2-Pyridinyl-Terminated Iminobenzoate: Type and Orientation of Mesogenic Core Effect, Geometrical DFT Investigation

Abstract

A new liquid crystal series of pyridin-2-yl 4-[4-(alkylphenyl)iminomethyl]benzoate was synthesized and characterized for their mesomorphic behavior. These compounds contain Schiff base and carboxylate ester mesogenic cores, in addition to terminal alkyl chains with a different number of carbons. The structures were confirmed via FT-IR, and ¹H NMR spectroscopy. The phase transitions were studied by differential thermal analysis (DSC) and the mesophase types were identified by polarized optical microscopy (POM). A comparative study was performed between the synthesized compounds and previously reported compounds. Density functional theory (DFT) calculations were included in the study to compute the dipole moment and the polarizability, as well as the frontier molecular orbitals and the charge distribution mapping, which impact the terminal and lateral interactions of the compounds. The theoretical results were discussed to confirm the experimental data and explain the mesomorphic behavior of the compounds. Finally, the energy gap, global softness, and chemical hardness were calculated to determine the suitability of the liquid crystalline compounds to be employed in applications.

1. Introduction

Liquid crystal (LC) represents a phase formed in certain compounds that intermediates solid and liquid states of matter. Therefore, the compounds that can form LC have two melting points: the one at which the LC is formed, which appears as a cloudy liquid, and the other one at which the clear liquid state is formed [1]. Liquid crystals are also known as mesophases. This phase is characterized by having unique properties, as it possesses properties from both solids and liquids. It can flow as a liquid but with certain crystalline physical properties [2], such as optical, electrical, and magnetic properties, in addition to the molecule’s arrangement in spatial directions [3]. In 1888, the botanist Friedrich Reinitzer worked on the cholesteryl benzoate, which is a crystal cholesterol derivative extracted from carrot; he found that it has two melting points. He explained that the cholesteryl benzoate crystals lose their rigidity and turn into a milky fluid at the first melting point, which is 145.5 °C, and this fluid finally turns into a clear liquid at the second melting point, which is 178.5 °C. He also discovered different optical properties as violet and blue colors appeared and then disappeared while cooling these crystals. This strange phase is then called liquid crystal [4].

LCs are divided into three classes, which are lyotropic, thermotropic, and polymeric. Lyotropic LCs are formed depending on the change in concentration of material in a certain solvent. One of the most commonly observed systems is the LC formed by water and amphiphilic molecules, forming micelles, such as soap, detergents, and lipids [5]. Polymeric LCs are high-molecular-weight liquid crystals with a degree of flexibility. They are classified according to the molecular structure arrangement of mesogenic monomers [2]. Thermotropic LCs are formed depending on the temperature change. This class is widely observed in televisions, laptops, and mobile phone displays. Thermotropic LCs have many phases, depending on the molecular arrangement and symmetry, which are divided into smectic, nematic, and cholesteric [3]. They are usually organic compounds similar to cholesteryl nonanoate and MBBA [6]. LCs that are either rod-like or disk-like in shape can undergo a nematic phase. In this phase, molecules are ordered in a long-range orientation parallel to each other. They are aligned in a certain direction, defined according to the medium director (the principal axis). Moreover, they lose their positional order upon melting [3,6]. There is another similar phase to the nematic, which is the cholesteric phase. It also has a long-range orientation, with no positional order, unlike the nematic phase. The director of the cholesteric phase varies periodically in solution [3,7]. On the other hand, the smectic phase is stratified, and these well-defined layers can slide over each other, unlike the nematic and cholesteric phases. The smectic phase has no long-range orientation but preserves the positional order, and according to this order, the smectic phase has many types. Smectic A is a phase through which a molecule is positioned perpendicular to the other layers. Smectic B is a phase that exhibits a hexagonal crystalline order within the layers. Smectic C is the reverse of smectic A as the molecules are not positioned perpendicularly [3].

Most thermotropic liquid crystals are mainly composed of an aromatic part that consists of two benzene rings or more and an aliphatic part that is flexible alkyl chains. The rings of the aromatic part are connected by linkage groups such as a Schiff base (-CH=N-). Therefore, it provides a stepped core structure and preserves the molecular linearity and develops high stability for the compound that enables mesophase formation [8,9]. Compounds containing Schiff bases have many applications; they act as systems with high thermal stability, corrosion inhibitors, and have some biological activity. In addition, they have been proven to have photo-efficiency, with the wavelength according to their structure [9,10]. Schiff base LCs have gained attention since the discovery of 4-methoxybenzylidene-4′-butylaniline, which is characterized by being in the nematic phase at room temperature [11,12]. Schiff base LCs that have low molecular weight or exhibit a twist-bend nematic phase have been synthesized and investigated [12]. Compounds containing ester group linking between two benzene rings obtain the characters of double bonds. The mesophase becomes more stable when the mutual conjugation increases between the ester carbonyl and the substituent [10]. Several studies have used LC compounds containing a Schiff base ester mesogenic core because of their remarkable properties and wide temperature range [8]. Schiff base ester LCs containing two or three aromatic rings were noticed to have interesting optical behavior. Moreover, LCs containing only one mesogenic core differ in behavior from other LCs having two or more mesogenic cores [13]. Furthermore, as the terminal substituent length increases, the molecules tend to arrange in a parallel orientation [8]. Moreover, to create new thermotropic LCs having phase transitions, it is important to select a proper mesogenic moiety, terminal groups, and flexible wings. Therefore, to design a new material, computational analysis is considered an excellent and helpful tool to do so. LCs must be investigated for their properties, such as MO energies, the frontier molecular orbitals’ energy difference, and the geometry of molecules [10].

Therefore, the aim of the current study was the synthesis of new liquid crystalline compounds A, B, and C, which were prepared from the reaction between 4-formylbenzoic acid and 4-alkylaniline. Their structure was confirmed with IR and NMR spectroscopy, and their mesomorphic behavior was inspected through POM and DSC. Then, a comparative study was performed of the newly synthesized compounds with previously reported ones, D [14], E [15], F [16], G, H [17], and I [18], to investigate the effect of the terminal chain—whether an alkyl chain or alkoxy chain—the effect of the mesogenic core—whether Schiff base/ester or azo/ester—and whether the aromatic ring is a benzene ring or heterocyclic ring. Figure 1 illustrates the structures of these compounds.

2. Materials and Methods

2.1. Materials

In this study, 4-Formylbenzonitrile (95%) and N,N′-dicyclohexylcarbodiimide (DCC) (99%) were purchased from Sigma Aldrich (Shanghai, China); 4-octyl aniline (97%) and 4-dodecylaniline (97%) were purchased from Sigma Aldrich (Verona, WI, USA); 4-tetradecylaniline (97%) was obtained from Sigma Aldrich (Delhi, India); 2-hydroxy pyridine (98%), 4-dimethylaminopyridine (DMAP) (98%), dichloromethane ($\ge$99.8), HCl (38%), and ethanol (95%) were purchased from Sigma Aldrich (Homburg, Germany).

2.2. Synthesis

The three compounds were prepared according to the following Scheme 1.

2.2.1. Synthesis of 4-Formylbenzoic Acid

First, 2 g of 4-formylbenzonitrile and 10 mL of hydrochloric acid in water were refluxed for four hours. Later, this mixture was left to cool and then the product (4-formylbenzoic acid) was separated by filtration.

2.2.2. Synthesis of 4(((4-Alkylphenyl)imino)methyl)benzoic Acid (X)

Equimolar amounts of 4-formylbenzoic acid (75 mg, 0.5 mmole) and 4-alkylaniline (0.5 mmole) in 15 mL ethanol were refluxed for four hours. Afterward, the mixture was left to cool and then the product was separated by filtration.

2.2.3. Synthesis of Pyridin-2-yl 4(((4-alkylphenyl)imino)methyl)benzoate (A, B, and C)

First, 4(((4-alkylphenyl)imino)methyl)benzoic acid (1 mmole) and 2-hydroxy pyridine (1.05 mmole) were dissolved in dichloromethane (10 mL). Then, 4 mmole of DCC and traces of DMAP (act as a catalyst) were added to the mixture. This mixture was left and stirred for 72 h at normal room temperature. After stirring, the reaction by-product, dicyclohexylurea, was filtered out. Then, the filtrate was evaporated and the obtained product was recrystallized from ethanol. The progress of the reaction was checked using thin-layer chromatography (TLC) on silica gel 60 F₂₅₄ E-Merck (layer thickness is equal to 0.2 mm) plates with hexane-ethyl acetate (1:1 v/v) mobile phase. The spots were observed under a UV lamp at λ = 254 nm.

Pyridin-2-yl 4-[4-(octylphenyl)iminomethyl]benzoate (A)

Yield: 67%, FTIR (ύ, cm⁻¹): 2946–2849 (CH₂ stretching), 1733 (C=O), 1605 (C=N), 1593 (C=C), 1460 (C-O_Asym), 1271 (C-O_Sym). ¹H NMR (400 MHz, CDCl₃): δ/ppm: 0.90 (t, 3 H, J = 6.0 Hz, CH₃(CH₂)₆CH₂), 1.31–1.65 (m, 12 H, CH₃(CH₂)₆CH₂), 2.63 (t, 2 H, J = 6.0 Hz, CH₃(CH₂)₆CH₂), 7.19 (d, 2 H, J = 8 Hz, Ar-H), 7.22 (d, 2 H, J = 8.0 Hz, Ar-H), 7.37 (d, 2 H, J = 8.0 Hz, Ar-H), 7.51 (dd, 1 H, J₁ = 8.0, J₂ = 4.0 Hz, Py-H), 8.02 (d, 2 H, J = 8.0 Hz, Ar-H), 8.48–8.52 (m, 2 H, Py-H), 8.90 (dd, 1 H, J₁ = 8.0, J₂ = 4.0 Hz, Py-H), 9.44 (s, 1 H, CH=N). ¹H NMR (100 MHz, CDCl₃): δ/ppm: 14.12, 22.68, 24.94, 29.53, 29.61, 31.43, 31.78, 33.89, 35.52, 120.83, 121.98, 123.54, 125.37, 129.14, 130.05, 134.54, 137.66, 141.18, 149.31, 151.30, 152.65, 154.19, 158.07, 163.60.

Pyridin-2-yl 4-[4-(dodecylphenyl)iminomethyl]benzoate (B)

Yield: 83%, FTIR (ύ, cm⁻¹): 2946–2849 (CH₂ stretching), 1730 (C=O), 1601 (C=N), 1591 (C=C), 1462 (C-O_Asym), 1269 (C-O_Sym). ¹H NMR (400 MHz, CDCl₃): δ/ppm: 0.89 (t, 3 H, J = 6.0 Hz, CH₃(CH₂)₁₀CH₂), 1.29–1.67 (m, 20 H, CH₃(CH₂)₁₀CH₂), 2.62 (t, 2 H, J = 6.0 Hz, CH₃(CH₂)₁₀CH₂), 7.21 (d, 2 H, J = 8 Hz, Ar-H), 7.23 (d, 2 H, J = 8.0 Hz, Ar-H), 7.38 (d, 2 H, J = 8.0 Hz, Ar-H), 7.51 (dd, 1 H, J₁ = 8.0, J₂ = 4.0 Hz, Py-H), 8.01 (d, 2 H, J = 8.0 Hz, Ar-H), 8.48–8.52 (m, 2 H, Py-H), 8.90 (dd, 1 H, J₁ = 8.0, J₂ = 4 Hz, Py-H), 9.44 (s, 1 H, CH=N). ¹H NMR (100 MHz, CDCl₃): δ/ppm: 14.12, 22.71, 24.93, 25.61, 29.54, 29.77, 31.56, 31.93, 33.94, 35.52, 120.80, 121.99, 123.53, 125.38, 129.14, 130.03, 134.53, 137.68, 141.19, 149.31, 151.30, 152.62, 154.18, 158.06, 163.59.

Pyridin-2-yl 4-[4-(tetradecylphenyl)iminomethyl]benzoate (C)

Yield: 85%, FTIR (ύ, cm⁻¹): 2946–2849 (CH₂ stretching), 1732 (C=O), 1601 (C=N), 1592 (C=C), 1462 (C-O_Asym), 1269 (C-O_Sym). ¹H NMR (400 MHz, CDCl₃): δ/ppm: 0.90 (t, 3 H, J = 6.0 Hz, CH₃(CH₂)₁₂CH₂), 1.28–1.67 (m, 24 H, CH₃(CH₂)₁₂CH₂), 2.6 (t, 2 H, J = 6.0 Hz, CH₃(CH₂)₁₂CH₂), 7.20 (d, 2 H, J = 8 Hz, Ar-H), 7.23 (d, 2 H, J = 8.0 Hz, Ar-H), 7.37 (d, 2 H, J = 8.0 Hz, Ar-H), 7.51 (dd, 1 H, J₁ = 8.0, J₂ = 4.0 Hz, Py-H), 8.01 (d, 2 H, J = 8.0 Hz, Ar-H), 8.48–8.52 (m, 2 H, Py-H), 8.90 (dd, 1 H, J₁ = 8.0, J₂ = 4.0 Hz, Py-H), 9.44 (s, 1 H, CH=N). ¹H NMR (100 MHz, CDCl₃): δ/ppm: 14.12, 22.69, 24.93, 25.61, 29.30, 29.54, 29.66, 29.78, 31.54, 31.93, 33.94, 35.52, 120.80, 121.99, 123.53, 125.39, 129.15, 130.02, 134.53, 137.66, 141.19, 149.31, 151.31, 152.62, 154.19, 158.07, 163.59. The ¹HNMR and C¹³ NMR of Pyridin-2-yl 4-[4-(tetradecylphenyl)iminomethyl]benzoate (C) are shown in Figures S1 and S2 (supplementary materials).

The phase transitions in the materials were determined via differential scanning calorimetry (DSC), DSC-60A, Shimadzu, Kyoto, Japan. Here, 2–3 mg specimens were encased in aluminum pans and heated or cooled in a dry nitrogen environment. The temperature was measured at a rate of 10.0 °C/min. Under an inert nitrogen gas atmosphere, samples were heated from room temperature to 200 °C and then cooled back to room temperature at the same rate. The endothermic peak minima of enthalpy in the heating curves were used to calculate the phase transition temperatures. Temperature monitoring accuracy was better than 1.0 °C.

A polarized optical microscope (POM, Wild, Germany) coupled to a Mettler FP82HT hot stage was used to check transition temperatures and identify phases for the prepared compounds.

3. Results and Discussion

3.1. Mesomorphic Behavior

The phase transitions of all synthesized compounds were studied by differential thermal analysis (DSC), and the mesophase types were recognized by observing their textures through polarized optical microscopy (POM). DSC was used to measure the phase transition temperatures (°C), their enthalpy of transition ΔH (kJ/mol), and their normalized entropy of transition ΔS/R. The results that were derived from DSC measurements and verified by POM are given in

Table 1. Phase transition temperatures (°C) at 10.0 °C/min, enthalpy of transition ΔH (kJ/mol), entropy of transition ΔS (J/mol.K), and normalized entropy of transition of compounds A, B, C, D, E, F, G, H, and I; all values have been calculated for the heating transition.

Compounds	TCr-SmA	TCr-N	TCr-I	TSmA-I	TN-I	ΔHCr-N	ΔSCr-N	ΔSCr-N/R	ΔHCr-SmA	ΔSCr-SmA	ΔSCr-SmA/R	ΔHCr-I	ΔSCr-I	ΔSCr-I/R	ΔHN-I	ΔSN-I	ΔSN-I/R	ΔHSmA-I	ΔSSmA-I	ΔSSmA-I/R
A	-	118.24	-	-	134.16	29.7	75.9	9.1	-	-	-	-	-	-	0.30	0.74	0.09	-	-	-
B	-	118.65	-	-	132.55	38.2	97.5	11.7	-	-	-	-	-	-	0.41	1.01	0.12	-	-	-
C	-	120.32	-	-	133.60	44.7	113.6	13.7	-	-	-	-	-	-	0.50	1.23	0.15	-	-	-
D	-	-	124.3	-	-	-	-	-	-	-	-	35.17	88.49	10.64	-	-	-	-	-	-
E	96.1	-	-	121.5	-	-	-	-	23.5	63.64	7.65	-	-	-	-	-	-	1.6	4.05	0.49
F	-	128.6	-	-	174.2	43.40	108.03	12.99	-	-	-	-	-	-	1.80	4.02	0.48	-	-	-
G	67.3	-	-	87.3	-	-	-	-	28.38	83.36	10.02	-	-	-	-	-	-	1.92	5.33	2.65
H	-	116.42	-	-	130.59	50.48	129.58	15.58	-	-	-	-	-	-	2.03	5.03	0.60	-	-	-
I	-	110.8	-	-	156.8	26.15	68.11	8.19	-	-	-	-	-	-	0.88	2.05	0.25	-	-	-

Cr-SmA = Crystal to smectic A transition; Cr-N = Crystal to nematic transition; Cr-I = Crystal to isotropic liquid transition; SmA-I = Smectic A to isotropic liquid transition; N-I = Nematic to isotropic liquid transition.

. A representative example demonstrating the DSC thermogram of heating/cooling processes for compound A is shown in Figure 2. The results of the POM showed enantiotropic nematic mesophases for all prepared compounds. The existence of sharp peaks in the DSC thermogram indicates the phase transition between the crystal phase and the liquid crystalline phase. Moreover, the small peaks signify the transition from the liquid crystalline phase to the isotropic liquid. The observed mesophase textures for compounds B and C are provided as examples in Figure 3.

A comparative study of the prepared compounds with the reportedly structurally similar compounds E, F, G, H, and I was carried out to explain the effect of the mesogenic core and its orientation on the mesomorphic behavior. It is clear from Figure 4 that compound D is non-mesomorphic. It is worth noticing that the mesophase ranges of compounds A (15.92 °C), B (13.9 °C), and C (13.28 °C) are decreased by increasing the length of the terminal alkyl chain. This could be attributed to alkyl group disorder. The longer alkyl group may result in insufficient space between the layers for the terminal chains to accommodate, causing the layers to be parted. This could reduce the overlap of aromatic rings and the interactions between distinct mesogenic groups, which are essential for the development of the nematic mesophase [19]. On the other hand, the absence of the mesophase for compound D could be due to the presence of the electronegative nitrogen heteroatom in the middle ring of the compound. Conversely, its analogue E compound, which has the nitrogen heteroatom in the terminal outer ring, exhibits a smectic A mesophase with a wide range of thermal stability (25.4 °C). It could be explained in terms of the higher degree of charge separation in compound E compared with its analogue D. The presence of the terminal pyridyl ring affords a higher packing pattern compared to that of the compound D to show a smectic mesophase. It is also observed that compounds F, H, and I possess nematic phases and their mesophase ranges are increased in order H (14.17 °C) < F (45.6 °C) < I (46 °C). Moreover, compound G has a high smectic A mesomorphic range (20 °C). On the other hand, it is worth noting the change in the nematic mesophase range by comparing all compounds discussed. For example, F and I have a broad nematic range, while A, B, and C have a short nematic mesophase range. It is clear that longitudinal diffusion is not favored for long alkyl chains.

3.2. DFT Calculations

3.2.1. Geometrical Structure

We utilized theoretical calculations to complement the experimental findings regarding the investigated compounds. The calculations were executed by Gaussian 09 Revision D.01 software [20] and performed using the DFT/B3LYP method at basis set 6–311G (d, p). The geometrical structures of compounds were subjected to an optimization process to develop a new lowest-energy geometrical structure, which is referred to as the convergence, by minimizing the energy of conformations regarding all geometrical parameters, without exerting any molecular symmetry constraints. All of the optimized compounds were stable due to lacking the imaginary frequency. The structures were drawn by GaussView 5.0.8 [21] and the optimized structures were captured by Chemcraft 1.8 [22]. Additionally, the optimized structures were subjected to a frequency process in the same level of theory and basis set to calculate some significant parameters. The optimized molecular structures are shown in Figure 5 and the twist angles of compounds’ rings are tabulated in

Table 2. The estimated twist angles for A, B, C, D, E, F, G, H, and I compounds.

Compounds	θI-II	θI-III	θII-III
A	27.28	11.20	37.04
B	11.86	26.17	37.57
C	15.15	22.16	36.80
D	0.06	0.06	0.01
E	0.01	0.04	0.05
F	6.25	22.51	28.70
G	48.91	79.97	31.42
H	26.41	6.63	30.57
I	40.15	57.07	16.96

θ_I-II = Twist angle between the planes of I ring and II ring; θ_I-III = Twist angle between the planes of I ring and III ring; θ_II-III = Twist angle between the planes of II ring and III ring.

. It can be noticed that the newly synthesized compounds A, B, and C are non-coplanar. Moreover, the length of the terminal alkyl chain has an insignificant effect on the twist angles of the rings of the compound, but it is worth mentioning that the D and E compounds are coplanar, which can be attributed to the presence of the azo group (-N=N-), which enhances the planarity of the compounds. This could be one of the factors responsible for the formation of the smectic A mesophase in the E compound, as the planarity increases the degree of molecular packing, which creates a more ordered smectic mesophase. Moreover, the presence of an alkoxy terminal chain in compound H may slightly enhance the planarity compared to the alkyl chain in the A compound.

The aspect ratio is another structural parameter that may explain the behavior of the mesophases as it can indicate the collision diameter of the compounds, so the intermolecular interaction increases when raising the aspect ratio values. Thus, we correlate in

Table 3. Mesomorphic parameters for heating cycle (°C), the calculated dimensions (Ǻ), and aspect ratios of A, B, C, D, E, F, G, H, and I compounds.

Compounds	Cr	TSmA	TN	ΔTC	ΔTSmA	ΔTN	Dimensions		Aspect Ratio (L/D)
							Length (L)	Width (D)
A	118.24	-	134.16	15.92	-	15.92	29.62	7.55	3.92
B	118.65	-	132.55	13.9	-	13.9	34.58	8.42	4.11
C	120.32	-	133.60	13.28	-	13.28	37.16	8.45	4.40
D	124.3	-	-	-	-	-	31.32	7.93	3.95
E	96.1	121.5	-	25.4	25.4	-	41.70	7.89	5.29
F	128.6	-	174.2	45.6	-	45.6	31.15	8.06	3.86
G	67.3	87.3	-	20	20	-	28.78	7.27	3.96
H	116.42	-	130.59	14.17	-	14.17	31.35	7.48	4.19
I	110.8	-	156.8	46	-	46	29.05	7.47	3.89

and Figure 6 the mesomorphic parameters of the investigated compounds with aspect ratio values and the ratio of the width to the height.

It is observed that the nematic mesophase ranges of the A, B, and C compounds are decreased upon increasing the aspect ratio. However, compound E has the highest aspect ratio and also possesses the more-ordered smectic A mesophase with a wide range (25.4 °C). The high aspect ratio could increase the space filling of the liquid crystal compounds, consequently enhancing terminal and lateral interactions, thus developing a smectic mesophase. On the other hand, the mesophase ranges decrease for the F, G, H, and I compounds upon the elevation of the aspect ratio. This finding may be explained in terms of the combined impact of various factors, such as the polarizability and dipole moment, that could contribute together to show the developed mesophase.

It is well known that the increment in the dipole moment has a major effect on mesomorphic behavior [23]. It may be rationalized in terms of the degree of molecular packing as it could permit π–π stacking interactions and other particular interactions, such as quadrupolar. Consequently, these may affect the mesophase stability and mesophase range of the liquid crystalline compounds. Thus, the dipole moments were estimated and results were correlated to the mesomorphic parameters of the investigated compounds, and they are tabulated in

Table 4. Mesomorphic parameters for heating cycle (°C), polarizability (Bohr), and dipole moment (Debye) of A, B, C, D, E, F, G, H, and I compounds.

Compounds	TSmA	TN	ΔTC	ΔTSmA	ΔTN	Polarizability	Dipole Moment
							(x)	(y)	(z)	Total
A	-	134.16	15.92	-	15.92	376.16	0.16	−1.45	−0.43	1.52
B	-	132.55	13.9	-	13.9	426.56	0.12	−0.59	−1.25	1.39
C	-	133.60	13.28	-	13.28	451.29	−0.12	0.79	−1.18	1.43
D	-	-	-	-	-	380.01	−1.81	0.76	0.00	1.96
E	121.5	-	25.4	25.4	-	475.63	−5.87	−2.63	0.00	6.43
F	-	174.2	45.6	-	45.6	377.90	3.28	3.27	0.11	4.64
G	87.3	-	20	20	-	368.62	−2.87	−1.86	0.51	3.45
H	-	130.59	14.17	-	14.17	391.56	−1.89	−0.14	−1.31	2.30
I	-	156.8	46	-	46	352.82	3.82	0.46	0.33	3.87

and graphically represented in Figure 7. It is worth noticing that the increase in the dipole moment may cause an increment in the temperature at which the nematic phase is formed for the A, B, and C compounds. Compound A (T_N = 134.16 °C) has a higher dipole moment (1.52 Debye) than compound C (T_N = 133.60 °C), which has a dipole moment value equal to 1.43 Debye. Furthermore, compound B (T_N = 132.55 °C) has the lowest dipole moment value (1.39 Debye) among the newly synthesized compounds. Through the same approach, it was noticed that the polar alkoxy terminal chain increases the dipole moment value to be higher than that of the non-polar alkyl chain. This slight change in the dipole moment may influence the competitive terminal and lateral intermolecular interactions of the compounds. Additionally, the formation of the smectic mesophase with a wide range (ΔT_SmA = 25.4 °C) in compound E might be caused by the high dipole moment (6.43 Debye), which allows a high degree of molecular ordering. Furthermore, the dipole moment increment resulted in an increment in the mesophase range for the F, G, H, and I compounds. These findings can be explained in terms of the high degree of molecular packing in the lattice, which may enhance the terminal intermolecular interactions.

The polarizability of the molecules could be highly influenced by the dimension and the electronic nature of the mesogenic core. Moreover, an increase in the polarizability of the compounds’ mesogenic cores may influence the mesophase ranges and stabilities. Thus, the polarizability of the investigated compounds was also estimated, and the values are included in Table 4 and illustrated in Figure 8. It is obvious that the polarizability increments with the chain length, as seen in the A, B, and C compounds. Moreover, compound E, which possesses a smectic A mesophase, has the highest polarizability (475.63 Bohr). This can be attributed to the presence of the mesogenic core (-N=N-) and the nitrogen atom in the terminal ring, as well as the large size of the compound, which facilitates the polarizability. The relatively low polarizability of the D compound compared to E can be attributed to the presence of a pyridine ring in the center of the compound. Moreover, compound H has slightly higher polarizability than F, which may be rationalized by the orientation of the mesogenic core (COO) and the position of the nitrogen atom in the heterocycle ring. Additionally, the change in the position of the mesogenic core (-CH=N-) and (COO) slightly affects the polarizability, as seen in compounds G and I. These findings accumulate together with other factors, such as dipole moment, to elucidate the mesomorphic behavior of the compounds.

3.2.2. Molecular Electrostatic Potential (MEP)

The electron density at the atomic sites of the studied compounds has a significant impact on the dipole moment, polarizability, and electronic structure. Moreover, the molecular electrostatic potential (MEP) is a useful tool to analyze the electron density distribution in the molecule and is considered an effective method for predicting intermolecular interactions. Thus, the charge distribution map of the compounds was computed by the same method, using the same basis set, according to molecular electrostatic potential (MEP) and is illustrated in Figure 9. The most negatively charged sites are colored in red, whereas the least negatively charged are colored in blue. It is obvious that the negatively charged sites are those with high numbers of electronegative nitrogen and oxygen atoms in the mesogenic cores, such as Schiff base portion (-CH=N-), azo group (-N=N-), and carboxylate ester (COO). On the other hand, the terminal chain is the least negatively charged site. It is noticed that the length of the terminal alkyl chain may have an insignificant effect on the charge distribution of the aromatic ring moieties, as seen in the A, B, and C compounds. Moreover, the charge map distribution is affected by the orientation and position of the mesogenic core, which could influence the type and the temperature range of the mesophase by varying the competitive interaction between terminal interaction and lateral interaction. The distribution of terminal charge for the F and I compounds permits the head-to-tail interaction to develop the nematic mesophase, whereas the high charge separation for the E compound enhances the side–side interaction, resulting in the formation of the more ordered smectic mesophase. In the same context, the accumulated negative charge in the center of the D compound reduces the lateral in addition to the terminal interactions, which could explain the non-mesomorphic behavior.

3.2.3. Frontier Molecular Orbitals (FMOs)

The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are the two frontier molecular orbitals (FMOs). The HOMO is an electron donor, whereas the LUMO is an electron acceptor. The energy gap is the difference in energy between them, which may indicate the electron transition capability. Furthermore, it is inversely proportional to the molecules’ reactivity. The energies of the compounds’ HOMO and LUMO orbitals were computed using the same method and the same basis set. The results are tabulated in

Table 5. The frontier molecular orbital energies (eV), global softness S, and chemical hardness η (eV) of A, B, C, D, E, F, G, H, and I compounds.

Compounds	ELUMO	EHOMO	ΔE	S	η
A	−2.38	−6.15	3.77	0.27	1.89
B	−2.39	−6.15	3.76	0.27	1.88
C	−2.39	−6.15	3.76	0.27	1.88
D	−2.75	−6.34	3.59	0.28	1.80
E	−2.75	−6.26	3.51	0.28	1.76
F	−2.33	−5.78	3.45	0.29	1.73
G	−2.28	−5.81	3.53	0.28	1.77
H	−2.30	−5.80	3.50	0.29	1.75
I	−1.97	−6.07	4.10	0.24	2.05

and the isodensity surface plots for the ground state FMOs are illustrated in Figure 10. One can conclude from the results that the polar alkoxy terminal chain significantly lowers the energy gap of FMOs compared to the non-polar alkyl chain, although it does not influence the location of the FMOs’ electron densities. Additionally, the aromatic rings have most of the electron densities of the sites that form the HOMOs and the LUMOs. In the same approach, the coplanarity of compound E caused by the azo group enhances the conjugation of the aromatic rings, thus decreasing the energy gap of FMOs. Furthermore, the energy gap is greatly affected by the position and the orientation of the mesogenic cores (-CH=N- and COO), as seen in the G and I compounds, which could be attributed to the high degree of conjugation in the G compound. On the other hand, the changing of the terminal chain length has no effect on the energy gap value, as seen in the A, B, and C compounds.

The energy gap values are important to calculate the global softness and chemical hardness of the investigated compounds. The global softness (S), which is equal to 1/(E_LUMO-E_HOMO), can predict the sensitivity of the compound for the photoelectric behavior [24]. The photoelectric sensitivity of the compound improves as its global softness increases. The chemical hardness (η), which is equal to (E_LUMO−E_HOMO)/2, indicates the resistance of the charge transfer. These parameters can determine the suitability of the liquid crystalline compounds to be employed in many applications as transistors. Some molecules that contain alkyl chains were functionalized to give smectic phases, which are optimized for applications as field-effect transistors [1]. In addition, LCs have been used in the manufacture of optical photovoltaic cells. The mechanism of OPV cell operation to exchange light energy into electrical energy is designed as follows: charge generation occurs between two different organic semiconductors, followed by separation and movement toward two opposite electrodes [25]. Recently, LCs served as active signal amplifiers in sensing and biosensing applications due to their high sensitivity properties, rapid response, and easy manufacturing. Thermotropic LC biosensors depend on the molecular interactions that lead to adsorption and desorption at the interface, and due to the optical anisotropy of LCs, the output optical signals will vary according to the samples analyzed [26].

4. Conclusions

Three new liquid crystalline compounds were synthesized and characterized, namely pyridin-2-yl 4-[4-(alkylphenyl)iminomethyl]benzoate. Each of the three compounds has an enantiotropic nematic mesophase. The enhancement of the nematic phase may be attributed to the alkyl group disorder. The longer alkyl group may result in insufficient space between the layers for the terminal chains to accommodate, causing the layers to be parted, which is essential in nematic phase development. A comparative study of the prepared compounds with their reported structurally similar compounds was carried out to explain the effect of the mesogenic core and its orientation on the mesomorphic behavior. The results reveal that the presence of the azo mesogenic core (-N=N-) might enhance the planarity of the compounds. This could be responsible for the formation of the smectic A mesophase in the E compound as the planarity could increase the degree of molecular packing, which creates a more ordered smectic mesophase. Moreover, the increment in the temperature at which the nematic phase was formed for the A, B, and C compounds may have caused the dipole moment to be increased. Additionally, the polar alkoxy terminal chain increases the dipole moment value to be higher than that of the non-polar alkyl chain. With the same approach, the dipole moment increment resulted in an increment in the mesophase range for the F, G, H, and I compounds. Meanwhile, the polarizability increases with the chain length, as seen in the A, B, and C compounds. Moreover, the relatively low polarizability of the D compound compared to E can be attributed to the presence of a pyridine ring in the center of the compound. In addition, it was found using the molecular electrostatic potential that the D compound has non-mesomorphic behavior due to the accumulated negative charge in the center of the compound, which reduces the lateral in addition to the terminal interactions.