Mesomorphic, optical, dielectric, and electro-optic properties of azo-ester materials: Effect of lateral methyl and terminal substituents

Article abstract of DOI:10.1016/j.molliq.2021.116308

The effect of introducing a lateral methyl substituent and the two different alkoxy terminal groups on the mesophase behavior of three-benzene-ring molecular core of azo-ester compounds bearing 2-methylbutoxy unit on the opposite terminal side (C1–C6) were explored. Their structures were confirmed by several spectroscopic techniques (Fourier transform infrared spectroscopy, ¹H and ¹³C nuclear magnetic resonance spectroscopy). The mesomorphic properties of these compounds were studied by differential scanning calorimetry, polarized optical microscopy, and small- and wide-angle X-ray scattering. Except for the un-substituted terminals (C1 and C4), all other derivatives are liquid crystalline, exhibiting enantiotropic nematic liquid crystal phase. The alteration of the alkoxy chain length from methoxy to butoxy at the phenyl ester terminal group (ring A) for each set of laterally-neat and laterally methyl-substituted mesogens decreases the nematic temperature range. A decrease in the mesophase temperature range is also observed when a lateral methyl substituent is incorporated into phenylazo ring of the mesogenic core (ring C). Optical study demonstrated that their absorption spectra are identical in shape due to the structural similarities of the mesogenic core. The corresponding fluorescence spectra exhibited blue-emission with the laterally methyl substituted compounds having a relatively higher fluorescence intensity compared to that of its laterally un-substituted analogues. Furthermore, the laterally methyl-substituted derivatives were subjected to the planar alignment and their dielectric and electro-optic properties were investigated. The nematic phase of the aligned samples shows antiferroelectric switching behavior with spontaneous polarization, P_s ~ 5 μC/cm².

Full text of DOI:10.1016/j.molliq.2021.116308

Journal of Molecular Liquids 336 (2021) 116308
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Mesomorphic, optical, dielectric, and electro-optic properties of
azo-ester materials: Effect of lateral methyl and terminal substituents
Salwa J. Kamal a,d, Noordini M. Salleh b, H.N.M. Ekramul Mahmud a, Iskandar Abdullah a, Sanjeev R. Gopal c,
Thamil Selvi Velayutham b,c, N. Idayu Zahid b,
⇑
a Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
b Centre for Fundamental and Frontier Sciences in Nanostructure Self-Assembly, Department of Chemistry, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
c Low Dimensional Materials Research Center, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur, Malaysia
d Department of Chemistry, College of Science, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, Dammam 31441, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of introducing a lateral methyl substituent and the two different alkoxy terminal groups on
the mesophase behavior of three-benzene-ring molecular core of azo-ester compounds bearing
2-methylbutoxy unit on the opposite terminal side (C1–C6) were explored. Their structures were
confirmed by several spectroscopic techniques (Fourier transform infrared spectroscopy, 1H and 13C
nuclear magnetic resonance spectroscopy). The mesomorphic properties of these compounds were
studied by differential scanning calorimetry, polarized optical microscopy, and small- and wide-
angle X-ray scattering. Except for the un-substituted terminals (C1 and C4), all other derivatives are
liquid crystalline, exhibiting enantiotropic nematic liquid crystal phase. The alteration of the alkoxy
chain length from methoxy to butoxy at the phenyl ester terminal group (ring A) for each set of
laterally-neat and laterally methyl-substituted mesogens decreases the nematic temperature range.
A decrease in the mesophase temperature range is also observed when a lateral methyl substituent
is incorporated into phenylazo ring of the mesogenic core (ring C). Optical study demonstrated that
their absorption spectra are identical in shape due to the structural similarities of the mesogenic core.
The corresponding fluorescence spectra exhibited blue-emission with the laterally methyl substituted
Received 29 December 2020
Revised 25 February 2021
Accepted 22 April 2021
Available online 27 April 2021
Keywords:
Azo-ester
Lateral methyl substitution
Terminal group
Mesomorphic behavior
Dielectric properties
Electro-optic properties
compounds having
a relatively higher fluorescence intensity compared to that of its laterally
un-substituted analogues. Furthermore, the laterally methyl-substituted derivatives were subjected
to the planar alignment and their dielectric and electro-optic properties were investigated. The
nematic phase of the aligned samples shows antiferroelectric switching behavior with spontaneous
polarization, Ps ~ 5
l
C/cm2.
Ó 2021 Elsevier B.V. All rights reserved.
1. Introduction
which is normally alkyl or alkoxy groups. The rigid unit and
flexible units must be balanced in order to demonstrate the liquid
crystal properties. The selection of the linking groups is highly
dependent on the required liquid crystals stability and the meso-
phase temperature range [9]. Consequently, many efforts have
been made to synthesize different mesogenic compounds contain-
ing azo linking groups due to their promising mechanical, optical,
and thermal properties [10]. In addition, the combination of the
azo central linkage together with ester groups in the liquid crystal
core, are commonly used to increase the polarizability of the com-
pounds and the stability of mesophase [11–13].
Extensive researches have been conducted to show ther-
motropic liquid crystal (LC) versatility for novel applications in
the field of optical organic transistors [1–3], liquid crystals display
[4,5], biomolecules fluorescent probes [6], sensors [7], and signal
processing and storage [8]. Most of the thermotropic LC com-
pounds are rigid rod-like molecules that possess two or more aro-
matic rings connected by a variety of linking groups such as ester
(ACOOA), stilbene (ACH@CHA), Schiff base (ACH@NA), acetylene
(AC„CA), and azo (AN@NA) in addition to a flexible spacer unit
Due to the interesting properties of azo-ester compounds
towards enhancing their mesomorphic behavior, considerable
efforts have been devoted to study the effect of alkyl chain length
and terminal groups on the mesomorphic properties of the
⇑
Corresponding author.
E-mail address: nooridayu@um.edu.my (N.I. Zahid).
https://doi.org/10.1016/j.molliq.2021.116308
0167-7322/Ó 2021 Elsevier B.V. All rights reserved.

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
modified molecules. Conjugation between the oxygen of alkoxy
group and the azo or ester carbonyl groups through the intermedi-
ate benzene ring increases the polarizability and polarity of the
molecule and thus enhances the stability of the mesophase. Any
difference in the electronic nature of the terminal polar group leads
to a significant variation in the mesophase behavior of the com-
pounds due to the extremes of the electronic interactions between
the substituent and the mesogenic core [14–19].
Moreover, many researchers synthesized liquid crystal com-
pounds bearing lateral fluorine, chlorine, or methyl substituents
in different positions and studied the effect of polarity and position
of the lateral group on the mesomorphic properties [20–23]. Lat-
eral substituents attached to the rigid mesogenic core of the liquid
crystal molecule have strongly affected their mesophase tempera-
ture range and stability. Steric or polar effects of the laterally pro-
truded substituent on the rigid core as well as the reduction in the
length-to-breadth ratio may lead to a reduction in the melting
temperature along with the mesophase thermal stability. The
incorporation of lateral groups to the molecule is an effective
way to obtain specific and beneficial properties required for the
desired applications [22,24,25].
Herein, we report the synthesis and characterization of new
calamitic liquid crystalline compounds of a three-benzene-ring
molecular core comprising both the azo and ester, as linking units.
As terminal groups, 2-methylbutoxy is attached to the phenylazo
ring (ring C) and methoxy or linear n-butoxy group, as the other
terminal substituent on the ester side of the molecule (ring A).
The effect of lateral methyl substitution at the 2-position of the
azo linkage (ring C) and two different alkoxy groups of the terminal
chain on the mesomorphic properties of these new azo-ester com-
pounds is investigated by identifying their transition temperatures,
the type of the mesophase, optical, dielectric, and electro-optical
properties.
2.2. Synthesis
The synthetic route of compounds C1 to C6 is illustrated in
Scheme 1. The intermediate compounds 1 (where X = H or CH3)
were synthesized via azo coupling reaction according to the meth-
ods reported in the literature [12,22]. Then, the terminal chain was
introduced by Williamson etherification of compounds 1 with
1-chloro-2-methylbutane to produce the second intermediate
compounds 2 [26]. Subsequently, sodium hydroxide solution was
added to compounds 2 in absolute ethanol and heated to give com-
pounds 3. Finally, compounds 3 were allowed to react with phenol
or substituted phenol i.e. 4-methoxyphenol and 4-butoxyphenol
by Steglich esterification to yield the target compounds C1–C6.
All the synthesized compounds were subjected to common spec-
tral techniques to characterize their structures and no attempts
were made to separate the R/S chiral isomers of 1-chloro-2-
methylbutane. Their common structural features are the 2-
methylbutoxy terminal groups on the phenyl azo side with
changes between lateral methyl substituents at the 2-position to
the azo linkage and the un-substituted lateral analogues. The sec-
ond terminal group within each set of laterally-neat analogues
(C1–C3) and their corresponding laterally methyl-substituted
derivatives (C4–C6) has different electron- donating alkoxy groups
(H, OCH3, OC4H9, respectively).
2.2.1. Synthesis of ethyl 4-(4-hydroxyphenylazo or 4-hydroxy-2-
methylphenylazo) benzoates (1)
Ethyl-4-aminobenzoate (30 mmol) was dissolved in
1 M
hydrochloric acid (HCl) solution (90 mmol) and the mixture was
stirred and cooled in an ice bath. 1 M sodium nitrite (NaNO2) solu-
tion (90 mmol) was added dropwise into the reaction mixture and
the temperature was controlled below 5 °C to obtain the diazo-
nium salt. Then, phenol (or m-cresol) (30 mmol) dissolved in 1 M
sodium hydroxide (NaOH) solution (90 mmol) was added dropwise
into the ice-cold diazonium salt solution. The cold-mixture was
further stirred for 1 h at a temperature below 5 °C, followed by
acidification with 1 M HCl solution. The solid separated was fil-
tered and recrystallized twice from ethanol to give a reddish
orange crystal.
2. Experimental
2.1. Materials and structural characterization
1-Chloro-2-methylbutane (96%), N,N0-dicyclohexylcarbodiimide
(99%), 4-dimethylaminopyridine (99%), and phenol (99%) were
supplied by Aldrich. Ethyl 4-aminobenzoate and 4-butoxyphenol
(99%) were acquired from Nacalai Tesque, Inc. and Acros Organics,
respectively. Potassium carbonate and sodium nitrite were pro-
cured from Friendemann Schmidt. 4-methoxyphenol (98%) and
m-cresol (98%) were purchased from Merck. Solvents which
include ethanol, ethyl acetate, hexane, N,N-dimethylformamide
(DMF), and dichloromethane (DCM) were of reagent grade and
purchased from various suppliers such as Merck, J.T. Baker, and
Friendemann Schmidt. DMF and DCM were kept anhydrous before
use.
Fourier transform infrared (FT-IR) spectra of the compounds
were recorded on a Perkin Elmer Spotlight 400 spectrometer (Wal-
tham, MA, USA) using the attenuated total reflectance (ATR)
method. All data were obtained in the 4000–450 cmꢀ1 range, with
a resolution of 4 cmꢀ1 and collected after 4 scans. 1H and 13C
nuclear magnetic resonance (NMR) spectra for all compounds were
recorded on a JEOL Lambda 400 MHz spectrometer (Tokyo, Japan)
and a Bruker AVN 400 MHz spectrometer (Fällanden, Switzerland)
at room temperature using deuterated chloroform (CDCl3) or
deuterated dimethyl sulfoxide (DMSO d6). Elemental analyses for
final compounds were carried out on Perkin Elmer CHNS/O 2400
Series II Analyzer, using the classical Pregl-Dumas method in
which samples were combusted in a pure oxygen environment,
with the resultant combustion gases measured in an automated
fashion. The calibration standard used was acetanilide.
2.2.2. Synthesis of (R,S)-ethyl 4-(4-(2-methylbutoxy)phenylazo or 2-
methyl-4-(2-methylbutoxy)phenylazo) benzoates (2)
Compound 1 (3 mmol), 1-chloro-2-methylbutane (4 mmol),
pinch of potassium iodide and potassium carbonate (7 mmol) were
dissolved in 25 mL anhydrous DMF and refluxed for 72 h. The reac-
tion mixture was diluted with ethyl acetate and the resultant mix-
ture was washed three times with distilled water. The organic
layer was separated, dried over anhydrous sodium sulphate (Na2-
SO4), and removed under reduced pressure. The red solid residue
was purified through column chromatography using silica gel as
a stationary phase and ethyl acetate:hexane (1:10) as eluent.
Finally, compound 2 was recrystallized from hexane.
2.2.3. Synthesis of (R,S)-4-(4-(2-methylbutoxy)phenylazo or 2-methyl-
4-(2-methylbutoxy)phenylazo) benzoic acids (3)
Compound 2 (3 mmol) and 10% wt. NaOH (6 mmol) were added
into a 250 mL round-bottom flask containing 50 mL absolute etha-
nol and the resultant mixture was refluxed for 6 h. The resultant
mixture was poured into ice-cold water and then acidified with
dilute HCl to give the desired compound 3.
2.2.4. Synthesis of (R,S)-phenyl or 4-alkoxyphenyl 4-(4-(2-
methylbutoxy)phenylazo or 2-methyl-4-(2-methylbutoxy)phenylazo)
benzoates (C1–C6)
Compound 3 (2 mmol) and phenol (or substituted phenol i.e. 4-
methoxyphenol and 4-butoxyphenol) (2 mmol) were dissolved in a
2

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
Scheme 1. Synthetic route of compounds C1–C6.
mixed solvent of DCM/DMF with 5:1 volumetric mixing ratio.
Then, N,N0-dicyclohexylcarbodiimide (3 mmol) and a small amount
of N,N0-dimethylaminopyridine as a catalyst were added to the
mixture and stirred at room temperature for 72 h. The resulted
white solid was filtered off and the solvent was removed under
reduced pressure. The solid residue was passed through column
chromatography using silica gel as a stationary phase and DCM:
hexane (10:5) as eluent. Finally, the compound was recrystallized
from a mixture of DCM and ethanol (1:1).
The details of the physical properties, yield, and spectroscopic
data for intermediates (1–3) and final (C1–C6) compounds are
shown in the Supplementary Material. The elemental analyses
for C1–C6 are also available in the Supplementary Material.
ried out on a SDT Q600 thermogravimetric analyzer (New Castle,
USA) at a heating rate of 20 °C minꢀ1 under nitrogen atmosphere.
2.4. Mesomorphic and structural studies
Differential scanning calorimetry (DSC) measurements were
recorded using a Mettler Toledo DSC 822e calorimeter (Columbus,
OH, USA) equipped with Haake EK90/MT intercooler. Indium was
used as the standard sample to calibrate the temperature and
enthalpy accuracy. Samples of 4–6 mg were placed into the 40-
mL sealed aluminium pans. The calorimetric measurements were
carried out for two cycles at heating and cooling scanning rates
of 10 °C minꢀ1 under nitrogen atmosphere. The data were analyzed
using STARe Thermal Analysis System software.
2.3. Thermal studies
The liquid crystalline textures and phase transitions were iden-
tified by polarized optical microscopy (POM) experiments using an
Olympus BX51 polarized light microscope (Tokyo, Japan) attached
to a Mettler Toledo FP82HT hot stage (Columbus, OH, USA) and a
The stability of the prepared compounds (C1–C6) was examined
via thermogravimetric analysis (TGA). The measurement was car-
3

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
temperature controller (FP90 central processor). The microscope
was connected to an Olympus camera for image capture, and cell-
Sens microscope imaging software was used for image analysis and
storage. The sample was placed between a non-treated glass slide
and coverslip. The sample was heated until it reaches its isotropic
phase and then cooled down to room temperature. The phase tran-
sition temperature was recorded during the second heating and
the liquid crystalline textures formed were taken upon the second
surement. The temperatures of the samples were regulated using
a temperature controller (Instec, USA) equipped with a hot stage.
The morphology of the compounds at the crystal and mesophases
before and during the switching measurement was observed and
captured via POM.
3. Results and discussion
cooling at a rate of 2 °C minꢀ1
.
Small- and wide-angle X-ray scattering (SWAXS) measure-
ments were performed to determine the structural properties of
the identified mesophases. An analytical SWAXS from SAXSpace
(Anton Paar, Austria) was equipped with an X-ray tube (DX-Cu
12 ꢁ 0.45, SERFERT) generating Cu-Kɑ radiation beam with wave-
length k = 0.1542 nm at 40 kV and 50 mA. The measurements were
carried out in line collimation mode under vacuum with the detec-
tor distance of 121 mm. Silver behenate (k = 5.84 nm) was used as
a calibrant for all measurements. The sample was transferred to a
solid sample holder and loaded into the X-ray machine. The sample
was then heated to its isotropic state and allowed to cool at room
temperature. The measurement at a selected temperature was con-
ducted upon second heating using a Peltier System (TCStage 300)
to control the sample temperature with an accuracy of 0.1 °C.
Each sample was equilibrated for 10 min at the designated temper-
ature followed by 10 min exposure time for the measurement. The
data were calibrated by normalizing the primary beam using SAXS-
treat software.
3.1. Synthesis and molecular structure determination
All compounds C1–C6 were successfully obtained in four steps:
diazotization, Williamson etherification, hydrolysis and Steglich
esterification (Scheme 1). The orange crystalline compounds were
produced in a relatively good yield (50–75%). The chemical struc-
tures of these newly prepared compounds were confirmed by FT-
IR, 1H and 13C NMR spectroscopic techniques as well as elemental
analyses. The spectra obtained were in good agreement with their
chemical structures. The FT-IR, 1H and 13C NMR spectra of C1–C6
are available in the Supplementary Material (Figs. S1–S18).
The main vibrational band frequencies for C1–C6 are listed in
Table 1. The FT-IR spectra of these materials exhibited absorption
bands between 2872 and 2962 cmꢀ1 which were corresponded
to the asymmetric and symmetric CAH stretching vibrations of
the methylene (ACH2A) groups in the alkyl chain. They showed a
sharp stretching peak between 1721 and 1728 cmꢀ1 which was
assigned to the C@O of the ester group. The un-substituted lateral
analogues (C1, C2, and C3) showed medium stretching vibration
bands for the C@C aromatic benzene groups that resonated within
the 1580–1599 cmꢀ1 region. As for the laterally methyl-
substituted analogues (C4, C5, and C6), the characteristic peak of
the C@C aromatic benzene groups appeared as a stretching peak
at the 1590–1594 cmꢀ1 region. Furthermore, all compounds
showed adsorption bands within the 1455–1505 cmꢀ1 region
which were corresponded to the AN@NA linkage. The similar
spectral behaviour of these compounds with slightly shifted vibra-
tional frequencies is due to the influence of terminal substituents
on the second terminal group and lateral methyl substituent at
the 2-position of the azo linkage (ring C).
2.5. UV–vis and fluorescence spectra
UV–vis absorption spectra were obtained in dilute chloroform
solution (5 ꢁ 10ꢀ5 M) with a Cary 60 UV–vis spectrophotometer.
The measurements were recorded from 200 to 600 nm. The corre-
sponding fluorescence spectra were obtained using Photon Tech-
nology International EL-1000 spectrofluorometer. In all the
experiments, samples were measured from 370 to 700 nm in a
1-cm path-length quartz cuvette at room temperature.
2.6. Dielectric measurement
The 1H and 13C NMR spectral data for C1-C6 are shown in Table 2
and Table 3, respectively. They showed the presence of aromatic
protons (Table 2) and aromatic carbons (Table 3) which were
assigned to the benzene moiety. The appearance of aliphatic pro-
tons (Table 2) and aliphatic carbons (Table 3) is further proof to
confirm the structure of the compounds. Additionally, for laterally
methyl-substituted compounds i.e. C4, C5, and C6, the peak due to
lateral methyl proton was observed around 2.7 ppm (Table 2),
while that of lateral methyl carbon appeared at 17.9 ppm (Table 3).
In general, all the ratios of the integrals corresponding to the ali-
phatic or aromatic protons/carbons agree well with the proposed
structures. Nonetheless, similar chemical shifts were observed for
protons and carbons that are close to the terminal substituents
and lateral methyl substituent at the 2-position of the azo linkage
(ring C).
The dielectric measurements were performed using a planar
commercial liquid crystal cell (Instec Inc. USA) with a cell gap of
9
l
m and an electrode area of 10 ꢁ 10 mm2. The cells were filled
by capillary action at a temperature above the liquid crystal–iso-
tropic transition temperature of the investigated compounds. The
dielectric measurements were conducted using an Agilent 4294
impedance analyzer. The dielectric response was measured in fre-
quency sweeps from 40 Hz to 15 MHz, obtained at isothermal
steps, on heating and cooling from 30 to 160 °C with an interval
of 5 °C. The results are expressed in terms of the complex permit-
tivity, eꢂ
:
eꢂ
¼
e0 ꢀ je00
ð1Þ
where e0 and e00 are the real and imaginary dielectric permittivity,
respectively and jis the imaginary unit.
3.2. Thermal behavior
2.7. Polarization measurement
The thermal stability of all azo-ester derivatives C1–C6 have
been investigated by TGA within a temperature range of 50–
900 °C under nitrogen atmosphere. Typical TGA curves and their
corresponding derivatives (DTG) are shown in Fig. 1. Table 4 sum-
marizes the TGA data of compounds C1–C6 which includes the
temperature at 5% of weight loss (Td, 5%) and the temperature of
The switching experiments were carried out by measuring the
voltage drop across a resistor (10 kX) connected in series with
the sample cell using an oscilloscope (Tektronix TDS2024B, USA)
by applying triangle voltage from a signal generator (FG110, Yoko-
gawa, Japan) and the signal was amplified by a high-speed bipolar
amplifier (HSA4051, NF Corporation, Japan). The samples were pre-
pared in the liquid crystal cell as specified for the dielectric mea-
maximum decomposition rate (Tmax). Taking Td,
as criteria for
5%
stability, the Td,
value for all the compounds were shown to be
5%
4

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
Table 1
FT-IR spectral data of C1–C6.
Compound
X
R
FT-IR (t )
, cmꢀ1
ACAH
C@O
C@C
AN@NA
C1
C2
C3
C4
C5
C6
H
H
H
CH3
CH3
CH3
H
2962, 2933, 2875
2962, 2924, 2875
2960, 2932, 2872
2961, 2929, 2874
2961, 2933, 2875
2962, 2939, 2873
1724
1727
1722
1721
1728
1723
1599, 1581
1599, 1582
1599, 1580
1590
1592
1594
1489, 1465
1499, 1466
1501, 1465
1488, 1456
1488, 1463
1505, 1455
OCH3
OC4H9
H
OCH3
OC4H9
Table 3
13C NMR spectral data of C1–C6.
Compound 13C NMR (100 MHz: CDCl3, d, ppm)
C1
C2
C3
164.8 (–C@O), 162.7, 155.8, 150.9, 146.9, 131.2, 130.5, 129.5,
125.9,125.3, 122.5, 121.7, 114.9 (Ar–C), 73.3 (Ar–OCH2–), 34.7
(–OCH2–CH), 26.1 (–CH–CH2), 16.5 (–CH–CH3), 11.3 (CH–CH2–
Table 2
1H NMR spectral data of C1–C6.
Compound 1H NMR (400 MHz: CDCl3, d, ppm, J in Hz)
CH3).
C1
C2
C3
8.27–8.25 (d, 2H, J = 8.7 Hz, Ar–H), 7.90–7.88 (dd, 4H, J = 2.4,
11.5 Hz, Ar–H), 7.39–7.35 (t, 2H, J = 7.8 Hz, Ar-H), 7.23–7.19 (t,
1H, J = 7.8 Hz, Ar–H), 7.18–7.16 (d, 2H, J = 8.7 Hz, Ar–H), 6.96–
6.94 (d, 2H, J = 8.7 Hz, Ar–H), 3.86–3.74 (q, 2H, J = 6.0 Hz, Ar–
OCH2–), 1.86–1.81 (m, 1H, OCH2-CH), 1.54–1.50, 1.25–1.21 (m,
2H, CH–CH2 R,S), 0.98–0.97 (d, 3H, J = 6.8 Hz, CH-CH3), 0.92–0.88
(t, 3H, J = 7.4 Hz, CH2-CH3).
165.2 (–C@O), 162.7, 157.4, 155.8, 146.9, 144.4, 131.2, 130.6,
125.3,122.57, 122.53, 114.9, 114.6 (Ar–C), 73.3 (Ar–OCH2–),
55.7 (O–CH3), 34.7 (–OCH2–CH), 26.2 (–CH–CH2), 16.6 (–CH–
CH3), 11.4 (CH–CH2–CH3).
165.2 (–C@O), 162.7, 157.07, 155.8, 146.9, 144.3, 131.2,130.6,
125.3, 122.5, 122.4, 115.2, 114.9 (Ar–C), 73.3 (Ar–OCH2–), 68.2
(Ar–OCH2-CH2), 34.7 (–OCH2–CH), 31.4 (Ar–OCH2-CH2), 26.2 (–
CH–CH2), 19.3 (CH2-CH3), 16.6 (–CH–CH3), 13.9 (CH2-CH3), 11.4
8.34–8.32 (d, 2H, J = 8.7 Hz, Ar–H), 7.99–7.97 (d, 4H, J = 8.7 Hz,
Ar–H), 7.19–7.16 (d, 2H, J = 9.2 Hz, Ar–H), 7.05–7.03 (d, 2H,
J = 8.7 Hz, Ar–H), 6.98–6.96 (d, 2H, J = 8.7 Hz, Ar–H), 3.95–3.83
(q, 2H, J = 6.0 Hz ,Ar–OCH2–), 3.84 (s, 3H, O–CH3), 1.95–1.90 (m,
1H, OCH2-CH), 1.63–1.60, 1.34–1.30 (m, 2H, CH–CH2 R,S), 1.07–
1.06 (d, 3H, J = 6.8 Hz, CH-CH3), 1.01–0.97 (t, 3H, J = 7.1 Hz, CH2-
CH3).
(CH–CH2–CH3).
C4
C5
C6
164.8 (–C@O), 162.7, 156.1, 150.9, 144.9, 142.1, 131.2, 130.2,
129.5, 125.9, 122.6, 121.7, 117.2, 115.9, 113.1 (Ar–C), 73.1 (Ar–
OCH2–), 34.7 (–OCH2–CH), 26.1 (–CH–CH2), 17.9 (Ar-C-CH3),
16.5 (–CH–CH3), 11.3 (CH–CH2–CH3).
165.2 (–C@O), 162.7, 157.4, 156.1, 144.9, 144.5, 142.2, 131.2,
130.4, 122.7, 122.5, 117.3, 116.0, 114.6, 113.2 (Ar–C), 73.2 (Ar–
OCH2–), 55.7 (O–CH3), 34.8 (–OCH2–CH), 26.2 (–CH–CH2), 17.9
8.35–8.32 (d, 2H, J = 9 Hz, Ar–H), 7.99–7.97 (d, 4H, J = 8.7 Hz, Ar–
H), 7.17–7.15 (d, 2H, J = 8.7 Hz, Ar–H), 7.05–7.03 (d, 2H,
J = 8.7 Hz, Ar–H), 6.97–6.95 (d, 2H, J = 8.7 Hz, Ar–H), 4.00–3.97
(t, 2H, J = 7.1 Hz, Ar–OCH2-CH2), 3.95–3.83 (q, 2H, J = 6.0 Hz, O–
CH2), 1.96–1.90 (m, 1H, OCH2-CH), 1.81–1.77 (m, 2H, CH2-CH2),
1.65–1.58, 1.36–1.30 (m, 2H, CH–CH2 R,S), 1.55–1.50 (m, 2H,
CH2-CH3), 1.08–1.06 (d, 3H, J = 8.7 Hz, –CH3), 1.01–1.0 (t, 3H,
J = 7.1 Hz,–CH3), 0.99–0.98 (t, 3H, J = 7.1 Hz,–CH3).
(Ar-C-CH3), 16.6 (–CH–CH3), 11.4 (CH–CH2–CH3).
165.2 (–C@O), 162.7, 157.0, 156.1, 145.0, 144.3, 142.2, 131.2,
130.4, 122.7, 122.5, 117.3, 116.0, 115.2, 113.2 (Ar–C), 73.2 (Ar–
OCH2–), 68.2 (Ar–OCH2-CH2), 34.8 (–OCH2–CH), 31.4 (Ar–OCH2-
CH2), 26.2 (–CH–CH2), 19.3 (CH2-CH3), 17.9 (Ar-C-CH3), 16.6 (–
CH–CH3), 13.9 (CH2-CH3), 11.4 (CH–CH2–CH3).
C4
C5
C6
8.26–8.24 (d, 2H, J = 8.7 Hz, Ar–H), 7.90–7.88 (d, 2H, J = 8.7 Hz,
Ar–H), 7.72–7.70 (d, 1H, J = 8.7 Hz, Ar–H), 7.40–7.36 (t, 2H,
J = 7.1 Hz, Ar–H), 7.24–7.20 (t, 1H, J = 7.8 Hz, Ar–H), 7.18–7.16
(d, 2H, J = 7 Hz, Ar–H), 6.79 (d, 1H, J = 2.3 Hz, Ar–H), 6.73–6.72
(dd, 1H, J = 2.4, 11.5 Hz, Ar-H), 3.84–3.72 (q, 2H, J = 6.0 Hz, Ar–
OCH2–), 2.68 (s, 3H, Ar-CH3), 1.85–1.80 (m, 1H, OCH2-CH), 1.57–
1.51, 1.24–1.19 (m, 2H, CH–CH2 R,S), 0.98–0.96 (d, 3H, J = 6.4 Hz,
CH-CH3), 0.92–0.88 (t, 3H, J = 7.8 Hz, CH2-CH3).
above 335 °C which indicate that the thermal stability of the syn-
thesized compounds is relatively high.
From the thermograms (Fig. 1), compounds C1–C6 showed
single-staged thermal decomposition. However, the azo-esters
without lateral methyl substitution (C1–C3) exhibited a slightly
different weight loss trend than that of with lateral methyl group
at position-2 with respect to the azo linkage (C4–C6). The un-
substituted derivative C1 gave the highest weight loss of 97%
around 270–450 °C (Tmax = 413 °C) with the remaining residue
completely decomposed around 500 °C. With the presence of
alkoxy substituent on the second terminal group i.e. OCH3 and
OC4H9 for C2 and C3, respectively, a weight loss of 75% was
observed around 260–456 °C (Tmax ~ 435 °C) for both compounds.
The decomposition is suggested due to the thermal cleavage of the
azo group [27,28] and aliphatic chains [29] located in the mesogen.
Furthermore, compounds C2 and C3 showed to exhibit a steady
weight loss of 23% until 900 °C due to the thermal degradation of
the heat resistant aromatic moiety [30,31] and the carbonyl group
[10]. As for the laterally methyl-substituted analogues, C4, C5, and
C6, a rapid weight loss of 61–68% occurred around 260–450 °C
with Tmax at 412, 424, and 413 °C, respectively. This result may
be attributed to the thermal break down of the azo group and ali-
phatic chains as observed in the un-substituted analogues. Subse-
quently, the remaining weight loss was taken place gradually until
8.34–8.32 (d, 2H, J = 8.7 Hz, Ar–H), 7.98–7.96 (d, 2H, J = 8.7 Hz,
Ar–H), 7.80–7.78 (d, 1H, J = 8.7 Hz, Ar–H), 7.19–7.16 (d, 2H,
J = 9.2 Hz, Ar–H), 6.98–6.96 (d, 2H, J = 8.7 Hz, Ar–H), 6.87 (d, 1H,
J = 2.4 Hz, Ar–H), 6.83–6.81 (dd, 1H, J = 2.4, 11.5 Hz, Ar–H), 3.93–
3.81 (q, 2H, J = 6.0 Hz, Ar–OCH2–), 3.85 (s, 3H, O–CH3), 2.77 (s,
3H, Ar-CH3), 1.94–1.88 (m, 1H, OCH2-CH), 1.64–1.60, 1.33–1.27
(m, 2H, CH–CH2 R,S), 1.07–1.05 (d, 3H, J = 6.4 Hz, –CH3), 0.99–
0.97 (t, 3H, J = 7.1 Hz, –CH3).
8.34–8.31 (d, 2H, J = 8.7 Hz, Ar–H), 7.98–7.96 (d, 2H, J = 8.7 Hz,
Ar–H), 7.81–7.78 (d, 1H, J = 8.7 Hz, Ar–H), 7.17–7.15 (d, 2H,
J = 8.7 Hz, Ar–H), 6.97–6.95 (d, 2H, J = 8.7 Hz, Ar–H), 6.88–6.87
(d, 1H, J = 2.4 Hz, Ar–H), 6.84–6.81 (dd, 1H, J = 2.4, 11.5 Hz, Ar–
H), 4.01–3.97 (t, 2H, J = 7.1 Hz, Ar–OCH2–), 3.93–3.81 (q, 2H,
J = 6.0 Hz, Ar–OCH2), 2.77 (s, 3H, Ar–CH3), 1.94–1.91 (m, 1H,
OCH2-CH), 1.83–1.76 (m, 2H, OCH2-CH2), 1.62–1.55, 1.34–1.28
(m, 2H, CH–CH2 R,S), 1.54–1.49 (m, 2H, CH2-CH2), 1.07–1.05 (d,
3H, J = 6.4 Hz, CH2-CH3), 1.01 (t, 3H, J = 7.1 Hz, –CH3), 0.97 (t, 3H,
J = 7.1 Hz, –CH3).
5

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
Fig. 1. (a) TGA and (b) DTG curves of C1–C6 at a heating rate of 20 °C minꢀ1 under nitrogen atmosphere.
Table 4
TGA data of C1–C6.
Compound
X
R
Td,
(°C)
Tmax (°C)
Weight loss (%)
Char yield
5%
at 850 °C (%)
C1
C2
C3
C4
C5
C6
H
H
H
CH3
CH3
CH3
H
338
351
373
345
377
349
413
437
435
412
424
413
97
75
75
63
61
68
0
4
5
3
8
2
OCH3
OC4H9
H
OCH3
OC4H9
900 °C due to the thermal cleavage of aromatic moiety which
include those containing lateral methyl substituent as well as
degradation of carbonyl group. Fig. 1 also shows that at 850 °C,
the decomposition processes was completed for un-substituted
compound C1 but not for the other derivatives C2, C3, C4, C5,
and C6 where the remaining residues are 4, 5, 3, 8, 2%, respectively.
further reduces the molecular length-to-width ratio, leading to a
pronounced decrease in the melting temperature in comparison
with that of the laterally un-substituted derivative C1 (see Table 5).
The replacement of terminal hydrogen on the phenyl ester moi-
ety by the electron-donating methoxy or butoxy groups in both the
laterally-neat (C1) and the laterally methyl-substituted (C4)
derivatives significantly increases the molecular length-to-width
ratio to give mesomorphic behavior in C2, C3, C5, and C6 [32,33].
During the second DSC heating scan, C2, C3, and C6 showed typical
two endothermic peaks (Fig. 2(b)), representing crystal to meso-
phase transition (melting) and mesophase to isotropic liquid tran-
sition (clearing). With respect to compound C5, during the second
heating cycle, C5 exhibited a recrystallization peak around 30 °C
followed by a step-like peak at ~60 °C, denoting a glass transition.
Upon further heating, melting and clearing transitions occurred at
105 and 160 °C, respectively. On cooling, two exothermic peaks
(Fig. 2(b)) were observed for C2, C3, and C6 which are attributed
to the isotropic to mesophase and mesophase to crystal transitions.
The absence of mesophase to crystal peak for C5 indicates that this
phase transition may happen at a much lower temperature region.
Additionally, Fig. 2(b) shows that C2, C3, C5, and C6 exhibited an
enantiotropic liquid crystal phase with marble or schlieren tex-
tures of nematic phase identified via POM on a surface-free sample
(Fig. 3).
As can be seen from Table 5, independent of the absence or
presence of the lateral methyl group, the effect of increasing alkoxy
chain length significantly affects the mesomorphic stability. The
melting temperature increases with the increase of the terminal
alkoxy-chain length from methoxy (e.g. 124 °C for C2) to butoxy
(e.g. 135 °C for C3) due to the higher van der Waals association
between the hydrophobic tails with an increase in the alkyl chain
length. This argument is valid for shorter alkoxy chains
(methoxy- and butoxy-chain), like in our case. However, such a
trend was not apparent for those with long alkoxy chains (n ꢃ 6)
as observed in other molecular cores [24,34,35]. This could be
explained on the basis that all the members of a homologous series
(n ꢃ 6) showed to possess the same dipole moments, irrespective
3.3. Mesomorphic behavior
The effect of introducing a lateral methyl group into the termi-
nal phenyl ring ortho with respect to the azo linkage and the differ-
ent alkoxy groups (OCH3 and OC4H9) including the un-substituted
homologues on the mesomorphic behavior of the azobenzene
derivatives was investigated using the DSC, POM, and SWAXS tech-
niques. Their phase transition temperatures and enthalpies were
determined by DSC and the texture of the mesophases was
observed by POM. Their mesomorphic properties were determined
from the second heating/cooling cycles to avoid the thermal his-
tory of compounds, and the results are summarized in Table 5.
The SWAXS measurements at selected temperatures were applied
to confirm the thermotropic behavior of these azo-ester
compounds.
Typical DSC thermograms of C1–C6 during the second heating
and cooling scans are given in Fig. 2. The DSC curves exhibited sim-
ilar behaviors when the samples were subjected to repeated heat-
ing and cooling cycles since the compounds synthesized in the
present work displayed good thermal stability in the mesophases
domains. Fig. 2(a) shows that on heating, compounds C1 and C4
only exhibited one endothermal transition peak which is attribu-
ted to the melting phase transition from solid into a liquid, without
the trace of phase transition into liquid crystalline phase. Upon
cooling, the single exothermal peak indicates the direct transition
from the isotropic liquid to the solid phase for C1 and C4, respec-
tively. The absence of the alkoxy group in C1 and C4 reduces the
molecular length-to-width ratio which consequently diminishes
their liquid crystalline phase [32,33]. On the other hand, the pres-
ence of a lateral methyl group in C4 widens the molecular core and
6

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
Table 5
Phase transition temperatures and enthalpy changes of C1–C6 upon the second heating and cooling runs at a scan rate of 10 °C minꢀ1 by DSC. The phase identity was determined
from POM texture. The mesophase range was obtained for the heating scan. The symbol ‘‘–” indicates the mesophase range does not exist.
Compound
X
R
Scan
Phase transitions temperature (°C)
Mesophase range (°C)
[Enthalpy changes (kJ molꢀ1)]
C1
C2
C3
C4
C5
C6
H
H
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Cr 128 [29.7] I
I 119[ꢀ31.1] Cr
–
H
OCH3
OC4H9
H
Cr 124 [35.5] N 206 [0.8] I
I 205 [ꢀ0.9] N 93 [ꢀ32.3] Cr
Cr 135 [36.9] N 207 [1.5] I
I 205 [ꢀ1.4] N 110 [ꢀ35.4] Cr
Cr 105 [33.7] I
82
72
–
H
CH3
CH3
CH3
I 72 [ꢀ29.3] Cr
OCH3
OC4H9
Cra 105 [36.2] N 160 [0.6] I
I 159 [ꢀ0.8] N
55
53
Cr 107 [26.5] N 160 [1.0] I
I 159 [ꢀ0.9] N 49 [ꢀ22.6] Cr
Abbreviations: Cr = crystalline phase; N = nematic phase; I = isotropic phase.
a
Upon heating, recrystallization and glass transition were detected at ~30 °C and ~60 °C, respectively.
of the alkoxy chain length [24,34]. Conversely, there is no consid-
erable change in the clearing point with the increase of the termi-
nal alkoxy chain length in both laterally-neat and laterally methyl-
substituted analogues. This is possibly due to the shorter alkoxy
chains used in the present work. Nonetheless, it has been reported
that the nematic-to-isotropic transition temperatures decrease
with increasing alkoxy-chain length due to the dilution of the
interaction between the mesogenic units [24,34,36]. For each set
of laterally-neat and laterally methyl-substituted analogues, those
of terminal methoxy-substituted derivatives (C2 and C5) have a
relatively wider nematic temperature range than that of their ter-
minal butoxy-substituted analogues (C3 and C6) as shown in
Table 5.
To investigate the effect of introducing the lateral methyl group
at the 2-position to the azo phenyl ring on their mesophase behav-
ior, a comparison was made between the liquid crystalline proper-
ties of C5 and C6 (with lateral methyl substituent) with those
without lateral methyl substituent i.e. C2 and C3. The attachment
of lateral methyl substituent to the molecular core is anticipated
to give a steric hindrance combined with a weak positive inductive
effect which influences the mesomorphic behavior of the laterally
substituted azo-ester compounds [37]. Again, from Table 5, the
incorporation of the lateral methyl substituent into the mesogenic
core of C5 and C6 considerably reduces the melting and clearing
temperatures than those without lateral methyl groups (C2 and
C3). This could be related to its steric effect; the introduction of lat-
eral methyl group widens the molecular core effectively and subse-
quently increases intermolecular separation, leading to the
reduction in the lateral associations of the rod-shape molecules
and, consequently, the mesophase stability is reduced [21,34]. Sim-
ilar behavior in the case of the laterally methyl-substituted com-
pounds with different molecular cores has already been reported
[18,19,22,35].
SWAXS measurements were used to confirm the thermotropic
behavior of C2, C3, C5, and C6. Initially, we measured the scattering
pattern at 25 °C, where the spectra showed a few reflections in the
small-angle region and multiple peaks in the wide-angle region,
typical for the crystalline state (Fig. 4(a)). The sample was then
heated to their respective isotropic temperature followed by cool-
ing to room temperature. Subsequently, the scattering profiles of
these azo-ester compounds were obtained at selected tempera-
tures upon the second heating and are illustrated in Fig. 4(b). All
the scattering patterns exhibited only a diffuse scattering of X-
rays at small- and wide-angles. Apart from the diffuse peak at
~1.6 nm (q ~ 4 nmꢀ1), there are no further peaks in the small-
angle region up to the maximum q-range we can study i.e.
Fig. 2. DSC thermograms of azo-ester compounds at the scanning rate of 10 °C
minꢀ1 for compounds (a) C1 and C4 and (b) C2, C3, C5, and C6.
7

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
Fig. 3. POM textures of azo-ester compounds upon cooling from the isotropic liquid using non-treated microscopic glass slides: (a) C2 marble texture at 200 °C. The inset
shows the nematic droplets separating from the isotropic phase at 206 °C; (b) C3 schlieren texture at 203 °C; (c) C5 schlieren texture at 155 °C; (d) C6 schlieren texture at
157 °C. All the microphotographs were taken with a magnification of 20ꢁ.
~0.5 nmꢀ1. The lack of Bragg scattering in SWAXS experiments con-
firms the nematic phase [38] for compounds C2, C3, C5, and C6.
250–270 nm, (ii) a high-energy absorption band around 300–
425 nm and (iii) a low-energy absorption band around 450–
500 nm. The shoulder-like absorption band appeared at the shorter
wavelength region arise from the HOMO ? LUMO transition and is
3.4. Optical properties
assigned as
whereas the high-energy absorption band at longer wavelength
region is attributed to a symmetry-allowed * transition involv-
ing the -electronic system throughout the whole molecule with
the characteristic for a large charge transfer character [18]. It is
noticed that the laterally-methyl substituted derivatives C5 and
C6 has a more pronounced shoulder-like absorption band at the
shorter wavelength region and are slightly red-shifted by 7 nm
p–p* transition within the phenyl rings [39,40],
The UV–vis absorption spectra of the mesogenic compounds C2,
C3, C5, and C6 in dilute chloroform solution of 5 ꢁ 10–5 M were
measured, and their absorption band are shown in Fig. 5(a). In gen-
eral, the absorption spectra of the compounds are very similar in
shape because of their structural similarity in the mesogenic unit
and each compound showed three absorption bands in their
p–p
p
UV–vis spectra: (i)
a shoulder-like absorption band around
Fig. 4. SWAXS pattern of compounds C2, C3, C5, and C6: (a) at 25 °C showing that it is in its crystalline state before the sample was heated to its isotropic state and (b) at
selected temperatures showing that it is in its nematic phase (obtained on the second heating scan).
8

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
Fig. 5. Compounds C2, C3, C5, and C6 in dilute chloroform solution of 5 ꢁ 10ꢀ5 M: (a) UV–vis absorption spectra and (b) fluorescence spectra.
for the strongest absorption bands, compared with that of the
laterally-neat analogues C2 and C3. Lastly, the small shoulder
to the DSC results upon heating (Fig. 2). The C5 compound showed
two distinct broad peaks at ~52 and ~102 °C while C6 displayed a
single broad peak at ~102 °C; indicating the phase transitions in
the samples parallel to the DSC results. Since there are molecular
arrangement changes at the phase transition, the changes in the
dielectric permittivity’s depend on the phases involved. The first
transition that is observed for C5 is due to the glass transition with
decreasing molecular order while the phase transition observed at
102 °C represents the melting of the crystal structure to a liquid
crystal phase (nematic). The latter is also observed for C6 at a sim-
ilar temperature. The transition temperatures are comparable and
in agreement with the temperature obtained from DSC measure-
ments and within the experimental error. Moreover, the dielectric
permittivity, e0 of both compounds in the crystal phase is below ~3.
This indicates that even though both compounds have similar
dipole moments, they are not influenced in terms of orientational
polarization in the solid-phase state which is not the case for the
nematic phase.
Fig. 7 shows the frequency response of C5 and C6 at 52 °C (crys-
tal phase) and 120 °C (nematic phase), respectively. The dielectric
spectra of both compounds showed almost a similar trend for the
nematic phase. On the other hand, the dielectric spectra of the
crystal phase are quite distinct for both samples with a clear relax-
ation is observed at 10 kHz for C5, while C6 showed relaxation at a
lower frequency <100 Hz. The only difference between the two
compounds is the terminal alkoxy-chain length, which is either
methoxy for C5 or butoxy for C6. Therefore, the relaxation is prob-
ably related to the distinct methoxy and butoxy groups motion.
Since the C6 compound contains a longer butoxy group chain com-
pared to C5, thus, rendering a slower dynamic than the C5.
appeared around 470 nm is corresponding to
forbidden n- * transition [40,41].
a symmetry-
p
The corresponding fluorescence spectra of compounds C2, C3
C5, and C6 are depicted in Fig. 5(b). Excitation at 365 nm gave a
broad unstructured blue emission band centred around 440 nm.
As observed in UV–vis spectra, their emission spectra were very
similar in shape because of the structural similarities in the meso-
genic unit. However, the laterally substituted compounds C5 and
C6 have a relatively higher fluorescence intensity compared with
that of the laterally neat analogues C2 and C3. Most of the azo com-
pounds are reported to exhibit a reversible trans-cis photoisomer-
ization process upon absorption of a photon within the absorption
band [37,42]. Nevertheless, suppression of the photoisomerization
movement in the azo compounds due to the conformational strain
or steric hindrance is considered as a key to providing them with a
fluorescent nature [43,44]. Hence, the presence of the lateral
methyl group ortho to the azo linkage in compounds C5 and C6
may quench or suppress their photoisomerization process which
leads to higher fluorescence intensity compared with that of the
lateral un-substituted analogues C2 and C3.
3.5. Dielectric properties
Among the other mesogenic compounds investigated, laterally
methyl-substituted derivatives i.e. C5 and C6 were chosen to be
loaded into liquid crystal cells which were treated with rubbed
alignment layers, giving planar oriented nematic phase. They were
selected because of their relatively lower clearing temperature val-
ues. The aligned C5 and C6 samples were subjected to the fre-
quency and temperature-dependent dielectric study using
impedance spectroscopy. The complex dielectric permittivity of
the materials shows a significant structural change at the phase
transition and it is illustrated in Fig. 6(a) and (b) for the C5 and
C6, respectively. The frequency spectra were recorded at a series
of increasing temperatures from 25 to 165 °C. The dielectric per-
mittivity increases with temperature due to the temperature-
driven mobility of the C5 and C6 molecules. The temperature plot
of the dielectric spectra shows a frequency-dependent behavior
where at the lower frequencies (100 Hz and 1 kHz) the dielectric
permittivity is much larger compared to those at higher frequen-
cies (10 and 100 kHz). The low-frequency spectra show a clear
phase transition behavior compared to the higher frequencies.
The high-frequency dielectric dispersion is generally referred to
as the reorientation of the molecules about their long axis [45].
The complex permittivity obtained for both samples corresponded
3.6. Polarization measurement
To investigate the polar properties of the nematic phase, the
electro-optical behavior of the aligned C5 and C6, as the represen-
tative case, has been investigated in the temperature range of
nematic mesophase i.e. 120 °C. Their electro-optical current
response trace, obtained under a triangular wave field (E = 16.7 V
/lm; f = 0.5 Hz) in the nematic phase, is shown in Fig. 8(a). A
prominent broad current peak per half cycle of the applied triangu-
lar voltage is observed for both compounds and indicated in black
arrows (see Fig. 8(a)). The broad peak is probably indicating the
overlapping of two peaks from different origin and implies antifer-
roelectric ordering. Fig. 8(b) and (c) show the current response at
various temperature ranges. It is observed that the peak is not dis-
cernable at the crystal phase (see Fig. 8(c)). The results of quantita-
tive analysis obtained by integrating the area under the current
9

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
Fig. 6. Temperature dependence of the complex permittivity, eꢂ of (a) C5 and (b) C6 at various frequencies. The top and bottom plots refer to imaginary dielectric permittivity,
e0 0 (top) and real dielectric permittivity, e0 (bottom).
Fig. 7. Frequency dependence of the complex permittivity, eꢂ of C5 and C6 in the crystal phase (solid lines) and the nematic phase (dashed lines): (a) real dielectric
permittivity, e0 and (b) imaginary dielectric permittivity, e00
.
response peak are proportional to the spontaneous polarization, Ps
and its temperature dependence is shown in Fig. 8(d). Previously, it
was reported that spontaneous polarization, Ps in the nematic
phase of the calamitic compound is attributed to the switching of
the cybotactic clusters [46]. The Ps decreases from the highest
ture with the applied electric field is observed as expected for the
crystal phase (see Fig. S19 in the Supplementary Material).
4. Conclusions
value in the nematic phase (~5
l
C/cm2) to saturated value (0.05
l
C/cm2) in the crystal phase indicating that a smaller number of
In this work, we have synthesized and characterized azo-ester-
based compounds composed of a three-benzene-ring molecular
core having a 2-methylbutoxy unit in one of the terminals and dif-
ferent alkoxy groups as other side terminal chain. The effect of lat-
eral methyl substitution at the 2-position to the azo linkage and
the different alkoxy groups at the terminal chain on the mesomor-
phic properties were investigated to understand the structure-
property relationship. The TGA analysis indicated that the thermal
stability of the synthesized compounds is relatively high i.e. above
330 °C. It is found that, except for the un-substituted terminal
group, all the azo-ester compounds are liquid crystalline, exhibit-
ing enantiotropic nematic mesophase. The change of the alkoxy
chain length at the terminal group for each set of laterally-neat
and laterally methyl-substituted mesogens decreases the nematic
temperature range. Similarly, the introduction of lateral methyl
substituent into the mesogenic core causing a decrease in the
clusters participated in the switching mechanism at the lower tem-
perature. In the crystal phase, the compounds are less viscous com-
pared to the nematic phase which acts as fluid-like behavior. The
voltage variation (not shown here) shows a typical enhancement
of Ps upon increasing applied voltage confirming the polar origin
of the current peaks.
By textural investigation of the switching process for the
aligned C5 and C6 samples, the difference between the texture of
the nematic phase at zero field (Eoff) as well as in the application
of electric field (Eon) is represented in Fig. 9. A uniform nematic
alignment with the optical axis oriented along the rubbing direc-
tion was seen at zero field for C5 and C6 (see Fig. 9(a) and (c)).
When an electric field was applied, striped filaments arranged
along the rubbing direction were observed clearly for both com-
pounds as shown in Fig. 9(b) and (d). No visible change in the tex-
10

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
Fig. 8. Investigation of current response applying triangular wave voltage 150 VPP at 0.5 Hz for the planar aligned samples of (a) C5 and C6 at 120 °C; (b) C5 at different
temperatures and (c) C5 at crystal phase (50–90 °C). (d) The temperature dependence of spontaneous polarization, Ps for C5 and C6.
Fig. 9. The textural changes of nematic phase (T = 120 °C) of the planar aligned samples between crossed polarizers: (a) C5 at zero field; (b) C5 with an electric field; (c) C6 at
zero field and (d) C6 with an electric field. The electric field applied is 16.7 V/
l
m at a frequency of 0.5 Hz. The arrow in the figure refers to the surface rubbing direction. All the
microphotographs were taken with a magnification of 10ꢁ.
11

S.J. Kamal, N.M. Salleh, H.N.M.E. Mahmud et al.
Journal of Molecular Liquids 336 (2021) 116308
[10] M.R. Karim, R. Yahya, M.R.K. Sheikh, N.M. Salleh, A. Hassan, H.N.M. Ekramul
Mahmud, Synthesis, thermal stability, optical and electrochemical properties
of halogen terminated azo-benzothiazole mesogen containing smectic side
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thermal stability of the nematic mesophase compared to the neat
lateral analogues. No significant changes in the UV–vis absorption
maxima were observed due to the structural similarities in the
mesogenic unit. All the mesogenic compounds exhibited blue-
emission with the laterally substituted compounds have a rela-
tively higher fluorescence intensity compared to that of the un-
substituted lateral analogues. Additionally, the dielectric and
electro-optical properties of laterally methyl-substituted deriva-
tives under planar alignment were investigated. The dielectric
study at low-frequency spectra shows a clear phase transition
behavior compared to the higher frequencies. From the electro-
optic measurements, the nematic phase of the aligned laterally
methyl-substituted derivatives shows antiferroelectric switching
behaviour, a suitable feature for use in passive matrix displays.
CRediT authorship contribution statement
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Salwa J. Kamal: Investigation, Writing - original draft. Noordini
M. Salleh: Conceptualization, Methodology, Supervision. H.N.M.
Ekramul Mahmud: Supervision. Iskandar Abdullah: Methodol-
ogy. Sanjeev R. Gopal: Investigation. Thamil Selvi Velayutham:
Writing - review & editing. N. Idayu Zahid: Visualization, Supervi-
sion, Writing - review & editing.
Declaration of Competing Interest
[21] N.H.S. Ahmed, G.R. Saad, M.M. Naoum, Effect of position of the lateral fluoro
substituent on the mesophase behaviour of aryl 4-alkoxyphenylazo benzoates
in pure and binary mixtures, Liq. Cryst. 45 (2018) 1487–1497.
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substitution on the mesophase behaviour of aryl 4-alkoxyphenylazo
benzoates, Liq. Cryst. 42 (2015) 1627–1637.
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.
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Acknowledgement
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The authors gratefully acknowledge financial support from the
University of Malaya Research University Grant–Faculty Program
(GPF060B-2018) and SATU Grant (ST001-2020). In appreciation
of their sponsorship and support, Salwa J. Kamal wishes to thank
Imam Abdulrahman Bin Faisal University and Saudi Arabian Cul-
tural Mission.
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Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molliq.2021.116308.
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