Synthesis and determination of thermotropic liquid crystalline behavior of cinnamaldehyde-based molecules with two schiff base linking units

Article abstract of DOI:10.3390/molecules25173780

A series of liquid crystal molecules with two Schiff base linking units and a cinnamaldehyde core with different terminal groups were synthesized and characterized. The intermediates of 4-heptyloxybenzaldehyde (1a) and 4-dodeyloxybenzaldehyde (1b) were synthesized through the alkylation of 4-hydroxybenzaldehyde with a series of bromoalkane. A condensation reaction of cinnamaldehyde, 1,4-phenylenediamine and a series of substituted benzaldehydes with different terminal groups such as bromo, chloro, hydroxy, cinnamaldehyde, hydrogen, methoxy, heptyloxy and dodecyloxy produced a series of new cinnamaldehyde-based compounds, 2-9, respectively. All these compounds were characterized using Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and CHN elemental analysis. The liquid crystal properties of these compounds were determined using polarized optical microscopy (POM), and their transitions were further confirmed using differential scanning calorimetry (DSC). Compounds with chloro, bromo, methoxy, heptyloxy, and dodecyloxy substituents are mesogenic compounds with nematic phase behavior. However, the other compounds were found to be non-mesogenic without any mesophase transitions. The structure-property relationship was investigated in order to study the effect of different terminal groups and Schiff base linking units on the liquid crystalline behavior of these compounds.

Full text of DOI:10.3390/molecules25173780

molecules
Article
Synthesis and Determination of Thermotropic Liquid
Crystalline Behavior of Cinnamaldehyde-Based
Molecules with Two Schiff Base Linking Units
Zuhair Jamain 1,2,
* *
, Nor Faizawani Omar 2 and Melati Khairuddean 2,
1
Faculty of Science and Natural Resources, Universiti Malaysia Sabah (UMS), Kota Kinabalu 88400, Malaysia
School of Chemical Sciences, Universiti Sains Malaysia (USM), Penang 11800, Malaysia;
fae_zawani@yahoo.com
2
*
Correspondence: zuhairjamain@ums.edu.my (Z.J.); melati@usm.my (M.K.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Pradip K. Bhowmik
Received: 9 July 2020; Accepted: 8 August 2020; Published: 20 August 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: A series of liquid crystal molecules with two Schiff base linking units and a cinnamaldehyde
core with different terminal groups were synthesized and characterized. The intermediates of
4-heptyloxybenzaldehyde (1a) and 4-dodeyloxybenzaldehyde (1b) were synthesized through the
alkylation of 4-hydroxybenzaldehyde with a series of bromoalkane. A condensation reaction of
cinnamaldehyde, 1,4-phenylenediamine and a series of substituted benzaldehydes with different
terminal groups such as bromo, chloro, hydroxy, cinnamaldehyde, hydrogen, methoxy, heptyloxy
and dodecyloxy produced a series of new cinnamaldehyde-based compounds, 2–9, respectively.
All these compounds were characterized using Fourier transform infrared (FTIR) spectroscopy,
nuclear magnetic resonance (NMR) spectroscopy, and CHN elemental analysis. The liquid crystal
properties of these compounds were determined using polarized optical microscopy (POM), and their
transitions were further confirmed using differential scanning calorimetry (DSC). Compounds with
chloro, bromo, methoxy, heptyloxy, and dodecyloxy substituents are mesogenic compounds with
nematic phase behavior. However, the other compounds were found to be non-mesogenic without
any mesophase transitions. The structure–property relationship was investigated in order to study
the effect of different terminal groups and Schiff base linking units on the liquid crystalline behavior
of these compounds.
Keywords: Schiff base; cinnamaldehyde; liquid crystal; nematic; mesogenic
1. Introduction
Nowadays, materials forming liquid crystal (LC) phases have found wide application in the
manufacturing of displays [
and molecular sensors and detectors [
synthesis of a wide range of molecules—in particular, thermotropic LC molecules [7,8
1
], spatial light modulators [
]. The strong demand for new LC applications has led to the
]. Thermotropic
2], optical connectors [3], switches [4],
5,6
LC is temperature dependent. Thermotropic LC can be characterized by the various phase transitions
that occur during heating, displaying temperature transitions between the crystal and the liquid
(isotropic) phases. The transition temperature from the crystal to the liquid crystal phase is known as
the melting temperature (Tm), while the temperature when the liquid crystal changes to an isotropic
liquid is known as the clearing temperature (Tc) [9]. Mesophases for thermotropic LC molecules occur
at a certain temperature range. The thermal motion destroys the delicate ordering of the liquid crystal
phase at high temperatures and causes the material to change into the isotropic phase.
Most thermotropic liquid crystals are calamitic molecules which have a rigid core with two or
more phenyl rings and one or more flexible terminal alkyl chains. A calamitic molecule must be
Molecules 2020, 25, 3780; doi:10.3390/molecules25173780
www.mdpi.com/journal/molecules

Molecules 2020, 25, 3780
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geometrically anisotropic, adopting a rod-shaped structure [10]. The shape and the resulting forces of
the molecule give rise to the formation of the liquid crystal phases. A calamitic liquid crystal usually
exhibits the nematic and smectic phases, while the discotic liquid crystal tends to form columnar and
nematic phases. The nematic phase is the least-ordered mesophase and thus resembles the isotropic
liquid state. The molecules have no positional order but possess an orientational order. On the
contrary, molecules in the smectic phase possess both positional and orientational orders due to their
arrangement in layers. Molecules in the smectic A phase have the director parallel to the layer normal,
whereas molecules in the smectic C phase are tilted with respect to the layer normal [11]. The smectic
A phase has a focal-conic fan texture, while the smectic C phase exhibits a broken focal-conic fan
texture which can be observed under POM. A typical calamitic liquid crystal molecule has an idealized
molecular structure, as shown in Figure 1.
R
A
B
C
L
Rʹ
L
R and Rʹ: Terminal units (alkoxy chains, OH, CN, NO2, etc)
A, B, and C: Ring system (phenyl, cyclohexyl, heterocyclic)
L: Linking units (C=N, N=N, C=C, etc)
Figure 1. Schematic representation of common rod-like calamitic molecules.
The linking unit between ring systems increases the length of the molecules, as well as altering
the polarizability and flexibility of the molecules. Linking units can impart polarity or act as non-polar
groups and, hence, increase or decrease polarizability. The Schiff base is one of the most well-known
linking groups used in connecting the rigid core groups. The Schiff base contains a carbon–nitrogen
double bond connected to an alkyl or aryl group but not the hydrogen. Although it provides a stepped
core structure, it maintains molecular linearity, and therefore provides better stability to induce the
formation of the mesophase [12
–14]. Since then, the Schiff base liquid crystals have been studied and
the area has been explored extensively [15
–
17]. A variety of molecules with Schiff base units have been
synthesized and their liquid crystal properties were determined [18,19].
Cinnamaldehyde is an interesting core used in the synthesis of liquid crystal molecules. Together
with a Schiff base unit, the molecule provides a conjugated system. However, not many works on such
conjugation have been documented. Cinnamic acid derivatives are known as mesogenic substances.
The 4-alkoxycinnamic acid compounds were among the first to be reported (Figure 2). Derivatives
with alkoxy chains of n = 1 to 16 were reported to exhibit liquid crystal properties in which the shorter
chains (n < 9) showed nematic phase behavior while the longer chains displayed (n > 9) both smectic
and nematic phase behavior. Interestingly, only the E isomers showed liquid crystal phase behavior
while Z isomers are non-mesogenic [20].
O
OH
RO
Figure 2. Structure of 4-alkoxycinnamic acids.
The synthesis of cinnamylidene-p-octyloxyaniline was reported but it was found to be
non-mesogenic. However, terminal nitro groups help to induce liquid crystal properties. Compounds
A
–
C
(Figure 3) with long flexible chains showed smectic phase behavior. Hydrogen bonding
in compound was investigated and higher enthalpy and melting temperatures were reported.
Compound possessed higher melting and clearing temperatures than compound . Another
interesting observation is the effect of the central spacer length in the molecules on the transition
D
E
A

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temperature. It was reported that longer tetraethylene glycol spacers (compound
H
) possessed lower
melting and clearing temperatures compared with those in compounds
G and F. This also lowered
the thermal transition temperatures [21]. No compounds similar to these have been reported on the
synthesis and properties of liquid crystals.
N
O (CH2)n CH3
O2N
O2N
n = 7 (A), 11 (B), 15 (C)
N
O (CH2)6 OH
(D)
N
N
O (CH2)8
(E)
O
N
O2N
O2N
NO2
O (CH2CH2O)n
n = 3 (F), 4 (G), 5 (H)
N
NO2
Figure 3. Previous study on the nitrocinnamylidene units.
Previous studies focused on the synthesis of a series of thermotropic rod-like liquid crystal
molecules with a cinnamaldehyde and aromatic cores attached by two Schiff base linking units. To date,
there has been much research on thermotropic liquid crystalline compounds. This research focused on
the optical measurements, ion-responsive properties, photoconductivity, and dielectric studies [22–25].
However, there are no studies on cinnamaldehyde-based compounds bearing different terminal groups
at the periphery. The rigidity of the cinnamaldehyde core system is able to increase the thermal
behavior of the molecules. As the thermal properties increase, this molecule can act as a fire retardant
which can withstand high temperature ranges [26]. The effects of terminal groups on liquid crystalline
behavior have also been investigated. Terminal groups that extend the molecular long axis without
increasing the molecular width increase the thermal stability of the nematic phase. The nature of the
terminal substituents or end groups in the mesogenic molecule has a profound influence on the liquid
crystal properties of the compound. Moreover, the synthesized molecules with conjugated Schiff base
units attached to a cinnamaldehyde core system have not been reported. The main interest of this
study is to understand the effect of the skeleton structure of these types of molecules and their ability
to induce the liquid crystal mesophase.
2. Results and Discussion
2.1. Synthesis of Intermediates and Cinnamaldehyde Compounds
The studies involved the alkylation reaction (Scheme 1) of p-hydroxybenzaldehyde with alkyl
bromide in the presence of potassium carbonate and potassium iodide to produce intermediates
1a
–
b. The reaction of 1,4-phenylenediamine and cinnamaldehyde with a series of benzaldehydes
produced eight Schiff base compounds (2–9) with different substituents (Scheme 2). The structures of
these intermediates and compounds were successfully characterized using Fourier transform infrared
(FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and CHN elemental analysis.

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The melting point and yield of compounds
2–9
are illustrated in Table 1. Meanwhile, as summary of
the data on these intermediates and compounds is presented in Section 3.3.
O
H
O
H
K2CO3, KI, DMF
reflux, 12 h
OH
O R
R-Br
+
R= C7H15, 1a; C12H25, 1b
Scheme 1. Alkylation reaction of compounds 1a–b [27].
O
O
X
+
H
+
NH2
H2N
H
MeOH
rt, stir
X
N
N
X =
Br
Cl
OH
H
2
3
4
5
6
OCH3
OC7H15
OC12H25
8
9
7
Scheme 2. Synthesis of Schiff base compounds 2–9 [28–30].
Table 1. Melting point and percentage yield of compounds 2–9.
Melting
Yield
(%)
Compound
Name of Compound
Point (◦C)
2
3
4
5
6
7
8
9
N-(4-bromobenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine
N-(4-chlorobenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine
N-(4-hydroxybenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine
N-(3-phenylallylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine
N-benzylidene-N0-(3-phenyl allylidene)benzene-1,4-diamine
N-(4-methoxybenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine
N-(4-heptyloxybenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine
N-(4-dodecyloxybenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine
208.2–210.4
211.1–213.4
196.8–198.1
190.5–193.3
204.5–206.3
174.8–177.5
166.4–169.8
162.1–165.9
89.1
87.8
81.5
76.9
72.6
74.3
74.9
76.9
2.2. FTIR Spectral Discussion
Intermediates 1a showed similar absorption bands in the IR data. The IR data displayed
–
b
absorption bands at approximately 2860 and 2922 cm−1 which are assigned for the symmetrical
and asymmetrical C-H (sp3) stretching, attributed to the methylene (CH2) and methyl (CH3) groups
of the heptyl and dodecyl chains. A small shoulder at 2701 cm−1 is the C-H stretching of the
aldehyde (H-C=O). Absorptions at 1691, 1600 and 1157 cm−1 are assigned for the C=O, C=C and
C-O stretching, respectively. No broad absorption at 3200–3300 cm−1 for O-H stretching is observed,
which confirmed that the p-hydroxybenzaldehyde used as starting material in the alkylation reaction

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has been completely utilized. FTIR data on p-hydroxybenzaldehyde, 1a, and 1b are provided in the
Supplementary Materials section.
A further reaction of cinnamaldehyde, 1,4-phenylenediamine and a series of p-substituted
benzaldehydes produced compounds 2–9, as shown in Scheme 2. Compounds 2–7 showed similar
absorption bands in the IR data. The absorptions in the region 3100 cm−1 refer to the C-H (sp2)
stretching, which is attributed to the C=C-H in the aromatic ring of the phenyl core, while the
aromatic C=C stretching is at 1610 cm−1. The absorption for C=N stretching is at 1630, indicating
that the formation of Schiff base linking units was a success. Compound
asymmetrical C-H (sp3) stretching at 2878 and 2930 cm−1, whereas the band at 1100 cm−1 was assigned
for C-O stretching. Meanwhile, compounds and showed an additional strong signal for the heptyl
and dodecyl terminal chains, respectively. The IR spectrum of compound will be used as an example
7 showed symmetrical and
8
9
8
(Figure 4). The IR spectrum showed an absorption band at 3080 cm−1, which is assigned for the C-H
(sp2) stretching, attributed to the aromatic and ethylene protons, while the bands at 2880 and 2936 cm−1
are assigned for the symmetrical and asymmetrical C-H (sp3) stretching, attributed to the methylene
(CH2) and methyl (CH3) groups of the heptyl chains. A band at 1637 cm−1 assigned for C=N stretching
confirmed the formation of the Schiff base units in compound 8. Meanwhile, the bands at 1617, 1251,
and 1140 cm−1 were assigned to C=C, C-N, and C-O stretching, respectively. The overall FTIR data on
compounds 2–9 are summarized in Table 2.
Figure 4. FTIR spectrum of compound 8.
Table 2. The FTIR data on compounds 2–9.
Vibrational (cm−1
Compound
)
IR
Linkage
2
3
4
5
6
7
8
9
Csp2-H stretch
3010
3050
3060
3075
3060
3070
3080
3070
2930,
2878
2936,
2880
2936,
2880
Csp3-H stretch
-
-
-
-
-
C=N stretch
C=C stretch
C-N stretch
C-O stretch
1637
1618
1251
-
1640
1618
1251
-
1634
1618
1253
-
1635
1600
1240
-
1637
1617
1248
-
1640
1618
1251
1100
1637
1617
1251
1140
1637
1617
1250
1170

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2.3. NMR Spectral Discussion
Compound
8 was used to represent the structure confirmation in this series. The structure of
compound 8 with complete atomic numbering is shown in Figure 5.
2
3
4
10 11
1
12
14
16
18
23
6
9
7
8
O
5
15
17
19
13
20
N
21 N
22
Figure 5. Structure of compound 8 with complete numbering.
1
The H-NMR spectrum of compound
8
(Figure 6) showed the presence of two azomethine
protons, six doublets for aromatic protons and a methyl proton in the upfield region. The presence of
12 methylene in compound is attributable to the heptyl chains. H7 and H8 appeared as one signal
8
at 7.28 ppm since they experience a similar electron environment. The doublet of doublet signal at
7.13 ppm is assigned to H5 because this proton has the potential to cause double splitting.
Figure 6. 1H-NMR spectrum of compound 8 (500 MHz, DMSO-d6).
The 13C-NMR spectrum (Figure 7) of compound
8 showed the presence of 23 carbons—one methyl,
six methylene, nine aromatic, five quaternary and two azomethine carbons. The DEPT experiment was
then conducted in order to confirm each type of carbon.
Figure 7. 13C-NMR spectrum of compound 8 (125 MHz, DMSO-d6).

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The DEPT-90 spectrum (Figure 8a) can distinguish the methyl carbon signal at
δ
13.1 ppm since this
signal disappeared in the spectrum but it appeared in the DEPT-135 spectrum (Figure 8b). The negative
signals in DEPT-135 are assigned for six methylene carbons in the heptyl chain. The signals assigned as
quaternary carbons at δ 161.3, 148.9, and 135.1 ppm did not appear in the DEPT spectra. The quaternary
carbon attached to the neighboring alkoxy chain is assigned as C19 at δ 161.3 ppm due to the deshielding
effect of the electronegative oxygen.
Figure 8. DEPT spectra of compound 8 in DMSO-d6: (a) DEPT-90; (b) DEPT-135.
Further confirmation of the protons and their correlations with the respected carbons was achieved
using 2D NMR of COSY (1H-1H) and HMQC (1H-13C) spectra, as shown in Figures 9 and 10, respectively.
The COSY (1H-1H) spectrum (Figure 9) revealed the connectivity between H1-H2, H4-H5-H6, H11-H10,
and H12-H13. Further confirmations of 1H-1H connectivity were also observed between the methylene
protons of the heptyl chain in the upfield region. Proton H9 did not give cross peaks, indicating the
presence of azomethine proton. The HMQC (1H-13C) spectrum in Figure 10 confirmed the connectivity
and correlations between protons and its carbon. These correlations are summarized in Table 3.

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Figure 9. The 2D COSY (1H-1H) NMR spectrum of compound 8.
Figure 10. The 2D HMQC (1H-13C) NMR spectrum of compound 8.

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Table 3. 1H-NMR, COSY (1H-1H) and HMQC (1H-13C) of compound 8.
δ (ppm) Multiplicity
Coupling Constant (Hz)
COSY (1H-1H)
(ppm)
Proton
HMQC (1H-13C) (ppm)
H1
H2
H3
H4
H5
H6
H7
H8
H9
H10
H11
H12
H13
H14
H15-16
H17
H18
7.35–7.38 (m)
7.44 (t, J = 7.5)
7.69 (d, J = 7.5)
7.35–7.38 (m)
7.13 (dd, J = 9.0, 15.0)
8.45 (d, J = 9.0)
7.25–7.28 (m)
7.25–7.28 (m)
8.57 (s)
H2
H1, H3
H2
H5
H4, H6
H5
H8
H7
-
C1 (128.3)
C2 (128.6)
C3 (126.5)
C4 (143.1)
C5 (121.2)
C6 (160.4)
C7 (121.2)
C8 (121.2)
C9 (158.2)
C10 (129.8)
C11 (114.3)
C12 (67.4)
C13 (27.9)
C14(24.8)
C15-16 (21.3)
C17 (36.6)
C18 (13.1)
7.87 (d, J = 8.5)
7.05 (d, J = 8.5)
4.08 (t, J = 6.5)
1.75 (t, J = 7.0)
1.44–1.46 (m)
1.31–1.37 (m)
1.31–1.37 (m)
0.89 (t, J = 6.5)
H11
H10
H13
H12, H14
H13, H15
H14, H17
H18, H16
H17
For other homologues, compounds
3–9 showed a slight difference in the chemical shift values
of each compound due to the different chemical environment, electronegativity effect, and bond
angles [31]. Only compounds 7 and 9 showed an additional signal for the methoxy and dodecyloxy
chains in the 1H and 13C NMR spectra, respectively. The overall chemical shift of compounds
summarized in Table 4.
2–9 are
Table 4. The chemical shift (1H and 13C) of compounds 2–9.
Chemical Shift (ppm)
1H
13C
Compound
CH=N (Schiff
C=C-H
(Methylene)
C=N
(Schiff Base)
C=C
(Methylene)
Base)
7.89 (d, J = 9.0 Hz, 2H), 7.74 (d,
J = 7.0 Hz, 2H), 7.68 (d,
149.5, 148.7, 144.1,
135.4, 135.2, 131.8,
130.3, 128.9, 128.4,
127.5, 124.8, 122.0,
121.8
8.67 (s, 1H), 8.47 (d,
J = 9.0 Hz, 1H)
J = 7.5 Hz, 2H), 7.44 (t,
2
3
4
161.4, 158.8
161.4, 158.6
161.1, 160.5
J = 7.0 Hz, 2H), 7.36-7.39 (m,
4H), 7.28 (t, J = 4.5 Hz, 2H),
7.18 (d, J = 5.0 Hz, 1H)
7.97 (d, J = 6.0 Hz, 2H), 7.67 (d,
J = 7.0 Hz, 2H), 7.60 (d,
149.5, 148.7, 144.1,
135.4, 135.2, 131.8,
130.3, 128.9, 128.4,
127.5, 124.8, 122.0,
121.8
8.70 (s, 1H), 8.47 (d,
J = 8.5 Hz, 1H)
J = 7.5 Hz, 2H), 7.44 (t,
J = 7.0 Hz, 2H), 7.36-7.39 (m,
4H), 7.29-7.35 (m, 2H), 7.18
(dd, J = 8.5, 12.0 Hz, 1H)
7.77 (d, J = 9.0 Hz, 2H), 7.66 (d,
J = 7.0 Hz, 2H), 7.44 (t,
162.1, 149.4, 143.9,
135.6, 130.5, 129.5,
128.9, 128.6, 127.5,
121.8, 121.7, 115.7
8.51 (s, 1H), 8.45 (d,
J = 9.0 Hz, 1H)
J = 6.0 Hz, 2H), 7.39 (t,
J = 7.5 Hz, 2H), 7.25 (s, 4H),
7.13 (dd, J = 8.5, 16.0 Hz, 2H),
6.88 (d, J = 6.5 Hz, 1H)

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Table 4. Cont.
Chemical Shift (ppm)
1H
13C
Compound
CH=N (Schiff
C=C-H
(Methylene)
C=N
(Schiff Base)
C=C
(Methylene)
Base)
7.66 (d, J = 7.5 Hz, 2H),
7.35-7.39 (m, 2H), 7.44 (t,
J = 6.0 Hz, 2H), 7.25 (s, 4H),
7.16 (dd, J = 9.0, 16.0 Hz, 1H)
8.45 (d, J = 8.5 Hz,
1H), 7.66 (d,
J = 7.5 Hz, 2H)
149.8, 143.3, 135.2,
129.0, 128.5, 128.2,
127.0, 121.3
5
6
160.6
7.96 (d, J = 7.0 Hz, 2H), 7.66 (d,
J = 6.5 Hz, 2H), 7.53 (d,
J = 1.5 Hz, 2H), 7.52 (t,
J = 1.5 Hz, 1H), 7.44 (t,
J = 8.5 Hz, 2H), 7.35-7.38 (m,
4H), 7.25 (s, 2H), 7.14 (dd,
J = 8.5, 15.0 Hz, 1H)
150.0, 149.9, 144.2,
136.8, 136.2, 131.7,
129.9, 129.4, 129.2,
129.1, 129.0, 128.0,
122.4, 122.2.
8.70, (s, 1H), 8.47 (d,
J = 8.5 Hz, 1H)
161.5, 160.3
161.6, 159.9
7.89 (d, J = 11.0 Hz, 2H), 7.68
(d, J = 8 Hz, 2H),7.46 (d,
J = 6.3 Hz, 2H), 7.35-7.41 (m,
2H), 7.27-7.30 (m, 4H), 7.18
(dd, J = 8.7, 15.0 Hz, 2H), 7.06
(d, J = 9.0 Hz, 1H)
162.2, 148.4, 143.1,
135.3, 131.3, 129.8,
128.4, 122.7, 115.1.
8.59 (s, 1H), 8.47 (d,
J = 9.0 Hz, 1H)
7
7.87 (d, J = 7.5 Hz, 2H), 7.69 (d,
J = 7.5 Hz, 2H), 7.44 (t,
J = 7.5 Hz, 2H), 7.35-7.38 (m,
2H), 7.27 (s, 4H), 7.13 (dd,
J = 9, 15.0 Hz, 1H), 7.05 (d,
J = 8.5 Hz, 2H)
161.1, 148.9, 143.1,
135.1, 129.8, 128.8,
128.6, 128.3, 128.1,
126.9, 121.2, 114.3
8.57 (s, 1H), 8.45 (d,
8
9
160.4, 158.2
160.4, 158.6
J = 9 Hz, 1H)
7.78 (d, J = 7.5 Hz, 2H), 7.66 (d,
J = 7.0 Hz, 2H), 7.35-7.44 (m,
4H), 7.28 (s, 4H), 7.13 (dd,
J = 10.0, 15.0 Hz, 1H), 7.05 (d,
J = 8.0 Hz, 2H)
162.3, 149.6, 143.2,
135.2, 130.0, 128.5,
128.3, 127.1, 121.3,
114.5
8.56 (s, 1H), 8.45 (d,
J = 9.0 Hz, 1H)
2.4. Determination of the Liquid Crystal Mesophase
The liquid crystal properties for compounds 2–9 were determined using polarized optical
microscopy (POM). POM displays the texture of liquid crystal properties under polarized light with
controlled temperature. In this work, compounds , and displayed liquid crystal textures,
making them as mesogenic molecules. However, compounds , and displayed transition textures
from the crystal phase to the isotropic phase. These compounds are non-mesogenic. All the mesogenic
compounds exhibited a thread-like schlieren texture with four brushes of nematic phase. Compounds
and with halogen substituents at the terminal end showed higher mesophase transition compared to
2,
3
,
7,
8
9
4,
5
6
2
3
compounds 7–9 with alkoxy terminal groups. This behavior was attributed due to the high polarity and
electronegativity of halogen groups which increase the lattice and melting temperatures. The liquid
crystal textures observed in POM photomicrographs for mesogenic compounds
2, 3, 7, 8, and 9 are
illustrated in Figure 11. Compounds with hydroxy, cinnamaldehyde, and hydrogen moieties only
4–6
displayed the crystal to isotopic transition and isotropic to crystal transition in both the heating and
cooling cycles, respectively.

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(a)
(b)
(c)
(d)
(e)
Figure 11. The optical photomicrographs of compounds 2, 3, 7, 8, and 9 in the heating cycle, showing
the thread-like schlieren texture of the nematic phase: ( ) compound ) compound
at 238.1 ◦C, (
at 242.3 ◦C, ( at 205.2 ◦C, ( at 185.1 ◦C, and ( at 182.8 ◦C
) compound ) compound ) compound
(magnification of 10 × 0.40).
a
2
b
3
c
7
d
8
e
9

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2.5. Determination of Thermal Transition
Further confirmation of the phase transition temperatures of the mesogenic compounds 2, 3, 7, 8,
and 9 was achieved using differential scanning calorimetry (DSC). DSC thermograms were recorded in
the heating and cooling cycles. The sample is heated with a scan rate of 10.0 ◦C/min and held at its
isotropic temperature for 2 min so as to attain the thermal stability. The cooling run is performed with
the same scan rate of 10.0 ◦C/min. The thermal enthalpy,
∆H (kJ/mol), of each phase transition was
calculated. All the data are summarized in Table 5.
Table 5. Differential scanning calorimetry (DSC) data for mesogenic compounds 2, 3, 7, 8, and 9.
Transition Temperature (◦C)
Compound
2
Mode
Enthalpy, ∆H (kJ/mol)
Heating
Cr
•
N
I
•
218.73
21.31
272.28
8.03
•
N
•
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
Heating
Cooling
I
Cr
216.70
−24.41
266.08
−7.63
•
•
Cr
N
I
220.45
14.56
282.78
3.43
3
7
8
9
•
•
•
I
N
•
Cr
209.85
−18.41
282.06
−4.00
•
Cr
N
I
•
181.98
15.40
222.58
2.58
•
•
I
N
•
Cr
177.63
−17.18
218.22
−1.65
•
•
Cr
N
I
172.03
28.25
209.63
3.49
•
•
•
I
N
•
Cr
168.03
−33.16
201.09
−3.66
•
•
Cr
N
I
163.45
34.94
200.54
1.19
•
•
•
I
N
•
Cr
158.96
−36.00
192.35
−0.22
•
•
Note: Cr = Crystal, N= Nematic, and I = Isotropic.
The DSC thermogram of compounds 2, 3, 7, 8, and 9 confirmed the existence of the nematic phase.
Two endotherms were observed in both the heating and cooling cycles which referenced one type of
liquid crystal phase. The first endotherm was attributed to the crystal to nematic phase transition, while
the other endotherm was attributed to the nematic phase before reaching the clearing point during the
heating cycle. The same trend was observed during the cooling cycle for the transitions from the I
→
N

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and the N
→
Cr phases. The clearing temperatures for compounds
2
,
3
,
7
,
8
, and
9
were observed at
272.28, 282.78, 222.58, 209.63, and 200.54 ◦C, respectively. DSC data on compounds
a wider nematic mesomorphic range as compared to compounds , and
2
and
3 showed
7
,
8
9 with alkoxy terminal
chains. The behavior was attributed to the high polarity of halogen groups which are able to reduce the
degree of molecular order and steric hindrance in the cinnamaldehyde central core [29]. Meanwhile,
the nematic mesophase range decreased as the number of alkoxy chains increased. This phenomenon
was observed in compound
between the side arms and the core of the molecules [15
and . In addition, longer terminal chains lower the melting and clearing temperatures [31]. The DSC
thermogram of compound is illustrated in Figure 12 as an example in this series, while the DSC
7
with a wider thermal temperature range due to the less cohesive forces
,
17]. The same trend occurred in compounds
8
9
8
thermograms for other mesogenic compounds (2, 3, 7, and 9) are provided in the Supplementary
Materials section.
Figure 12. DSC thermogram of compound 8.
2.6. Structure–Property Relationship
In general, a rod-like molecule is composed of flat, rigid cores, linking units, and terminal groups
of long flexible chains [32]. Molecular shape is important in the self-assembly of a molecule and
impacts the ordering abilities of mesogenic molecules. It is important to choose a suitable core, linking
unit or terminal group to obtain a mesogenic molecule. It was reported that molecules without
distinct hydrophilic and hydrophobic regions are unlikely to self-assemble into the liquid crystal phase.
The π–π interactions also influence the ability of the molecules to self-assemble [33]. The synthesis
of calamitic molecules with a liquid crystal mesophase requires at least two aromatic rings—either
cycloaliphatic or a combination of one aromatic and one cycloaliphatic ring—which are connected
directly or through a suitable linking unit. Calamitic molecules containing a rigid core have been
regarded as the most suitable geometry to exhibit mesogenic behavior [34]. Therefore, the study of the
structure–property relationship is very important in designing new liquid crystal materials.
The Schiff base is one of the most well-known linking units used in connecting the rigid core
groups. Schiff base molecules provide a stepped core structure which can maintain molecular linearity

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to provide high stability and enable mesophase formation [29
,
31]. Liquid crystal molecules with
Schiff base linking units, with two cinnamaldehyde cores attached, did not show any liquid crystal
properties. However, replacing one of the cinnamaldehyde cores with a p-substituted benzene ring
helps to induce the liquid crystal properties. It was found that substituents such as the halogen
(2,3) and alkoxy groups (7–9) displayed liquid crystal properties of the nematic phase. Halogen
groups are polar substituents possessing strong dipole moments, and thus have the ability to promote
mesomorphic properties. The increased dipole moment can enhance the stability of lattice and melting
temperatures [34]. Moreover, the electronegativity of the halogen atom reduces the degree of molecular
order, which enhances a higher clearing temperature [31,35]. As a result, compounds 2 and 3 showed
nematic phase behavior at high temperatures. In terms of the alkoxy substituents, the long alkyl chains
add flexibility to the rigid core and stabilize the molecular interactions needed for the formation of
a liquid crystal mesophase. This in turn, reduces the melting temperature, Tm [29]. Compound 7
with a methoxy group displayed nematic phase behavior as the molecules tend to orientate in parallel
arrangement with an increase in the ionic radius of the terminal substituents [35]. Further, the length
of the alkoxy side chain showed a strong influence on mesophase formation [36
–38]. Compounds 8
and did not show smectic phase transition even though these compounds attached to long terminal
9
alkyl chains. This behavior was due to the high longitudinal polarizability and conjugation between
the terminal chains and the aromatic system in cinnamaldehyde structure. Hence, only the nematic
phase was facilitated in compounds 8 and 9.
In this research, the behavior of compounds 4–6 attached to small terminal substituents such as
hydroxy, cinnamaldehyde, and hydrogen groups did not show any liquid crystal phases. Terminal
substituents can both attract and repel one another in different molecules. They can also affect the
polarizability of the aromatic rings to which they are attached [39]. Hydrogen and cinnamaldehyde
groups are non-polar compounds, which decreased the molecular aromaticity and polarizability of
the molecules. As a result, the mesophase transition cannot be induced. Meanwhile, the lone pair of
the hydroxy groups caused the cancellation of dipole moments of the molecules, which reduced the
polarizability and molecular interactions needed for the formation of the mesophases [31,40]. The POM
observation of compounds 2–9 is summarized in Table 6.
Table 6. POM observation of compounds 2–9.
Material
POM Observation
Material
POM Observation
Compound 2
Compound 3
Compound 4
Compound 5
Nematic
Nematic
Non-mesogenic
Non-mesogenic
Compound 6
Compound 7
Compound 8
Compound 9
Non-mesogenic
Nematic
Nematic
Nematic
3. Materials and Methods
3.1. Chemicals
The chemicals and solvents used in this study are cinnamaldehyde, 1,4-phenylenediamine,
4-chlorobenzaldehyde, benzaldehyde, 4-bromobenzaldehyde, 4-hydroxybenzaldehyde,
4-methoxybenzaldehyde, dimethylformamide, 1-bromoheptane, 1-bromononene, 1-bromododecane,
potassium carbonate, potassium iodide, methanol, ethyl acetate, and anhydrous sodium sulphate.
These chemicals and solvents were purchased from Merck, Sigma-Aldrich, Fluka, System, and Qrëc
(Asia). All chemicals were used as received without further purifications.
3.2. Instruments
In this research, thin layer chromatography (TLC) is a technique used to monitor the reaction
progress and the purity of the products. TLC identifies a compound in a mixture using a thin stationary
phase supported by an inert backing. The eluent used was 20:80 of ethyl acetate:hexane. Meanwhile,

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FTIR spectroscopy (PerkinElmer, Waltham, MA, USA) is a technique used to determine the functional
groups in a sample. Samples are scanned at a range of 700 to 4000 cm−1. The structure elucidation
of all compounds was determined using NMR spectroscopy (Bruker, Coventry, UK), and the purity
of compounds was confirmed using CHN elemental analysis (PerkinElmer, Waltham, MA, USA).
Moreover, the liquid crystalline behavior of these compounds was determined using POM (Linkam,
London, UK). The sample was tested using an Olympus system model bx53 linksys32 with a scan
rate of 10 ◦C and held at 2 ◦C as the sample reached the liquid crystal state. Finally, the mesophase
transition of these compounds was confirmed using DSC (PerkinElmer, Waltham, MA, USA).
3.3. Synthesis Method
3.3.1. Synthesis of 4-heptyloxybenzaldehyde, 1a
The 4-Hydroxybenzaldehyde (0.10 mol) and 1-bromoheptane (0.10 mol) were dissolved in 20 mL
of DMF each and both solutions were mixed in a 250 mL round-bottomed flask. Potassium carbonate
(K2CO3) (0.15 mol) and potassium iodide (KI) (0.01 mol) were added into the mixture. The mixture
was refluxed at 80 ◦C for 12 h and the reaction progress was monitored by TLC. Upon completion,
the reaction mixture was poured into 250 mL of cold water and was extracted by 30 mL ethyl acetate
(2 ). These combined organic layers were dried in anhydrous sodium sulphate. The crude extract was
×
filtered and evaporated to give a yellowish oil product. The same method was used to synthesize 1b.
Yield: 15.1 g (72.7 %), light-yellow oil. IR (cm−1) 2928 and 2734 (Csp3-H stretch), 2700 (H-CO, aldehydic),
1687 (C=O stretch), 1599 (C=C stretch), and 1157 (C-O stretch). 1H-NMR (500 MHz, DMSO-d6)
δ,
ppm: 9.85 (s, 1H), 7.82 (d, J = 8.5 Hz, 2H), 7.03 (d, J8.5 Hz, 2H), 3.98 (t, J6.5 Hz, 2H), 1.64–1.70 (m, 2H),
1.31–1.37 (m, 2H), 1.20–1.27 (m, 6H), and 0.81 (t, J7.0 Hz, 3H). 13C-NMR (125 MHz, DMSO-d6)
δ, ppm:
190.6, 163.6, 131.4, 129.5, 114.6, 67.9, 31.2, 28.5, 28.5, 25.4, 22.1, and 13.7.
The 4-Dodecyloxybenzaldehyde, 1b; Yield: 20.0 g (72.1%), dark yellow oil. FTIR (cm−1) 2922 and 2733
(Csp3-H stretch), 2701 (H-CO, aldehydic), 1691 (C=O stretch), 1600 (C=C stretch), and 1157 (C-O stretch).
1H-NMR (500 MHz, DMSO-d6)
δ
, ppm: 9.81 (s, 1H), 7.76 (d, J = 8.5 Hz, 2H), 6.96 (d, J = 8.5 Hz, 2H),
3.93 (t, J = 6.5 Hz, 2H), 1.61–1.67 (m, 2H), 1.31–1.35 (m, 2H), 1.15–1.22 (m, 16H), and 0.78 (t, J = 7.0 Hz,
3H). 13C-NMR (125 MHz, DMSO-d6)
, ppm: 191.4, 163.5, 131.4, 129.5, 114.4, 67.8, 31.4, 29.1, 29.2, 29.2,
29.1, 28.9, 28.9, 28.6, 25.5, 22.1, and 13.6.
δ
3.3.2. Synthesis of N-(4-bromobenzylidene)-N0-(3-phenylallylidene)-benzen-1,4-diamine, 2
mixture of cinnamaldehyde (0.004 mol), 1,4-phenylenediamine (0.004 mol),
A
and 4-bromobenzaldehyde (0.004 mol) was stirred in 15 mL methanol at room temperature.
The reaction was monitored using TLC. After 5 h, the reaction was stopped, filtered, and washed with
cold methanol. The crude extract was dried and recrystallization from methanol gave a yellowish
powder. The same method was used to synthesize compounds 3–9. Yield: 1.4 g (89.1%), mp:
208.2–210.4 ◦C, light yellow powder. FTIR (cm−1): 3010 (Csp2-H stretch), 1637 (C=N stretch), 1618 (C=C
1
stretch), 1251 (C-N stretch). H-NMR (500 MHz, DMSO-d6)
δ, ppm: 8.67 (s, 1H), 8.47 (d, J = 9.0 Hz,
1H), 7.89 (d, J = 9.0 Hz, 2H), 7.74 (d, J = 7.0 Hz, 2H), 7.68 (d, J = 7.5 Hz, 2H), 7.44 (t, J = 7.0 Hz, 2H),
7.36–7.39 (m, 4H), 7.28 (t, J = 4.5 Hz, 2H), and 7.18 (d, J = 5.0 Hz, 1H). 13C-NMR (125 MHz, DMSO-d6)
δ
,
ppm: 161.4, 158.8, 149.5, 148.7, 144.1, 135.4, 135.2, 131.8, 130.3, 128.9, 128.4, 127.5, 124.8, 122.0, and 121.8.
CHN elemental analysis: calculated for C22H17N2Br: C: 67.88%, H: 4.37%, and N: 7.20%. Found: C:
67.79%, H: 4.31%, and N: 7.16%.
N-(4-chlorobenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine,
3; Yield: 1.3 g (87.8%), mp:
211.1–213.4 ◦C, yellow powder. FTIR (cm−1): 3050 (Csp2-H stretch), 1640 (C=N stretch), 1618 (C=C
1
stretch), and 1251 (C-N stretch). H-NMR (500 MHz, DMSO-d6)
δ, ppm: 8.70 (s, 1H), 8.47 (d, J = 8.5 Hz,
1H), 7.97 (d, J = 6.0 Hz, 2H), 7.67 (d, J = 7.0 Hz, 2H), 7.60 (d, J = 7.5 Hz, 2H), 7.44 (t, J = 7.0 Hz,
2H), 7.36–7.39 (m, 4H), 7.29–7.35 (m, 2H), and 7.18 (dd, J = 8.5, 12.0 Hz, 1H). 13C-NMR (125 MHz,

Molecules 2020, 25, 3780
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DMSO-d6) δ, ppm: 161.4, 158.6, 149.5, 148.7, 144.1, 135.4, 135.2, 131.8, 130.3, 128.9, 128.4, 127.5, 124.8,
122.0, and 121.8. CHN elemental analysis: calculated for C22H17N2Cl: C: 76.63%, H: 4.93%, and N:
8.15%. Found: C: 76.55%, H: 4.82%, and N: 8.13%.
N-(4-hydroxybenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine,
196.8–199.1 ◦C, yellow powder. FTIR (cm−1): 3414 (O-H stretch), 3060 (Csp2-H stretch), 1634 (C=N
stretch), 1618 (C=C stretch), and 1253 (C-N stretch). 1H-NMR (500 MHz, DMSO-d6)
, ppm: 8.51
4; Yield: 1.1 g (81.5%), mp:
δ
(s, 1H), 8.45 (d, J = 9.0 Hz, 1H), 7.77 (d, J = 9.0 Hz, 2H), 7.66 (d, J = 7.0 Hz, 2H), 7.44 (t, J = 6.0 Hz,
2H), 7.39 (t, J = 7.5 Hz, 2H), 7.25 (s, 4H), 7.13 (dd, J = 8.5, 16.0 Hz, 2H), and 6.88 (d, J = 6.5 Hz, 1H).
13C-NMR (125 MHz, DMSO-d6)
δ, ppm: 162.1, 161.1, 160.5, 149.4, 143.9, 135.6, 130.5, 129.5, 128.9, 128.6,
127.5, 121.8, 121.7, and 115.7. CHN elemental analysis: calculated for C22H18N2O: C: 80.98%, H: 5.52%,
and N: 8.59%. Found: C: 80.91%, H: 5.49%, and N: 8.57%.
N-(3-phenylallylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine,
5; Yield: 1.0 g (76.9%), mp:
190.5–193.3 ◦C, yellow powder. FTIR (cm−1): 3075 (Csp2-H stretch), 1635 (C=N stretch), 1600 (C=C
1
stretch), and 1240 (C-N stretch). H-NMR (500 MHz, DMSO-d6)
δ
, ppm: 8.45 (d, J = 8.5 Hz, 1H), 7.66 (d,
J = 7.5 Hz, 2H), 7.35–7.39 (m, 2H), 7.44 (t, J = 6.0 Hz, 2H), 7.25 (s, 4H), and 7.16 (dd, J = 9.0, 16.0 Hz, 1H).
13C-NMR (125 MHz, DMSO-d6)
δ, ppm: 160.6, 149.8, 143.3, 135.2, 129.0, 128.5, 128.2, 127.0, and 121.3.
CHN elemental analysis: calculated for C24H20N2: C: 85.71%, H: 5.81%, and N: 9.03%. Found: C:
85.69%, H: 5.84%, and N: 8.97%.
N-benzylidene-N0-(3-phenylallylidene)-benzene-1,4-diamine,
6
;
Yield:
0.9 g (72.6%), mp:
201.5–203.3 ◦C, light brown powder. FTIR (cm−1): 3060 (Csp2-H stretch), 1637 (C=N stretch), 1617 (C=C
1
stretch), and 1248 (C-N stretch). H-NMR (500 MHz, DMSO-d6)
δ
, ppm: 8.70, (s, 1H), 8.47 (d, J = 8.5 Hz,
1H), 7.96 (d, J = 7.0 Hz, 2H), 7.66 (d, J = 6.5 Hz, 2H), 7.53 (d, J = 1.5 Hz, 2H), 7.52 (t, J = 1.5 Hz, 1H),
7.44 (t, J = 8.5 Hz, 2H), 7.35–7.38 (m, 4H), 7.25 (s, 2H), and 7.14 (dd, J = 8.5, 15.0 Hz, 1H). 13C-NMR
(125 MHz, DMSO-d6) δ, ppm: 161.5, 160.3, 150.0, 149.9, 144.2, 136.8, 136.2, 131.7, 129.9, 129.4, 129.2,
129.1, 129.0, 128.0, 122.4, and 122.2. CHN elemental analysis: calculated for C22H18N2: C: 85.16%, H:
5.81%, and N: 9.03%. Found: C: 85.09%, H: 5.79%, and N: 8.99%.
N-(4-methoxybenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine,
7; Yield: 1.0 g (74.3%),
174.8–177.5 ◦C, yellow powder. FTIR (cm−1): 3070 (Csp2-H stretch), 2930 and 2878 (Csp3-H stretch), 1640
1
(C=N stretch), 1618 (C=C stretch), 1251 (C-N stretch), and 1100 (C-O stretch). H-NMR (500 MHz,
DMSO-d6)
7.46 (d, J = 6.3 Hz, 2H), 7.35–7.41 (m, 2H), 7.27–7.30 (m, 4H), 7.18 (dd, J = 8.7, 15.0 Hz, 2H), 7.06 (d,
J = 9.0 Hz, 1H), and 3.84 (s, 3H). 13C-NMR (125 MHz, DMSO-d6)
, ppm: 162.2, 161.6, 159.9, 148.4,
δ, ppm: 8.59 (s, 1H), 8.47 (d, J = 9.0 Hz, 1H), 7.89 (d, J = 11.0 Hz, 2H), 7.68 (d, J = 8 Hz, 2H),
δ
143.1, 135.3, 131.3, 129.8, 128.4, 122.7, and 115.1. CHN elemental analysis: calculated for C23H20N2O:
C: 81.18%, H: 5.88%, and N: 8.23%. Found: C: 81.08%, H: 5.82%, and N: 8.18%.
N-(4-heptyloxybenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine,
8; Yield: 1.3 g (74.9%), mp:
166.4–169.8 ◦C, yellow powder. FTIR (cm−1): 3080 (Csp2-H stretch), 2936 and 2880 (Csp3-H stretch), 1637
1
(C=N stretch), 1617 (C=C stretch), 1251 (C-N stretch), and 1140 (C-O stretch). H-NMR (500 MHz,
DMSO-d6) δ, ppm: 8.57 (s, 1H), 8.45 (d, J = 9 Hz, 1H), 7.87 (d, J = 7.5 Hz, 2H), 7.69 (d, J = 7.5 Hz, 2H),
7.44 (t, J = 7.5 Hz, 2H), 7.35–7.38 (m, 2H), 7.27 (s, 4H), 7.13 (dd, J = 9, 15.0 Hz, 1H), 7.05 (d, J = 8.5 Hz,
2H), 4.08 (t, J = 6.5 Hz, 2H), 1.75 (t, J = 7Hz, 2H), 1.44–1.46 (m, 2H), 1.31–1.37 (m, 6H), and 0.89 (t,
J = 6 Hz, 3H). 13C-NMR (125 MHz, DMSO-d6)
δ, ppm: 161.1, 160.4, 158.2, 148.9, 143.1, 135.1, 129.8,
128.8, 128.6, 128.3, 128.1, 126.9, 121.2, 114.3, 67.4, 30.6, 28.1, 27.7, 24.8, 21.3, and 13.1. CHN elemental
analysis: calculated for C29H32N2O: C: 82.08%, H: 7.55%, and N: 6.60%. Found: C: 82.06%, H: 7.48%,
and N: 6.56%.
N-(4-dodecyloxybenzylidene)-N0-(3-phenylallylidene)-benzene-1,4-diamine,
9; Yield: 1.5 g (76.9%),
mp: 162.1–165.9 ◦C, yellow powder. FTIR (cm−1): 3070 (Csp2-H stretch), 2936 and 2880 (Csp3-H stretch),
1
1637 (C=N stretch), 1617 (C=C stretch), 1250 (C-N stretch), and 1170 (C-O stretch). H-NMR (500 MHz,

Molecules 2020, 25, 3780
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DMSO-d6) δ, ppm: 8.56 (s, 1H), 8.45 (d, J = 9.0 Hz, 1H), 7.78 (d, J = 7.5 Hz, 2H), 7.66 (d, J = 7.0 Hz, 2H),
7.35–7.44 (m, 4H), 7.28 (s, 4H), 7.13 (dd, J = 10.0, 15.0 Hz, 1H), 7.05 (d, J = 8.0 Hz, 2H), 4.08 (t, J = 6.0 Hz,
2H), 1.75 (t, J = 6.0 Hz, 2H), 1.43–1.45 (m, 2H) 1.27–1.42 (m, 16H), and 0.87 (t, J = 2.0 Hz, 3H). 13C-NMR
(125 MHz, DMSO-d6) δ, ppm: 162.3, 160.4, 158.6, 149.6, 143.2, 135.2, 130.0, 128.5, 128.3, 127.1, 121.3,
114.5, 67.6, 30.8, 28.5, 28.2, 25.0, 21.5, and 13.3. CHN elemental analysis: calculated for C34H42N2O: C:
82.59%, H: 8.50%, and N: 5.67%. Found: C: 82.54%, H: 8.45%, and N: 5.63%
4. Conclusions
Thermotropic compounds 2–9 with a rod-like structure of cinnamaldehyde and phenyl cores,
attached with Schiff base linking units and varieties of terminal groups, were successfully synthesized
and characterized. Structural characterization was confirmed using FTIR, 1D and 2D NMR, and CHN
elemental analysis. Based on the POM observation, compounds
with nematic phase behavior. Compounds and with polar groups (Br and Cl) at the terminal end
possessed strong dipole moments and had the ability to promote mesomorphic nematic properties.
Meanwhile, compounds , and attached to alkoxy terminal chains (CH3, C7H15, and C12H25) were
able to add flexibility to the rigid core and stabilize the molecular interactions needed for the formation
of a liquid crystal mesophase. However, compounds , and are non-mesogenic due to the low
2, 3, 7, 8, and 9 were mesogenic
2
3
7,
8
9
4,
5
6
polarizability and molecular interactions in the molecules. The presence of a cinnamaldehyde core
system led to high rigidity and resulted in high mesophase transitions, and could therefore be used to
display liquid crystalline behavior at high temperature ranges.
Supplementary Materials: The following are available online, Table S1: FTIR data of p-hydroxybenzaldehyde,
1a, and 1b; Figure S1–S4: DSC thermogram of compounds 2, 3, 7, and 9, respectively.
Author Contributions: Conceptualization, Z.J. and M.K.; methodology, Z.J. and N.F.O.; validation, Z.J., M.K.
and N.F.O.; formal analysis, Z.J. and N.F.O; investigation, Z.J. and N.F.O; resources, M.K. and Z.J.; data curation,
Z.J. and N.F.O; writing—original draft preparation, Z.J. and N.F.O; writing—review and editing, M.K. and Z.J;
visualization, Z.J. and N.F.O; supervision, M.K.; funding acquisition, Z.J. and M.K. All authors have read and
agreed to the published version of the manuscript.
Funding: This research was funded by the Universiti Sains Malaysia, grant number 1001/PKIMIA/811332;
Universiti Malaysia Sabah, grant number SGA0037-2019 and SPB0004-2020; and The APC was funded by the
Universiti Malaysia Sabah.
Acknowledgments: The authors would like to thank the Universiti Sains Malaysia (USM) and the Universiti
Malaysia Sabah (UMS) for the lab facilities.
Conflicts of Interest: The authors declare no conflict of interest.
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