The influence of core fluorination on the phase behaviour of rod-like mesogens

ABSTRACT The effect of core fluorination on the mesomorphic properties of rod-like Schiff base-derived liquid crystalline materials was reported. For this purpose, two new series of laterally fluorinated Schiff base liquid crystals are synthesised and investigated in detail. The two series differ from each other’s in the length of the terminal alkoxy chain attached to one end of the aromatic core, where in the first series an octyloxy chain is used and in the second series it is replaced by a longer dodecyloxy chain. At the other end in both series of compounds, in addition to the nonsubstituted end (X = H) four different substituents were used (X = CH3O, C6H13O, CH3 and F). The molecular structures of all synthesised compounds were confirmed using different spectroscopic techniques. The molecular self-assembly was investigated using differential scanning calorimetry (DSC) and polarised optical microscope (POM). Depending on the length of terminal alkoxy chain at one end and nature of the substituent X at the other end, different types of mesophases were observed, including nematic and two different types of smectic phases. GRAPHICAL ABSTRACT


Introduction
Liquid crystalline (LCs) materials are not only used in liquid crystal displays (LCDs), but also in optical switches, molecular detectors and sensors and spatial light modulators [1][2][3][4][5][6], During designing of LC it is important to choose the correct linking groups between the aromatic rings.Besides ester (-COO-) and azo (-N=N-) linkages, one of the most applied linkages is the imine unit (-CH =N-) resulting in materials known as Schiff bases.
Depending on its nature, a lateral substituent could induce or suppress the mesomorphic behaviour of the resultant material.For example, a lateral substituent in the rigid core can effectively improve LC solubility by destroying molecular closed packing and lowering the melting temperatures [28,29].Due to its unique combination of high polarity and low polarisability in addition to its steric and conformational effects insertion of fluorine atom into the molecular structure causes major changes in the chemical and physical characteristics of organic molecules.Therefore, fluorine is one of the most used lateral substituents in designing LCs [30][31][32][33][34].
Mirror-symmetry breaking in soft matter is an interesting area of research [35].Such a phenomena was found in the fluidic developing as the nematic phase, which exhibits molecular orientational order.Despite being composed of achiral molecules, the phase can also display structural chirality, with the average molecular direction following a short-pitch helix.A group of achiral asymmetric dimers with an odd number of atoms in the spacer have been found to produce twisted forms in both nematic and lamellar phases.Both the heliconical tilted smectic C phase and the tight pitch heliconical nematic phase are produced [35].
The production of fluorinated functional materials has received a lot of attention from researchers over the past few decades [31][32][33].As a result, many fluorinated compounds have been produced and used as dyes, liquid crystals, photoluminescence molecules, and medicinal medicines [34][35][36].
The aim of the present work is twofold.Firstly, to design and synthesise new Schiff base-derived LCs and secondly to investigate the effect of an inner aromatic core fluorination on the phase behaviour of the resulting materials.Therefore, two new series of LC compounds (IFna-e), namely, (E)-2-fluoro-4-((phenylimino)methyl)phenyl 4-(alkoxy)benzoate, are designed and synthesised (Scheme 1).The two different series have the same aromatic core unit and a variable substituent X at one end (X = CH 3 O, C 6 H 13 O, CH 3 , H, and F).They differ from each other in the length of the terminal alkoxy chain connected to the other end, where an octyloxy chain is used in the first series (IF8a-e) and is replaced by a dodecyloxy chain in the second series (IF12a-e).The liquid crystal self-assembly of all synthesised compounds is investigated using differential scanning calorimetry (DSC) and polarised optical microscope (POM).

Synthesis
The synthesis of the target compounds IFna-e was performed according to the synthetic route shown in Scheme 1.The synthesis was straightforward and starts from commercially available materials.The synthesis details of the intermediate hydroxy compounds AX as well as for the final compounds are given below.

Synthesis of compounds AX
2.1.1.1.General procedure.To 4-hydroxy-2-fluorobenzaldehyde (10 mmol) the 4-substituted aniline (10 mmol) was added followed by addition of absolute ethanol (20 mL).The reaction mixture was refluxed with stirring for two hours.At the end of the reaction as indicated from TLC, the mixture was cooled to room temperature and the solid materials were filtered off, washed with cold ethanol, and recrystallised twice from hot ethanol to give pure imine compounds (AX).
As an example, the analytical data of (E)-

Synthesis of final compounds (IFna-e)
2.1.2.1.General procedure.To the appropriate imine compounds AX (10 mmol) dissolved in dry methylene chloride (DCM, 25 mL) the appropriate 4-alkoxy benzoic acid derivative Bn (10 mmol) was added followed by addition of N,N′-dicyclohexylcarbodiimide (DCC, 10 mmol) and few crystals of 4-dimethylaminopyridine (DMAP), as catalyst.The reaction mixture was stirred for 72 hours at room temperature.The formed solid materials were filtered off and washed several times with DCM.After removing the solvent under reduced pressure, the obtained solid residue was recrystallised from ethanol to give the target materials.The purity of the final compounds was checked with thin-layer chromatography (TLC) using CH 2 Cl 2 /CH 3 OH (9:1) as eluent and was confirmed by spectral techniques.

Characterisation methods
Thin layer chromatography (TLC) was performed on aluminium sheet precoated with silica gel.Analytical quality chemicals were obtained from commercial sources and used as obtained.The solvents were dried using the standard methods when required.The purity and the chemical structures of all synthesised materials were confirmed by the spectral data.The structure characterisation of the prepared materials is based on 1 H-NMR and 13 C-NMR (Varian Unity 500 and Varian Unity 400 spectrometers, in CDCl 3 solutions, with tetramethylsilane as internal standard).Microanalyses were performed using a Leco CHNS-932 elemental analyser.
The mesophase behaviour and transition temperatures of the synthesised compounds were measured using a Mettler FP-82 HT hot stage and control unit in conjunction with a Nikon Optiphot-2 polarising microscope.The associated enthalpies were obtained from DSC-thermograms which were recorded on a Perkin-Elmer DSC-7, heating and cooling rate: 10°C min −1 .The samples were measured from room temperature up to 250°C.Two heating/cooling scans were performed, and the transition temperature was read at the top of the peak.The optical micrographs were taken with a Leica MDN20 HD camera.

Mesophase and optical behaviour
The transition temperatures and related enthalpies as recorded during DSC measurements for all synthesised materials IFna-e are collected in Table 1 and represented graphically in Figure 1.The thermal stability of all derivatives was confirmed by the reproducibility of the heating and cooling DSC curves.As an example Figure 2 shows the DSC heating and cooling traces of compound IF8d.For additional DSC curves see Figure S10 in the SI.

Compounds IF8a-e
As can be seen from Table 1 all derivatives from IF8a-e series display enantiotropic LC phases.Moreover, all of them exhibit exclusively the same type of the highertemperature LC phase in different temperature ranges depending on the nature of the terminal substituent X.For example, on heating the first member of this series IF8a (with X= OCH 3 ) under crossed polarisers a direct transition from the birefringent crystalline state takes place to a typical schlieren texture observed for a nematic (N) phase exhibited by conventional rod-like LCs (see Figure 3(a)).Therefore, the LC phase exhibited by IF8a is assigned as N phase.On further heating of IF8a, the N phase remains over~111°C which is relatively wide range till a direct transition to the isotropic liquid state occurs at T ~ 219°C.The recorded enthalpy value for the N-Iso transition (ΔH~1.5 kJ/mol) is also in the typical range for the N phases formed by rod-like LCs (Table 1).On cooling IF8a from the isotropic liquid state the same N phase is observed but remains for a wider range of temperature~180°C (see Figure 2) without formation of any additional types of LC phases till crystallisation.
For the next derivative IF8b having X = OC 6 H 13 instead of CH 3 the melting temperature is reduced, and the N phase is retained as the higher-temperature LC phase.However, additional LC phase is observed below the N phase on heating from the crystalline state or cooling from the schlieren texture of the N phase.This lower-temperature LC phase is characterised by higher viscosity compared to the N phase, meaning higher degree of order (Figure 3(b)) confirming the presence of an unknown smectic (SmX) phase.This was also supported by the higher value of transition enthalpy recorded for the SmX-N transition (ΔH~2.3kJ/mol) compared to that recorded for the N-Iso transition (ΔH~1.3kJ/mol).Replacing the oxygen atom in the substituent X of compound IF8a results in compound IF8c with X = CH 3 .Surprisingly, this slight modification widens the range of the SmX phase on the expense of the N phase.Therefore, the N phase of I8c is observed only for~1°C, while the SmX phase is retained for~90°C.
For compounds IF8d and IF8e having X= H and F, respectively only N phases were observed.For IF8d the mesophase range is the smallest among the synthesised materials indicating that the presence of either a small polar group or a terminal chain is essential for inducing mesomorphism in such materials.Compound IF8e (X= F) has a comparable melting temperature to IF8b (X= OC 6 H 13 ) but narrower N phase range without formation of SmX phase.

Compounds IF12a-e
Increasing the terminal alkoxy chain from the octyloxy chain to the dodecyloxy chain leads to formation of the second series of materials IF12a-e.As can be seen from Figure 2(b), the melting temperatures are reduced for all derivatives because of the dilution effect caused by the longer alkoxy chain.Essentially, the same phase sequences and phase types observed in IF8a-e are also exhibited by IF12a-e except compound IF12e with X= F. For this material additional metastable LC phase is observed only on cooling from the N phase, i.e. a monotropic LC phase.This phase is characterised by its homotropic appearance (Figure 4(a)).On applying shearing stress on this texture an oily streak texture is observed (Figure 4

LC phases in relation to the substituent X
The data presented in Table 1 and Figure 1 reveal that the melting temperatures (Cr -N) of methoxy and unsubstituted derivatives declined with increasing the alkoxy chain length from n = 8 to 12, except the methyl and hexyloxy derivatives which showed irregular trends.Melting points increase as polarisability of the compounds within the same series increases.Additionally, all the members of the homologous series are enantiotropic with high mesophase thermal stability and broad temperature mesomorphic range.For electron-donating terminal CH 3 O group (compounds IFna) with n = 8 or n = 12 only enantiotropic N phases are formed.The N phase range (ΔT N = T N -T cr ) is decreased from 111 to 108°C with increasing alkoxy chain length from n = 8 to 12.For relatively longer substituent X= OC 6 H 13 (IFnb), both enantiotropic SmX and N phases are detected.The SmX range (ΔT SmX = T SmX -T cr ) is increased from 11 to 42°C with increasing n, while the N phase range (ΔT N = T iso -T SmX ) decreased from 104 to 54 K with elongation of n from 8 to 12. Compounds with terminal CH 3 substituent (IFnc) are also found to be dimorphic possessing SmX and N mesophases.ΔT SmX is slightly increased from 90 to 91°C with increasing n from 8 to 12, also ΔT N enhanced from 1.0 to 2 K.For X= H (IFnd), both compounds exhibit purely monomorphic N phases, irrespective of alkoxy chain length, with relatively less thermal stability compared to IFna-c derivatives.ΔT N is reduced from 28.0 to 14.0°C for IF8d and IF12d, respectively.Electron-withdrawing F derivatives (IFne) have broad mesomorphic range.Compound IF8e is monomorphic exhibiting only N phase, while compound IF12e is dimorphic displaying both SmA and N phases.ΔT N is decreased from 87 to 35°C with n increasing from 8 to 12, while ΔT SmA was observed for~17°C with n = 12 only.
Regardless the different polarity of the substituent X, for all compounds, the stability of the nematic phase decreases as one would expect with lengthening the terminal alkoxy chain [56,57].The decreasing the rigid/flexible ratio is responsible for the downtrend in the thermal transition of the N phase.Nevertheless, as the length of the alkoxy chain is increased, the smectic phase range decreased.This is most definitely caused by an increase in the Van der Waals forces between long alkoxy chains, which leads to their interweaving and enables lamellar packing, which is essential for the formation of the smectic phase, more feasible.
According to the above results, the thermal stability of the N phase (N-isotropic transition) declined in the following order: X= CH 3 O > C 6 H 13 O > CH 3 > F > H.The stability of the LC phases and their textures are generally attributed to essential factors such as the polarity of the substituents, polarisability, aspect ratio, rigidity and molecular architecture.The LC behaviour is influenced by these components to various extents.It is known that any increment in the polarity and/or polarisability of the mesogenic core, which is affected by the polarity of the substituent and consequently affects the polarity of the entire molecular structure, increases the stability of a mesophase of a given mesomorphic compound.
The estimated entropy changes of mesophase transitions (∆S/R) for the reported compounds, are shown in Table 2. Small magnitudes of the ∆S/R associated with the Sm-N, N-isotropic are detected, along with an irregular trend that is independent of the molecules' terminal alkoxy chain length (n).The found minor values in all derivatives, however, may be caused by the ester linkage group's slight promotion of molecular biaxiality and the relatively high clearing temperature values, which in turn inhibit Sm-N and N-isotropic entropy changes [58][59][60].It is possible to attribute the variation and complexity in the entropy change with compact terminal group X and alkoxy chain length to changes in the molecular interactions between molecules, which are influenced by the dipole moment, polarisability, rigidity, aspect ratio (length/breadth ratio), and geometrical shape of molecules.The conformational, orientational, and translational entropies of the molecule may be affected by these factors to varying degrees.Although the lengthening of the alkoxy chain reduces the strength of core-core interactions, it raises the polarisability of the entire molecule, which enhances the forces of intermolecular adhesion between neighbouring molecules and enhances the degree of molecular ordering.
Terminally substituted polarisable hexyloxy, methyl and F groups increase the dipole moment of the molecule which enhances the lateral interaction and consequently allow the molecules to pack more efficiently in the LC phase, resulting in higher ∆S Sm-N /R magnitude, especially associated with Sm-N transition as observed in IF8b, IF8c, IF12b, IF12c and IF12e derivatives.

Comparison with related materials
To understand the effect of the lateral fluorine substitution on the phase behaviour of the investigated compounds, it is interesting to compare them with related materials reported before but without any lateral substitution.Table 3 shows the phase transition temperatures and phase types of the neat compounds, having the same aromatic core of compounds IFna-e but without fluorine substitution (Ina-d) [61].The number of carbon atoms in the terminal chain (n) in compounds Ina-d is also 8 and 12 as those used in compounds IFna-e, while X = OCH 3 , CH 3 and H.
As can be seen from Tables 1 and 3 all compounds are mesomorphic having either one or two LC phases depending on the nature of the substituent X.For X= OCH 3 and n = 8, the lateral fluorine substitution reduces the melting temperature as well as the mesophase range but does not change the type of the mesophase as only N phase is observed for both homologues (compare compounds IF8a and I8a, Tables 1 and 3).The nematic phase is retained with increasing n from 8 to 12 (compounds IF12a and I12a).However, in the case of the fluorinated compound IF12a the melting point is greatly reduced, resulting in a wider range of N phase in case of IF12a (~108 K) compared to that of I12a (~97 K).Similar trends were observed for the melting and clearing temperatures for derivatives with X = H and CH 3 , but more interesting is the strong dependence of the LC phase type on the lateral substitution.Therefore, for fluorinated and nonfluorinated compounds with n = 8 and X= CH 3 only N phases were observed and on increasing n to 12 the SmA observed in case of I12b is totally removed and the N phase is stabilised.
For compounds with n = 8 and X = H a SmX phase is induced in case of the fluorinated material IF8c, which replaces the SmA observed in case of the nonfluorinated derivative I8c.On increasing n from 8 to 12, the SmA observed for the neat derivative I12c is totally removed and only N phase is exhibited by IF12c.
Overall, this comparison indicates that lateral F results in less ordered mesophases due to its steric effect and  therefore the SmA phases are replaced with SmX phases in addition to the stabilisation of the nematic phases in most cases.

Summary and conclusions
Two new series of laterally fluorinated Schiff base liquid crystals were synthesised and thermally as well as optically investigated.Various spectroscopic methods were used to confirm their molecular structures.DSC and POM were used to study their molecular self-assemblies.Three different types of LC phases, including nematic and two smectic phases, were recorded depending on the length of the terminal alkoxy chain at one end and the nature of the substituent X at the other end.Moreover, all prepared derivatives exhibit high thermal stability with wide enantiotropic temperature mesomorphic ranges.Furthermore, small magnitudes of the entropy changes associated with the Sm-N, N-isotropic are detected, along with an irregular trend independently on terminal alkoxy chain length (n).That may be caused by the ester linkage group's slight promotion of molecular biaxiality and the relatively high clearing temperature values, which in turn inhibit Sm-N and N-isotropic entropy changes.Finally, a comparison between the investigated materials and their related neat compounds reported in the literature proved that the lateral fluorine substitution induces the formation of nematic phases.

Figure 1 .
Figure 1.(Colour online) The phase behavior of the investigated materials as recorded on heating; (a) for If8a-e and (b) for If12a-e.

Figure 2 .
Figure 2. (Colour online) DSC thermograms of If8d: (a) recorded from second heating scan and (b) from second cooling scan with a rate of 10°C/min.

Figure 3 .
Figure 3. (Colour online) Optical textures observed under polarised optical microscopy under crossed polarisers for compound IF8b in: a) the N phase at 115°C and b) the SmX phase at 90°C.

Figure 4 .
Figure 4. (Colour online) Optical textures observed under polarised optical microscopy under crossed polarisers for compound IF12e in: a) the N phase at 130°C; b) the SmA phase at 95°C; c) the oily streak texture of SmA phase at 95°C after applying a shearing stress.

Table 1 .
Transition temperatures (°C) and enthalpy of transitions in kJ/mol IFna-e a .