Synthesis, mesomorphic and computational characterizations of nematogenic schiff base derivatives in pure and mixed state

Article abstract of DOI:10.3390/molecules26072038

Homolog series based on three aromatic rings bearing terminal alkoxy chain of various lengths named 4-(4-(alkoxy)phenylimino)methyl)phenyl nicotinate (An) were synthesized. The alkoxy-chain length changed between 6, 8 and 16 carbons. Mesomorphic and optical properties were carried out via differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Elemental analyses, FT-IR and NMR spectroscopy were carried out to elucidate the molecular structures of the prepared derivatives. Mesomorphic results indicated that all the synthesized homologs (An) are monomorphic possessing the nematic (N) phase enantiotropically with wide thermal stability. Computational simulations were measured via density functional theory (DFT) theoretical calculation tool. The estimated thermal and geometrical parameters are in agreement with the experimental data. By discussing the estimated parameters, it was found that the molecular architecture, dipole moment and the polarizability of the investigated compounds are highly affected by the length of the attached terminal flexible chain and the location of the nitrogen atom in the other terminal aromatic ring. Binary phase diagrams of two corresponding homologs with different proportionating terminals were constructed, and their binary phase physical properties were discussed in terms of the temperature range and stability of the N phase.

Full text of DOI:10.3390/molecules26072038

molecules
Article
Synthesis, Mesomorphic and Computational Characterizations
of Nematogenic Schiff Base Derivatives in Pure and Mixed State
Laila A. Al-Mutabagani , Latifah A. Alshabanah , Hoda A. Ahmed 2,3,* , Hafsa H. Alalawy 2
1
1
and Mayada H. Al alwani 3
1
Chemistry Department, College of Science, Princess Nourah bint Abdulrahman University,
Riyadh 11671, Saudi Arabia; Laalmutbagani@pnu.edu.sa (L.A.A.-M.); laalsabanah@pnu.edu.sa (L.A.A.)
Department of Chemistry, Faculty of Science, Cairo University, Cairo 12613, Egypt; hafsahamdy@gmail.com
Chemistry Department, College of Sciences, Yanbu, Taibah University, Yanbu 30799, Saudi Arabia;
2
3
tu4053550@taibahu.edu.sa
*
Correspondence: ahoda@sci.cu.edu.eg
Abstract: Homolog series based on three aromatic rings bearing terminal alkoxy chain of various
lengths named 4-(4-(alkoxy)phenylimino)methyl)phenyl nicotinate (An) were synthesized. The
alkoxy-chain length changed between 6, 8 and 16 carbons. Mesomorphic and optical properties were
carried out via differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Ele-
mental analyses, FT-IR and NMR spectroscopy were carried out to elucidate the molecular structures
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
of the prepared derivatives. Mesomorphic results indicated that all the synthesized homologs (An
)
ꢀ
ꢁꢂꢃꢄꢅꢆ
are monomorphic possessing the nematic (N) phase enantiotropically with wide thermal stability.
Computational simulations were measured via density functional theory (DFT) theoretical calculation
tool. The estimated thermal and geometrical parameters are in agreement with the experimental
data. By discussing the estimated parameters, it was found that the molecular architecture, dipole
moment and the polarizability of the investigated compounds are highly affected by the length
of the attached terminal flexible chain and the location of the nitrogen atom in the other terminal
aromatic ring. Binary phase diagrams of two corresponding homologs with different proportionating
terminals were constructed, and their binary phase physical properties were discussed in terms of
the temperature range and stability of the N phase.
Citation: Al-Mutabagani, L.A.;
Alshabanah, L.A.; Ahmed, H.A.;
Alalawy, H.H.; Al alwani, M.H.
Synthesis, Mesomorphic and
Computational Characterizations of
Nematogenic Schiff Base Derivatives
in Pure and Mixed State. Molecules
2
021, 26, 2038. https://doi.org/
1
0.3390/molecules26072038
Academic Editors:
Keywords: azomethine liquid crystals; nicotinate derivative; nematic phase; geometrical structure;
Agostina-Lina Capodilupo and
Roberto Termine
DFT; optimized structure; binary phase diagram
Received: 13 March 2021
Accepted: 30 March 2021
Published: 2 April 2021
1
. Introduction
Nowadays, liquid crystals (LCs) based on organic derivatives prove to have broad
areas of industrial applications as optical technology and photoconductors [ ]. Structural–
characteristic relationships are important approaches to prepare compatible geometrical
shapes to achieve the essential properties for recent technological applications [ ]. Selec-
1–3
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
4–7
tion of attached substituents, flexible chains and mesogenic spacers are important criteria in
the designing of proper thermotropic LCs for applications. Modifications in the molecular
architectures lead to change in the mesomorphic behavior and play important roles in the
formation, kind and thermal stability of the produced mesophase [8–10].
Insertion of heterocyclic pyridine ring in the LCs’ skeletons strongly impacts their
polarizability and/or polarity as well as their geometric structures. Consequently, it exten-
sively affects the phase transition temperature, types of mesophase and other physical and
geometrical parameters essential for better properties of the LC materials [11]. Incorpora-
tion of Schiff base and ester linking groups widely improves many properties of LCs. This
could be attributed to their rigidity that enhances the stability of LC mesophases [12,13].
These modifications in the properties of the LCs may be beneficial for mesomorphism as
well as the physical properties necessary for technical uses. Schiff base linkages have been
Copyright: © 2021 by the authors.
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Molecules 2021, 26, 2038
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broadly employed in the preparation of numerous LC derivatives [14
–
17]. Most reports
0
are focused on Schiff bases since the discovery of nematogenic 4-methoxybenzylidene-4 -
butylaniline at room temperature [18]. Recently, several thermotropic Schiff base/ester
liquid crystals have been documented [19–21].
Today, the designing of LC compounds according to theoretical prediction has at-
tracted high interest [22–25]. Understanding many optical parameters requires increased
knowledge about the molecular geometries and energies of molecular orbitals of the meso-
morphic compounds. Recently, density functional theory (DFT) has become an effective
tool for its excellent performance and computational data being consistently correlated
with the experimental findings [22–26].
Decrement of the melting temperatures of the LCs and broadening their mesophase
range are important goals for many practical applications. One way to achieve a low
melting point is the mixing of compounds possessing various molecular shapes and
characteristics in binary systems [26–33]. The mesomorphic properties are greatly modified
upon the mixing of individual components. These components may be symmetric, with
identical mesogenic moieties; or non-symmetric, with different mesogenic linkers. In
both cases, the specific interactions between the two mesogenic compounds do lead to a
significant variation in the mesophase behavior of such materials [34–39].
Recently, our team has centered its interest on the computational simulations of new
synthesized LC materials to experimentally illustrate their mesophase properties in terms
of theoretical estimations [26]. These investigations were focused on the obtained me-
somorphic transition results and the evaluated theoretical calculations correlations for
investigated compounds. The geometrical conformations of LCs depend on their structural
shapes that play an essential role in the stability and formation of their mesophases. More-
over, it has been found that the possible orientation of hetero-atoms in pyridines results
in the modification of the existing functions, thus offering new geometrical properties to
the organic compound [40]. Further, the length of flexible terminal chains plays important
roles in the formation, stability, type and mesomorphic range of LC compounds. The
mesomorphic molecules tend to be more oriented in a parallel arrangement as the length
of the terminal moiety increases [41].
The goal of our investigation was to synthesize three-ring Schiff base/ester liquid
crystals with a terminal heterocyclic ring (Scheme 1) and to investigate the effect of different
proportionating of the terminal alkoxy chain length on the mesomorphic and optical
behavior of synthesized derivatives. The evaluation was carried out by experimental and
computational tools aiming to investigate their thermal and physical behaviors in pure and
mixed states and to correlate the computational DFT simulations with the experimental
variables data to explain the experimental results’ outcome.
Scheme 1. Investigated homologues series, An.
2. Results and Discussion
2.1. Mesomorphic Characterizations
The phase transition temperatures, enthalpies and normalized entropies, as derived
from DSC measurements, of synthesized homolog series An are collected in Table 1. DSC

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thermograms are represented in Figure 1 upon heating/cooling for A8 derivative as an
example and the others displayed in Figures S1 and S2 in supplementary data. Mesophase
textures were identified by polarized optical microscopy (POM). POM images showed
schlieren texture of nematic (N) phase covering all members of present group An upon
heating and cooling (Figure 2). DSC measurements were performed for second heating
scans to ensure the thermal stabilities of the investigated compounds. Moreover, DSC
results were confirmed by the POM image observations. All members of the series showed
to exhibit only two transition peaks upon heating and cooling cycles of DSC thermograms
which are assigned to Cr-to-N (during heating) and N-to-Cr (upon cooling). In order to
investigate the impact of terminal flexible alkoxy chain length on the mesomorphic proper-
ties of synthesized series, the graphical transition temperature relationship of DSC data is
illustrated in Figure 3. Table 1 and Figure 3 reveal that all compounds of the homologs An
are purely nematogenic possessing only the N phase enantiotropically. Moreover, there is,
as usual, irregular change of their melting points with the increment of the terminal alkoxy
chain length (n). It was previously documented that the N stability decreases gradually
with n [36
,
37]. Thus, the homolog with the shortest alkoxy-chain (A6) exhibits the highest
◦
thermal nematic stability, 184.5 C with a wide nematogenic temperature range of 54.1
◦
C. Meanwhile, the higher homolog A16 possesses nematogenic stability and temperature
◦
range between 146.6 and 24.4 C, respectively. The heterocyclic nicotinic moiety enables en-
hancement of the mesophase properties without any steric disruption, and consequently, N
mesophases can still be observed covering all terminal lengths. Polarity of whole molecule
also serves to enhance the optical transitions and other physical parameters. It was doc-
umented that [42] the type of mesophase and its stability are mainly dependent on the
dipole moment of the mesogenic core of the molecule which varies according to the incor-
porated terminal groups. In general, the polarity and/or polarizability of the mesogenic
moieties have a main role in improving the mesophase stability that will be proved in the
theoretical calculation part. It could be seen also from Table 1 that all normalized entropies
of transitions (∆S/R) are of lower values, irrespective of the length of the alkoxy chain. This
is in complete agreement with previous investigations [43].
◦
Table 1. Phase transition temperatures (
T
C), enthalpy of transition,
∆
H, kJ/mol, normalized
entropy, ∆S, and mesophase temperature range, ∆T, for groups An.
Comp.
TCr-N
∆HCr-N
TN-I
∆HN-I
∆SN-I
∆T
A6
A8
A16
130.4
128.6
122.2
37.59
43.40
55.25
184.5
174.2
146.6
1.94
1.80
1.66
0.51
0.48
0.48
54.10
45.60
24.40
Cr-N denotes transition from solid to the nematic mesophase. N-I denotes transition from nematic to
the isotropic mesophase.

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◦
Figure 1. DSC thermograms of A8 derivative upon second heating-cooling scan with a rate of 10 C/min.
◦
Figure 2. POM images of N mesophase upon heating of compound A8 at (
a
) 158.0 C and (
b
)
◦
1
72.0 C.

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Figure 3. DSC transitions of synthesized groups An.
A comparison is established between the present A6 derivative and their correspond-
ing isomer (B6, Scheme 2) bearing terminal benzene ring [16]. The results reveal that the
type and stability of the observed mesophase are dependent on the enhanced dipole mo-
ment of the molecular mesogenic part which is mainly dependent on the hetero-ring moiety.
In addition, the introduction of the pyridyl moiety rather than benzene ring disrupts the
smectic A molecular packing and giving only the N phase. Another comparison is made
between the present An series and their corresponding homologs Cn series (Scheme 3) [26
]
bearing terminal cinnamate moiety. The comparison revealed that the mesophase thermal
stability of the formed N phase is dependent on the enhanced conjugation of C=C of the
cinnamate group. Moreover, the insertion of the pyridyl group rather than the cinnamate
moiety decrements by a small extent the nematic range and stability.
Scheme 2. B6 derivative.
Scheme 3. Cn homologues series.
2.2. DFT Calculations
2.2.1. Geometrical Simulations and Thermal Parameters
In order to study the effect of linking and terminal alkoxy chain groups on the me-
somorphic properties of the present homologs An, computational calculations for each

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member of the investigated series were studied. The correlations were recognized between
the quantum chemical parameters, derived by DFT theoretical calculations, and the experi-
mental data variables for the synthesized series An. Calculations were estimated in the gas
phase at B3LYP/6-311g(d,p) for predicting the optimum and most stable molecular geome-
tries. Computational calculations were performed by Gaussian 09W package (Gaussian,
Inc., Wallingford CT, USA) [44]. Data revealed that all compounds exhibit planar geometry
with semi-linear molecular structure. Moreover, the length of terminal flexible chain highly
impacts the structural geometry of the molecule (Figure 4).
Figure 4. Atomic charges and dipole moment vector for the optimized structure of present series An, calculated at
B3LYP/6-311g(d,p) level.
The calculated total energy of the synthesized homologs series An and the other
thermal parameters (thermal energy, enthalpy, Gibb’s free energy and entropy) at room
temperature were estimated at the same level of theory and are summarized in Table 2.
Moreover, other predicted parameters are collected in Table 3. Tables 2 and 3 data show
that the thermal and geometrical parameters predicted from DFT calculations are highly
affected by the electronic conjugation of the linking groups. Moreover, the energy gap (∆E)
between HOMO and LUMO levels is an indicator of the chemical reactivity of compounds.
The lower its value, the more reactive the molecule would be. The predicted energy gap
collected in Table 3 affirms the A6 to be more reactive than others. It is also softer than
others as the energy gap is inversely related to the softness. On the other hand, the lower
ionization potential (I.E) calculated for the A16 derivative indicates a more basic nature
than others in the series.

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Table 2. ZPE, thermal energy, enthalpy, Gibb’s free energy and entropy of the series An at room temperature calculated at
B3LYP/6-311g(d,p) level.
Thermal
Energy, kcal/mol
Enthalpy,
kcal/mol
Gibb’s Free
Energy, kcal/mol
Comp.
ZPE, kcal/mol
Entropy, cal/molK
A6
A8
A16
288.066
323.766
466.039
305.566
342.923
492.042
306.158
343.515
492.635
247.348
280.483
412.108
197.254
211.414
270.094
Table 3. HOMO energy, LUMO energy, orbital energy gap, ionization energy, electron affinity, dipole moment and
polarizability calculated at B3LYP/6-311g(d,p) level.
Dipole
Moment, Debye
Polarizability,
Bohr
Comp.
EHOMO, eV ELUMO, eV
∆E, eV
I.E, eV
E.A, eV
A6
A8
A16
−5.669
−5.667
−5.664
−2.097
−2.096
−2.079
3.572
3.571
3.586
5.669
5.667
5.664
2.097
2.096
2.079
0.896
0.911
0.800
354.830
379.780
477.790
Aspect ratio, polarity, polarizability, rigidity of the mesogenic cores and the molecular
shape of the molecules, as well as the attached terminal chains, are essential parameters
to enhance the type and thermal mesomorphic stability of the formed mesophase [45].
The competition between the intermolecular lateral and end-to-end interactions affects
the mesomorphic properties of the resulted structure. The experimental and theoretical
correlations revealed a pronounced increment of the mean polarizability with an increment
of the length of terminal alkoxy chain (n) and a decrease of the nematic stability. As the
chain length of the terminal group increases, the polarizability increases; this could be
attributed to the increment in the aspect ratios. The space filling of the mesomorphic
compounds increases as the aspect ratios of the molecular geometry increase, and this
results in an enhancement of the polarizability. Moreover, the predicted polarizability
shows an increment in its values as the nematic range (∆T) decreases which is in agreement
with our previous studies [26]. Furthermore, constant relation of the dipole moment and
the N range and stability was observed. As the length of the alkoxy chain (n) elongates,
the week van der Waals intermolecular attractions have an important role in destabilizing
the observed N mesophase. The lower values of the estimated dipole moments allow the
terminal aggregations to predominate more than the lateral one, which influences the N
mesophase formation. In Table 3, the ionization energy and electron affinity, also calculated
as I.E = −EHOMO and E.A = −ELUMO, respectively, are included [46].
2.2.2. Frontier Molecular Orbitals (FMOs)
Optical investigations of the non-linear optical (NLO) materials are highly enhanced
by the energy difference between the FMOs, HOMO (highest occupied molecular orbital)
and LUMO (lowest unoccupied molecular orbital) [47]. The estimated ground state density
surface plots for the FMOs of present groups An are simulated in Figure 5. The results
of the FMO energy gaps are also collected in Table 3. As shown from Table 3 and Figure
5
, the electron densities of the sites that contribute to the formation of the HOMOs and
the LUMOs are localized on each of the azomethine linkage and the hetero-ring moiety.
Moreover, the energy gap of FMOs is slightly dependent on the length of the alkoxy chain
(n). It was reported that [26] the long terminal chain in addition to the conjugation within
the mesogenic cores leads to a pronounced decrease in the FMO energy gap. Finally,
the mesomorphic behavior of the rod-like mesogens develops the molecular–molecular
interactions that essentially depend on the molecular geometry of prepared compounds,
polarizability of the terminals and attached substituents, as well as the stereoelectronic
characteristics of the whole molecule.

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Figure 5. Estimated geometry for frontier molecular orbitals (FMOs) of the investigated homolog, An.
2.2.3. Molecular Electrostatic Potential (MEP)
Charge distribution maps for the investigated group An were predicted under the
same basis sets according to molecular electrostatic potential (MEP, Figure 6). The neg-
atively charged atomic sites (red regions) were localized on the oxygen atoms and the
nitrogen atom of the Schiff base linkage as well as the nitrogen of the hetero-terminal ring.
While the alkyl chain showed the least negatively charged atomic sites (blue regions). As
shown from Figure 6, the length of the terminal alkoxy group and the conformation of
the attached aromatic ring are more effective on the orientation of the charge distribution
map; this could affect the thermal nematic stability of the mesophase by alteration of the
competitive interaction between end-to-end and side-side interaction. The measurements
of theoretical charge distribution and the mesomorphic properties correlation were recently
documented [26]. The alteration of the charge distribution on the whole molecules due
to mesomeric interactions enhances the terminal aggregation to induce the N mesophase.
Furthermore, the geometrical structure of the attached flexible chain affects the charge
distribution map orientation. It was found that the longest terminal alkoxy group (A16) of
the present homologs has the highest effect on the localization of the isoelectron density of
the electron-rich and electron-deficient regions.

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Figure 6. Molecular electrostatic potentials (MEPs) of the investigated group, An.
2.3. Binary Mixture
From DSC measurements, binary phase diagram examples of terminally different
lengthened alkoxy chains prepared from a combination of the homologs A6 and A8 as well
as A16 are presented in Figure 7a,b. As can be seen from these figures, the constructed
binary mixtures from the shortest and longest terminals (A6/A8 and A6/A16) showed to
exhibit the N phase over the entire composition range of the binary mixtures. Figure 7a
(
A6/A8 homolog) shows nearly ideal behavior as observed according to nematic stability
for both derivatives. Meanwhile, a negative decrease from the ideality is observed for
A6/A16 homolog (Figure 7b). The nematic molecular association disruption in the longer
chain length mixtures (A6/A16) is attributed to the non-similarity of the terminal flexible-
chain length (n = 6 and 16), which is widely different between the two constructed
derivatives. Figure 7a,b also show that the solid mixtures of A6/A8 and A6/A16 homologs
◦
possess eutectic melting points at 124.7 and 117.0 C, respectively. Moreover, their eutectic
compositions formed at 60 and 23 mol% of A6 with N temperature ranges at the eutectic
◦
compositions
≈
50.1 and 38.7 C for A6/A8 and A6/A16 binary mixtures, respectively.
Furthermore, the molecular geometry (which is affected by the stereo and/or mesomeric
configurations) impacts the molecular-molecular interactions. It can be concluded that the
terminal length of the flexible chain has a pronounced effect that controls both conformation
and steric effect for pure and mixed states. This is consistent with our previous work [26,39].

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Figure 7. Binary phase diagram of (
nematic–isotropic liquid transition.
a)
A6/A8 and (
b)
A6/A16. “#
” denotes crystal–nematic transition, and “ ” denotes
•
3. Experiment
Synthesis
The investigated compounds An were synthesized according to the following Scheme 4
.
Scheme 4. Preparation of 4-(4-(alkoxy)phenylimino)methyl)phenyl nicotinate (An).
◦
(
(
Hexyloxy)phenylimino)methyl)phenyl nicotinate (A6) [45]. Yield: 94.1%; mp. 131.0 C, FTIR
−
1
υ´ , cm ): 2959–2833 (CH2 stretching), 1729 (C=O), 1591 (C=C), 1473 (C–OAsym), 1266 (C–
1
OSym). H-NMR (400 MHz, CDCl3)
δ (ppm) 10.08 (s, 1H, CH=N), 9.59 (s, 1H, pyr),
9
7
.11–9.04 (m, 2H, pyr), 8.10–8.05 (m, 3H, pyr, ArH), 7.43 (d, J = 8.5 Hz, 2H, ArH),
.01 (d, J = 8.1 Hz, 2H, ArH), 6.83 (d, J = 8.1 Hz, 2H, ArH), 9.93 (t, J = 6.2 Hz, 2H,
CH (CH ) CH CH ), 1.92–1.75 (m, 2H,CH (CH ) CH CH ), 1.56–1.51 (m, 6H,
3
2 3
2
2
3
2 3
2
2
CH (CH ) CH CH ), 0.92 (t, J = 6.3 Hz, 3H, CH (CH ) CH CH ). Elemental analyses:
3
2 3
2
2
3
2 3
2
2
Found (Calc.): C, 74.58 (74.60); H, 6.50 (6.51); N, 6.94 (6.96).
◦
(
Decyloxy)phenylimino)methyl)phenyl nicotinate (A8). Yield: 91.0%; mp. 129.0 C, FTIR (υ´
,
−
1
cm ): 2931–2849 (CH stretching), 1735 (C=O), 1607 (C=C), 1473 (C-OAsym), 1268 (C–
2
1
OSym). H-NMR (400 MHz, CDCl3)
δ
(ppm) 10.05 (s, 1H, CH=N), 9.59 (s, 1H, pyr), 9.08 (m,
2
H, pyr), 8.11–8.05 (m, 3H, pyr, ArH), 7.41 (d, J = 8.5 Hz, 2H, ArH), 7.01 (d, J = 8.1 Hz, 2H,
ArH), 6.80 (d, J = 8.1 Hz, 2H, ArH), 9.93 (t, J = 6.2 Hz, 2H, CH (CH ) CH CH ), 1.91–1.76
3
2 3
2
2
(
m, 2H, CH (CH ) CH CH ), 1.56–1.51 (m, 6H, CH (CH ) CH CH ), 0.94 (t, J = 6.3 Hz,
3 2 3 2 2 3 2 3 2 2

Molecules 2021, 26, 2038
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3
H, CH (CH ) CH CH ). Elemental analyses: Found (Calc.): C, 75.30 (75.32); H, 7.00 (7.02);
3 2 3 2 2
N, 6.51 (6.51).
◦
Hexadecyloxy)phenylimino)methyl)phenyl nicotinate (A16). Yield: 92.7%; mp. 122.0 C, FTIR
(
(
(
−
1
υ´ , cm ): 2921–2850 (CH stretching), 1727 (C=O), 1599 (C=C), 1473 (C-OAsym), 1268
2
1
C–OSym). H-NMR (400 MHz, CDCl3)
δ
(ppm) 10.05 (s, 1H, CH=N), 9.57 (s, 1H, pyr), 9.11–
9
2
.06 (m, 2H, pyr), 8.08 (m, 3H, pyr, ArH), 7.43 (d, J = 8.5 Hz, 2H, ArH), 7.02 (d, J = 8.1 Hz,
H, ArH), 6.80 (d, J = 8.1 Hz, 2H, ArH), 9.93 (t, J = 6.2 Hz, 2H, CH (CH ) CH CH ), 1.94–
3
2 3
2
2
1
.76 (m, 2H,CH (CH ) CH CH ), 1.5–1.50 (m, 6H, CH (CH ) CH CH ), 0.93 (t, J = 6.3 Hz,
3 2 3 2 2 3 2 3 2 2
3
H, CH (CH ) CH CH ). Elemental analyses: Found (Calc.): C, 77.44 (77.45); H, 8.51 (8.54);
3
2 3
2
2
N, 5.14 (5.16).
Nearly identical FT-IR spectra were observed for all investigated An series (Figures
S3–S5, Supplementary Materials) that may be attributed to the length of the terminal
alkoxy chain and does not significantly affect the location of the main characteristic FT-IR
absorption bands of the investigated compounds. The effect on the C=O moiety with the
−
1
slight change in the polarity character of the terminal chains was in the range of 8.0 cm
− −1 −1
1
(
1727 cm for A16, 1729 cm for A6 and 1729 cm for A8), while that for the C=N was
6 cm (1591–1607 cm ). It can be concluded that the little change of the polarity of the
−1 −1
1
terminal alkoxy chains has a small effect on the polarization of the ester C=O and –CH=N–
groups.
4
. Conclusions
Series based on three-ring azomethine liquid crystal derivatives, 4-(4-
alkoxy)phenylimino)methyl)phenyl nicotinate (An), were synthesized and evaluated
(
by experimental and computational approaches. Molecular structures were elucidated via
elemental analyses, FT-IR and NMR spectroscopy. Mesomorphic characterization of its
behavior was conducted by DSC, POM. Theoretical simulation of geometrical structure
investigations was carried out by the DFT calculation method.
The study revealed that:
1
.
Independent on the terminal alkoxy chain length, all synthesized groups (An) are
monomorphic exhibiting enantiotropic wide nematic thermal stability.
The structural parameters, dipole moment and the polarizability of the present com-
pounds are highly affected by the length of the attached terminal flexible chain as
well as the N atom in the heterocyclic terminal ring.
2
.
3
4
.
.
Binary phase diagrams constructed between different homologs showed to possess
low melting temperature with wide N mesophase at the eutectic composition.
The different extent of the physical and structural parameters are sharing together to
impact the N temperature range and the thermal stability of present compounds in
their pure and mixed states.
Supplementary Materials: The following are available online, Figure S1, DSC thermograms of A6
derivative upon second heating/cooling scan with rate 10 C/min; Figure S2, DSC thermograms
of A16 derivative upon second heating/cooling scan with rate 10 C/min; Figure S3, FT-IR spec-
°
°
trum of A6 derivative; Figure S4, FT-IR spectrum of A8 de-rivative; Figure S5, FT-IR spectrum of
A16 derivative.
Author Contributions: Conceptualization, L.A.A.-M. and H.A.A.; methodology, H.A.A. and M.H.A.a.;
software, H.H.A.; validation, H.H.A., H.A.A. and L.A.A.-M.; formal analysis, H.H.A. and H.A.A.;
investigation, H.A.A.; resources, L.A.A.; data curation, H.H.A.; writing—original draft preparation,
H.A.A., H.H.A. and M.H.A.a.; writing—review and editing, L.A.A.-M. and H.A.A., H.H.A., L.A.A.;
visualization, H.H.A.; supervision, H.A.A.; project administration, L.A.A.; funding acquisition,
L.A.A.-M. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by the Deanship of Scientific Research at Princess Nourah bint
Abdulrahman University through the Fast-track Research Funding Program.

Molecules 2021, 26, 2038
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Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds An are available from the authors.
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