Liquid crystalline compounds containing lateral thiol group: synthesis, characterisation, its mesomorphic properties and DFT studies

ABSTRACT One new homologous series of mesogenic compounds containing imine (-CH = N-) and ester (-COO-) linkages as well as lateral thiol (−SH) group and terminal alkoxy chain was synthezised. Molecules in the present work was convinced with respective right structure confirmed with FT-IR, 1H-NMR and 13C-NMR spectroscopy. Thermotropic properties of the compounds were investigated on a hot-stage plate of a polarising optical microscope and further by differential scanning calorimetry. All the compounds of the series exhibited excellent liquid crystalline nature. Structure-property relationships, the influence of different flexible spacers and the mesophase ranges were studied. Computational studies and the electron density method was performed with B3LYP method for all molecular structures in present work. The mesophase ranges were found to be dependent on the length and flexibility of the alkoxy chain, as well as the presence of the lateral thiol group. The comparative study of the present series with the geometrically related series provided valuable insights into the effect of molecular shape on the mesomorphic properties. GRAPHICAL ABSTRACT


Introduction
Anisotropic materials with different properties in different direction can be created using liquid crystalline (LC) materials.Overall, LC materials have a wide range of applications in various fields, from electronics to optics, and continue to be an area of active research and development [1][2][3].Understanding the relationship between molecular structure and mesomorphic behaviour is crucial for designing new materials with desired properties because its geometry alters its mesogenic properties [4].For example, the length of the alkyl chains, presence of polar groups, lateral substituents, etc. in a LC molecule can affect its melting point and the temperature range of its mesophase [4,5].To create a novel geometry for LC material that might produce compounds with varying mesomorphic characteristics and other technological uses, the imine (-CH = N-) group is regarded as a worthy connecting group between two aromatic rings.The imine group possesses unique characteristics such as planar geometry, strong π-conjugation and the ability to maintain the rigidity and linearity of the molecular geometry, hence promoting the stability of mesomorphic properties of LC materials.Several studies conducted by different researchers on various LC compounds containing esters, azoesters, chalcones, Schiff's base moieties and the benzylidene phenyl benzoate fragment have attracted much interest in general and are central to the study of twist bend phases [6][7][8][9][10][11][12][13][14].Schiff bases have been used as effective corrosion inhibitors [15], some Schiff bases have shown promising antibacterial and antifungal properties, making them potentially valuable for use in medicine and agriculture [16] and also in thermostable systems [17,18].Furthermore, the presence of the ester group can affect the reactivity and stability of the molecule.Additionally, the ester linkage can influence the solubility, melting point and polarity of the molecule, which are important factors for its applications in different fields.Because of this, it has recently been particularly important in ferroelectric nematic liquid crystals [19][20][21][22].The LC Schiff base ester incorporating a phenolic thiol (−SH) lateral group is rare.In summary, the presence of conjugative interactions within the ester moiety and the benzene rings lead to certain double bond features and increases the phase stability of the mesophase.
Yang et al. [23] describe the synthesis and characterization of a main-chain chiral smectic LC thiol-ene polymer system.Beckel et al. investigated the electro-optic properties of polymer stabilised ferroelectric liquid crystals (PSFLC) with thiol-ene polymers and found that varying concentrations of linear or crosslinked thiol-ene polymer affected the properties of the PSFLCs [24].A number of thiol-isocyanate-ene ternary networks in polymer dispersed liquid crystal (PDLC) films with good electrooptical and adhesion properties have been reported [25,26].The synthesis of terminally thiol-functionalized alkoxycyanobiphenyls has also been reported [27].Bubnov et al. synthesized new chiral thiols with biphenyl and phenyl moieties and ester linkage groups in the molecular core [28].Novel phenyl benzoate-based and biphenyl-based LC thiol-ene monomers with mesogenic properties were reported earlier [29].The presence of sulphur-containing spacers had a significant impact on the mesophase formation in low molecular mass compounds [30].There have been reports of liquid crystal dimers with an S-S link in the flexible spacer.The introduction of sulphur-containing spacers has been found to enhance the mesomorphic properties of liquid crystal dimers [31][32][33][34][35][36][37].The Schiff base-ester central linkage also contributes to the stability of the liquid crystal phase, allowing for improved thermal and mechanical properties [38].Another example of a thiol liquid crystal derivative is 1,3,5-tris[4-(4'-octyloxybenzoyloxy)benzoyloxy]benzene-2-thiol, which contains a discotic mesogenic unit.This compound has been used in the preparation of highly ordered self-assembled monolayers on gold surfaces, which are important for biosensing applications [39].Many other reports are present on the LC alkylthio derivatives [40][41][42][43][44]. Hagar et al. [45] studied the mesophase stability of new Schiff base ester liquid crystals with different polar substituents.We have previously reported [46] Schiff base esters with lateral thiol group.The effects of these substituents on the thermal and mesomorphic properties of the compounds were investigated, revealing that the nature and position of the substituents can have a significant impact on the LC behaviour of the molecules [47].Some recent reports reported that thioethers can give rise to chalcogen bonding in liquid crystals [48].
The lateral thiol group is known to significantly affect the mesomorphic properties of the compounds due to its strong intermolecular interactions.Overall, the synthesis and investigation of novel LC compounds with various spacers and substituents has led to a better understanding of the factors that influence their mesomorphic properties and potential applications, so here we report the synthesis and characterisation of novel Schiff base ester with lateral thiol group.The synthesis of the homologous series involved the gradual increase in the length of the alkoxy chain.The synthesized compounds were characterized using various techniques, such as Polarizing Optical Microscopy (POM) and Differential Scanning Calorimetry (DSC), to determine their mesomorphic properties.The effect of the terminal alkoxy chain length and lateral thiol group on mesophase stability were studied.Density Functional Theory (DFT) calculations were used to study various theoretical parameters for the series we have synthesized, and theoretical findings were then compared to experiment results.

Measurements
Aluminium sheets pre-coated with Kiesel 60F254 silica gel were found to be an effective stationary phase for organic compound separation using thin layer chromatography (TLC).A Bruker spectrometer was used to acquire Fourier Transform Infrared Spectroscopy (FTIR) spectra of compounds as potassium bromide (KBr) pellets.Using deuterated chloroform (CDCl 3 ) solvent and internal standard tetramethyl silane (TMS), proton nuclear magnetic resonance ( 1 H-NMR) and carbon-13 Nuclear Magnetic Resonance ( 13 C NMR) spectrum data were obtained on an Advance Bruker 400 spectrometer (400 MHz).J values are given in Hz.The optical texture of various substances was examined using Polarizing Optical Microscope (POM), the Leica DM 2500P POM.Differential Scanning Calorimeter (DSC-822)Mettler Toledo with Stare software was used to calculate the transition temperatures and enthalpies (in kJ mol −1 ) of novel mesogenic compounds.DSC measurements were investigated using aluminium pans and sample quantities of 2-3 mg.All measurements were taken in a nitrogen atmosphere at 30 ml min −1 with a heating rate of 10°C min −1 , and all transitions were recorded starting with the first heating and cooling cycle.The molecular properties in the current work are calculated with the help of the Gaussian 09 software using the Density Functional Theory (DFT) with method B3LYP, basis set 6-31 G (d, p), polarised, and diffuse functions.The geometrical optimization was carried out to minimize the energy of the system, and all calculations were done in their respective equilibrium geometry.

Synthesis
The lateral thiol group-containing LC compounds (LSHn, where n = 3-8, 10, 12, 14, and 16) were synthesized using the synthetic pathway depicted in Scheme 1. Alkylation of 4-hydroxybenzoic acid with n-alkyl bromide in the presence of KOH was the first step in the synthesis, which resulted in the intermediate 4-n-alkoxybenzoic acid (n-AA) [49].Through the Steglich esterification of 4-hydroxybenzaldehyde and 4-n-alkoxybenzoic acid in the presence of DCC and DMAP, intermediates 4-formylphenyl 4-(alkoxy)benzoates (n-AAL), were synthesized [50].The condensation reaction between n-AAL and 2-aminobenzenethiol was the last step, and it produced the final compounds, LSH-n.

Synthesis of (E)-4-(((2-mercaptophenyl)imino) methyl)phenyl 4-(alkoxy)benzoates
In a round-bottom flask filled with 60 mL of ethanol and heated to reflux, 1 mol of 4-formylphenyl 4-n-alkoxybenzoates (n-AAL) was added.Dropwise, an ethanolic solution of 1 mol of 2-aminobenzenethiol was added to the reaction mixture.Glacial acetic acid was added in small amounts as a catalyst.During the 24-hour reflux process, the mixture was constantly stirred.The crude product was then purified through filtration and recrystallization with ethanol to obtain a white solid of LSH-n.As a representative we have discussed characterization data of only one compound LSH-6.

Chemistry
The synthetic route (Scheme1) describes the preparation of (E)-4-(((2-mercaptophenyl)imino) methyl)phenyl-4-alkoxybenzoate (LSH-n) through a condensation reaction between 4-formylphenyl 4-(alkoxy)benzoates (n-AAL) and 2-aminobenzenethiol in the presence of glacial acetic acid.LSH-n showed three peaks in their FTIR spectra for the alkyl chain between 3065 and 2840 cm −1 , while the carbonyl group's -C = O-stretching frequency was found between 1720-1735 cm −1 .Imine group (-CH = N-) shows stretching frequency around 1600-1610 cm −1 .The ether group experienced -C-O-Cstretching between 1245-1265 cm −1 .Thiol (−SH) group shows peaks in the region of 2360-2430 cm −1 .Methylene protons (-CH 2 -CH 2 -) in the chain were seen as a multiplet at δ 1.2-2.0 in the 1H-NMR spectra of various substances.When methylene protons are connected to oxygen, triplets are seen in around δ 4.05-4.4ppm.Other methylene protons of the chain were seen in the region of 0-2 ppm, and terminal methyl group of alkoxy chain show triplet around 0.7-1.0ppm.The aromatic protons of the phenyl ring had a chemical shift range of δ 6.8-8.4 ppm.One proton in the imine linkage (-CH = N-) shows singlet around δ 8.2-8.4 ppm.Alkyl carbon peaks were found in the range of 14 to 30 ppm in the 13C-NMR spectra dimers, while peaks for methylene carbons, which are directly connected to oxygen (-O-C-), were found at 68-70 ppm.All aromatic carbons were found around δ value of 110-170 ppm.

Mesomorphic properties
The transition temperatures of all the compounds of present series (LSH-n) are given in the Table 1.The lower members of the present series LSH-n (n = 3-8,10) are showing enantiotropic nematic mesophase.As the number of carbons increases in terminal alkoxy chain, it shows both nematic and smectic phase (n = 12, 14).The higher members of the series show smectogenic behaviour (n = 16).
The compounds with LSH-n, where n = 3,5,7, which have odd number of carbons in its terminal alkoxy chain, have comparatively lower transition temperatures than even number of carbons (n = 4, 6, 8) in its terminal alkoxy chain.Thus, it shows pronounced odd-even effect.This odd-even effect has been observed in other liquid crystal systems as well, and it is believed to be due to the packing arrangement of the molecules in the mesophase.The odd-numbered compounds have a slightly different packing arrangement than the even-numbered ones, leading to differences in their transition temperatures.It is clear that the length and evenness of the alkoxy chain play a significant role in determining the transition temperatures and mesophase behaviour of these compounds.Further research in this area could lead to the development of new liquid crystal materials with tailored properties for specific applications.
The mesogenic properties of the compounds were observed using Polarizing Optical Microscope(POM), and the separate heating plate allowed for different temperatures to be achieved and monitored for each sample.As a preliminary investigation, the mesophases exhibited by compounds of present mesogenic homologous series (LSH-n where n = 3-8, 10, 12, 14, 16) were examined using polarising optical microscope.(Figure 1).
On heating, crystalline solid (Figure 1(a)) of LSH-6 shows transitions to nematic mesophase and shows marble texture (Figure 1(b)) at 163°C.On further heating at 207°C, it shows transition from nematic phase (N) to isotropic liquid (Iso) (Figure 1(c)).On cooling from isotropic liquid, it gives small droplets (Figure 1(d)) that coalesced to the classical marbled texture, which is characteristic of nematic mesophase.On cooling, nematic phase of n-dodecyloxy and n-tetradecyloxy derivatives shows focal conic texture, which is characteristic of Smectic A mesophase (Figure 1(e)), was crystallized on further cooling (Figure 1(f)).On cooling from isotropic liquid, n-hexadecyloxy derivative shows only focal conic texture, characteristic of Smectic A mesophase.
The thermogram of LSH-6 as a representative compound of series (Figure 2) was calculated utilising DSC    crystalline phase at 135.37°C with enthalpy change of 1.22 kJ mol −1 and 50.74 kJ mol −1 , respectively.

Structure-mesomorphic property relationship
The relationship between phase-transition temperatures and the number of carbons in the alkoxy tail allowed the effects of the terminal chain on the mesomorphic properties to be determined.The relationship was established using two plots (Figures 3 and Figure 4).Figures 3 and Figure 4 shows the phase-transition temperatures of the alkoxy tails as a function of the number of carbons in the chain.
Figure 3 shows that all the compounds show enantiotropic mesophase, and it can be observed that the phasetransition temperature decreases with an increase in the number of carbons in the alkoxy tail.As the chain length increases, the molecules become more flexible and the intermolecular interactions become weaker, resulting in a decrease in the phase-transition temperature.The clearing temperature decreases because the mole fraction of alkyl chain increases, diluting the interactions between the cores [51].Figure 4 shows the mesophase stability of the compounds of present series LSH-n.
From the data presented in both graphs, it is clear that the length of the alkoxy tail has a significant impact on the phase-transition temperatures and thermal stability of the mesophase.Considering these observations, it appears that the pronounced alternation seen in the middle of the series (between odd and even-numbered compounds) is not unusual but can be explained by the combined influence of the odd-even effect and the decrease in clearing temperature.

Comparison of present series LSH-n with the structurally related series
Table 2 summarize the transition temperatures (℃) and comparative geometry of the present series and the structurally related series [31,32]   We can see from the Table 2 that the compounds of series A have higher thermal stability than that of series LSH-n.As we can see in LSH-n and series A, the only difference is the aromatic part.LSH-n has a phenyl ring, whereas series A has a naphthalene moiety.This may be attributed to the fact that it has moiety which increases the length and breadth of molecule, as well as the polarizability of the molecule also increases.The increase in length and polarisability of the molecule overcomes the increase in breadth of the molecule, which reflects in higher thermal stability and greater mesophase length.The only difference between LSH-n and series B lies at the ortho position of the terminal phenyl ring.The present series contains a thiol group at the ortho position of the azomethine central linkage, whereas the phenyl ring does not contain any lateral substituents.It seems that the polar thiol group in the present series LSH-n increases the overall polarizability as compared to series B, which may be responsible for the higher thermal stability and greater mesophase range of the present  series.This highlights how small variations in chemical structure can significantly impact a compound's thermal and mesophase stability.

DFT studies
The aim of this DFT study is to investigate the effects of different alkoxy chain on the properties of thiol derivatives and to identify any trends or patterns that may exist.This study utilized DFT analysis to gain insights into the properties and behaviour of thiol derivatives, specifically focusing on molecular conformation, electrostatic potential distribution and molecular polarizability.The Gaussian 09 programme package was used with the B3LYP and 6-31 G (d, p) basis set for full geometry optimization.The ball and stick model can be used to represent the optimized molecular structures of compounds in series LSH-n (Figure 5).
The spheres represent the atoms like oxygen in red, nitrogen in blue, hydrogen in white, sulphur in yellow and carbon in grey colour.The sticks connecting the atoms show how they are bonded together.The optimized energy minima for LSH-n can be found in Table 3. Dependence of the terminal alkoxy chain on the energy minima is shown in Figure 6.As depicted in Figure 6, the energy minima (in Hartree) decreases as the number of carbons in the alkoxy tail increases.
A precise description of Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO), including information on atomic orbital composition, absolute energy and relative energy gap, offers crucial knowledge about mesogenic properties and aids in the creation of new molecules.The 3D isosurface plots of the HOMO and LUMO are shown in Figure 7, which clearly illustrate their atomic orbital composition.
These plots give a visual representation of the electron density distribution in the molecule, highlighting the regions of high and low electron density.The 3D isosurface plots of the HOMO and LUMO energies of LC Schiff bases (LSH-n) have been studied to understand their mesogenic properties.The plots showed that the electron density was concentrated around the Schiff base group and the aromatic rings, which are responsible for the mesogenic behaviour.The HOMO-LUMO energy gaps for LSH-n were found to be in the range of 3.7 eV to 4.08 eV.In summary, the HOMO-LUMO energy gaps for LSH-n indicate that the compounds are fairly stable and their mesogenic properties are  attributed to the electron density concentrated around the Schiff base group and aromatic rings.Table 4 contains the calculated and displayed HOMO and LUMO energies.Furthermore, the frontier molecular orbital analysis provided insight into the reactivity of the compounds.The HOMO-LUMO energy gap (ΔE) was calculated, which indicates the ease of electron transfer between the highest occupied and lowest unoccupied molecular orbitals.A smaller ΔE value suggests a more reactive compound, while a larger value indicates a more stable compound [52].Overall, this study utilized various chemical descriptors, including global softness, electronegativity, global hardness, electrophilicity and dipole moment, to gain a better understanding of the properties and potential interactions of the compounds studied [53].These calculations are performed on isolated molecules, and the differences in energy are relatively small.In a condensed environment like a liquid crystal phase,these energy differences are likely to differ [54].
In addition to the global softness, other chemical descriptors were also calculated from the FMOs energy levels in this study (Table 4).The electronegativity (χ) is a measure of the Lewis acidity ability of the compounds, and the global hardness (η) reflects the magnitude of the charge transfer hindrance.The electrophilicity (ω), which represents the amount of energy involved in electronic transitions, could also be estimated from the values of the electronegativity and chemical hardness.These descriptors provide valuable insights into the electronic properties of the synthezised compounds and can aid in the design of new materials [55].Table 5 is showing the optical properties of compounds of series LSH-n.The dipole moment components μ x , μ y , μ z and modulus (μ) for compounds were calculated along the three cartesian directions.The calculated values of μ x , μ y and μ z were found to be in the range of 3.3-6.7 D, 3.3-4.2D and 0.2-0.9D, respectively.The dipole moment component along the X-axis being larger than the transverse axes suggests that the molecules align themselves with their long axis parallel to the director axis of the mesophase [56].
Additionally, the even number of carbon atoms in the alkoxy tail of LSH-n (n = 4, 6, 8,10, 12, 14, and 16) seems to play a role in increasing the dipole moment of the compounds, compared to those with odd number of carbon atoms.Overall, these findings provide insights into the molecular properties of LC Schiff bases and their potential applications in the development of new mesogenic materials.The electronic properties of the molecule can also affect its polarizability [57].For example, the presence of electron-donating or electron-withdrawing groups can  alter the distribution of electrons within the molecule, leading to changes in its polarizability.
Understanding the factors that influence molecular polarizability is important for predicting and interpreting the behaviour of molecules in various contexts.
In spectroscopy, the vibrational modes in a molecule can be studied by analysing the wave numbers of the absorbed or emitted light.Figure 8 is showing the comparison between experimental and theoretical FTIR spectra of LSH-6.The absence of negative wave numbers indicates that the proposed compounds are stable and do not undergo any decomposition reactions.The agreement between the computed and experimental wave numbers further validates the accuracy of the theoretical calculations and supports the proposed structures of the compounds [58].
The charge distribution map for the prepared compounds LSH-n was calculated with the B3LYP-6-31 G basis sets according to molecular electrostatic potential (MEP) (Figure 9).MEP analysis [59] revealed that the LSH-n compounds exhibited a higher positive charge density on their aromatic rings, whereas the sulphur atom of the thiol group had a negative charge density.Regions of wavelengths are inversely proportional to potential sequentially with colour accordingly as blue > green > yellow > red.The molecular structures of the thiol derivatives were found to be planar, with the thiol group oriented perpendicular to the aromatic ring.So, the MEP calculations showed that the thiol group at ortho position of the phenyl ring had a significant influence on the charge distribution of LSH-n compounds.
The superior value of positive and negative atomic charge in LSH-6 confirms the improved LC nature.Therefore, the distribution of atomic charges in LSH-6 plays a crucial role in determining its LC nature, with oxygen atoms showing a greater negative charge and hydrogen and carbon atoms showing a higher positive charge [60].This information can be visualized in Figure 10, which represents the atomic charge distribution of LSH-6.

Conclusion
The new homologous series of mesogenic compounds with lateral thiol group have synthesized, and they were found to exhibit a wide range of mesomorphic behaviour, including nematic and smectic phases, and provided important insights into the structure-property relationships of these compounds.The use of both experimental techniques and computational studies allowed for a comprehensive understanding of the thermotropic properties of the compounds.All the compounds show enantiotropic mesophase, and it can be observed that the phasetransition temperature decreases with an increase in the number of carbons in the alkoxy tail.The lower members of the series are nematogenic, whereas higher members show smectogenic behaviour.The mesomorphic behaviour of the compounds can be attributed to the presence of a rigid core in the molecule.The rigidity of the core is maintained by the presence of aromatic rings and imine and ester linkages in the molecule.The DFT calculations also revealed that the introduction of the lateral thiol group led to changes in the electronic properties of the compounds, which may have contributed to their mesomorphic behaviour.The results suggest that the presence of lateral polar thiol substituents and longer alkoxy chains can enhance the polarizability of the compounds, leading to stronger intermolecular attractions and more stable mesophases.The electrostatic potential distribution revealed the presence of both positive and negative charges in the molecules, indicating their potential for chemical reactivity.Overall, this study provides valuable insights into the properties and behaviour of thiol derivatives, which could be useful in designing new molecules with specific properties for various applications.In comparison of the thermal stability of series A, series B and LSH-n compounds, it was found that the series A exhibit higher thermal stability than series LSH-n, attributed to the presence of a naphthalene moiety that increases molecule size and polarizability.The polar thiol group in LSH-n enhances its overall polarizability, resulting in higher thermal stability compared to series B. Small structural variations greatly influence thermal and mesophase stability.

(
DSC-822, Mettler Toledo having Stare software).During both heating and cooling cycles, all phase transitions were monitored at a rate of 10°C min −1 .Figure2represents the DSC thermogram of representative member LSH-6.In heating cycle, LSH-6 shows two exotherms corresponding to a change in phase at 163.38°C from crystal-line to nematic phase followed with another phase change from nematic to isotropic phase at 207.79°C.The enthalpy change of Cr-N transition is −46.02 kJ mol −1 and −1.57kJ mol −1 for N-Iso.In cooling cycle, the same compound shows two endotherms for a phase change from isotropic to nematic phase at 205.88°C, followed with another phase change from nematic to

Figure 1 .
Figure 1.(Colour online) Optical microphotograph of LSH-6 (a) Crystalline solid (b) marble texture on heating at 163°C (c & d) N-Iso transition at 207°C (e) nematic droplets at 206°C on cooling (f) marble texture on further cooling at 204°C.

Figure 3 .
Figure 3. (Colour online) Line graph of transition temperature against no. of carbons (a) during heating cycle and (b) cooling cycle.

Figure 4 .
Figure 4. (Colour online) Bar graph of transition temperature against no. of carbon showing thermal stability of the mesophase in the (a) heating and (b) cooling cycle.

Table 1 .
Transition temperatures (in ℃) of all the compounds (LSH-n) in heating and cooling cycle.

Table 3 .
Optimized energies (in hartree) of the prepared compounds.

Table 4 .
Quantum mechanical descriptors of compounds.

Table 5 .
Optical properties of the compounds of compounds.