
gel (Merck, Kiesel 60F254 pre-coated). Shimadzu IR-408 spec-
trophotometer was used to record the infrared spectra (IR) using
potassium bromide (KBr) pellets. Proton nuclear magnetic reso-
Fig. 1. k-shape molecular of series I1 and T-shape molecular of series II16.
4

Y. Yao, R.R. Koshti, A. Vyas et al.
Journal of Molecular Liquids 336 (2021) 116863
on water bath for approximately 30–45 min and was allowed to
stand overnight. It was acidified with cold 1:1 hydrochloric acid
and precipitates obtained were filtered and washed with water,
followed by washings of cold dilute sodium hydroxide solution
and cold water. The 3-hydroxy-4-((4-nitrophenyl) diazenyl) phe-
nyl 4–4-methyl (–CH3)/methoxy (–OCH3) benzoate was crystal-
lized two to three times from glacial acetic acid until constant
transition temperatures were (Cr 186 ℃ N 236 ℃ I) obtained. Yield:
72%. Elemental analysis: Found C 63.51, H 3.86, N 11.523%,
1342, 1251, 1238, 1168, 1102, 1064, 968, 890, 856, 749, 684. 1H
NMR spectrum (400 MHz): d 0.92 (t, 3H, –CH3), 1.25–1.56 (m,
10H, 5 ꢁ –CH2-), 1.80 (quant., 2H-O-C-CH2-), 2.47(s, 3H, Ar-CH3),
4.04 (t, 2H, Ar-O-CH2-), 7.05 (d, 2H, ArH), 7.11 (s ,1H, ArH), 7.22
(d ,2H, ArH), 7.26 (d ,1H, ArH), 7.80 (d, 2H, ArH), 8.03 (d, 2H,
ArH), 8.05 (d, 1H, ArH), 8.15–8.30 (d, 4H, ArH).
2.3.1. 3-((4-dodecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl)
phenyl 4-methyl benzoate
C
V
20H15N3O5 requires C 63.67, H 3.98, N 11.15%. IR Spectrum (KBr)
max/cm-1: 3200–3600 (–OH phenolic), 2921, 1749 (–COO-), 1605
(-N = N-), 1512 (–NO2), 1344, 1255, 1168, 1010, 916, 896, 840,
758, 658 cmꢀ1
UV spectrum (MDC) nm: 225 and 365. IR spectrum (KBr)
V-m1ax/cm:2958, 2920, 1742 (–COO-), 1605 (-N = N-), 1521, 1509,
(–NO2), 1342, 1251, 1238, 1168, 1102, 1064, 968, 890, 856, 749,
684. 1H NMR spectrum (400 MHz): d 0.91 (t, 3H, –CH3), 1.24–
1.56 (m, 18H, 9 ꢁ –CH2-), 1.78 (quant., 2H-O-C-CH2-), 2.48(s, 3H,
Ar-CH3), 4.02 (t, 2H, Ar-O-CH2-), 7.06 (d, 2H, ArH), 7.12 (s ,1H,
ArH), 7.23 (d ,2H, ArH), 7.26 (d ,1H, ArH), 7.80 (d, 2H, ArH), 8.02
(d, 2H, ArH), 8.05 (d, 1H, ArH), 8.13–8.30 (d, 4H, ArH).
.
2.2.4. 4-n-Alkoxybenzoic acids [d]
4-n-alkoxybenzoic acids [D] were synthesized from 4-
hydroxybenzoic acid by employing a Williamson’s ether synthesis
protocol [79].
2.3.2. 3-((4-tetradecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl)
phenyl 4-methyl benzoate
2.2.5. 3-((4-alkoxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl)
phenyl 4-methyl (–CH3) /methoxy (–OCH3) benzoate (series-I/
Series-II)
UV spectrum (MDC) nm: 225 and 365. IR spectrum (KBr)
V-m1ax/cm: 2960, 2920, 2850, 1741 (–COO-), 1604 (-N = N-), 1527,
1510 (–NO2), 1471, 1450, 1342, 1259, 1168, 1093, 1051, 844,
800, 759, 688. 1H NMR spectrum (400 MHz): d 0.89 (t, 3H, –CH3),
1.25–1.57 (m, 22H, 11 ꢁ –CH2-), 1.80 (quant., 2H, -O-C-CH2-),
2.49 (s, 3H, ArCH3), 4.04 (t, 2H, Ar-O-CH2-), 7.03 (d, 2H, ArH),
7.12 (s ,1H, ArH), 7.22 (d ,2H, ArH), 7.25 (d ,1H, ArH), 7.81 (d, 2H,
ArH), 8.02 (d, 2H, ArH), 8.06 (d, 1H, ArH), 8.14–8.31 (d, 4H, ArH).
0.01 Mole of 3-hydroxy-4-((4-nitrophenyl) diazenyl) phenyl 4–
4-methyl (–CH3)/methoxy (–OCH3) benzoate [C], 0.01 mol of 4-n-
alkoxy benzoic acids [D], 0.01 mol of DCC [80] and 0.001 mol of
DMAP were dissolved in dry THF and stirred at room temperature
for 36 h. The insoluble solid was removed through filtration. The
solution was chromatographed on silica gel using petroleum ether
(60–80 °C) ethyl acetate mixture (96:4) as eluent. Removal of sol-
vent from the eluate afforded a solid material which was crystal-
lized two–three times from methanol. The purities of all these
synthesized compounds were checked by thin layer chromatogra-
phy (Merk-kiesel gel 60F254 pre-coated plates). Yield: 36–41%. The
elemental analysis of all the synthesized compounds was found to
be satisfactory.
2.3.3. 3-((4-hexadecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl)
phenyl 4-methyl benzoate
UV spectrum (MDC) nm: 225 and 365. IR spectrum (KBr)
V-m1ax/cm: 2960, 2920, 2850, 1741 (–COO-), 1606 (-N = N-), 1527,
1512 (–NO2), 1471, 1450, 1342, 1259, 1168, 1093, 1051, 844,
800, 759, 688. 1H NMR spectrum (400 MHz): d 0.89 (t, 3H, –CH3),
1.25–1.58 (m, 26H, 13 ꢁ –CH2-), 1.80 (quant., 2H, -O-C-CH2-),
2.49 (s, 3H, ArCH3), 4.04 (t, 2H, Ar-O-CH2-), 7.05 (d, 2H, ArH),
7.10 (s ,1H, ArH), 7.21 (d ,2H, ArH), 7.25 (d ,1H, ArH), 7.81 (d, 2H,
ArH), 8.04 (d, 2H, ArH), 8.07 (d, 1H, ArH), 8.11–8.30 (d, 4H, ArH).
The UV, IR and 1H NMR spectral data of representative com-
pounds were found to be consistent with the proposed structure.
2.3. Spectroscopic characterization
2.3.4. 3-((4-pentyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl)
phenyl 4-methoxy benzoate
3-((4-octyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phe-
nyl 4-methyl benzoate
UV spectrum (MDC) nm: 266 and 368. IR spectrum (KBr)
V-m1ax/cm: 2920, 2832, 1732 (–COO-), 1606 (-N = N-), 1525, 1510,
(–NO2), 1344, 1255, 1168, 1105, 1010, 916, 898, 840, 758, 658.
1H NMR spectrum ((400 MHz): d 0.93 (t, 3H, –CH3), 1.25–1.57 (m,
4H, 2 ꢁ –CH2-), 1.80 (quant., 2H, -O-C-CH2-), 4.00–4.10 (m, 5H,
Ar-O-CH2- and Ar-OCH3), 7.10 (d, 2H, ArH), 7.30 (d, 2H, ArH),
7.40 (s, 1H, ArH), 7.52 (d, 1H, ArH), 7.80 (d, 2H, ArH), 8.04 (d, 1H,
ArH), 8.15 (d, 4H, ArH), 8.30 (d, 2H, ArH).
UV spectrum (MDC) nm: 225 and 365. IR Spectrum (KBr)
V-m1ax/cm: 2920, 1734 (–COO-), 1604 (-N = N-), 1521, 1508, (–NO2),
2.3.5. 3-((4-heptyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl)
phenyl 4-methoxy benzoate
UV spectrum (MDC) nm: 266 and 368. IR (KBr) V-m1ax/cm: 2941,
2920, 2832, 2852, 1734 (–COO-), 1608 (-N = N-), 1525, 1510
(–NO2), 1475, 1396, 1342, 1238, 1165, 1105, 1008, 916, 898, 840,
758, 684. 1H NMR spectrum (400 MHz): d 0.92 (t, 3H, –CH3),
1.25–1.57 (m, 8H, 4 ꢁ –CH2-), 1.81 (quant., 2H, O-C-CH2-), 4.00–
4.15 (m, 5H, Ar-O-CH2- and Ar-OCH3), 7.10 (d, 2H, ArH), 7.30 (d,
2H, ArH), 7.40 (s, 1H, ArH), 7.54 (d, 1H, ArH), 7.70(d, 2H, ArH),
8.05 (d, 1H, ArH), 8.15 (d, 4H, ArH), 8.30 (d, 2H, ArH).
2.3.6. 3-((4-dodecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl)
phenyl 4-methoxy benzoate
UV spectrum (MDC) nm: 266 and 368. IR spectrum (KBr)
Fig. 2. UV–Vis absorption spectra of compounds I12 and II12 in THF.
V-m1ax/cm: 2920, 2832, 1736 (–COO-), 1606 (-N = N-), 1525,1510
5

Y. Yao, R.R. Koshti, A. Vyas et al.
Journal of Molecular Liquids 336 (2021) 116863
Table 1
Transition temperatures (°C) of the series I compound.
Compound No.
R = –CnH2n+1 n =
Cr
SmC
SmA
N
I
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
10
12
14
16
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
191
181
186
175
170
160
80
77
85
83
80
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
(ꢂ
110)
(ꢂ
(ꢂ
(ꢂ
(ꢂ
50)
57)
80)
79)
9
10
11
12
ꢂ
105
72)
82
(ꢂ
() = monotropic value.
1510 (–NO2), 1471, 1342, 1317, 1255, 1234, 1141, 1093, 1051,
1006, 900, 804. 1H NMR spectrum (400 MHz): d 0.86 (t, 3H,
–CH3), 1.27–1.49 (m, 26H, 13 ꢁ –CH2-), 1.81 (quant., 2H, -O-C-
CH2-), 4.00–4.15 (m, 5H, Ar-O-CH2- and Ar-OCH3), 7.05 (d, 2H,
ArH), 7.30 (d, 2H, ArH), 7.35 (s, 1H, ArH), 7.54 (d, 1H, ArH), 7.70
(d, 2H, ArH), 8.05 (d, 1H, ArH), 8.15(d, 4H, ArH), 8.36 (d, 2H, ArH).
3. Results and discussion
3.1. Effect of molecular constitution on mesomorphism
In all trans conformation, from the molecular shape of the series
I and II it was observed that the molecules are neither rod-like nor
disc-like but of k-shape to T-shape. In thermotropic liquid crystal
formation of mesophase is highly sensitive to the molecular config-
uration which can be correlated with thermal stability and meso-
phase length. Fig. 1 summarizes the molecular structure and
dimensions of series I and II.
The molecular geometry of series I and II contains three phenyl
rings linked by ester (–COO-) and azo (-N = N-) with central rigid
ring, lateral n-alkoxy phenyl group and terminal methyl phenyl/
methoxy phenyl groups are attached to the meta and para position
of central core ring with reference to the azo (-N = N-) group. The
terminal methyl (–CH3) /methoxy (–OCH3) branch in series I and
II seems to be affects the immergence of the mesophase and its
thermal stability to a greater extent than the lateral n-alkoxy
group, presumably due to the effect of its increased width. The L/
D ratio in series I (1.596) is lower than that in series II (1.629)
due to the presence of methyl (–CH3) group in series I and methoxy
(–OCH3) group in series II. Representative k-shape molecule of ser-
ies I and T-shape molecule of series II with length and diameter are
shown in Fig. 1. As the lateral n-alkoxy chain length increases the
Fig. 3. The phase behavior of series I.
(–NO2), 1475, 1396, 1253, 1165, 1008, 916, 898, 758, 658. 1H NMR
spectrum (400 MHz): d 0.83 (t, 3H, –CH3), 1.18–1.49 (m, 18H, 9 ꢁ –
CH2-), 1.74 (quant., 2H, -O-C-CH2-), 4.00–4.10 (m ,5H, Ar-O-CH2-
and Ar-OCH3), 7.04 (d, 2H, ArH), 7.25 (d, 2H, ArH), 7.40 (s, 1H,
ArH), 7.52 (d, 1H, ArH), 7.71 (d, 2H, ArH), 7.95 (d, 1H, ArH), 8.17
(d, 4H, ArH), 8.32 (d, 2H, ArH).
2.3.7. 3-((4-hexadecyloxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl)
phenyl 4-methoxy benzoate
UV spectrum (MDC) nm: 266 and 368. IR spectrum (KBr)
V-m1ax/cm: 2920, 2832, 2850, 1741(–COO-), 1604(-N = N-), 1527,
Table 2
Transition temperatures (°C) of the series II compound.
Compound No.
R = –CnH2n+1 n =
Cr
SmC
SmA
I
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
10
12
14
16
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
175
160
145
137
129
118
107
102
101
100
98
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
ꢂ
(ꢂ
(ꢂ
72)
77)
9
(ꢂ
(ꢂ
(ꢂ
(ꢂ
79)
85)
89)
60)
10
11
12
100
() = monotropic value.
6

Y. Yao, R.R. Koshti, A. Vyas et al.
Journal of Molecular Liquids 336 (2021) 116863
Fig. 7. n-hexadecyloxy derivative showed focal conic smectic A mesophase at 68 ℃
on cooling from the isotropic liquid.
Fig. 4. The phase behavior of series II.
Fig. 8. n-tetradecyloxy derivative on cooling from the isotropic liquid showed focal
conic smectic A mesophase at 81 ℃ .
Fig. 5. n-hexyloxy derivative on cooling from isotropic liquid showed classical
schlieren nematic texture at 100 ℃
and layered smectic C/ smectic A structure is maintained, giving
rise to a smectic mesophase.
Lateral methoxy derivative of both the series I and II were found
to be of k-shape, as the number of carbon increases in lateral
alkoxy chain the L/D ratio of the ascending molecules decreases
and shape of the molecule changes from k-shape to T-shape. For
present series I it was observed that at about 1.351 L/D ratio meso-
morphism immerges due to the mesogenic core and the lateral
alkoxy chain interaction, whereas for series II threshold L/D ratio
was about 1.442. Whereas for lower members I1-I5 and II1-II6
inheriting k-shape, due to the short lateral alkoxy chain length
weak intermolecular interaction between mesogenic core and the
lateral alkoxy chain length as well as terminal nitro (–NO2) and
methyl (–CH3)/methoxy (–OCH3) prevents the formation of inter-
calation which in result prevents formation of mesophase. Increas-
ing the lateral alkoxy chain length changes the molecular changes
from k-shape to T-shape and increase the interaction between ter-
minal nitro (–NO2), methyl (–CH3)/methoxy (–OCH3), mesogenic
core and the lateral alkoxy chain, which favors the formation of
intercalated structure.
Fig. 6. n-dodecyloxy derivative showed smectic C mesophase on cooling from
isotropic liquid at 75 ℃ .
width of the molecules increases and the shape of molecule
changes from k-shaped to T-shaped which lowers the L/D ratio.
This decrease in L/D ratio affects the formation of mesophase and
thermal stability. Terminal attraction between the molecules pre-
vails the smectic C/smectic A and nematic mesophase formation
due to intermolecular attraction. As the lateral alkyl chain length
increase the total polarizability enhances and the terminal attrac-
tion to lateral attraction ratio decreases. Under such conditions
the molecules will adhere and strictly maintains the layered struc-
ture, although they might moderately acquire a fluid condition,
3.2. Spectroscopic investigation
The presence of azo (-N = N-) groups leads to the possibly of
photo-isomerization and photochromic behaviour. Preliminary
studies of the photochemical properties were therefore conducted
in solution. Chloroform solutions of concentration, 1 ꢁ 10ꢀ6 for
dodecyl derivative of both the series were prepared for determina-
tion of UV–Vis absorption spectra. The k-shaped to T-shaped meso-
genic compounds of series I displayed three primary absorption
7

Y. Yao, R.R. Koshti, A. Vyas et al.
Journal of Molecular Liquids 336 (2021) 116863
Table 3
DSC data for series I and II compounds.
Series-I
Heating Temp ℃ [
D
H/Jgꢀ1
Iso
,
D
S/Jgꢀ1 Kꢀ1
]
Cooling Temp ℃ [
D
H/Jgꢀ1
,
D
S/Jgꢀ1 Kꢀ1
SmA
]
SmA
N
SmC
Cr
6
7
8
10
12
14
16
159.6[13.09, 0.0302]
80.2[16.74, 0.0473]
76.8[10.27, 0.0293]
84.9[21.32, 0.0595]
83.3[17.02, 0.0472]
105.6[14.81, 0.0391]
81.9[19.46, 0.2307]
110.3[6.83, 0.0178]
78.6[8.05, 0.0228]
34.4[10.43, 0.0336]
37.7[13.61, 0.0438]
61.8[11.97, 0.0357]
64.7[13.14, 0.0397]
73.8[19.04, 0.0549]
50.9[7.94, 0.02450]
49.7[5.09, 0.0157]
57.1[6.01, 0.0182]
79.8[8.97, 0.0254]
79.3[3.01, 0.0085]
80.4[0.73, 0.0020]
104.2[2.89, 0.0076]
72.5[8.55, 0.0247]
Series-II
7
8
10
12
14
16
106.3[33.59, 0.0885]
102.6[40.11, 0.1067]
101.1[42.79, 0.1137]
99.7[29.67, 0.0715]
98.0[35.92, 0.0968]
100.4[19.04, 0.0509]
71.8[5.18, 0.2082]
77.4[1.73, 0.0049]
51.1[23.82, 0.0734]
55.7[35.01, 0.1065]
62.4[36.17, 0.1082]
64.3[29.34, 0.0869]
75.5[15.62, 0.0448]
40.9[36.69, 0.1168]
78.9[6.17, 0.0175]
84.7[7.82, 0.0218]
89.2[5.23, 0.0144]
60.1[9.12, 0.0273]
470–475 nm due to n–p* transition. The trans-form is much more
stable than the cis-form, but each isomer can be transformed into
the other by irradiation of light of the appropriate wavelength. In
general, on irradiation with UV light azobenzene compounds are
characterized by a reversible transformation from the more stable
trans- to the less stable cis-form. Photochemical trans–cis-trance
isomerization by reorientation of azobenzene group can be
induced by means of linearly polarized light [81-84].
The IR spectrum (kmax/cmꢀ1) show absorption bands in the
region of 2960–2920 due to C-H stretching, in region of 1732–
1741 due to carbonyl (C = O of ester) stretching vibration. The
strong intensity stretching band in the region of 1604–1608 arising
due to azo (-N = N-) group. Strong intensity C-O stretching band in
the region of 1278–1282 and 1251–1269 arising in series I and II
respectively, due to aromatic ester (O = C-O-Ar). In Series I there
is only one the lateral alkoxy group (-OR) while in series II there
is one the lateral alkoxy (-OR) and one terminal methoxy (–
OCH3), which in para direction to azo group (-N = N-) of central
ring. This methoxy (–OCH3) group creates asymmetric ether sys-
tem which gives rise to additional frequency in the region of
2832 in series II. In the 1H NMR spectrum, fifteen aromatic protons
appear in the form of seven doublets (d) in the region of 8.11–8.31,
8.05–8.07, 8.02–8.04, 7.80–7.81, 7.25–7.26, 7.21–7.23 and 7.02–
7.06, while one proton appear in the form of one singlet (s) in
the region of d 7.10–7.12 for series I, 7.52–7.40 for series II. One
oxy-methylene of the lateral alkoxy chain (-OR) and terminal
methyl (–CH3) group resonate in the region of d 4.02–4.04 as triplet
and d 2.47–2.49 as singlet in series I. Oxy-methylene of the lateral
alkoxy chain (-OR) and terminal methoxy (–OCH3) protons res-
onate in the region of d 4.00–4.15 as a multiplate in series II. One
methylene (O-C-CH2) protons resonate in the region d 1.74–1.81
as a quartered. One methyl group (–CH3) attached to chiral carbon
(C*) of ester chain (–COO-) and rest of the methylene protons of the
lateral alkoxy chain resonate in the region d 1.18–1.58 as a multi-
plate. Methyl group (–CH3) of the lateral alkoxy chain (-OR) res-
onate in the region of d 0.83–0.93 as triplet.
Fig. 9. DSC plots of series I, compound 12.
As a preliminary investigation, the mesophases exhibited by
compounds of series I and II were concluded by the optical micro-
scopic studies.
Fig. 10. DSC plots of series II, compound 12.
3.3. The phase behavior and mesomorphic properties of series I and II
maxima, at 225 nm, 365 nm and 470 nm; compounds of series II
also showed three primary absorption maxima at 266 nm,
368 nm and 475 nm (Fig. 2). The k-shaped to T-shaped azo (-
N = N-) compounds in the trans-form showed a strong band at
3.3.1. 3-((4-alkoxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl
4-methyl benzoate
In the series I methoxy to n-pentyloxy derivatives are non-
mesogenic. n-Hexyloxy derivative exhibits only monotropic
nematic mesophase. n-Heptyloxy to n-dodecyloxy derivatives
365–368 nm attributed to
p–p* transition, and a weak band at
8

Y. Yao, R.R. Koshti, A. Vyas et al.
Journal of Molecular Liquids 336 (2021) 116863
Table 4
The comparison of mesophase length (℃), thermal stabilities (℃) and molecular structure of n-dodecyloxy derivative of series I and II, Series A to D.
Series
Mesophase length
SmA
Thermal stability
SmA-I
Commencement of smectic phase
N
N-I
I
21
23
37
–
23.3
21
–
–
–
9
26
–
79
85
105
–
41.4
58
–
–
–
80
–
C7
C7
C7
–
C12
C8
II
A
B
C
D
–
exhibit monotropic smectic
C
mesophase. n-tetradecyloxy
tions are also observed. Data of these DSC analyses are recorded in
table 3. Representative thermogram of n-dodecyloxy of series I is
shown in Fig. 9 where peak of crystal to isotropic appeared at
derivative exhibits enantiotropic SmA mesophase whereas
n-hexadecyloxy derivatives exhibits monotropic Smectic A meso-
phase. The transition temperatures are recorded in table 1. The plot
of transition temperatures against the number of carbon atoms in
the alkoxy chain is given in Fig. 3 from which it can be noticed that
the SmC-I transition temperatures increase as the series ascended
which levels off slightly for n-dodecyloxy derivative, whereas it
exhibits a falling tendency for SmA-I transition temperatures in
the higher homologues.
83.3 ℃ with a
ing cycle, while peak of isotropic to Smectic C appeared at 79.3 ℃
with a
H of 3.01 Jgꢀ1 and S of 0.0085 Jgꢀ1 Kꢀ1 and peak of Smec-
tic C to crystal appeared at 64.7 ℃ with a
H of 13.14 Jgꢀ1 and
of 0.0397 Jgꢀ1 Kꢀ1 on cooling cycle, similarly peak of crystal to iso-
tropic appeared at 99.7 ℃ with a S of 0.0715
H of 29.67 Jgꢀ1 and
Jgꢀ1 Kꢀ1 on heating cycle, while peak of isotropic to Smectic A
appeared at 84.7 ℃ with a
H of 7.82 Jgꢀ1 and S of 0.0218 Jgꢀ1
Kꢀ1 and peak of Smectic A to crystal appeared at 64.3 ℃ with a
H of 29.34 Jgꢀ1 and S of 0.0869 Jgꢀ1 Kꢀ1 on cooling cycle ther-
D D
H of 17.02 Jgꢀ1 and S of 0.0472 Jgꢀ1 Kꢀ1 on heat-
D
D
D
DS
D
D
D
D
3.3.2. 3-((4-alkoxybenzoyl) oxy)-4-((4-nitrophenyl) diazenyl) phenyl
4-methoxy benzoate
D
D
mogram of n-dodecyloxy of series II which is shown in Fig. 10.
Enthalpy changes of the various transitions agree well with the
existing related literature value.
In the series II methoxy to n-hexyloxy derivatives are non-
mesogenic. n-Heptyloxy and n-octyloxy derivatives exhibit mono-
tropic smectic C mesophase. Smectic A mesophase commences
from n-decyloxy derivative as a monotropic and persist up to the
last member synthesized. The transition temperatures are
recorded in table 2. The plot of transition temperatures against
the number of carbon atoms in the alkoxy chain is given in Fig. 4
from which it can be noticed that the crystal-isotropic transition
temperatures exhibit a tendency to decrease with increase in the
length of terminal alkoxy tail except for n-tetradecyloxy derivative.
Smectic C to isotropic transition temperatures exhibited little ris-
ing tendency. SmA-I transition temperatures exhibited rising ten-
dency which levels off for the last homologue synthesized.
3.5. Mesogenic properties and molecular constitution:
Table 4 summarizes comparison of smectic mesophase range,
transition temperatures and molecular structure of n-dodecyloxy
derivative of the series I and II respectively as well as structurally
related known compounds A [73], B [74], C [69] and D [43]. All the
compound (n = 7–16) of series I and II are monotropic smectogenic
with the exception of the n-hexyloxy derivative of series I which is
monotropic nematogenic because of the presence of less polar
methyl (–CH3) terminal group. The smectic range of series I
(n = 12) is about 21 °C whereas that of series II (n = 12) is 23 °C.
However, the Sm-I transition temperatures of both the series are
almost the same. This is understandable as the molecular constitu-
tion of series I and II differ only at one terminus. The molecule of
series I has methyl group (–CH3) at one terminus whereas mole-
cule of series II has methoxy group (–OCH3) at the same terminus.
Reference to table 4 also shows that the smectic mesophase
length of series I and II is lower by 16 °C and 14 °C and smectic
mesophase thermal stabilities is lower by 26 °C & 20 °C, respec-
tively when compared that of series A. The molecular structure
of series I, II and series A differ at one terminus only. The molecules
of series I and II have terminal methyl group (–CH3) and methoxy
group (–OCH3) respectively at one terminus whereas molecules of
series A have –OR (-OC12H25) group at the terminus. Gray [85] has
explained that the addition of each methylene group increases the
polarizability of the molecules. Hence the higher mesophase length
and the thermal stabilities of series A may be due to the presence
of longer alkoxy chain (-OR).
3.3.3. Microscopic observation for series I and II
All mesogenic compounds of series I was found monotropic
except n-tetradecyl derivative SmA mesophase which was found
enantiotropically mesogenic. n-hexyloxy derivative (n = 6) formed
small droplets that coalesced to the classical schlieren texture of
the nematic phase. n-heptyloxy to n-dodecyloxy derivatives exhib-
ited schlieren texture of smectic C mesophase. Rest of the higher
homologues of series I (n = 14 and 16) gave only focal-conic texture
characteristic of smectic A mesophase. n-hexyloxy derivative on
cooling from isotropic liquid showed classical schlieren nematic
texture at 100 ℃ (Fig. 5), n-dodecyloxy derivative showed smectic
C mesophase on cooling from isotropic liquid at 75 ℃ (Fig. 6),
and n-hexadecyloxy derivative showed focal conic smectic A meso-
phase at 68 ℃ (Fig. 7) on cooling from the isotropic liquid.
Mesogens of series II were also found to be monotropically
mesogenic where methoxy to n-hexyloxy derivatives found non
mesogenic, n-heptyloxy and n-octyloxy derivative exhibited
schlieren texture of smectic C mesophase and rest of the higher
homologues n-decyloxy to n-hexadecyloxy exhibited only focal-
Series I and II are smectogenic whereas series B is nematogenic.
Moreover, the smectic mesophase length & thermal stabilities of
series I and II are much higher as compared to the nematic meso-
phase length and thermal stabilities of series B. The main differ-
ence in the molecular structures of series I & II and series B is
that the series I and II all are having less polar methyl (–CH3)
and methoxy (–OCH3) group terminal group whereas series B has
no terminal substituent at one end. Probably the presence of less
polar methyl group (–CH3) and methoxy group (–OCH3) dipole in
series I and II are responsible for the smectogenic properties as
conic texture characteristic of smectic
A
mesophase. n-
tetradecyloxy derivative on cooling from the isotropic liquid
showed focal conic smectic A mesophase at 81 ℃ (Fig. 8).
3.4. Calorimetric studies
Thermograms for all mesogenic compounds of series I and II
were recorded to confirm the microscopic reading and the type
of mesophase. The enthalpy changes with respect to phase transi-
9

Y. Yao, R.R. Koshti, A. Vyas et al.
Journal of Molecular Liquids 336 (2021) 116863
well as lower mesophase length & thermal stabilities of the present
series I and II as compared to series B. This is also reflected in com-
parison of series I and II with series C. Table 4 indicates that the
thermal stabilities of series I and II are higher compared to series
C even though it contains longer dodecyloxy chain. However, the
smectic mesophase length of series I and II are little lower as com-
pared to series C. One should keep in mind that the mesophase
length is also partly depend on the crystal-mesophase transitions.
It’s also seen that the smectic mesophase length of series I and
II is higher by 0 °C and 2 °C and smectic mesophase thermal stabil-
ities is higher by 21 °C & 27 °C, respectively when compared that of
series D. The molecular structure of series I, II and series D differ at
one terminus and lateral chloro substituent only. The molecules of
series I and II have methyl group (–CH3) and methoxy group (–
OCH3) respectively at one terminus whereas molecules of series
D have alkoxy (-OR) (-OC12H25) group at the terminus and lateral
chloro group (–Cl) meta to the nitro group (–NO2).
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This work was supported by National Natural Science Founda-
tion of China (Project No.81903623 and 22007086), the China Post-
doctoral
Science
Foundation
(No.
2019 M652586
and
2020M670083ZX), Henan science and technology key project
(No. 202102310148) and Henan Medical Science and Technology
Program (No. 2018020601 and SBGJ202003051). Authors are
thankful to Kadi Sarva Vishwavidhyalaya for providing laboratory
facility.
Due to the presence of terminus the n-alkoxybenzoyl sub-
stituent, which may be responsible for the slightly lower smectic
A to isotropic transition temperature of series D. The range and
thermal stability of the mesophase is the more important factor
in relating mesomorphic behaviour to chemical constitution, since
the chemical grouping gives rise to intermolecular attractions,
which in turn determine the lower mesophase range and thermal
stability. Gray [85] has explained that the addition of each methy-
lene group increases the polarizability of the molecules. Hence the
higher mesophase length and lower the thermal stabilities of series
D may be due to the presence of longer alkoxy chain.
lateral chloro group (–Cl) meta to the nitro group (–NO2),
whereas the position meta to the nitro group (–NO2) in series I
and II is unsubstituted. The lateral substitution in the molecule
normally decreases the thermal stability of both the smectic and
nematic mesophases, as a result of the broadening effect and forc-
ing the molecules apart. It seems that in series D. Lateral chloro (–
Cl) substituents which do not broaden the molecule enhance liquid
crystalline thermal energy; the thermal stability of the smectic
phase is properly enhanced when the substituent is dipolar. Gray
[85] has explained that a compound which requires more thermal
energy to disorganize the molecular arrangement of the smectic
mesophase will be more thermally stable.
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