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. 2024 Jul 12;9(29):31601–31610. doi: 10.1021/acsomega.4c01564

Mesophase Behavior of Molecules Containing Three Benzene Rings Connected via Imines and Ester Linkages

Fowzia S Alamro †, Hoda A Ahmed ‡,*, Nuha Salamah Alharbi §, Nada S Al-Kadhi †, Omaima A Alhaddadd §, Magdi M Naoum ‡, Mohamed A El-Atawy ∥,⊥,*
PMCID: PMC11270723  PMID: 39072071

Abstract

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Ten new compounds based on the methineazo-azomethine (CH=N–N=CH) and ester linking groups were prepared and investigated for their mesophase behavior and optical stability, and liquid crystals of 4-substituted phenyl methineazo-azomethine phenyl 4-alkoxybenzoates, Ina–e, were investigated. An alkoxy group with a length between 8 and 12 carbons is attached to the phenyl eater wing, while the other terminal ring is substituted in its 4-position with one of the polar NO2, F, Cl, CH3O, and N(CH3)2 groups. The molecular structures of the newly prepared compounds were verified by using 1H NMR, 13C NMR, and elemental analysis. Differential scanning calorimetry and polarized optical microscopy were applied to investigate their mesophase behavior. All members of the prepared homologous series showed excellent thermal mesomorphic stability over wide temperature ranges. The geometrical and thermal properties of the investigated compounds were verified via density functional theory (DFT). The theoretical results revealed that all of the compounds are almost planar. Finally, the experimentally established values of the mesophase data were correlated with the predicted quantum chemical characteristics evaluated by DFT.

1. Introduction

Liquid crystals are materials that exist in a state of matter that exhibits the properties of both liquids and crystals. They have attracted considerable attention due to their unique optical, electrical, and mechanical properties. Liquid crystals have numerous applications in liquid crystal displays (LCDs), optical sensors, telecommunication devices, and many others.1−7 Bis-Schiff base is an important linker group that links two different types of aromatic rings together, producing a linear molecular structure.8 Thus, it can be used in mesogenic molecules, particularly in liquid crystals. One of the most significant properties of the bis-Schiff base linker group is its ability to form strong intermolecular interactions.8 This property is crucial for the formation of ordered structures in liquid crystals, where intermolecular interactions between mesogens are important for maintaining the liquid crystal phase. Moreover, the aromatic rings of the bis-Schiff base can also participate in π–π stacking interactions, which further enhance the ordering ability of the mesogen.8 Another criterion of bis-Schiff base as a linker group is its ability to tune the physical properties of the liquid crystal by varying the nature of the substituents attached to the aromatic rings.8 For example, the inclusion of electron-donating or -withdrawing moieties can modify the strength of the intermolecular interactions and tune the mesogen’s melting point, viscosity, and birefringence. Furthermore, bis-Schiff bases are known for their relative mesomorphic stability and robust nature, which are important for the fabrication of LCDs.9 Due to their high mesomorphic stability, these Schiff base-based mesogens would improve their temperature regions according to their packing behavior, which makes them ideal for various applications in modern technology.10

Many different kinds of liquid crystal compounds have been synthesized using Schiff bases as a connecting group.11−15 The primary forms of connectors within the cores are functional groups with dipole associations, such as ester or amido groups.16,17 The production, kind, mesomorphic stability, and temperature range of the mesomorphic material have all been observed to be significantly influenced by the central bridges and the end groups.18−23 Numerous thermotropic liquid crystals of the Schiff base type and low molar masses have been created and studied.24−26 Additionally, it has been noted that several mesogenic homologous series include two central connections, one of which may be an ester and the other an azomethine.27,28 Sometimes, traditional methods of new material design and development may not yield satisfactory results. Therefore, finding an alternative route becomes an urgent need. Due to their crucial significance in the discovery of novel materials, the reduction of the high costs and lengthy time requirements of materials research, and the quick transformation of new materials into products, theoretical calculations have recently become a crucial component of material design.29−32

To make molecular synthesis and device manufacturing easier, it is crucial to have an extensive understanding of the structures and properties of newly created materials.33−35 Several computational approaches have been used in facilitating the designing of layered and inorganic crystals,36 two-dimensional compounds for energy-related materials29,33,36 functional organic materials,29,35,37 and polymeric materials.29,38 In addition, the combination of theoretical computation and experiment is widely used to obtain an in-depth understanding of the synthesized material structure and properties and generate directional design strategies for experiments.39

The influence of terminal flexible chain lengths on the mesophase behavior using novel homologous series carrying the bis-azomethine central linkage, ((1E,10E)-hydrazine-1,2-diylidenebis(methanylylidene)) bis(4,1-phenylene) dialkanoate, has been reported.8 Herein, the current work aims to prepare (E)-4-((4-(trifluoromethyl)phenyl)diazenyl)phenyl 4-alkoxybenzoate (Inx, Figure 1) to investigate the correlation between the molecular structure and property of mesomorphic compounds with bis-Schiff base central linkages.

Figure 1.

Figure 1

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Structure of 4-substituted phenyl methine/azo-azo/methine phenyl 4-alkoxybenzoates Ina–e.

2. Results and Discussion

2.1. Liquid Crystalline Characteristics of Series Inx

Table 1 and Figure 2 provide the differential scanning calorimetry (DSC) measurements of the transition temperatures and associated enthalpies and entropies for all synthetic series Inx. All of the derivatives were found to be mesomorphically stable, as demonstrated by the consistency of the heating and cooling DSC curves. The DSC heating and cooling traces for compound I12e are shown in Figure 3. The transition temperatures and enthalpy values are calculated using the results of the second heating scan.

Table 1. Transition Temperatures (°C), Enthalpy of Transitions (in kJ/mol), Mesophase Range, and Normalized Entropy for Inx, upon the Heating Cyclea.

comp. X T/°C (ΔH/kJ mol–1) ΔT ΔS/R
I8a NO2 Cr 108.1 (47.7) N 189.0 (1.9) 80.9 0.49
I8b F Cr 111.4 (43.2) N 144.8 (2.1) 33.4 0.60
I8c Cl Cr 97.6 (41.4) N 187.4 (1.8) 89.8 0.47
I8d OCH3 Cr 120.0 (53.8) N 245.0 (1.7) 125.0 0.39
I8e N(CH3)2 Cr 127.5 (41.6) N 200.9 (1.4) 73.4 0.36
I12a NO2 Cr 104.5 (41.6) N 171.2 (2.4) 66.7 0.65
I12b F Cr 103.2(39.7) SmA 167.4 (3.3) 64.2 0.90
I12c Cl Cr 101.3 (37.8) N 178.1 (2.2) 76.8 0.59
I12d OCH3 Cr 110.0 (51.5) N 232.9 (2.0) 122.9 0.48
I12e N(CH3)2 Cr 135.8 (49.0) SmA 220.7(1.9)N 228.7 (1.2) 92.9 0.29
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a

Cr, crystal; N, nematic; SmA, smectic A; T, temperature; ΔH, enthalpy; ΔT, mesomorphic range; ΔS/R, normalized entropy.

Figure 2.

Figure 2

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Mesomorphic behavior of the prepared materials Inx as observed after the second heating rate.

Figure 3.

Figure 3

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DSC thermograms of I12e: recorded from second heating and cooling scans at a rate of 10 °C/min.

Following heating, the example compound, I12e, in Figure 3’s DSC thermogram was found to exhibit three endotherms that are assigned to the crystal–smectic, smectic–nematic, and nematic–isotropic liquid transitions. The compound exclusively exhibits the nematic and smectic phases during the heating and cooling cycles; however, their transitions shifted slightly at lower temperatures in the heating cycle. Mesophase textures were confirmed by polarized optical microscopy (POM) measurements (Figure 4). It was clear from this that the material has enantiotropic monomorphic properties. To evaluate how the length of the terminal alkoxy chain impacts their behavior in the mesophase, Figure 2 graphically represents the transition temperatures of all of the derivatives that have been evaluated.

Figure 4.

Figure 4

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POM textures during heating of compound l12e: (a) SmA phase at 180 °C and (b) N phase at 224 °C.

The melting transitions (Cr/SmA) of the studied derivatives display a different pattern as the alkoxy chain length is increased from n = 8 to 12, as can be seen in Table 1 and Figure 2. The melting point typically rises with the increased polarizability of derivatives within the same series. However, the observed trend did not conform to this common rule. However, such a tendency is not consistent with this overarching rule as reported in our previous work.40−44 Additionally, each homologue of the homologous series is found to be enantiotropic and possesses excellent thermal stability in the mesophase, with a broad temperature mesomorphic range. Thus, for electron-withdrawing terminal NO2-substituted homologues (Ina), their molecules are monomorphic exhibiting only the enantiotropic N phase. Their N temperature ranges (ΔT = TN – TCr) decreased from 80.9 to 66.7 °C with increasing alkoxy chain length from n = 8 to n = 12. Compounds with terminal F-substituent (Inb) are also found to be monomorphic possessing purely nematogenic mesophase for the short terminal chain length (n = 8) and show the smectogenic phase (SmA phase) for n = 12. Its thermal stability increases from 144.8 to 167.4 °C with increasing n from 8 to 12 as well as ΔT is enhanced from 33.4 to 64.2 °C as n increases from 8 to 12. For X= Cl (Inc), all homologues exhibit only the N phase, irrespective of alkoxy chain length, with relatively high thermal stability compared with those of Inb. ΔT is reduced from 89.8 to 76.8 °C. The Cl-substituted analogues possess the highest mesomorphic temperature ranges for electron-withdrawing substituted derivatives. Compounds with electron-donating group CH3O (Ind) possess an N mesophase, with high thermal stability and a broad range. ΔT is slightly decreased from 125.0 to 122.9 °C with increasing n from 8 to 12 carbons. N(CH3)2 substituted derivatives (Ine) were shown to exhibit the N phase for n = 8 and are dimorphic possessing both the SmA and N mesophases for n = 12 with high thermal stabilities. The stability of the nematic phase diminishes for all series, regardless of the polarity of substituent X, while the SmA phase increases as normal with the lengthening of the terminal alkoxy chain.45,46 The downward trend in the thermal transition of the nematic (N) phase is due to the dilution of the rigid mesogenic core. However, as the length of the alkoxy chains increases, the smectic A (SmA) phase appears, decreasing the temperature range of the nematic phase. This likely occurs because increasing the length of the terminal alkyl chain widely promotes microphase separation and facilitates the lamellar structure required for the formation of the smectic phase.47

Based on the results mentioned earlier, the thermal stability of the formed mesophase for n = 8 declines in the following order: CH3O > N(CH3)2 > NO2 > Cl > F, while for n = 12, it decreases in the order: CH3O > N(CH3)2 > Cl > NO2 > F. Their mesomorphic range ΔT is found to decrease in the following order: CH3O > Cl > NO2 > N(CH3)2 > F for n = 8, while for n = 12, the mesophase temperature range decreases in the following order: CH3O > N(CH3)2 > Cl > NO2> F.

Several factors are usually cited as critical in determining the stability of mesophases and their textures, including the polarity of substituent groups, polarizability, aspect ratio, stiffness, and molecular structure. These factors influence the mesophase behavior to varying degrees. It is generally understood that the stability of a mesophase in a given mesomorphic compound improves with enhanced polarity or polarizability in the mesogenic core, which is typically driven by the polarity of substituent groups, thereby affecting the polarity of the entire molecule.

For the present series of compounds, the estimated entropy change of the mesophase transitions (ΔS/R) is shown in Table 1. An uneven trend and modest magnitudes of ΔS/R are seen, independent of the length of the molecules as a function of the terminal alkoxy chain. Changes in molecular interactions between molecules may be responsible for the variance and complexity in the entropy change with compact terminal group X and alkoxy chain length. Although the lengthening of the alkoxy chain reduces the strength of core–core contacts, it raises the polarizability of the entire molecule, which strengthens the forces of intermolecular adhesion between neighboring molecules and enhances the degree of molecular ordering. The removal of the lengthy orientational order and the rise in the number of conformational distributions during the mesophase transitions are likely to be blamed for the increase in ΔS/R values with increasing number of carbons in the alkoxy chain. Higher ΔS/R magnitude, especially when associated with SmA–isotropic transition as in the case of I12b derivatives, is the result of terminally substituting with a polarizable F atom. This process causes the dipole moment of molecules to increase, which improves the lateral interaction and subsequently allows the molecules to pack more efficiently in the liquid crystal phase. Furthermore, the effect of shape and specifically the biaxiality of the molecule play important roles in the entropy change results. The greater the biaxiality, the smaller the orientational order of the nematic phase, and this reduces the N–I entropy changes as reported in previous research studies.48

2.2. DFT Theoretical Studies

Quantum chemical calculations provide a powerful tool for understanding the behavior and properties of liquid crystals and are essential for developing new materials with improved performance characteristics. Optimized molecular structures of the synthesized compounds I12a–e are presented in Figure 5. The thermodynamic parameters as well as the energetics are listed as examples in Table 2.

Figure 5.

Figure 5

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Optimized geometries of series I12x at the B3LYP/6-31+G(d,p) level of theory.

Table 2. Energy and Thermodynamical Properties of the Optimized Structures (in Hartree) for I12a–e.

  E0 E298 H298 G298 S (cal/mol-kelvin)
I12a –1820.522458 –1820.481448 –1820.480504 –1820.608539 269.473
I12b –1715.298659 –1715.259409 –1715.258465 –1715.380935 257.759
I12c –2075.657559 –2075.617863 –2075.616919 –2075.741194 261.558
I12d –1730.526692 –1730.485687 –1730.484743 –1730.610732 265.165
I12e –1749.930912 –1749.888167 –1749.887223 –1750.017333 273.840
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Generally, the molecular structures of liquid crystals are typically characterized by a high degree of planarity, meaning that the molecules largely lie flat and are oriented in specific directions relative to each other. This planarity is essential for the specific alignment of the liquid crystal molecules and consequently for the creation of the ordered phases that are characteristic of such materials. The degree of planarity varies depending on the specific type of liquid crystal molecules and accordingly is influenced by factors such as the molecular structure and temperature and external influences such as electric or magnetic fields. Overall, the high degree of planarity in liquid crystals plays a critical role in their unique properties and makes them useful in a wide range of applications, including in displays, sensors, and optical devices.

As illustrated in Figure 5, all molecular configurations are almost planar, as per the terminal aryl ring that bearing the dodecyloxy group possesses a dihedral angle of ≈0.1°, concerning the central ring. On the other side, the other terminal ring has been proven to lie in the same plane as the central ring; thus, both rings form a dihedral angle of ≈0.1–0.2° concerning the molecular plane of the bis-Schiff base central linking group. Such an extremely small value of the dihedral angle indicates that the entire molecule deviates barely from planarity. Furthermore, the results indicated that the molecular geometry and molecular planarity are not significantly affected by changing the polar X group from an electron-withdrawing group (NO2, F, and Cl) to an electron-donating group (OMe and NMe2). As a result, molecular packing in the condensed liquid crystalline phase is enhanced and improved, depending on the molecular planarity of the investigated compounds 1a–e. Even though our results offer a reliable estimate of the necessary molecular structure in the gas phase, it should be noted that the presence of these compounds in the condensed liquid crystalline phases might show some variations.

Dipole moment and polarizability are important factors of liquid crystals that allow them to be used in a variety of applications and affect their behavior in electric and magnetic fields. The dipole moment is a measure of the polarity of the molecules, which determines the orientational order of the liquid crystal.49 The dipole moment enables the external electric field to influence the liquid crystal’s alignment, resulting in the formation of distinctive features in the material, such as LCDs and photonic crystal fibers. Moreover, the polarity and interactions between adjacent liquid crystal molecules influence the mesophase temperature range, which affects the stability of the liquid crystal.50 Polarizability, on the other hand, refers to the ability of a material to be deformed by an electric field. Liquid crystals have high polarizability, which means that they can be easily distorted by an external electric field. This property is important in the electro-optical response of liquid crystals, where the alignment of the molecules can be changed by applying an electric field. Furthermore, the polarizability of the molecule contributes to the magnitude of the intermolecular interactions and provides stability to the structure, and stability leads to durability and enhanced functionality. The density functional theory (DFT)-calculated dipole moments and molecule polarizability for the titled compounds I12a–e at the same level of theory are displayed in Table 3. The results showed that changing the terminal X group from an electron-donating group (OMe and NMe2) to an electron-attracting group (Cl, F, and NO2) increases the molecular polarity, ranging from 2.619 to 10.727 D.

Table 3. Quantum Chemical Characteristics for the Optimal Structures of the Investigated Molecules, I12a–ea.

E (a.u) I12a I12b I12c I12d I12e
dipole moment (D) 10.727 6.394 6.910 2.619 3.991
polarizability (α) 512.54 471.969 490.685 501.426 539.989
EHOMO (eV) –6.533 –6.227 –6.274 –5.841 –5.225
ELUMO (eV) –3.334 –2.303 –2.385 –2.055 –1.871
ΔE (eV) 3.199 3.924 3.889 3.786 3.354
IP (eV) 6.533 6.227 6.274 5.841 5.225
EA (eV) 3.334 2.303 2.385 2.055 1.871
μ (eV) –4.934 –4.265 –4.330 –3.948 –3.548
χ (eV) 4.934 4.265 4.330 3.948 3.548
η (eV) 1.599 1.962 1.944 1.893 1.677
σ (eV–1) 0.625 0.510 0.514 0.528 0.596
S (eV–1) 0.313 0.981 0.972 0.946 0.838
ω (eV) 7.609 4.636 4.821 4.117 3.754
ΔNmax 3.085 2.174 2.227 2.086 2.116
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a

EHOMO: energy of the highest occupied molecular orbital; ELUMO: energy of the lowest unoccupied molecular orbital; ΔE: energy gap between the HOMO and LUMO; IP: ionization potential; EA: electron affinity; χ: absolute electronegativity; μ: chemical potentials; η: absolute hardness; σ: absolute softness; ω: global electrophilicity; S: global softness; and ΔNmax: additional electronic charge.

The results revealed that the molecule with an electron-donating methoxy-substituted compound, I12d, is less polar than the other compounds in the series I12a–e. However, the highest value of the dipole moment was observed for the electron-donating nitro-substituted molecule, I12a. On the other hand, polarizability has different values according to terminal group X. Compound I12e is more polarizable than the other compounds (see Table 3). We can conclude that the dipole moment and polarizability with different extents affect the stability of the resulting liquid crystal phase. The presence of nematic and smectic phases for compound I12e could be explained in terms of the increment of polarizability.

Quantum chemical parameters are vital for the study of liquid crystals.49,51,52 These parameters help us to predict the stability of the molecule, understand molecular interactions, estimate the optical properties, and design new liquid crystal materials. One of the most important quantum chemical parameters in the study of liquid crystals is the energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). These parameters help to predict the stability of liquid crystals.53 Additionally, the absorption and emission spectra of liquid crystals are influenced by their molecular structure and electronic transitions. The HOMO–LUMO energy gap (ΔE) can be used to predict the electronic transitions that occur in the UV–vis region of the spectrum.54 The DFT calculations of investigated compounds I12a–e revealed that the energy values of HOMO or LUMO are affected by the X group of the terminal ring. Accordingly, the X group has an impact on the HOMO–LUMO energy gap (ΔE), which varies between 3.199 and 3.924 eV and follows the order I12b > I12c > I12d > I12e > I12a. Additionally, the relatively modest energy gaps for all of the compounds under investigation show how soft and reactive these molecules are. The HOMO and LUMO electron densities are primarily confined over the central ring, methineazo-azomethine linker, and the terminal ring containing the X group, as shown in Figure 6. Moreover, the terminal dodecyloxy chain does not participate in the HOMO or LUMO electron densities. Furthermore, absolute electronegativity, χ, chemical potentials, μ, absolute hardness, η, absolute softness, σ, global electrophilicity, ω, global softness, S, and additional electronic charge, ΔNmax, which provide clues regarding the reactivity and stability of the molecules under study, have been determined using the HOMO and LUMO energies (Table 3). These properties are critical for the development of innovative liquid crystal technologies because they influence the materials’ stability, reactivity, and susceptibility to external fields.

Figure 6.

Figure 6

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FMOs for the prepared series I12a–e were calculated at the B3LYP/6-31+G(d,p) level of theory.

The study of molecular electrostatic potential (MEP) is an important component of liquid crystal research, providing valuable information about the chemical and physical properties of these materials and enabling the design of new liquid crystal materials with improved properties. MEP is a measure of the electric potential energy that exists around a molecule and is determined by the distribution of its electrons.55

One of the key properties of liquid crystals is their ability to align themselves in a particular direction in response to an external electric field. This property, known as an electro-optical response, is critical for the operation of LCDs used in televisions, smartphones, and other electronic devices. MEP plays a crucial role in understanding the electro-optical properties of liquid crystals.56 By modeling the MEP of liquid crystal molecules, researchers can predict how these molecules will behave in an electric field and design new liquid crystal materials with improved properties. Furthermore, MEP is also important for understanding the behavior of liquid crystals in solution.13 The interaction between liquid crystal molecules and solvents can affect their stability, solubility, and other physical properties. By measuring the MEP of the solvents, researchers can determine which solvents are most compatible with certain liquid crystal molecules and optimize the formulation of liquid crystal solutions. MEP of compounds 112a–e has been calculated using geometries of compounds optimized at B3LYP/6-31+G(d,p). The MEP’s negative sites, which appear red, have a larger electron density and are more reactive as nucleophiles. Positive sites (green or blue) are electrophilic sites and have a low electron density. The potential energy range of our potential map is found to be −5.407e–2 to +5.407e–2 esu. In all the I12a–e series, as shown in Figure 7, the carbonyl oxygen of the ester linking group and oxygen of the nitro group represent the nucleophilic centers with high electron density, while low electron density is observed near the dodecyl chain. The configuration of the linked alkoxy chain influences the orientation of the charge distribution map, which can impact the type of mesophase through changes in the competitive interactions between end-to-end and side-by-side interactions. The correlation between theoretical charge distribution and observed mesophase types has been reported.44,57,58

Figure 7.

Figure 7

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MEPs of the homologous series I12a–e.

To study how the size of the terminal substituent influences the mesophase behavior of synthesized compounds, we examined the relationship between the nematic–isotropic transition temperature (TN–I) and the van der Waals radius of the terminal substituent, X. This relationship is depicted for compounds I8a–e in Figure 8. It was evident that the transition temperatures do not follow a linear correlation with the shape of the terminal substituent regardless of the length of the alkoxy chain. Compounds with electron-withdrawing moieties (F, NO2, and Cl) and terminal substituents displayed lower nematic–isotropic transition temperatures (TN–I) compared to their corresponding terminal electron–donating groups (OCH3 and N(CH3)2). This indicates that the shape of the terminal substituent influences the mesophase behavior, although the relationship is more or less linearly dependent on the size of the terminal substituent.59,60

Figure 8.

Figure 8

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Relationship between the nematic–isotropic transition temperature and the van der Waals radius of the terminal substituent, X, for compounds I8a–e.

3. Experimental Section

3.1. Materials

4-Hydroxybenzaldehyde, 4-nitrobenzaldehyde, 4-fluorobenzaldehyde, 4-chlorobenzaldehyde, 4-anisaldehyde, 4-(dimethylamino)benzaldehyde, hydrazine hydrate, 4-octoyloxybenzoic acid, and 4-dodecyloxybenzoic acid were purchased from Sigma-Aldrich (Germany). Dichloromethane, N,N′- dicyclohexylcarbodiimide (DCC), ethanol, and 4-dimethylaminopyridine (DMAP) were purchased from Aldrich (Wisconsin, USA).

3.2. Synthesis

The mesomorphic derivatives Ina–e were synthesized as detailed in Scheme 1.

Scheme 1. Synthesis of 4-Substituted Phenyl Methineazo-Azomethine Phenyl 4-Alkoxybenzoates, Ina–e.

Scheme 1

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3.2.1. Synthesis of 4-((E)-((E)-(4-Substitutedbenzylidene)hydrazono)methyl)phenol

  • A.

    To a stirred hot ethanolic solution of hydrazine hydrate (10.1 mmol, 15 mL), an ethanolic solution of 4-hydroxybenzaldehyde (10 mmol, 10 mL) was added dropwise for 15 min. The reaction was completed after heating under reflux for a further 60 min, and the precipitates of the desired product were obtained, which were filtered. Moreover, these were utilized in the next step without further purification.

  • B.

    An ethanolic solution of 4-substitutedbenzaldehyde (10 mmol, 10 mL) was added dropwise for 20 min to a stirred ethanolic solution of the product from step A (10.1 mmol, 12 mL). After 90 min of continued heating under reflux, the reaction was finished. The precipitates of the products Ina–e were obtained, and then filtered, washed, and dried. The product was crystallized from hot ethanol.

3.2.2. Synthesis of 4-((E)-((E)-(4-Substitutedbenzylidene)hydrazono)methyl)phenyl 4-Alkoxybenzoate, Ina–e

Molar equivalents of 4-((E)-((E)-(4-substitutedbenzylidene)hydrazono)methyl)phenol and 4-alkoxybenzoic acid (0.01 mol) were dissolved in 25 mL of dry dichloromethane. DCC (0.02 mol) and a few crystals of DMAP, as catalysts, were added. The solution was left to stand for 70 h at room temperature with continuous stirring. The byproduct, dicyclohexylurea, separated was then filtered off, and the filtrate was then evaporated. The obtained solid residue was recrystallized twice from ethanol to give TLC pure products. Full characterization data are given in the Supporting Information (Figures S1–S21).

3.2.2.1. 4-((E)-((E)-(4-Fluorobenzylidene)hydrazono)methyl)phenol

1H NMR (400 MHz, DMSO) two isomeric mixtures δ 10.20 (brs, 1H, OH), 10.13 (brs, 1H, OH), 8.72 (d, J = 1.9 Hz, 1H, CH=N), 8.68 (d, J = 1.5 Hz, 1H, CH=N), 8.61 (s, 1H, CH=N), 8.56 (s, 1H, CH=N), 8.03–7.86 (m, 4H, Ar–H), 7.78–7.67 (m, 4H, Ar–H), 7.35 (q, J = 8.2 Hz, 4H, Ar–H), 6.89 (d, J = 7.4 Hz, 4H, Ar–H).13C NMR (101 MHz, DMSO) δ 165.63 (C), 165.44 (C), 163.15 (CH), 162.96 (C), 162.20 (C), 161.05 (C), 160.93 (CH), 160.72 (C), 159.78 (CH), 131.24 (CH), 131.15 (CH), 131.10 (C), 130.97 (CH), 130.89 (CH), 130.80 (C), 130.60 (CH), 125.53 (C), 125.22 (C), 116.67 (CH), 116.59 (CH), 116.45 (CH), 116.37 (CH), 116.30 (CH), 116.24 (CH). C14H11FN2O requires: C, 69.41; H, 4.58; N, 11.56; % found C, 69.23; H, 4.66; N, 11.74%.

3.2.2.2. 4-((E)-((E)-(4-Chlorobenzylidene)hydrazono)methyl)phenol

1H NMR (400 MHz, DMSO, D2O) δ 8.67–8.53 (4s, 2H, CH=N), 7.91–7.80 (m, 2H, Ar–H), 7.70 (dd, J = 12.9, 8.7 Hz, 2H, Ar–H), 7.55 (t, J = 7.9 Hz, 2H, Ar–H), 6.93–6.83 (m, 2H, Ar–H). C14H11ClN2O requires: C, 65.00; H, 4.29; Cl, 13.70; N, 10.83; % found: C, 64.80; H, 4.49; N, 10.87%.

3.2.2.3. 4-((E)-((E)-(4-Methoxybenzylidene)hydrazono)methyl)phenol

1H NMR (400 MHz, DMSO) two isomeric mixture δ 10.12 (s, 1H, OH), 8.65–8.59 (3s, 4H, CH=N), 7.87–7.79 (m, 4H, Ar–H), 7.72 (d, J = 8.6 Hz, 2H, Ar–H), 7.08–7.04 (m, 4H, Ar–H), 6.89–6.87 (m, 2H, Ar–H), 3.84 (s, 6H, 2OCH3). 13C NMR (101 MHz, DMSO) δ 162.16 (C), 162.07 (C), 161.28 (CH), 160.98 (CH), 160.84 (C), 160.58 (CH), 130.67 (CH), 130.48 (CH), 130.39 (CH), 127.11 (C), 127.02 (C), 125.49 (C), 116.25 (CH), 114.88 (CH), 114.86 (CH), 55.85 (OCH3), 55.84(OCH3) C15H14N2O2 requires: C, 70.85; H, 5.55; N, 11.02; % found: C, 70.75; H, 5.43; N, 11.22%.

3.2.2.4. 4-((E)-((E)-(4-(Dimethylamino)benzylidene)hydrazono)methyl)phenol

1H NMR (400 MHz, DMSO) δ 10.06 (s, 1H, OH), 8.55–8.51 (3s, 2H, CH=N), 7.70–7.64 (m, 4H, Ar–H), 6.87 (d, J = 8.6 Hz, 2H, Ar–H), 6.78 (d, J = 7.9 Hz, 2H, Ar–H), 3.00 (2s, 6H, N(CH3)2). 13C NMR (101 MHz, DMSO) δ 161.21 (CH), 160.53 (C), 159.88 (CH), 152.48 (C), 130.37 (CH), 130.16 (CH), 125.85 (C), 121.69 (C), 116.20 (CH), 112.15 (CH), 40.19 (CH3). C16H17N3O requires: C, 71.89; H, 6.41; N, 15.72; % found: C, 71.67; H, 6.33; N, 15.87%.

3.2.2.5. 4-((E)-((E)-(4-Fluorobenzylidene)hydrazono)methyl)phenyl 4-(Dodecyloxy)benzoate

1H NMR (400 MHz, CDCl3) δ 8.66–8.60 (m, 2H, CH=N), 8.13 (dd, J = 8.8, 1.9 Hz, 2H, Ar–H), 7.89 (dd, J = 8.5, 1.9 Hz, 2H, Ar–H), 7.86–7.80 (m, 2H, Ar–H), 7.32–7.28 (m, 2H, Ar–H), 7.12 (t, J = 7.9 Hz, 2H, Ar–H), 6.96 (dd, J = 8.7, 1.9 Hz, 2H, Ar–H), 4.03 (t, J = 5.3 Hz, 2H, OCH2), 1.90 (m, 4H, 2CH2), 1.73 (m, 4H, 2CH2), 1.55 (m, Hz, 2H, CH2), 1.43–1.11 (m, 10H, 5CH2), 0.87 (t, J = 6.2 Hz, 3H, CH3). 13C NMR (101 MHz, CDCl3) δ 165.56 (d, 1JC,F = 253.5 Hz, CF), 164.52 (C=O), 163.70 (C), 160.92 (d, 2JC,F = 28.5 Hz, CH),, 153.39 (C), 149.63 (CH), 139.77 (C), 132.32 (CH), 131.52 (C), 130.51 (d, 3JC,F = 8.7 Hz, CH), 130.37 (C), 129.72 (CH), 122.29 (C), 115.95 (d, 2JC,F = 21.9 Hz, CH), 114.34 (CH), 68.33 (OCH2), 34.90 (CH2), 31.90 (CH2), 29.64 (CH2), 29.54 (CH2), 29.33 (CH2), 29.07 (CH2), 25.96 (CH2), 25.44 (CH2), 24.66 (CH2), 22.67 (CH2), 14.10 (CH3). C33H39FN2O3 requires: C, 74.69; H, 7.41; N, 5.28; % found: C, 74.63; H, 7.55; N, 5.39%.

3.2.2.6. 4-((E)-((E)-(4-Nitrobenzylidene)hydrazono)methyl)phenyl 4-(Dodecyloxy)benzoate

1H NMR (400 MHz, CDCl3) δ 8.72 (s, 1H, CH=N), 8.70 (s, 1H, CH=N), 8.32 (d, J = 8.7 Hz, 2H, Ar–H), 8.16 (d, J = 8.8 Hz, 2H, Ar–H), 8.03 (d, J = 8.7 Hz, 2H, Ar–H), 7.95 (d, J = 8.6 Hz, 2H, Ar–H), 7.35 (d, J = 8.5 Hz, 2H, Ar–H), 7.00 (d, J = 8.9 Hz, 2H, Ar–H), 4.06 (t, J = 6.5 Hz, 2H, OCH2), 3.21 (m, 2H, CH2), 1.93–1.32 (m, 18H, 9CH2), 0.90 (t, J = 6.7 Hz, 3H, CH3). 13C NMR (101 MHz, CDCl3) δ 164.56 (CO), 163.79 (C), 162.68 (CH), 159.39 (CH), 153.84 (C), 149.09 (C), 139.98 (C), 132.39 (CH), 131.09 (C), 130.11 (CH), 129.06 (CH), 124.03 (CH), 122.47 (CH), 121.06 (C), 114.30 (CH), 68.40 (OCH2), 34.93 (CH2), 31.93 (CH2), 29.64 (CH2), 29.57 (CH2), 29.36 (CH2), 29.09 (CH2), 25.99 (CH2), 25.46 (CH2), 24.62 (CH2), 22.70 (CH2), 14.13(CH3). C33H39N3O5 requires: C, 71.07; H, 7.05; N, 7.53; % found: C, 71.23; H, 7.25; N, 7.53%.

3.2.2.7. 4-((E)-((E)-(4-Methoxybenzylidene)hydrazono)methyl)phenyl 4-(Octyloxy)benzoate

1H NMR (400 MHz, CDCl3) δ 8.68 (s, 1H, CH=N), 8.65 (s, 1H, CH=N), 8.17 (d, J = 8 Hz, 2H, Ar–H), 7.92 (d, J = 8 Hz, 2H, Ar–H), 7.82 (d, J = 8 Hz, 2H, Ar–H), 7.32 (d, J = 8 Hz, 2H, Ar–H), 6.99 (d, J = 8 Hz, 4H, Ar–H), 4.06 (t, 2H, OCH2), 3.88 (s, 3H, OCH3), 3.24–320 (m, 1H), 1.95–1.32 (m, 12H, 6CH2), 0.92 (t, 3H, CH3). 13C NMR (101 MHz, CDCl3) δ 164.59 (C), 163.71 (C), 162.19 (C), 161.92 (CH), 160.39 (CH), 153.22 (C), 132.36 (CH), 131.83 (C), 130.31 (CH), 129.63 (CH), 126.82 (C), 122.28 (CH), 121.22 (C), 114.37 (CH), 114.30 (CH), 68.37 (CH2), 55.35 (CH2), 34.94 (CH2), 31.82 (CH2), 29.23 (CH2), 26.00 (CH2), 22.67 (CH2), 14.12 (CH3). C30H34N2O4 requires C, 74.05; H, 7.04; N, 5.76; % found: C, 74.08; H, 7.22; N, 5.87%.

3.2.2.8. 4-((E)-((E)-(4-Methoxybenzylidene)hydrazono)methyl)phenyl 4-(Dodecyloxy)benzoate

1H NMR (400 MHz, CDCl3) δ 8.66 (s, 1H, CH=N), 8.63 (s, 1H, CH=N), 8.15 (d, J = 8.8 Hz, 2H, Ar–H), 7.90 (d, J = 8.6 Hz, 2H, Ar–H), 7.80 (d, J = 8.7 Hz, 2H, Ar–H), 7.30 (d, J = 8.5 Hz, 2H, Ar–H), 6.97 (dd, J = 8.8, 2.1 Hz, 4H, Ar–H), 4.04 (t, J = 6.5 Hz, 2H, OCH2), 3.87 (s, 3H, OCH3), 3.26–3.12 (m, 6H, 3CH2), 1.99–1.13 (m, 14H, 7CH2), 0.89 (t, J = 6.7 Hz, 3H, CH3). 13C NMR (101 MHz, CDCl3) δ 164.57 (C), 163.69 (C), 162.17 (C), 161.88 (CH), 160.35 (CH), 153.20 (C), 132.33 (CH), 131.82 (C), 130.28 (CH), 129.59 (CH), 126.80 (C), 122.25 (CH), 121.19 (C), 114.34 (CH), 114.27 (CH), 68.21 (CH2), 55.64 (OCH3), 34.92 (CH2), 31.91 (CH2), 29.65 (CH2), 29.55 (CH2), 29.34 (CH2), 29.08 (CH2), 25.93 (CH2), 25.45 (CH2), 24.68 (CH2), 22.68 (CH2), 14.12 (CH3). C34H42N2O4 requires: C, 75.25; H, 7.80; N, 5.16; % found: C, 75.43; H, 7.65; N, 5.26%.

3.2.2.9. 4-((E)-((E)-(4-(Dimethylamino)benzylidene)hydrazono)methyl)phenyl 4-(Octyloxy)benzoate

1H NMR (400 MHz, CDCl3) δ 8.72 (s, 1H, CH=N), 8.61 (s, 1H, CH=N), 8.17 (d, J = 8.9 Hz, 2H, Ar–H), 7.91 (d, J = 8.6 Hz, 2H, Ar–H), 7.77 (d, J = 8.5 Hz, 2H, Ar–H), 7.31 (d, J = 8.6 Hz, 2H, Ar–H), 7.00 (d, J = 8.9 Hz, 2H, Ar–H), 6.75 (d, J = 8.9 Hz, 2H, Ar–H), 4.06 (t, J = 6.5 Hz, 2H, OCH2), 3.08 (s, 6H, N(CH3)2), 1.89–1.78 (m, 2H, CH2), 1.55–1.43 (m, 2H, CH2), 1.34 (m, 8H, 4CH2), 0.91 (t, J = 6.8 Hz, 3H, CH3). 13C NMR (101 MHz, CDCl3) δ 164.80, 163.68, 162.84, 161.21, 160.31, 159.17, 152.55, 132.36, 130.46, 129.80, 129.44, 122.22, 121.26, 114.36, 111.79, 68.37 (OCH2), 40.19 (N(CH3)2), 31.93 (CH2), 29.57 (CH2), 29.37 (CH2), 29.10 (CH2), 25.99 (CH2), 22.71 (CH2), 14.15 (CH3). C31H37N3O3 requires C, 74.52; H, 7.46; N, 8.41; % found: C, 74.74; H, 7.67; N, 8.55%.

3.2.2.10. 4-((E)-((E)-(4-(Dimethylamino)benzylidene)hydrazono)methyl)phenyl 4-(Dodecyloxy)benzoate

1H NMR (400 MHz, CDCl3) δ 8.65 (s, 1H, CH=N), 8.60 (s, 1H, CH=N), 8.14 (d, J = 8.8 Hz, 2H, Ar–H), 7.88 (d, J = 8.6 Hz, 2H, Ar–H), 7.72 (d, J = 8.8 Hz, 2H, Ar–H), 7.31–7.27 (m, 2H, Ar–H), 6.97 (d, J = 8.9 Hz, 2H, Ar–H), 6.72 (d, J = 8.9 Hz, 2H, Ar–H), 4.04 (t, J = 6.5 Hz, 2H, OCH2), 3.03 (s, 6H, N(CH3)2), 1.92–1.12 (m, 20H, 10CH2), 0.90 (t, J = 6.7 Hz, 3H, CH3). 13C NMR (101 MHz, CDCl3) δ 164.52, 163.60, 162.82, 159.04, 152.86, 152.47, 139.75, 132.26, 130.20, 129.32, 122.12, 114.27, 111.59, 77.38, 77.06, 76.74, 68.28 (OCH2), 40.06 (CH2), 34.86 (N(CH3)2), 31.74 (CH2), 29.27 (CH2), 29.16 (CH2), 29.08 (CH2), 25.93 (CH2), 25.39 (CH2), 24.47 (CH2), 22.60 (CH2), 14.05 (CH3). C35H45N3O3 requires C, 75.64; H, 8.16; N, 7.56; % found: C, 75.33; H, 8.24; N, 7.76%.

4. Computational Methods

Computational methods are provided in the Supporting Information.

5. Conclusions

The effective synthesis of ten new derivatives with bis-Schiff base linking units, ((E)-4-((4-(trifluoromethyl)phenyl)diazenyl)phenyl 4-alkoxybenzoate, provided for experimental and theoretical investigation of both thermal and optical properties. 1H NMR, 13C NMR, and elemental studies were used to confirm their molecular structures. By using DSC and POM instruments, the mesomorphic and optical activity of produced compounds were examined. The study revealed that all synthesized homologous series exhibit excellent thermal stability over a wide temperature range with different mesomorphic types depending on the attached terminal polar group and the alkoxy chain length. Moreover, the DFT results demonstrated that none of the compounds exhibit twisting moieties in the bis-Schiff base region and are all planar. Finally, the investigated compounds showed correlations between the experimentally determined values of the mesophase data and the projected quantum chemical properties.

Acknowledgments

The authors extend their sincere appreciation to “Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2024R107), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia”.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.4c01564.

  • Full characterization information for prepared materials, including 1H NMR, 13C NMR data, and computational methods (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao4c01564_si_001.pdf (926.7KB, pdf)

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