Synthesis, mesomorphic and magnetic properties at room temperature of biphenyl ester-aromatic imine with terminal alkoxy chains and methoxy substituent

ABSTRACT A series of calamitic biphenyl ester-aromatic imine derived from the reaction of 4-formylphenyl-4’-(alkyloxy)-[1,1’-biphenyl]-4-carboxylate and p-anisidine have been successfully synthesised and characterised. The general molecular structures of ultimate compounds show the central fragment made up by a hybrid core of biphenyl ester-aromatic imine in which the terminal alkoxy chains, CnH2n + 1 in which n = 7–12 were connected to biphenyl while the other end consists of methoxy moiety attached to a phenyl ring. All the target compounds under polarised lights exhibit enantiotropic nematic phase of which the temperature range was further supported by DSC analysis. It can be summarised that the lengthening of terminal alkoxy chains has contributed to the lowering of melting and clearing temperatures as well as the thermal stability of nematic phase. A dedicated magnetic characterisation analysis disclosed that apart from a dominant diamagnetic character these compounds possess a noticeable magnetic interaction despite knowing that the compounds belong to the non-metal and non-radical containing species. GRAPHICAL ABSTRACT


Introduction
Liquid crystals (LCs) are fluid materials made of anisotropic molecules, exhibiting as an intermediary state between isotropic liquid and crystalline solid [1][2][3].LCs typically have low viscosities (~0.1 P) and the ordered phase is characterised by fascinating optical properties like low-threshold laser emission, Bragg reflection and strong Rayleigh scattering [4,5].Interestingly, they are found in everywhere and utilised in numerous appliances, display devices, manufacturing facilities, automotive and medical technology [6][7][8][9][10].
The orientation of LCs materials is effectively controlled by weak electric or magnetic field owing to the anisotropy of the electrical and magnetic properties [11,12].The calamitic mesogens have commonly been associated with the rod-like (elongated) molecule.It is one of the most influential molecular types in the formation of liquid crystals.The existence of this matter can be explained based on the anisotropic interactions and steric packing of the molecules [13,14].In general, the molecular length (l) of rod-like molecule is significantly greater than the molecular breadth (h).It has well been reported that rod-like molecules packed together in parallel to maximise the use of available volume and thus minimise the free volume [15,16].The typical formulation of the calamitic mesogens can be exemplified with the presence of aromatic/nonaromatic rigid core and flexible hydrocarbon chains.The introduction of the flexible terminal chains on the rigid core is very useful to offset the molecular structure in promoting the liquid crystalline properties [17][18][19][20].Fornasieri and co-workers reported three series of rod-shaped LCs comprising a mesogenic core (monophenyl, biphenyl or phenyl benzoate groups) bonded to a perflourinated chain through thioester linkage and a hydrocarbon chain with terminal double bond [21].Throughout their observation, the increase of the aromatic rings in the core structure of the LCs compound as well as elongating the hydrocarbon chains gave significant effects on the liquid crystalline behaviour and transition temperature wherein the long hydrocarbon chains led to a reduction in the mesomorphic properties whilst increasing the number of aromatic rings in the core fragments resulted in the increase of the transition temperature [21].
Apart from behaving as liquid crystal, many researchers prompted to innovate and synthesise new materials which possess the potential to show the magnetic properties.The magnetic interaction occurred when the materials exert either attractive or repulsive forces on another.These magnetic properties typically lie in the orbital and spin motions of electrons [22,23].The magnetic properties of liquid crystalline materials can be classified into several main categories, which are diamagnetism, paramagnetism and ferromagnetism [24][25][26].Many studies have claimed that the requirement of a material to possess magnetic behaviour depends on the structure of compound, usually associated with the rare-earth elements, metal-complexes and organic radical compounds [27,28].One of these works focussed on the magnetic behaviour that was discovered for nitronyl nitroxide and verdazyl derivatives [29].Besides, the magnetic behaviour of the materials had been linked to the influence brought about by the introduction of specific substituent in the compound.For instance, the influence of the different fluorosubstituted phenylacetates on co-ligands can improve the structural topologies and subsequently affect the magnetic properties of azido-Cu(II) complexes [30].The experimental studies have shown the unusual intermolecular magnetic interaction in the SmC phase of organic radical compound [31].To better understand the effect of the macroscopic structures on the magnetic properties, Umeta and co-workers analysed the magnetic properties of organic radical fibres aligned self-assembled in the LCs.The organic radicals are anticipated as organic magnetic compounds because of the magnetic moments obtained from the odd electrons [32].Magnetic LCs containing rare-earth complex are attracting widespread interest due to their large magnetic anisotropy, hence the external magnetic field is required to align these paramagnetic LCs [33].Although substantial research has been conducted on metal and organic radical materials with magnetic and liquid crystal properties, but there is only minimal study on non-metal or non-radical compounds that exhibit magnetic and liquid crystal properties.Our group has recently discovered the presence of magnetism in non-radical or non-metal materials on a liquid crystalline compound of tri-substituted phenyl derivatives [34].
Subsequent to this, we are prompted to investigate a new series of mesomorphic properties of molecules containing four phenyl rings of which the rings made up by hybrid molecular segments from biphenyl ester and aromatic imines wherein various terminal alkoxy chains, OC n H 2n + 1 (n = 7-12) connected to the biphenyl but the other end of aromatic imine is having a paramethoxy group.The supporting magnetic characterisation has been used to detect and quantify the presence of weak magnetic properties existing among the title compounds.

Characterization
All compounds obtained were characterised by numerous methods.The elemental analyses of the reported compounds were carried out using a PerkinElmer 2400 LS Series CHNS/O analyser.The functional group of all compounds was recorded by a PerkinElmer 2000 FT-IR spectrophotometer in the frequency range 600-4000 cm −1 .The 1 H and 13 C-NMR spectra for the synthesised compounds were measured on the Bruker-Avance 500 MHz spectrometer equipped with ultra-shield magnets in deuterated chloroform (CDCl 3 ) and dimethyl sulphoxide (DMSO) using tetramethylsilane (TMS) as the internal standard.Thin-layer chromatography (TLC) was conducted using TLC plate coated with silica gel and was examined under the short wave ultraviolet (UV) light irradiation.The measurements of phase transition temperatures and associated enthalpy values were established using Seiko DSC120 model 5500 differential scanning calorimeter (DSC) under a flow of dry nitrogen with a heating and cooling rate of ±2°Cmin −1 at the Faculty of Engineering, Soka University, Japan.The liquid crystals texture identification of all samples was observed under a Carl Zeiss Axioskop 40 polarising microscope (POM) equipped with a Linkam LTS350 hot stage and TMS94 temperature controller.Magnetic studies are conducted using superconducting quantum interference device (SQUID) magnetometer MPMS XL5 of Quantum Design.(San Diego, CA, USA).Two experimental approaches are applied and compared.In the first one, as in refs [34,35], the powder sample is stabilized on the clean Si rectangular support plate by means of a strongly ethanol-diluted GE-varnish.In the second one a singlemilligrams-sized powder sample has been transferred into a polycarbonate capsule (available also from Quantum Design).In this case the in situ compensation method of the magnetic signal of the capsules has been applied to reduce the magnitude of the effective signal and large misalignments errors brought about the use of polycarbonate capsules [36].Isothermal magnetization, M(H) curves have been measured at room temperature with the magnetic field H range up to 40 kOe following the code elaborated for high sensitivity studies of minute magnetic signals [37].We confirm that the results obtained by these two approaches are consistent within 5 to 10%, despite the miniscule weight of the specimens (a mere few milligrams).

Synthesis
The aforementioned homologous 3a-3f were synthesised according to the synthetic scheme as depicted in Scheme 1.
Scheme 1. Synthetic pathway towards the preparation of final compounds 3a-3f.

Synthesis of 4'-alkoxy-4-biphenylcarboxylic acid, 1a-1f
The synthesis of intermediary compounds, 1a-1f follow a similar reported procedure [34,38] and it can be described in detail by referring to a representative compound 1f.In a round bottom flask, a mixture of 4'hydroxy-4-biphenylcarboxylic acid (9.34 mmol) and potassium hydroxide, KOH (18.67 mmol) in ethanol/ water (9/1) were stirred for 20 minutes.1-bromoheptane (23.34 mmol) was then added dropwise and the mixture was refluxed overnight.Upon completeness of the reaction, one equimolar of KOH was added and the mixture was refluxed for another 4 hours.The resulting mixture was left to evaporate at room temperature and the mixture was poured into distilled water and acidified with glacial acetic acid (pH = 2-3).The solid precipitate thus obtained was filtered off and washed twice with distilled water and diethyl ether.The crude product was recrystallised twice from glacial acetic acid [38,39]

Synthesis of 4-formylphenyl-4'-(alkyloxy)-[1,1'biphenyl]-4-carboxylate, 2a-2f
Compounds 2a-2f were prepared according to the reported procedures [40][41][42].The same method was employed to form the homologous 2a-2f.A detailed synthetic procedure was based on a representative compound 2f.4-hydroxybenzaldehyde (1.12 mmol) was mixed with compound 1f (1.12 mmol) and DMAP (0.36 mmol) with adequate amount of DCM.A solution of DCC (1.34 mmol) in DCM was then added dropwise and the mixture was stirred overnight at room temperature.The N,N'-dicyclohexylurea (DCU) thus formed was isolated by filtration and the filtrate was evaporated under reduced pressure.The white precipitate was collected by vacuum filtration and recrystallised twice from ethanol.

Synthesis of 4-(((4-methoxyphenyl)imino) methyl)phenyl-4'-(alkyloxy)-[1,1'-biphenyl] -4-carboxylate, 3a-3f
The title compounds 3a-3f were obtained by following the same synthetic pathway.Thus, detailed description with respect to the formation of 3f was given as representative compound.In a round bottom flask, an ethanolic solution of 4-methoxyaniline (1.20 mmol) was added dropwise to a stirring hot ethanolic solution of compound 2f (1.20 mmol).The mixture was refluxed overnight at 70°C.Upon completion of the reaction, the resulting solution was evaporated under reduced pressure.The white precipitate thus obtained was then collected by vacuum filtration and recrystallised from absolute ethanol.

Mesomorphic behaviour
All the homologues in this series under the polarised lights exhibited enantiotropic nematic (N) phase.The nematogenic phase was identified by the appearance of N droplets and the schlieren texture with two-brush and four-brush defects [43].It can clearly be observed for a representative compound 3d under the POM that the crystal-crystal phase (Cr 1 -Cr 2 ) appeared at 165.6°C upon the heating process.The crystal-crystal phase transitions are usually formed through the nucleation and growth process of a new crystal phase.There is a noticeable interface between the old and new crystal phases, and atoms in the old phase will cross the interface singly and disorderly to form into the new crystalline phase [44][45][46][47].
The N phase begins to appear at 273.5°C before it completely melted into isotropic liquid at 299.8°C.On cooling, the N droplets were formed, which then coalesced into the schlieren texture with two-brush and four-brush defects (Figure 1) before it transformed into crystal phase.
The corresponding phase transition temperatures and associated enthalpy values obtained from the DSC thermograms of all title compounds 3a-3f are shown in Table 1.The thermal characteristics on cooling and heating as shown by compound 3d are also shown in Figure 2. As the sample was heated, a sharp peak was found at 271.5°C which corresponds to the crystalnematic (Cr-N) transition with an enthalpy value of ΔH = 29.7 kJmol −1 .The relatively small peak was then detected at 299.4°C with enthalpy value of ΔH = 3.2 kJmol −1 that can be assigned to the nematic-isotropic (N-I) transition.On cooling two exothermic peaks attributed to I-N and N-Cr transitions were found at 297.7°C (ΔH = 1.9 kJmol −1 ) and 269.2°C (ΔH = 31.9kJmol −1 ), respectively.The transition peaks of Cr 1 -Cr 2 during cooling and heating processes were found to be weak and therefore unobservable in the DSC thermogram.However, this transition can be detected under the polarised light which resembles the phenomenon reported in literature [13,48].The entropy changes associated with the transition between I and N are presented in Table 1 and expressed as dimensionless quantity ∆S I-N /R, where ∆S values were derived 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 .Table 1 shows that the ∆S I-N /R for compounds 3a-3c are in the range of 0.32-0.41,indicating a rising trend when the length of terminal alkoxy chains, n, increases from 7 to 9.However, increasing the length of terminal alkoxy chains to 12 carbon atoms reduces the ∆S I-N /R value.The sudden reduction in entropy values could be attributed to an increase in cohesive energy density, which can be influenced by increasing a compound's molecular size as more carbon atoms are added [49].
The correlation between the phase transition temperatures and the number of carbon atoms, n in the terminal alkoxy chains for all members 3a-3f was depicted in Figures 3. From 3, it is obvious that the clearing temperatures decrease with the lengthening of the terminal alkoxy chains from n = 7 to n = 12.This phenomenon occurred owing to the influence by the flexibility of the longer terminal alkoxy chains leading to the dilution of the mesogenic unit [50,51].Besides, upon elongating the terminal alkoxy chains, the N phase temperature range decreased from 38.8°C (3a) to 24.7°C (3f).In addition, there is a decrease in the melting temperatures when the length of terminal alkoxy chains increased wherein compound 3a showed highest melting temperature at 334.5°C while compound 3f exhibited lowest melting temperature at 264.3°C.This observation can probably be attributed to the less rigidity of the molecule and weak molecular packing interaction on increasing the terminal alkoxy tails, thus affected the molecular linearity [13,39,52].Therefore, the members with higher terminal alkoxy chains possess lower phase transition temperatures as well as lower mesophase stability.It can also be inferred from Figure 3 that a sharp decline was observed when changed from 3c (n = 9) to 3d (n = 10).This observation can probably be resulted from a sudden decrease in Van der Waals forces interaction that occurred between nonyloxy chains and decyloxy chains [53,54].
The mesomorphic behaviour of the present compounds 3a-3f was then compared with the previously reported analogous o-ethoxy substituted [34].The compounds from previous series were the analogues of compounds 3a-3f.From the observation, the nematogenic phase was also spotted in the compounds reported previously.The appearance of terminal chains in these compounds indicates also a similar trend as observed in homologues 3a-3f, of which the melting temperatures were found decreasing upon increasing lengths of the terminal alkoxy chains.A comparative study between homologues 3a-3f and their reported analogues shows that o-ethoxy substituted analogues displayed lower transition temperatures than compounds 3a-3f due to the existence of peripheral o-ethoxy substituent at the central phenyl ring of reported compounds.For example, the melting and clearing temperatures for reported analogues with heptyloxy derivative were observed at 249.3°C and 266.1°C while for homologue 3a at 334.5°C and 373.3°C, respectively.Furthermore, the presence of ortho substituent of the reported analogues results in narrower N phase temperature range than compounds 3a-3f.It can be seen that compound 3a possesses larger N phase temperature range (Δ N = 38.8°C)as compared to reported heptyloxy analogue with Δ N = 16.8°C.This could be ascribed to the change in the length-to-breadth ratio of the molecules in which the presence of o-ethoxy substituent enhances the molecular breadth of the compounds leading to a lower N phase temperature range [55,56].

Magnetic properties
The established by us magnetisation curves M(H) for the samples studied follow the same general pattern exemplified in Figure 4(a) for the compound 3f.This nearly linear slope of M(H) yields magnetic susceptibility χ mol = −3.9(3)× 10 −4 emu/mol, where the uncertainty given in the parenthesis reflects the spread of the results obtained from the two experimental  approaches to the magnetisation measurements of such miniscule powder specimens.The magnitude of χ mol obtained here compares favourably with the rule of thumb estimation χ mol = −(molecular weight)/2 × 10 −6 emu/mol [57,58], which for compound 3f of molecular weight of 591.8 gives about −3 × 10 −4 emu/mol.
The more insight into the data reveals an existence of a weak nonlinear component to linear diamagnetic M dia (H) = χ dia H.We subtract numerically the magnitudes of M dia (H) and plot the resulting ΔM(H) = M(H) − χ dia (H) in Figure 4(b).The existence of such a strongly nonlinear component is indicative of a presence of small magnetic entities, which as an ensemble exhibit a Langevine like M(H), typical to a superparamagnets [59,60], yet very weak and visible only after a high degree of numerical compensation of the dominant diamagnetic background in the studied compounds.This observation can also be claimed as very unique because the compounds investigated here do not have any magnetic species like metal or organic radical elements intentionally added.On the other hand, we strongly point out that this contribution is brought to the magnetic lab within the investigated specimens.We confirm, beyond any doubt, that the appearance of such a ΔM(H) as presented in Figure 4(b) has not resulted from any technical glitch in the experimental procedure [36,37].It therefore remains to be seen whether this weak superparamagnetic-like contribution is due to a tiny amount of ferrous contaminants present in the reagents used in the synthesis, or, due to a presence of some large clusters of locally broken bonds with uncompensated spins.
Similar observation by naked eyes on analogous o-ethoxy substituted compounds has been reported earlier by our group [34].Hence, a comparison in term of the magnetic strength between the present materials 3a-3f and their analogous members reported earlier (acronyms used earlier were 5a-5f but in this paper we change to 3a*-3f*) can be made without much ambiguity.Although both series were attracted to the permanent magnet but it can be seen very clearly that the attraction for the current compounds without o-ethoxy substituent 3a-3f only lasted for a few seconds before it disconnected from the magnet.The compounds from previous series 3a*-3f* consisting of o-ethoxy substituent seem to exhibit stronger magnetic behaviour than their unsubstituted analogues 3a-3f.One of the probable reasons could be attributed to the presence of o-ethoxy substituent in compounds 3a*-3f* which possesses the tendency to give exhibit stronger magnetic behaviour in comparison to the unsubstituted analogues 3a-3f.
The magnetic interaction of present compounds and their substituted analogues were further confirmed by SQUID magnetometer.As can be seen in Figure 5 , the presence of ethoxy substituent at the ortho position apparently give rise to the stronger magnetic property in comparison to the unsubstituted compound 3f wherein the signal from substituted analogue 3f* appeared stronger than the unsubstituted compound 3f.

Conclusions
A series of new mesogens 3a−3f which consist of hybrid biphenyl ester-aromatic imine backbone were synthesised and characterised in a comprehensive manner.All the compounds 3a-3f exhibited the domains characteristic of nematic phase on heating and cooling.Comparison of the nematogenic temperature range among all the members within this series revealed the flexible terminal alkoxy chains attached to the biphenyl unit gave a remarkable influence on the thermal transition temperatures and mesophase stability wherein the long terminal alkoxy chains led to a reduction in melting and isotropization temperatures as well as the thermal stability associated with the emergence of nematic phase.Magnetic characterisation of these compounds confirms their predominantly diamagnetic nature.The magnitude of the diamagnetic susceptibility is found to scale reasonably well with a simple scaling rule.An small admixture of a superparamagnetic-like component has been found despite the fact that the synthetised compounds do not nominally possess any magnetic species like metal or organic radical elements.Its origin remains yet to be identified.

Figure 2 .
Figure 2. (Colour Online) DSC thermogram of compound 3d during heating and cooling processes.

Figure 4 .
Figure 4. (Colour Online) Magnetization as a function of magnetic field H, M(H), of 3f compound at room temperature.(a) The dominant diamagnetic component with a slope yielding molar diamagnetic susceptibility χ mol = −3.9× 10 −4 emu/mol.(b) A nonlinear part of M(H) form (a) obtained upon numerical subtraction of the linear diamagnetic component, ΔM(H) = M(H) − χ dia (H).