New symmetrical U- And wavy-shaped supramolecular H-bonded systems; geometrical and mesomorphic approaches

Article abstract of DOI:10.3390/molecules25061420

New mesomorphic symmetrical 2:1 supramolecular H-bonded complexes of seven phenyl rings were prepared between 4-n-alkoxyphenylazobenzoic acids and 4-(2-(pyridin-3-yl) diazenyl)phenyl nicotinate. Mesomorphic studies of the prepared complexes were investigated using differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Fermi bands of the formed H-bonded interactions were confirmed by FT-IR spectroscopy. Geometrical parameters for all complexes were performed using the density functional theory (DFT) calculations method. Theoretical results revealed that the prepared H-bonded complexes are in non-linear geometry with U-shaped and wavy-shaped geometrical structures; however, the greater linearity of the wavy-shaped compounds could be the reason for their stability with respect to the U-shaped conformer. Moreover, the stable, wavy shape of supramolecular H-bonded complexes (SMHBCs) has been used to illustrate mesomeric behavior in terms of the molecular interaction. The experimental mesomorphic investigations revealed that all complexes possess enantiotropic smectic C phase. Phases were confirmed by miscibility with a standard smectic C (SmC) compound. A comparison was constructed to investigate the effect of incorporating azophenyl moiety into the mesomeric behavior of the corresponding five-membered complexes. It was found that the addition of the extra phenylazo group to the acid moiety has a great increment of the mesophase stability (T_C) values with respect to the monotropic SmC phase of the five aromatic systems to the high stable enantiotropic SmC mesophase.

Full text of DOI:10.3390/molecules25061420

molecules
Article
New Symmetrical U- and Wavy-Shaped
Supramolecular H-Bonded Systems; Geometrical and
Mesomorphic Approaches
Laila A. Al-Mutabagani 1, Latifah Abdullah Alshabanah 1, Hoda A. Ahmed 2,3,
* ,
Mohamed Hagar 3,4,
*
and Khulood A. Abu Al-Ola 5
1
College of Science, Chemistry Department, Riyadh, Princess Nourah bint Abdulrahman University,
Riyadh 11671, Saudi Arabia; Laalmutbagani@pnu.edu.sa (L.A.A.-M); Laalsabanah@pnu.eud.sa (L.A.A.)
Faculty of Science, Department of Chemistry, Cairo University, Cairo 12613, Egypt
Chemistry Department, College of Sciences, Yanbu, Taibah University, Yanbu 30799, Saudi Arabia
Faculty of Science, Chemistry Department, Alexandria University, Alexandria 21321, Egypt
Chemistry Department, College of Sciences, Al-Madina Al-Munawarah, Taibah University, Al-Madina 30002,
Saudi Arabia; KOla@taibahu.edu.sa
2
3
4
5
*
Correspondence: ahoda@sci.cu.edu.eg (H.A.A.); mhagar@taibahu.edu.sa (M.H.)
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 17 February 2020; Accepted: 11 March 2020; Published: 20 March 2020
Abstract: New mesomorphic symmetrical 2:1 supramolecular H-bonded complexes of seven
phenyl rings were prepared between 4-n-alkoxyphenylazobenzoic acids and 4-(2-(pyridin-3-yl)
diazenyl)phenyl nicotinate. Mesomorphic studies of the prepared complexes were investigated using
differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Fermi bands of the
formed H-bonded interactions were confirmed by FT-IR spectroscopy. Geometrical parameters for all
complexes were performed using the density functional theory (DFT) calculations method. Theoretical
results revealed that the prepared H-bonded complexes are in non-linear geometry with U-shaped and
wavy-shaped geometrical structures; however, the greater linearity of the wavy-shaped compounds
could be the reason for their stability with respect to the U-shaped conformer. Moreover, the stable,
wavy shape of supramolecular H-bonded complexes (SMHBCs) has been used to illustrate mesomeric
behavior in terms of the molecular interaction. The experimental mesomorphic investigations revealed
that all complexes possess enantiotropic smectic C phase. Phases were confirmed by miscibility with
a standard smectic C (SmC) compound. A comparison was constructed to investigate the effect of
incorporating azophenyl moiety into the mesomeric behavior of the corresponding five-membered
complexes. It was found that the addition of the extra phenylazo group to the acid moiety has a great
increment of the mesophase stability (TC) values with respect to the monotropic SmC phase of the
five aromatic systems to the high stable enantiotropic SmC mesophase.
Keywords: phenyl nicotinate; supramolecular H-bonding complexes; U-shaped; wavy-shaped;
smectic phase; DFT calculations
1. Introduction
Geometrical conformations of the liquid crystalline (LC) materials depend on their molecular
structures [
crystalline complexes were reported for the first time by Bradfield, Gray, and Jones [
liquid crystals based on intermolecular hydrogen-bonded interactions are mostly used in display
devices as well as sensor applications [ 13]. Most binary H-bonded mixtures are based on pyridine
and carboxylic derivatives [14 24]. Recently, our research group [25 28] started a new research line of
LCs by illustrating the behavior of the supramolecular H-bonded (SMHB) complexes of carboxylic
1–
5], which could impact mesophase stability. Hydrogen-bonded (H-bonded) liquid
6–8
]. Thermotropic
9
–
–
–
Molecules 2020, 25, 1420; doi:10.3390/molecules25061420
www.mdpi.com/journal/molecules

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acids and pyridine derivatives in the mesophase. These studies focused on the relation between
the computational calculations that were estimated from molecular structure predictions and the
experimental data evaluated from SMHB mesomorphic behavior. A slight change in the molecular
shape of organic materials may change their optical properties and offer new transitions. Moreover,
the importance that arises from the possible orientation of heteroatoms in pyridines is to modify
existing functions and introduce new desired geometrical characteristics to the organic base molecule.
Geometry improvement could be achieved through the introduction of one or two heteroatoms
into their architectures. It is known that the heteroatom effect is responsible for changes in the
molecular characteristics so that it responds to many demands relevant to optics [29–31], material
science [32], and electronics [33], as well as the mesomorphic properties of the LCs materials. The ability
of 4,4’-Dipyridine to form novel networks through H-bonded complexes has a potential for wide
study [34–38]. Further, various changes of spacers [39,40] introduce a conformational flexibility that
affects the mesophase type and stability. Previous studies [40] on the thermal and photo-switching
characteristics of mesomorphic complexes formed via hydrogen bonding between 4,4’-azabipyridine
and 4-alkyloxybenzoic acids were conducted. The presence of dinitrogen atoms with a different position
and orientation leads to the redistribution of electron density and changes in the possible aromatic
character [41], which allows for various geometrical parameters, such as optoelectronic phenomena.
Mesogenic cores, flexible chains, and terminal groups play an important role in the formation,
thermal stability, type, and mesomorphic range of LC compounds. The molecules tend to be oriented in
a parallel arrangement as the length of the terminal substituent increases [42]. In addition, the role of the
terminal chains influences the heliconical and twist-bend nematic phases [43,44]. Moreover, H-bonded
interactions play a significant role in the association of molecules. Recently, density functional theory
(DFT) has become an effective tool due to its excellent performance and consistent results with respect
to experimental evidence. Furthermore, the molecular geometries and thermal parameters, such as the
dipole moment, polarizability, and energy difference between the molecular orbitals of the compound
are interesting investigations [5,45–54].
The goal of our present work is to design new structural seven-ring 2:1 supramolecular H-bonded
complexes, as well as to conduct a theoretical (DFT) and experimental investigation on the effect of the
possible geometrical orientations of prepared binary mixtures. Moreover, our goal is to analyze their
mesomorphic, optical, and photophysical properties and correlate the evaluated variables with the
predicted geometrical parameters. Finally, we study the effect of incorporating an additional phenylazo
group into the previously estimated data of a five-ring system.

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OCnH2n+1
N
N
OCnH2n+1
O
O
H
N
N
O
N
O
O
H
N
O
O
O
N
N
N
N
N
N
H
O
O
H
O
O
N
N
N
N
OCnH2n+1
H2n+1CnO
Syn -
I/An
Anti -
I/An
2. Experimental Section
Preparation of 2:1 Symmetrical Supramolecular Complexes
Symmetrical supramolecular complexes I/An were prepared from 1:2 molar ratios of dipyridine
I
and 4-n-alkoxyphenylazobenzoic acids An, respectively. The solid mixture was melted by stirring to
form an intimate blend and then allowed to cool to room temperature (Scheme 1).

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N
COOH
N
N
N N
N N
N
O
or
+
O
O
O
N
N
OCnH2n+1
1 mole
2 mole
An
I - Syn
I - Anti
OCnH2n+1
Melting with stirring then
cooling to room temperature
N
N
OCnH2n+1
O
O
H
N
N
O
N
O
O
H
N
O
O
O
N
N
N
N
N
N
H
O
O
H
O
O
N
N
N
N
OCnH2n+1
H2n+1CnO
Syn -
I/An
Anti -
I/An
Scheme 1. Preparation of 2:1 symmetric supramolecular H-bonded complexes I/An.

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3. Results and Discussion
3.1. Theoretical DFT Calculations
3.1.1. Molecular Geometry
DFT calculations were carried out using the DFT/B3LYP method at basis set 6-311G for the base
and all H-bonded supramolecular complexes I/An. The dinitrogen base was considered to have
two conformers, syn and anti-one, and consequently, all H-bonded compounds could exist in both
conformers. Figure 1 shows the proposed geometrical structures of both conformers of the H-bonded
complexes of the base I with the azo acid A6 of six carbon atoms in the alkoxy chain.
Figure 1. The calculated molecular geometry of Syn-I, Anti-I, Syn-I/A6 and Anti-I/A6.
As shown in Figure 1, the orientation of the nitrogen atom of the symmetrical dinitrogen base has a
high impact on the molecular structure of the base as well as the H-bonded complexes. The dinitrogen
base I is present in two possible conformers, syn and anti. Even though the base I as well as the
acid An are completely linear, the molecular geometry of the H-bonded complexes are non-linear
for both conformers. The formed, supramolecular H-bonded complexes, I/An, derived from the
syn-isomer of the base, exhibit a non-linear geometry with a U-shaped geometrical structure; however,
their corresponding conformational isomeric anti-isomers are also non-linear in geometry with a
wavy-shaped structure.

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3.1.2. Thermal Parameters
All thermal parameters were calculated with the same method of calculations under the same set
for both conformers of the base I as well as H-bonded complexes (I/An); the data are summarized in
Table 1. The syn-isomer of the base is more stable than that of anti-one, with ∆ .
H = 0.2788 kcal mol−1
The small value of enthalpy change between the isomers is evidence of the equilibrium existence
of both isomers. This higher stability of the syn-isomer could be explained in terms of the better
conjugation of the three rings. However, the syn-isomer H-bonded complex I/An is less stable than
that of anti-one. The enthalpy difference between the same chain length of the acid (n = 8) is only
0.7222 kcal mol−1. The higher stability of anti-isomers of the H-bonded complexes could be illustrated
in terms of a greater planarity of the anti-isomer with respect to syn-one.
Table 1. Thermal parameters (Hartree/Particle) of both conformers of the base I as well as the H-bonded
complexes I/An.
Conformer
Parameter
Ecorr
Syn
I
Anti
I
Syn
Anti
I/A6
1.0
I/A8
I/A10
I/A12
I/A6
I/A8
I/A10
I/A12
0.262
0.262
1.1
1.3
1.4
1.0
1.1
1.3
1.4
ZPVE
Etot
−1024.112
−1024.094
−1024.093
−1024.163
−1024.112
−1024.093
−1024.092
−1024.163
−3168.2
−3168.1
−3168.1
−3168.3
−3325.3
−3325.2
−3325.2
−
3325.4
−
3482.4
3482.4
3482.4
3482.6
−
−
−
−
3639.6 3168.2
−
−
−
−
3325.3
3325.2
3325.2
3325.4
−
3482.4
3482.4
3482.4
3482.6
−
3639.6
3639.5
3639.5
3639.7
−
−
−
3639.5
3639.5
3639.7
−3168.1
−3168.1
−
3168.3
−
−
−
−
−
−
H
G
3.1.3. Frontier Molecular Orbitals Dipole Moment and Polarizability
Figures 2 and 3 show the estimated plots for frontier molecular orbitals HOMO (highest
occupied) and LUMO (lowest unoccupied) of conformers of the base I, as well as the H-bonded
complexes and I/An. The figures emphasize that the electron densities are mainly localized on the
4-n-alkoxyphenylazobenzoic acids for all HOMOs while they shifted to the base in the case of LUMOs.
The magnitude of energy difference between the Frontier molecular orbitals (FOMs) could be used in
the prediction of the capability of the electron to transfer from HOMOs to LUMOs during any electronic
excitation process. The global softness (S) = 1/∆E is the parameter that predicts the polarizability as
well as the sensitivity of the compounds for the photoelectric effects. The higher global softness of
the compounds enhances their photoelectric sensitive as well as their polarizability. As shown in
Table 2 and Figure 2, the orientation of the nitrogen atoms of the dinitrogen base has little effect on the
FMOs’ energy gap as well as on the polarizability and the dipole moment of the H-bonded complexes.
The more stable anti-conformer of the H-bonded complex derived from base I is softer than that of the
other conformer. Moreover, the lower energy difference of the anti-isomer increases its polarizability.
Additionally, the length of the alkoxy chains of the acid component highly affects the polarizability.
It has been reported that the more polarizable the compound is, the better the characteristics of the
liquid crystalline are for certain applications [55–57]. Another important parameter that affects the
type of the mesophase and its stability is the dipole moment. It is obvious from Table 2 that the dipole
moment of the U-shaped H-bonded complexes is higher than that of the wavy shape derived from the
anti-isomer of the dinitrogen base I.

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Figure 2. The estimated plots for frontier molecular orbitals of both conformers of dinitrogen base I.
Figure 3. The estimated plots for frontier molecular orbitals of both conformers of the H-bonded
complexes I/An.
Table 2. FMO energies (a.u.), polarizability,
α
(Bohr3), and dipole moment
µ
(Debye) of both conformers
of the supramolecular H-bonded (SMHB) complexes I/An.
Syn
Anti
Parameter
I/A6
I/A8
I/A10
−0.12055
−0.22068
0.10013
2.7941
I/A12
−0.12056
−0.22066
0.1001
I/A6
I/A8
I/A10
−0.12080
−0.22131
0.10051
2.0176
I/A12
−0.12078
−0.22126
0.10048
2.0260
ELUMO
EHOMO
−0.12030
−0.22075
0.10045
2.6897
−0.12035
−0.22073
0.10038
2.7634
−0.12081
−0.21410
0.09329
1.9935
−0.12080
−0.22135
0.10055
1.4576
∆EHOMO-LUMO
Total µ
2.8109
Polarizability α
893.01
941.85
988.48
1036.07
902.07
951.13
999.38
1047.24
3.1.4. Molecular Electrostatic Potential
The charge distribution map for both conformers of the H-bonded complexes I/An was calculated
with the same method for the same basis sets according to molecular electrostatic potential (MEP)
(Figure 4). The negatively charged atomic sites (the red region) were estimated to be localized on
carboxylate moiety of the alkoxy azo acid, while the moiety of the dinitrogen base as well as the alkyl
chain were predicted to show the least negatively charged atomic sites (blue regions). As shown in
Figure 4, the orientation of the charges was affected by the orientation of the nitrogen atoms of the
base I, and consequently, this could affect the variation of the mesophase with the orientation of the
dinitrogen bases.

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Figure 4. Molecular electrostatic potentials (MEP) of both conformers of the H-bonded complexes I/An
.
3.2. Experimental Results
3.2.1. FT-IR Conformation of Prepared SMHBLCs
The experimental vibrational bands were examined using the FT-IR absorption spectrum of the
dinitrogen base and its H-bonded complex I/A10 (Figure 5).
Figure 5. FT-IR spectra of I, A10, and I/A10.
For C-H vibrations, the C-H vibrational stretching bands for aromatic rings and aliphatic moiety
are located at 3100–2800 cm−1. For the C=O of the free alkoxy azobenzoic acid, their vibrational bands
are situated at 1680 cm−1, and it has been reported that the length of the alkoxy chain has no significant
effect of on the vibrational frequency of the C=O group’s stretching vibration. The signal at 1680 cm−1
was the stretching vibration of the C=O group of the dimeric form of the 4-alkoxyphenyl azo benzoic
acid. The dihydrogen bond between the nitrogen of the dipyridine (I) and the 4-alkoxyphenyl azo
benzoic acid (An) of the supramolecular complexes I/An replaces the bis H-bonds of the dimeric form

Molecules 2020, 25, 1420
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of the acid. For the C=O group of the dipyridine, the formation of the complex has an intensive effect
on the stretching vibration of the C=O group of the ester linkage of the dipyridine part; its wave
number increases by 16 cm−1 from 1727 to 1743 cm−1. For Fermi resonance vibration bands, it has been
reported [22
,58–63] that they are an important evidence on the H-bond formation. There are three
Fermi bands of the H-bonded OH groups, A-, B-, and C-types. The vibrational band assigned to the
A-type Fermi band of complex I/A10 is located under the C-H vibrational peaks at 2935 to 2856 cm−1
However, the band at wave numbers lying in the span of 2356 cm−1 could be identified as the in-plane
bending vibration of the OH group as well as its fundamental stretch (B-type). The stretching bands
that are observed at 1906 cm−1 were assigned to C-type Fermi bands due to the interaction between
the fundamental stretching vibrations of the OH and the overtone of the torsional effect.
.
3.2.2. Mesomorphic and Optical Investigations
Interestingly, we must study the mesomorphic and optical behavior of the prepared supramolecular
H-bonded complexes, which have seven aromatic rings (I/An). Investigations of complexes were
carried by DSC, and textures under polarizing optical microscopy (POM) confirmed the data evaluated
from DSC. Results of the transition temperatures that associate enthalpy and normalized entropy
of mesophase transitions for all the analyzed di-symmetric SMHB complexes I/An, as derived from
DSC measurements, are collected in Table 3. Mesomorphic transition temperatures are graphically
represented in Figure 6 in order to study the effect of the terminal alkoxy chain length of the acid
component on the mesophase behavior. During the second heating and cooling scans, all SMHB
mixtures (I/An) showed an enantiotropic smectic C phase with a good range of thermal stabilities.
The POM investigations confirmed the smectic C (SmC) mesophase textures (Figure 7). The data
values of Table 1 and Figure 4 reveal that enantiotropic smectic C mesophase was only observed for
all investigated 2:1 mixtures, and their thermal stabilities increased by increasing the terminal alkoxy
acid chain length. Moreover, irregular trends were observed in the melting point transitions of the
complexes. Further, all investigated complexes exhibit two solid crystalline phases (given as Cr1 and
Cr2), followed by an SmC phase. In order to confirm that the SmC phase is the only mesophase
exhibited by all complexes, a binary phase diagram was constructed between I/A6 and the smectogenic
4-hexadecyloxy benzoic acid as an example. The diagram is depicted in Figure S4 as supplementary
data; the phase diagram exhibits only the enantiotropic SmC phase.
Figure 6. Graphical DSC transitions of 2:1 SMHB complexes, I/An.

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Figure 7. Polarizing optical microscopy (POM) smectic C (SmC) phase texture upon heating for 2:1
supramolecular complex: (a) I/A8 at 185 oC and (b) I/A12 at 197 oC.
Table 3. Phase transition temperatures (◦C), enthalpy of transitions (kJ/mol), and normalized transition
entropy of supramolecular H-bonded complexes I/An.
Compound
I/A6
TCr1-Cr2
130.4
TCr2-SmC
139.7
∆HCr2-SmC
38.67
TSmC-I
183.2
191.5
203.7
209.4
∆HSmC-I
3.91
∆S/R
1.03
0.77
0.68
0.92
I/A8
124.0
147.0
27.21
2.98
I/A10
150.2
184.2
21.99
2.70
I/A12
144.7
169.2
28.24
3.68
Abbreviations: TSmC-I = Smectic C to isotropic liquid transition. ∆HSmC-I = Smectic C to isotropic liquid phase
transition. ∆S/R = Normalized entropy transition.
It was reported [64] that the dipyridine derivative I is non-monomorphic and converted directly
from a crystalline solid state to isotropic liquid states at 150.7 ◦C without displaying any liquid crystal
phase. Meanwhile, the pure alkoxyphenylazobenzoic acids An exhibited an smectic C mesophase,
followed by a narrow range of nematic phase (N) [28]. However, all prepared 2:1 supramolecular
complexes I/An showed only the SmC phase with relatively higher ranges of mesophase stability for
the complex I/A8 (~44.5 ◦C) and the lower value for I/A10 (~19.5 ◦C). In addition, the SmC transition
stability decreases with the increment of the acid chains (n), and thus, the terminal length and the
mesogenic core of the H-donor component have an important role in enhancing the stability of the
formed phase. Moreover, it was found that the polarity difference between H-donors and H-acceptor
affects the hydrogen bonding strength and increment of the molecular anisotropy, and it promotes
broadening of the mesophase range [28]. However, the polarity of both components of the mixture is
not affected by the length of the terminal alkoxy chain. From a theoretical calculations study, it can be
concluded that the higher dipole moment increases the lateral interaction with respect to the terminal
one, and this could be another good explanation of the smectic mesophase formation for the H-bonded
complexes I/An. Furthermore, the wavy shape of the anti-isomer of the H-bonded complexes I/An
explains the smectic C mesophase. The lateral interaction of the wavy shape could be higher than the
terminal interaction; consequently, the slightly ordered smectic phase is a predominating phase. As the
terminal chain length increases, the lateral interaction will be enhanced, and the stability of the SmC
phase increases. This geometry permits the maximum terminal alkoxy chain aggregations to show the
smectogenic mesophase. Moreover, the increment of the chain length highly enhanced the estimated
stability of the complexes, whereas the predicted energy deceases by lengthening the chains; however,
the difference in the energy between the proposed complexes is the same.
In order to investigate the effect of the addition of an extra phenylazo group moiety into the
previously investigated complexes, I/Bn [64], on the mesophase thermal stability (TC), a comparison
was made with the data reported before [64] and is illustrated here in Figure 8. As can be seen from
Figure 8, the addition of the extra phenylazo group into the acid component resulted in a great increase
of the Tc values from the monotropic SmC phase of I/Bn to the enantiotropic high stable SmC mesophase
for I/An. These increases are attributed to the increase of polarizability of the whole molecule, as well

Molecules 2020, 25, 1420
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as the increase of rigidity and the aspect ratio, which in turn lead to the increase of intermolecular
interactions between molecules.
O
H2n+1CnO
O
O H
OCnH2n+1
N
O
O
H O
N
N
N
I/Bn
240
200
160
120
80
I/An
I/Bn
6
8
10
12
Alkoxy chain length (n)
Figure 8. Comparison between present SMHB complexes I/An
thermal mesophase stability.
(
•
) and five-ring complexes I/Bn
(
#
) in
3.2.3. Entropy Changes
Normalized entropy of smectic C transitions (∆S/R), driven from DSC, were calculated for the
present symmetrical supramolecular H-bonded complexes I/An; the results are collected in Table 3.
The results indicate that a random trend of the entropy changes with the terminal alkoxy chain lengths.
The variation in the entropy change with acid terminals may be attributed to a change of molecular
interactions between molecules, which are affected by the dipole moment, polarizability, rigidity,
aspect ratio (length/breadth ratio), and geometrical shape of molecules. These factors may contribute
to the conformational, orientational, and translational entropies of the molecule in different amounts.
Moreover, the relationship between the change in the entropy, ∆S/R, and the type of the mesophase
could be explained in terms of the wavy shape of the H-bonded complexes I/An, as previously discussed
from the DFT calculations. Moreover, it could be emphasized from DFT that the increment of the alkoxy
chain length increased the predicted total electronic stability and resulted in enhancing the smectic
mesophase stability effect of the prepared H-bonded complex I/An. These results could be explained in
terms of the enhancement of the alkoxy chain wing aggregation as well as the higher degree of parallel
interactions. The longer the terminal lengthens, the more conformational orientations of the molecules,
the higher the total thermodynamic energy and the smectic C stability (TC) are. Finally, incrementing
the alkoxy chain length of I/An does not impact either the orientation or the amount of the charges in
MEP calculations, and consequently, it could be used to illustrate the smectic C phase observed for all
H-bonded complexes with little increment of the mesophase stability of a longer chain length.
4. Conclusions
Here, we reported the symmetrical 2:1 supramolecular H-bonded complexes of the seven phenyl
ring system. Mesomorphic and optical investigations were carried out by DSC and POM. H-bonded
interactions were confirmed via the formation of Fermi bands using FT-IR spectroscopy. DFT theoretical

Molecules 2020, 25, 1420
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calculations were performed to estimate the thermal and geometrical parameters for the present SMHB
complexes and their individual components. Results revealed that the geometrical structures of
the present complexes are non-linear and have U-shaped and wavy-shaped structures. Moreover,
the wavy-shaped structure is more stable than the U-shaped conformer. The mesomorphic study
revealed that the symmetric orientation of dinitrogen atoms in the base moiety has an important role in
the observation of the enantiotropic SmC mesophase with a broad range of stability than the previously
reported five-ring complexes system. Furthermore, the wavy shape of the SMHB complexes has been
used to illustrate the mesomeric behavior in terms of the molecular interaction.
Supplementary Materials: The following are available online, Scheme 1. Synthesis of 4-(2-(pyridin-3-yl)diazenyl)
phenyl nicotinate (I). Figure S1: 1H-NMR of 4-(2-(pyridin-3-yl)diazenyl)phenyl nicotinate (I) Figure S2: DSC
thermograms upon second heating /cooling cycles for supramolecular complex I/A10. Figure S3: DSC thermograms
upon second heating /cooling cycles for supramolecular complex I/A12. Figure S4: Binary phase diagram of the
complex I/A6 with 4-hexadecyloxybenzoic acid; Cr1 to: smectic C transition (
#
); Cr2 to smectic C transition ( );
•
and SmC to isotropic liquid transition (ꢀ).
Author Contributions: Data curation, L.A.A.-M., H.A.A. and M.H.; formal analysis, L.A.A.-M., L.A.A., and M.H.;
funding acquisition, K.A.A.A.-O., L.A.A.-M., L.A.A., and H.A.A.; methodology, L.A.A.-M., L.A.A., H.A.A., and
M.H.; project administration, M.H.; resources, L.A.A. and H.A.A.; software, M.H.; writing—original draft, H.A.A.
and M.H.; writing—review and editing, H.A.A., M.H., and K.A.A.A.-O. All authors have read and agreed to the
published version of the manuscript.
Acknowledgments: This research was funded by the Deanship of Scientific Research at Princess Nourah bint
Abdulrahman University through the Fast-track Research Funding Program.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds IAn are available from the authors.
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