Fluorinated esters with a very broad temperature range of the antiferroelectric phase

ABSTRACT Antiferroelectricity is a desirable property of liquid crystalline materials. Therefore, we synthesised and studied two new chiral rod-like mesogens with a molecular core based on two biphenyls connected via ester linkage, with the phenyl substituted by a fluorine atom. The studied mesogens are characterised by nuclear magnetic resonance spectroscopy and mass spectrometry analysis. They have methyl heptyl in the chiral chain and exhibit an antiferroelectric phase in a very broad temperature range. We investigated their mesomorphic properties and confirmed the phase identification by differential scanning calorimetry and broad-frequency dielectric spectroscopy measurements. Additionally, we compared the studied mesogens with previously synthesised analogous materials. Two mixtures were formulated using a base mixture and new mesogens. The helical pitch of the synthesised mesogens and formulated mixtures was estimated using the selective reflection method. Graphical abstract


Introduction
Since the discovery of antiferroelectricity in liquid crystals [1][2][3][4], this topic has become very interesting from both scientific and application points of view.Antiferroelectric liquid crystals (AFLCs) are a subclass of chiral tilted smectics in which the director (n) in adjacent layers tilts in opposite directions relative to the layer normal (z).The anticlinic order is shown in Figure 1.In addition to this anticlinic double-layer structure, the molecules form a helical superstructure along the layer normal with a period usually of about 1 µm.In the surface-stabilised state used in electro-optical devices, the superstructure is absent.The anticlinic molecular arrangement results in an antipolar order where the direction of local polarisation alternates between adjacent layers.
The most exciting antiferroelectric liquid crystals are orthoconic ones (OAFLCs) [5,6].This group of materials has unique optical properties resulting from a high tilt angle θ ð Þ and is characterised by a perfect dark state and high contrast [7][8][9].The chemical structure plays an essential role in the design of antiferroelectric liquid crystalline compounds.The main factors determining the antiferroelectric properties are the number of aromatic rings, the type of linkage units, the presence of aromatic ring substituents, and the type of terminal aliphatic chains.Our research group synthesised several dozen rod-like antiferroelectric liquid crystals with a different number of aromatic rings, with -COOand/or -CH 2 O-groups, with or without aromatic ring substituents, with varying lengths of aliphatic chains and with (S) or (R) configuration [7,8,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25].We also synthesised racemic mixtures with the anticlinic smectic phase [26].These materials were used to design and formulate dozens of antiferroelectric mixtures with excellent properties [15,16,20,22,25,[27][28][29][30][31][32][33][34][35][36][37].These mixtures were obtained by calculating the eutectic composition or by doping previously formulated mixtures.We decided to synthesise two new molecules with (S) configuration and different lengths of the oligomethylene spacer, with three benzene rings, -COO-groups, the fluorine atom in the phenyl ring, and a chiral centre based on (S)-(+)-2-octanol.The motivation of the work was to show how the structure of the rod-like molecules affects their properties and how synthesised mesogens change mesomorphic properties and helical pitch when doped into the base mixture.

Experimental details
Two compounds of the following general formula and acronym rPhPhCOOPhF were synthesised, for which r=3 or 7.
COOCH(CH 3 )C 6 H 13 (S) The general synthesis pathway of new compounds is given in Figure 2.They were prepared in the same classical pathway by treating chiral phenol with benzoic acid chloride in the presence of pyridine.An efficient method of synthesis of high optical purity fluorinated hydroxyester has been given earlier in Ref [14].The perfluoroalkoxyalkoxy benzoic acids were synthesised as described in Ref [15].The reagents used for the reactions were commercially available, and only toluene was dried by distillation over diphosphorus pentaoxide.
Proton ( 1 H) and carbon ( 13 C) nuclear magnetic resonance (NMR) spectra in CDCl 3 were collected using a Bruker, model AvanceIII spectrometer (Bruker, Billerica, MA, USA).The spectra of all of the samples were measured at room temperature.A comparison of NMR spectra confirmed the compliance of real structures with the planned structures. 1 H NMR and 13 C NMR spectra of the new compounds are presented in the Appendix (see Supplementary materials).Phase transition temperatures and enthalpies were determined by a differential scanning calorimeter with 2.0°C/min scanning rate during heating/cooling cycles using the DSC 204 F1 Phoenix instrument (Netzsch, Selb, Germany).The samples of about 10 mg, hermetically sealed in aluminium pans, were placed into the calorimeter chamber filled with nitrogen.Mesophases were identified by conventional observation of characteristic texture patterns under a polarising optical microscope (OLYMPUS BX51, Shinjuku, Tokyo, Japan) equipped with a heating/cooling stage unit (THMS-600, Linkam Scientific Instruments Ltd., Tadworth, United Kingdom) and a temperature controller (TMS-93, Linkam Scientific Instruments Ltd., Tadworth, United Kingdom).The liquid crystal specimen was placed between microscopic glass slides without any alignment layers or spacers for thickness control.
Impedance spectroscopy is a valuable method to characterise the electric properties of liquid crystals.This experimental technique confirmed the liquid crystal phases expected in the two compounds under study.We use an impedance analyser by Hewlett Packard: HP 4192A (Tokyo, Japan) in the frequency measuring range from 100 Hz to 10 MHz.For measurements, we used selfmade cells with gold electrodes to avoid high-frequency parasitic effects [39].Such cells can be used for frequencies up to 5À 6 MHz.The thickness of the used cells was around 3 µm while the alignment of the cells was planar (the aligning layer was polyimide SE130 by Nissan Chem.).Liquid crystals were heated and put into a measuring cell in the isotropic phase using capillary action.The temperature controller TMS 92 and the hotstage THMSE 600 (Linkam Scientific Instruments Ltd., Tadworth, United Kingdom) were used to control the temperature.All measurements were performed on the cooling cycle.We performed measurements without and with a 6 V DC field for better characterisation of the electric properties of mesophases.
The helical pitch length measurements were performed according to the procedure described in Refs [31][32][33] using a UV-Vis-NIR spectrophotometer (UV-3600 Shimadzu, Kyoto, Japan).The spectrophotometer had a temperature controller U7 MLW with a Peltier element.Before measurements, a thin layer of orienting surfactant was applied to the glass plate to force the required homeotropic alignment of the LC molecules.After baseline collection, LC samples were placed on the surface of the slide, and the wavelength of selectively reflected light was recorded.The measurements were performed in the cooling cycle.

Phase transitions
The measured phase transition temperatures and related enthalpies for new mesogens are listed in Table 1.
The synthesised compounds have the antiferroelectric phase (SmC A *) in a very broad temperature range (above 100°C in the cooling cycle).This phase occurs in a broader temperature range for the compound with the acronym 7PhPhCOOPhF.This compound also has a monotropic ferroelectric phase (SmC*) in a very narrow temperature range (below 1°C).In addition, both compounds have the SmA* phase in a narrow or medium temperature range.The compound with the shorter oligomethylene spacer (r = 3) has a higher crystallisation point than the compound with the longer oligomethylene spacer (r = 7) as shown in Figure 3.The clearing points are below 140°C.Characteristic microscopic patterns for the observed phases are shown in Figure 4(a,b).
Phase transition temperatures for the compounds with exchanged PhPh and PhF groups (the acronym for this type of the compounds -rPhFCOOPhPh, see their general formula below) are shown in Figure 5 [15].For these compounds, we observe various mesomorphic properties, for the compound with r = 3, we observe a direct transition from the SmC A * phase to the isotropic phase, and for the compound with r = 7, we also have the ferroelectric and smectic A* phases.
Comparing the phase transition temperatures, it can be seen that the compounds presented in this paper have a broader temperature range of the antiferroelectric phase and higher melting and clearing points than the compounds synthesised earlier [15].As shown in Figure 5, the modification of the molecular structure by changing the order of the groups: PhF and PhPh affects the temperature range and the occurrence of the liquid crystalline phases.

Dielectric measurements for the compound 3PhPhCOOPhF
In Figure 6, the real part ε 0 ? of permittivity, measured on cooling, is shown.The SmA* or the SmC* phase appears from isotropic liquid at 121°C (red arrow).The SmC A * phase nucleates from the SmA* or the SmC* phase at 115°C (black arrow).To determine precisely the phase/phases above the SmC A * phase, the spectra of the imaginary part ε 00 ? of permittivity for several temperatures (without and with the 6 V DC field) where the SmA*/SmC* and the SmC A * phases exist, were measured, and are presented in Figures 7-9.When we identify the dielectric modes typical for particular smectic phases, the phases will be confirmed.
In Figure 7(a), two dielectric modes are presented: the soft mode (with maximum amplitude at 116°C).This mode is typical for the SmA* phase.The next mode is seen for temperatures 115°C and lower.This mode can be identified as P L mode -typical for SmC A *. Unfortunately, the ions do not allow us to investigate the low-frequency electric response.The 6 V DC field solves this problem (Figure 7(b)).Two modes are seen in the electric response below the SmA* phase.The lowfrequency mode (with the relaxation frequency, which does not change with the temperature) is the Goldstone one.It is detectable for temperatures 116°C and lower.The Goldstone mode appears when the soft mode exhibits a maximum amplitude.The mode seen in the middle-frequency range is P L one.P L mode is amplified by the DC field.It is characteristic for the SmC A * phase.The coexistence of Goldstone mode and P L mode means that compound 3PhPhCOOPhF below the SmA* phase exhibits two phases: the SmC* and SmC A *.In Figure 7(b), both modes: Goldstone and P L, exhibit similar dielectric strength.This coexistence is also seen in Figure 8.
Figure 8 presents the imaginary part ε 00 ? of permittivity for lower temperatures.The Goldstone mode is still detectable at 90°C, and its relaxation frequency (,2 kHz) does not depend on temperature.P L mode relaxation frequency decreases, and at 80°C, both modes are merged.Because the P L mode is stronger, the Goldstone mode is covered by P L one.Next, mode -P H one appears at high frequencies (500 kHz À 2 MHz).The next figure presents the spectra for lower temperatures.
Figure 9 shows that P L , P H, and the high-frequency S-molecular mode (molecular rotation around the short molecular axis) are thermally activated.Their relaxation frequencies decrease when the temperature decreases.All three modes are typical in the antiferroelectric phase [40][41][42].The electric response (Figures 7-9) suggests that below the SmA* phase, both SmC* and SmC A * phases coexist.The Goldstone mode is weak, but it is detectable for temperature range: 116-80°C.The antiferroelectric phase gradually displaces the ferroelectric one.It is worth underlining that to see all relaxations, the 6 V DC field was necessary to apply.

Dielectric measurements for the compound 7PhPhCOOPhF
In Figure 10 the real part ε 0 \ of permittivity measured on cooling is shown.The SmA* phase creates from isotropic liquid at 137°C (red arrow).The SmC A * phase nucleates definitely from the SmC* phase while the SmC* from the SmA* one.However, it is not easy to  ? for several temperatures (with the 6 V DC field) were measured and are presented in Figures 11-13.
Figure 11 shows that soft mode reaches the maximum amplitude at 125°C and its relaxation frequency of around 150 kHz.This mode behaves classically: the relaxation frequency of soft mode goes up, and amplitude goes down when the temperature moves away from 125°C.This is the phase transition temperature SmA*-SmC* phase.At 123°C and lower temperatures, one relaxation mode nucleate with a relaxation frequency of around 56 kHz.This is the P L mode typical for the SmC A * phase.It seems that the SmC* and SmC A * phases again coexist as it was found in the compound 3PhPhCOOPhF.Remembering that the DC field suppresses the Goldstone mode, we prepared Figure 12, hoping that the Goldstone mode will appear.
Figure 12 shows that the soft mode disappears from the electric response (for temperatures 121°C and lower), while the low-frequency mode appears (at 120°C) with a relaxation frequency of around 1-2 kHz, and this relaxation does not move with the temperature.It is the residual Goldstone mode.This mode is typical for the SmC* phase.P L mode still exists and it is typical for the SmC A * phase.We observe two modes which are observed in two different smectic phases.It seems, that below the SmA* phase, the compound 7PhPhCOOPhF "is not sure" what phase to create: the synclinic or anticlinic.We see that the relaxation frequency of P L mode decreases with the temperature decrease, and finally, the weak Goldstone mode is covered by P L mode.At 85°C, the Goldstone mode is practically not detectable.Additionally, P H antiferroelectric mode appears (with a relaxation frequency of around 1 MHz).Its     relaxation frequency decreases with temperature decrease, as is seen in the SmC A * phase.
Figure 13 presents the situation at lower temperatures (85-15°C).It is seen that the well-defined SmC A * phase exists.Two collective modes (P L and P H ) and one molecular mode (S-mode -rotation around short molecular axis) are detectable.The last mode exhibits the highest relaxation frequency of more than 1 MHz in this temperature range.In Figure 12, S-mode is not seen because it is too fast.P L mode slowly leaves the measuring range (at temperatures 60°C and lower) -its relaxation frequency becomes lower than 600 Hz.A few additional figures showing electric properties are included in the supplementary materials.

Helical pitch
The results of the helical pitch measurements for new compounds are presented in Figure 14.For the compound with the acronym 3PhPhCOOPhF, the helical pitch in the SmC A * phase is longest at 95°C and is over 1350 nm, while for the compound with the acronym 7PhPhCOOPhF, the helical pitch is longest at 47°C and is almost 1200 nm.The pitch increases with increasing temperature for both compounds, and the compound with the shorter oligomethylene spacer (r = 3) has a longer pitch.

Properties of mixtures
Two mixtures were formulated by adding 25 wt% of new mesogens to the eutectic mixture W-450 (base mixture); see Tables 2 and 3.The composition of the eutectic mixture was calculated from equations given by Le Chatelier, Schrӧder, and van Laar.These equations combine component concentrations in the individual phases from temperature and parameters of the pure compounds (temperature and enthalpy of the phase transition), assuming that the system is in the isobaric conditions, as well as constant of the  [29] 42.0 thermal capacity of components in the phases being in the phase equilibrium, and lack of chemical reaction of components in the system [43][44][45].The components of the mixture W-450 are structurally similar to the compounds presented in the paper but have different chiral centers [18,25,29].
The eutectic mixture's theoretically calculated melting and clearing points are −44°C and 88.2°C, respectively.
Phase transition temperatures for the base mixture and the formulated mixtures are presented in Table 4 and Figure 15.
All mixtures have the antiferroelectric phase in a very broad temperature range.In the case of the base mixture, a direct transition from this phase to the isotropic phase is observed.The mixture W-450D is the only one with the ferroelectric phase in a narrow temperature range.The doped mixtures also have the SmA* phase in a narrow (W-450C) or very narrow (W-450D) temperature range.The mixtures do not crystallise despite being cooled to temperatures below −10°C.The clearing points are below 100°C.
The results of the helical pitch measurements for all mixtures are presented in Figure 16.
The helical pitch in the SmC A * phase is the longest for the mixture W-450D and reaches a maximum value of 900 nm at 59°C.For the base mixture, this value is about 775 nm at 86°C.For the mixture W-450C, the helical pitch reaches a value of about 750 nm at 78°C.Adding 25% of the synthesised compounds to the base mixture makes the helical pitch longer.

Conclusions
The paper shows how the modification of the molecular structure changes the occurrence and the temperature range of individual liquid crystalline phases.The replacement of biphenyl and phenyl fragments significantly affects the mesomorphic properties, which can be used when planning new structures of antiferroelectric liquid The first row is the temperatures from DSC obtained in the cooling cycle; the second row is the temperatures from DSC obtained in the heating cycle; the third row is the enthalpies.
Table 3.The compositions of the formulated mixtures.crystals.The substituents of the benzene ring also have a great influence on the properties.Newly synthesised fluorinated chiral esters are characterised by the antiferroelectric phase presence in a very broad temperature range (above 65°C in the heating cycle) and a long helical pitch (above 1000 nm) in this phase which increases with increasing temperature, which was particularly important to us.The presence of this phase was confirmed by dielectric spectroscopy.In addition, it is planned to study the electrooptical properties of new mesogens, as they are most certainly orthoconic compounds.The OAFLCs differ in optical properties from regular (low-tilted) AFLCs and generate new possibilities for their applications [5, 6, 8, 10-12, 14, 15, 17, 22, 24, 25, 28, 29, 31, 32, 34, 36, 37].
A wide temperature range of the antiferroelectric phase, as well as a low melting point and a long helical pitch, are the main application requirements, which were achieved by doping the base mixture with the synthesised mesogens.Because the LC materials presented in this paper reveal good chemical stability and appropriate physical properties, therefore, can be used to design further multicomponent mixtures targeted for optoelectronics and photonics.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 1 .
Figure 1.(Colour online) Scheme of the anticlinic order.Director n, tilt angle θ, layer normal direction z.

FFigure 2 .
Figure 2. Scheme of the synthesis of the final compounds.

Figure 5 .
Figure 5. (Colour online) Phase transition temperatures observed in the heating cycle for the compounds synthesised earlier with the acronym: rPhFCOOPhPh.

Figure 3 .Figure 4 .
Figure 3. (Colour online) Phase transition temperatures observed in the cooling cycle for the synthesised compounds.

Figure 6 .
Figure 6.(Colour online) Real part ε 0 \ of permittivity versus temperature T (measurements in the planar cell, on cooling and without DC field) for 12 frequencies of the measuring signal.

Figure 8 .
Figure 8. (Colour online) Imaginary part ε 00 ? of permittivity versus frequency f (measurements in the planar cell, on cooling and with 6 V DC field) for 13 temperatures (110°C-65°C).Three detectable modes are listed.

Figure 7 .
Figure 7. (Colour online) Imaginary part ε 00 ? of permittivity versus frequency f (measurements in the planar cell, on cooling and without (a) and with (b) 6 V DC field) for 13 temperatures (121°C-110°C).Three detectable modes are listed.

Figure 10 .
Figure 10.(Colour online) Real part ε 0 \ of permittivity versus temperature T (measurements in the planar cell, on cooling and without DC field) for 12 frequencies of the measuring signal.

Figure 9 .
Figure 9. (Colour online) Imaginary part ε 00 ? of permittivity versus frequency f (measurements in the planar cell, on cooling and with 6 V DC field) for 13 temperatures (65°C-25°C).Three detectable modes listed.

Figure 12 .
Figure 12. (Colour online) Imaginary part ε 00 ? of permittivity versus frequency f (measurements in the planar cell, on cooling and with 6 V DC field) for 13 temperatures (121°C-85°C).

Figure 11 .
Figure 11.(Colour online) Imaginary part ε 00 ? of permittivity versus frequency f (measurements in the planar cell, on cooling and with 6 V DC field) for 16 temperatures (135°C-121°C).

Figure 14 .
Figure 14.(Colour online) Temperature dependence of the helical pitch p in the SmC A * phase for both synthesised compounds.
*The first row is the temperatures from differential scanning calorimetry obtained in the cooling cycle; the second row is the temperatures from differential scanning calorimetry obtained in the heating cycle; the third row is the enthalpies.

Table 2 .
The composition of the base mixture W-450.

Table 4 .
Phase transition temperatures [°C] and enthalpies [J•g −1 ] obtained by differential scanning calorimetry for all mixtures.