Mesomorphic, magnetic and DFT studies of biphenyl-based molecule with various substituted anilines

ABSTRACT A new series of liquid crystals consisting of biphenyl unit connected to phenyl core system have been successfully prepared wherein various substituents X= OCH3, F, Cl, Br and I were introduced in the para position of the imine fragment while the length of terminal alkoxy chain of seven carbon atoms was intact. The mesomorphic and thermal studies by POM and DSC exhibit dominant enantiotropic nematic (N) phase. A relatively weak magnetism was detected with vibrating sample magnetometer (VSM) for compounds 3F and 3I at room temperature, wherein the magnetic strength of fluoro-substituted compound appeared stronger than the iodo-substituted compound. The connectivity between the substituents X and their kinetic stability associated with the changes in HOMO-LUMO energy gap value (ΔEgap) supports that the terminal substituents X have great influence on the ΔEgap which can be ascribed to the chemical activity of the compounds. Present study shows an increase in the ΔEgap from OCH3 < I < Br < Cl < F, where the largest ΔEgap indicates high molecular kinetic stability and low chemical reactivity because adding electrons to high-lying LUMO is energetically unfavourable, resulting in disruption of the molecular packing and reduction of nematogenic, which were validated by DSC and POM. GRAPHICAL ABSTRACT


Introduction
Biphenyl or phenylbenzene can be regarded as an aromatic hydrocarbon that possesses the chemical formula C 12 H 10 in which two phenyl rings connected by a single covalent bond [1,2].The biphenyl compound exists in nature as white crystalline solid or monoclinic prismatic crystals or plates that can be separated from solvents.The discovery of biphenyl compound has attracted the interest of many researchers owing to its applications in various areas including agro-industrial, pharmaceutical, technological development, scientific and manufacturing [3][4][5][6][7].Besides, some of the biphenyl derivatives could be well associated to the mesogenic behaviour since these compounds exhibited rich mesomorphism and generate various mesophases such as nematic, smectic A, smectic C, smectic X and blue phase [8][9][10][11].It also can act as a building block to construct various functional molecules [12,13].
According to the study by Goodby et al., the rod-like molecules would pack together to maximise utilisation of the available space and therefore minimise the free volume.In this study, the inclusion of lateral groups or/ and terminal chains in the rod-like compounds such as sexiphenyl, quinquephenyl, terphenyl and biphenyl resulted in a reduction in clearing temperatures.They also discovered that adding bulky terminal groups reduced the transition temperatures as the size of the molecules increases, which can improve the misalignment and self-rectification following shock damage.These bulky terminal groups sterically hindered the ends of the molecules from joining together, preventing splaying between the molecules [18].
Besides, azo-cinnamate central linkages also display higher thermal stability and mesophase range than the azo-ester central linkages.The variations of molecular structure gave rise to distinctive mesomorphic behaviour of triazole-based compounds possessing the biphenyl moiety [19,20].
The present paper focuses on the synthesis and characterisation of a new series of biphenyl compounds in which the lateral-ethoxy phenylimino fragment possessing various terminal substituents X (X = OCH 3 , F, Cl, Br and I) at the para position along the long axis of this fragment and heptyloxy terminal chains resided at the biphenyl moiety.The thermal stability, optical behaviour and magnetic properties of the target compounds are investigated along with an attempt to establish the effect of terminal substituents X of the title compounds based on the experimental and theoretical calculation utilising density functional theory (DFT).

Physical measurement
All target materials thus obtained were elucidated using CHN analyses (Perkin Elmer 2400 LS Series CHN/O analyser), FT-IR spectroscopy (Perkin Elmer FT-IR ATR Frontier spectrophotometer via attenuated total reflectance (ATR) method in the frequency range of 600-4000 cm −1 ) and NMR spectroscopy (Bruker Avance 500 MHz Ultrashield FT-NMR using appropriate deuterated dimethylsulfoxide (DMSO-d6) with tetramethylsilane (TMS) as the internal standard).The melting point of the synthesised compounds were observed under Gallenkamp melting point apparatus.The phase transition temperatures and enthalpy values were measured by Seiko DSC120 Model 5500 differential scanning calorimeter with heating and cooling rates of ± 2°-Cmin −1 , respectively.The optical observations were studied on a Carl Zeiss Axioskop 40 polarising optical microscope equipped with a Linkam LTS350 hot stage and TMS94 temperature controller.The textures of the samples were prepared in thin films sandwiched between glass slide and cover slip.The density functional theory (DFT) calculations for all target compounds were carried out using the Gaussian 16 software [21,22].For all computations, Lee-Yang-Peer (LYP) correlation function with the LanL2DZ basis set was used in conjunction with Becke's three-parameter (B3) exchange function [23][24][25][26][27].The geometries were optimised by minimising the energies with respect to all geometrical parameters without imposing any molecular symmetry constraints.The structures of the optimised geometries were drawn with the Gauss View 6.1.1 [28][29][30].Additionally, calculation frequencies were performed with the same level of theory, wherein all structures were stationary points in the geometry optimisation method, with no imaginary frequencies.The NBO 7.0 programme was used to compute natural bond orbitals for optimal geometries [31].The magnetic behaviours of the title compounds were measured at room temperature using a Lakeshore 7407 vibrating sample magnetometer (VSM) in the Department of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia.

Synthesis
The preparation of the title compounds 3X were carried out according to the synthetic routes as illustrated in Scheme 1.

Synthesis of 4'-alkoxy-4-biphenylcarboxylic acid, 1
The intermediate 1 was synthesised using the same method as detailed in previously reported paper [32].

Liquid crystalline behaviours
The mesophase properties of the synthesised compounds, 3X were validated by synthesised optical microscope (POM) and differential scanning calorimetry (DSC).The transition temperatures and associated enthalpy values for homologues 3X are summarised in Table 1.From the POM observation, all the target compounds were found to display enantiotropic nematogenic liquid crystals.The N phase was identified based on the emergence of the small droplets which has subsequently merged into the schlieren texture with twoand four-fold brushes (Figure 1(a,b)).Since the series of title compounds 3X show identical characteristics, therefore, the detailed discussion can be based on a representative member 3Cl.Inspection from POM shows that the additional feature of crystal-crystal phase (Cr 1 -Cr 2 ) was observed at 113.3°C.The Cr 2 phase (Figure 1(d)) forms a tiny number of substantially larger crystallites than the Cr 1 phase (Figure 1(c)), which appears as numerous tiny crystallites.This Cr 1 -Cr 2 phase transition occurred owing to the nucleation and growth process of a new crystalline phase.There is a significant interface between the old and new crystal phases and atoms in the old phase will cross the interface singly and disorderly to transform into the new crystalline phase [34][35][36][37].On further heating, Cr 2 phase has subsequently undergone the transition to N phase at 212.1°C prior to a complete melt into isotropic liquid at 225.3°C.On cooling compound 3Cl underwent the transition from isotropic to nematic phase at 222.2°C before crystallisation.
The representative DSC thermogram of 3Cl is shown in Figure 2. On heating the compound 3Cl displays two endothermic peaks of Cr-N and N-I transitions at 211.2°C (∆H = 32.1 kJmol −1 ) and 223.7°C (∆H = 1.4 kJmol −1 ), respectively.On cooling the first exothermic transition was recorded at 221.9°C with enthalpy value (∆H) of 1.5 kJmol −1 during the formation of the intermolecular forces which corresponds to the I-N transition and followed by the emergence of an intense peak associated with N-Cr transition at  209.0°C (∆H = 6.8 kJmol −1 ).However, the transition of Cr 1 -Cr 2 was not detected by DSC upon both heating and cooling despite knowing that this transition was detectable under the polarised light.
The introduction of different terminal substituents (X) in the molecular structure has been identified as one of the contributing factors that influenced the mesomorphic behaviour and thermal stabilities.Among the members 3X (X= F, Cl, Br, I and OCH 3 ), compound 3OCH 3 was found to display highest clearing and melting temperatures at 266.1°C and 249.3°C with respective ∆H of 1.9 and 54.3 kJmol −1 .This observation can be explained by the presence of oxygen lone pairs which is shielded by methyl group [38].The repulsive forces implicating the lone pairs of oxygen significantly decreased and allow a close approach of the neighbouring molecules, thus increasing the bonding forces which will enhance the transition temperatures as well as the thermal stability.In addition, the inclusion of methoxy group may result in a structural change and enhance the shape anisotropy.As such, the OCH 3 substituent with greater shape anisotropy results in longer elongated structures [39].On the other hand, the compound 3F has the lowest clearing and melting points at 181.7°C (∆H = 1.0 kJmol −1 ) and 149.3°C (∆H = 25.6 kJmol −1 ), respectively.This observation can probably be due to the fluoro-substituted compound, 3F which shows low polarisability and results in weak dispersive forces [40].By comparing the transition temperatures among the halogen atoms, the order appears to follow the ordinary trend across a series of ordered groups in which the iodo-substituted compound, 3I exhibits highest clearing and melting temperature due to the biggest molecular radius, thus enhanced the molecular attractive forces [41].As such, the ordering of the terminal substituents in raising the transition temperatures of the compounds 3X can be arranged as follows: The contribution of different terminal substituents also affects the thermal stability of liquid crystal properties in which the N temperature range for compound 3F is found as 32.4°C which is the widest among all the members within this series.This phenomenon can be associated with the high electronegativity of fluorine atom which tends to decrease the degree of molecular order.As a result, the F-substituted compound has lower polarisability than the other members of Cl, Br, I and OCH 3 , leading to disrupt the molecular packing and reduce the degree of molecular order in the N phase [38,42,43].The trend of N phase temperature range in the correlation with the terminal substituents follows the order as demonstrated below: Table 1 shows that on heating the substantial ∆H values of compounds 3X were observed for Cr-N phase transition.However, upon cooling the ∆H values detected for N-Cr phase transition are lower as compared to the ∆H values reported during heating process.This can be attributed to the strong Van der Waals interactions between the end units of adjacent molecules, which keep them static in the crystalline state until the change to liquid crystal begins in the heating cycle [44].
The entropy changes associated with the transition between I and N are listed in Table 1 and expressed as dimensionless quantity ∆S I-N /R, where ∆S values were obtained from ∆H/T in which T is the corresponding phase transition temperature in unit Kelvin, K and R is 8.314 J K −1 mol −1 .As inferred from Table 1, the ∆S I-N /R for title compounds 3X are found to be in the range of 0.36-0.58.These values are lower than commonly reported data for liquid crystalline materials [39].It is noteworthy to state that the ∆S I-N /R for halogen atoms are lower as compared to methoxy group.This can be attributed to their lower anisotropy resulted from their molecular geometry and molecular biaxiality [39,[45][46][47][48]. Besides, the conjugation forces, induction, specific dipolar and π-π stacking interactions all play crucial roles in molecular orientation and subsequently in molecular arrangement and mesophase formation [49].

Computational study via density functional theory
The DFT calculation using Gaussian 16 [21,22] and NBO 7.0 program package [31] was employed to acquire a better understanding of the molecular geometry and optimised structure with minimal energy conformations of all title compounds.The optimised molecular structures of compounds 3F, 3Cl, 3Br, 3I and 3OCH 3 are depicted in Figure 3 wherein the compounds are slightly bent and the two biphenyl rings are not coplanar.The terminal substituents X (X = F, Cl, Br, I and OCH 3 ,) at the para position of aromatic imine fragment are almost aligned with the adjacent phenyl ring.
The following equation is used to compute the energy gaps between HOMO-LUMO levels based on the expected molecular conformation for all title compounds 3X at the B3LYP/LanL2DZ level [50].The results are summarised in Table 2.
Table 2 reveals that compound 3F displays largest HOMO-LUMO energy gap of ΔE gap = 0.14405 Hartree whereas compound 3OCH 3 has the smallest ΔE gap = 0.13989 Hartree.The largest ΔE gap of compound 3F implies high molecular kinetic stability and low chemical reactivity because it is energetically unfavourable to add electrons to high-lying LUMO [51].Therefore, it might lead to the disruption of the molecular packing and reduce the nematogenic domains [50,52].This is substantiated by the detection of the lowest transition temperatures among the members by DSC and POM.Meanwhile, the lowest ΔE gap of compound 3OCH 3 is generally associated with a high chemical reactivity and low kinetic stability as well as increased the transition temperatures.The energy gap between the HOMO and LUMO decreases according to F > Cl > Br > I > OCH 3 .Figures 4 and 5 illustrate the HOMO and LUMO molecular orbital for title compounds 3X, measured at an isovalue of 0.02 and recorded at B3LYP/6-31 G+(d,p) level.

Magnetic properties
Researchers have been inspired to develop materials that can exhibit various unique features including magnetic properties in addition to their liquid crystal behaviour.The introduction of the specific lateral substituents imparts significant influence on mesophase  development and could induce the formation of magnetic properties.These lateral alkyloxy groups are currently in the spotlight due to their role in the design of ferroelectric nematic materials.The discovery of ferroelectric nematic, N F phase is of enormous fundamental and technological significance since it has a very high response sensitivity to electric and magnetic fields and has the potential to significantly improve the performance of LCDs [53].Recently, Cruickshank et al. observed that the lateral alkyloxy chain may adopt conformations in which it lies along the major axis of the mesogenic unit.They also discovered that adding fluorine substituent decreases the packing efficiency of molecules in the Cr phase, whereas increasing the stability of the N F phase reflects changes in polarity and polarisability.It is noteworthy that changing the position of the lateral alkyloxy chain changes the electron distribution of the aromatic rings [54].
In 2020, another anomalous feature has been reported earlier by our group on a compound of trisubstituted phenyl derivatives.These non-metal or nonradical materials show a magnetic response that can be observed by the naked eye, in which they were attracted to the magnet both in the solid state and in the presence of a water surface at room temperature.This observation is further supported by the evidence from SQUID magnetometer analysis [33].Interestingly, the current study of the liquid crystalline materials has also been found with relatively weak magnetic behaviours at room temperature.A vibrating sample magnetometer (VSM) was used to measure the magnetic behaviours of fluoroand iodo-substituted compounds at room temperature [55,56].The raw data collected has been corrected by subtracting the diamagnetic contribution of the sample holder.Figure 6 illustrates the corrected hysteresis loops of representative compounds 3F and 3I.The hysteresis curve proves that the compounds exhibit magnetic properties.From Figure 6, the narrow hysteresis loops were observed with coercivity values of 135 Oe and 198 Oe for compounds 3I and 3F, respectively.This indicates the compounds have soft magnetic materials properties.It also can be seen that both compounds showed very small magnetisation values.The compound 3F showed higher value of 0.016 emu/g than compound 3I (0.0074 emu/g).This indicates that compound 3F appears to have a larger magnetic moment than the other member.However, the observed magnetisation value is much lower compared to any magnetic material-based ceramics, alloy, or metal.It is well understood that the magnetic properties of a substance were contributed by electron spin, nuclear spin and electron orbital motion in the atom or molecule [57,58].In this study, the compounds 3X behave like soft magnet with low magnetic properties and they only retain their magnetic properties under certain conditions.However, these compounds are inclined to show magnetic interaction in the presence of strong magnetic field which exhibits low coercivity [59].Therefore, it is suggested that the magnetic properties exhibited by compounds 3F and 3I were seemingly contributed by unpair spin which aligned in the same direction when exposed to a magnetic field, leading to a weak magnetisation [60].

Conclusions
The homologues 3X were found to be nematogenic.The N phase was observed during both heating and cooling cycles.The incorporation of various terminal substituents in homologues 3X (X= F, Cl, Br, I and OCH 3 ) have shown an influence on the thermal stabilities and mesomorphic behaviour.The N phase temperature range for compound 3F was found to be the largest.This occurrence can be rationalised by the low polarisability and high electronegativity attributed by fluorine atom which is inclined to reduce the degree of molecular order as a result of the disruption of the molecular packing and predisposition to reduce the molecular order of N phase.
The strength of terminal substituents in stabilising the N phase follows the order of F > OCH 3 > Cl > Br > I. Besides, compound 3OCH 3 showed highest melting and clearing temperature.This implied that the homolog 3OCH 3 was the most thermally stable among this series.Thus, the trend of clearing and melting temperatures related to different polar substituents are arranged in the following order: OCH 3 > I > Br > Cl > F. The calculation based on DFT supports an increase in the HOMO-LUMO energy gap, ΔE gap , which is equivalent to the reduction in chemical reactivity leading to lower transition temperatures.Besides, the comparatively weak magnetic strengths of these compounds have been validated through further investigation using VSM.It demonstrates that fluoro-substituted compound possesses greater magnetic strength than iodo-substituted compound.

Scheme 1 .
Scheme 1. Synthetic route towards the preparation of final compounds 3X.

Figure 1 .
Figure 1.(Colour online) Photomicrographs presenting (a,b) nematic droplets merged into the schlieren texture with two-brush and four-brush defects of compound 3Cl at (a) 222.2°C and (b) 220.8°C on cooling cycle, whereas (c,d) Cr 1 -Cr 2 phase texture at 113.3°C during heating.

Figure 2 .
Figure 2. (Colour online) DSC thermogram of compound 3Cl on heating and cooling runs.
a Transition temperatures are detectable under POM but not DSC.