Synthesis and properties of ferro- and antiferroelectric esters with a chiral centre based on (S)-(+)-3-octanol

ABSTRACT A homologous series of chiral three ring esters were synthesised and their properties were studied. The materials possess an antiferroelectric phase (SmCA*) or/and a ferroelectric phase (SmC*), which was confirmed by observations of microscopic textures using a polarising optical microscope. The phase transition temperatures and enthalpies were checked by differential scanning calorimetry. The helical pitch was measured by a spectrophotometry method. For all cases the helix is right-handed, in the antiferroelectric phase the helical pitch increases with temperature, and in the ferroelectric phase the helical pitch is short. Graphical Abstract


Introduction
Antiferroelectric liquid crystals have been the subject of much research since their discovery in 1989 [1]. Useful applications of antiferroelectric liquid crystals require them to have low melting points, a long helical pitch, a wide temperature range for this phase and to exhibit a whole host of other desirable physical properties. Compounds having a high tilted (up to 45°-orthoconic) antiferroelectric phase which produces a perfect dark state and by consequence a high contrast are most suitable for many applications [2][3][4]. These materials with the direct SmC A *-Iso transition are especially sought out. Many homologous series of antiferroelectric three ring liquid crystals have been prepared and tested earlier by our [5][6][7][8][9][10][11][12] and other research groups [13][14][15][16][17]. Compounds with a partially fluorinated alkoxyalkoxy terminal chain and chiral centre based on (S)-(+)-2-octanol (see general formula (I) below) have very interesting properties [10][11][12]. These compounds have a very broad range of the antiferroelectric phase, good chemical stability, a high tilt angle (some are orthoconic), and their helical pitch increases or decreases with temperature.
(I) COOC*H(CH 3 )C 6 H 13 (S) C F CH 2 O(CH 2 ) O X 1 X 2 COO r n 2n+1 with n = 3 or 4; r = 2-7 and X 1 = H, X 2 = F or X 1 = F, X 2 = H or X 1 = X 2 = H or X 1 = X 2 = F. Since the shape and chemical architecture of the molecule has a significant effect on its self-assembling behaviour, even a minor change of its parts can cause a drastic change in the resulting properties. While designing chiral molecules, special care should be taken with the type of attached chiral centre, substitution of the benzene ring, as well as length and type of the terminal chains [18]. The main objective of this work is to design and study new compounds similar to compounds (I) but with different chiral centre based on (S)-(+)-3-octanol.

Synthesis of compounds
The synthesis of the esters was carried out by treating chiral phenol with benzoic acid chloride, see Figure 1.
All reagents were used for the reactions as purchased, only toluene was dried by distillation over diphosphorus pentaoxide. The purity of the liquid crystalline esters was recorded using a Schimadzu prominence chromatograph with an HPLC MS (API-ESI) detector 2010EV. The strong molecular ion with a captured sodium atom [M+ Na + ] was observed in all of the compounds. The purity of the chiral phenol was checked using a Hewlett-Packard HP-6890N chromatograph with the MS detector HP5973N.
The structure of the final compounds was confirmed by 1 H NMR and 13 C NMR nuclear magnetic resonance. NMR spectra were obtained with a Bruker AvanceIII HD 500 MHz spectrometer (field 11.7 T) operating at 500 MHz ( 1 H) and 125 MHz ( 13 C) with CDCl 3 as the eluent [19]. NMR spectra of all samples were measured at room temperature. NMR spectra of the new compounds are presented in the Supplemental online material.
Examples of the synthesis are given below. Ethyl 4-benzyloxybiphenylcarboxylate Ethyl 4-hydroxybiphenylcarboxylate (121 g; 0.5 mol), potassium carbonate (104 g; 0.75 mol), 2-butanone (1100 ml) were heated to a boil (79°C) and benzyl chloride (76 g; 0.6 mol) was added dropwise. During heating, the mixture turned from white to yellow. The mixture was stirred for about 24 h. The progress of the reaction was monitored by chromatographic analysis (GC), then the reaction mixture was cooled down and poured into water (3000 ml). The precipitate that formed was filtered and washed with plenty of water and allowed to dry. Then the precipitate was crystallised from 2-butanone. Yield: 145.32 g.
4-benzyloxybiphenyl-4ʹ-carboxylic acid Ethyl 4-benzyloxybiphenylcarboxylate (145.32 g; 0.44 mol), ethylene glycol (1300 ml) and water (26 ml) were stirred and then potassium hydroxide (49.28 g; 0.88 mol) was added. The mixture was then heated slowly until boiling. During heating, the mixture turned from white to beige. The temperature stabilised at 165°C. The reaction lasted about 16 h. After the reaction, the mixture was poured into water (1500 ml) and hydrochloric acid (150 ml) and was stirred for 4 h. After this time, the formed precipitate was filtered and washed abundantly with water, and then three times with ethanol and finally with acetone. The precipitate was allowed to dry for 2 days. The precipitate was crystallised from a mixture of 2200 ml of 2-butanone, 35 ml of hydrochloric acid and 15 ml of acetic acid per 10 g of precipitate. Afterward, filtration of the solvent was recovered using a vacuum evaporator and then the crystallisation process was repeated for the next portions of precipitate. After eight crystallisations, 56.88 g of 4-benzyloxybiphenyl-4ʹ-carboxylic acid were obtained.
4-benzyloxybiphenyl-4ʹ-carboxylic acid chloride 4-benzyloxybiphenyl-4ʹ-carboxylic acid (56.88 g; 0.187 mol), oxalyl chloride (17.9 ml; 0.21 mol), a few drops of N,N-dimethylformamide and toluene (1700 ml) were stirred and heated to 65°C. During the reaction 1 ml of oxalyl chloride and 200 ml of toluene were added; the reaction was carried out for 8 h. The mixture was then heated to the boiling point of toluene and the excess oxalyl chloride was distilled off with toluene.
(S)-4-benzyloxy-4ʹ-(1-ethylhexyloxycarbonyl) biphenyl After distilling off the chloride and cooling the reaction, (S)-(+)-3-octanol (24 g; 0.187 mol) and pyridine in double excess (31 ml; 0.374 mol) were added and heated to 65°C. The reaction was carried out for 16 h. After the reaction, the mixture was poured into water (2000 ml) with 10% hydrochloric acid (35 ml) and was stirred for 2 h. Then it was filtered off by a layer of active carbon. The organic layer was separated and dried over anhydrous magnesium sulphate. Then toluene was distilled off until dry. The crude yellow oil (22.1 g) was crystallised from 250 ml of acetone. 20 g of white crystals were obtained.
(S)-4-hydroxy-4ʹ-(1-ethylhexyloxycarbonyl)biphenyl (S)-4-benzyloxy-4ʹ-(1-ethylhexyloxycarbonyl)biphenyl (20 g; 0.048 mol), 300 ml of acetone and 2.5 g of catalyst (10% palladium on carbon) were used in the reduction reaction. The reaction did not start therefore the mixture was heated to 40°C. The reduction ended after 6 h. Then the mixture was heated to a boil, cooled down and purged with nitrogen. The catalyst was then filtered off and the filtrate was concentrated until dry. The resulting yellow oil was crystallised from hexane (200 ml). 15.7 g of (S)-4-hydroxy-4ʹ-(1-ethylhexyloxycarbonyl)biphenyl were obtained with a purity of 99.9% (GC). ( To a suspension of 4-[6-(2,2,3,3,4,4,4-heptafluorobutoxy)hexyl-1-oxy]benzoic acid (2.1 g; 5 mmol) in dry toluene (65 ml), oxalyl chloride (0.5 ml; 5.5 mmol) and one drop of N,N-dimethylformamide were added. A Vigorous reaction occurred. When the evolution of gases stopped, the mixture was heated to 30°C with continual stirring for 2 h. The clear solution was then refluxed and the excess of oxalyl chloride was distilled off with toluene (~15 ml) using a Vigreux column. Then (S)-4-hydroxy-4ʹ-(1-ethylhexyloxycarbonyl)biphenyl (1.63 g; 5 mmol) and pyridine (0.8 ml; 10 mmol) were added to the cold solution. The mixture was stirred at 60°C for 2 days, and then it was cooled down to room temperature and poured into a solution prepared with concentrated hydrochloric acid (2 ml) and water (500 ml). The layers were separated and the organic layer was washed twice with water. The extract was filtered off with a layer of active carbon, then it was dried over anhydrous magnesium sulphate and the solvent was evaporated until dry. The crude product was purified using column chromatography on silica gel with methylene chloride as the eluent. Then the ester was crystallised from anhydrous ethanol (50 ml). Yield: 0.

Mesomorphic properties of compounds
Phase transitions and phase sequences were observed using a polarising optical microscope (OLYMPUS BX51, Japan) equipped with a heating stage (Linkam THMS-600) and a temperature controller (Linkam TMS-93). Observations were performed over a heating and cooling cycle at a rate of 2°C/min. The temperatures and enthalpies of phase transitions were recorded by a DSC "Setaram" 141 microcalorimeter both in heating and cooling cycles with a scan rate of 2ºC· min −1 .
The mesomorphic measurements of compounds are compared in Table 1 and visualised in Figure 3(a-c). Four types of phase transition sequences are observed: Cr-SmC A *-SmC*-SmA*-Iso Cr-SmC A *-SmA*-Iso, Cr-SmC*-SmA*-Iso and Cr-SmC A *-Iso.
The lowest melting points (below 0°C) are seen in compounds II.6.(FH) (S) and II.4F3(HF) (S). The clearing points of all the compounds are below 100°C. Unsubstituted compounds -(HH) have higher clearing points. Characteristic microscopic patterns for the observed phases are shown in Figure 4(a-c).
Four compounds with oligomethylene spacers equal to 3 and one with r = 5 (II.5.(HF)) have a direct transition from the antiferroelectric phase to the isotropic phase. The broadest temperature range (higher than 50°C) for the chiral anticlinic smectic C (SmC A *) phase is observed for compound II.4F3(HF) (S). Three compounds with r = 5, besides chiral anticlinic smectic C, also possess the smectic A* phase. The SmC A * phase exhibits a medium temperature range for compounds II.5.(HH) (S) and II.5(FF) (S) and a short temperature range for compound II.5 (FH) (S). Phase transition temperatures for analogous compounds with a chiral center based on (S)-(+)-2octanol, are shown in Figure 5(a-c) with acronyms: I. r.(X 1 X 2 ) (S) for compounds with a C 3 F 7 CH 2 O-chain and 4Fr(X 1 X 2 ) (S) for compounds with a C 4 F 9 CH 2 Ochain.
Three previously described compounds in [10] with r = 3 have a direct transition SmC A *-Iso. One of the compounds, 4F3(HH) (S), also has a ferroelectric phase at a very short temperature range. All previously synthesised compounds with r = 5 possess the phase sequence: Cr-SmC A *-SmC*-SmA*-Iso.
Three compounds with r = 6 have ferroelectric and smectic A* phases, one also has the antiferroelectric phase-II.6.(HH) (S), see Figure 3(c). The smectic C*   phase is observed at a broad temperature range for compounds II.6.(FH) (S) as well as II.6.(HH) (S) and a medium temperature range for compounds II.6.(FF) (S) and II.6.(HF) (S). The smectic A* phase appears in a short or a very short temperature range for these compounds. All compounds with r = 6 and a chiral centre based on (S)-(+)-2-octanol have antiferroelectric, ferroelectric and smectic A* phases, see Figure 5(c) [10]. The older compounds with r = 6 have SmC A * and SmC* phases in a medium temperature range. For the new compounds, we observe the disappearance of the antiferroelectric phase for three compounds.
For other compounds obtained by our group with the acronym 3XOmC2 (prepared also from (S)-(+)-3octanol), we observed the destabilisation of the SmC* phase. These compounds also have lower melting points than analogous compounds prepared from (S)-(+)-2-octanol -3XOmC1 [20].

Helical parameters of compounds
The helical pitch measurements were done using a selective light reflection phenomenon [21]. The pitch p was calculated for antiferroelectric phase from the dependence: and for the ferroelectric phase from the dependence: where n is the average refractive index. The value: n = 1.5 was used for calculations [22]. The transmission spectra were acquired using a Shimadzu UV-Vis-NIR spectrometer in the wavelength range of 360-3000 nm. An AMLWU7 temperature controller with Peltier element was used for changing the temperature within the range 2-110°C with an accuracy of 0.1°C. All measurements were done during cooling cycles. The helical twist sense for the chiral phases was determined by the polarimetry method [23]. The temperature of the helical twist sense inversion was established by the analysis of temperature dependent transmission of light through a homeotropically aligned sample under a polarising optical microscope where the brightest texture indicates an unwound structure [24][25][26][27][28].
Comparisons of helical pitch versus temperature for individual compounds are shown in Figure 6-8.
The helical pitch in the antiferroelectric phase for all cases increases with temperature (the highest observed value of helical pitch is 928 nm (at 65°C) for the compound II.5.(FH) (S). In the ferroelectric phase, the helical pitch is short; the lowest value of helical pitch (138 nm at 2°C) was detected for the compound II.6.(FH) (S). For all cases the helix is right-handed in the SmC A * phase as well as in the SmC* phase. Dependencies of helical pitch upon temperature referred to the phase transition between the SmC A * phase and low ordering phases for unsubstituted compounds prepared in this work as well as previously described in [10] are presented in Figure 9. This figure should help to better compare the helical parameters of the investigated compounds.
It can be seen that the increase in the length of the oligomethylene spacer of the investigated compounds causes an increase of the helical pitch without changing its helix handedness. Importantly, the behaviours of the helix parameters is the same for compounds based on (S)-(+)-3-octanol as well as (S)-(+)-2-octanol. The difference between these two kinds of compounds is that compounds previously synthesised and described in [10] possess higher values of helical pitch in the SmC A * phase at the same temperature distance from the phase transition to a low ordering phase.
In the case of compounds 3XOmC2, we observed longer helical pitch than for compounds 3XOmC1 [20].

Conclusions
Several new materials with a chiral centre based on (S)-(+)-3-octanol have been designed, synthesised and characterised. (S)-(+)-3-octanol was very rarely used by our research group to obtain a chiral centres. The most popular alcohol to obtain different chiral liquid crystals was (S)-(+)-2-octanol. The obtained results confirm that the change of the chiral centre significantly affects the mesomorphic properties of the compounds, i.e. the types of phases, their range, melting and clearing points. Eight of the studied compounds show antiferroelectric phase and four exhibit ferroelectric phase. The SmA* phase is observed for six compounds. The results also indicate that the length of the oligomethylene spacer is important for the formation of different liquid  crystal phases. Changing the methyl group in the chiral carbon atom on the ethyl group causes more segregation of the ferro-and antiferroelectric phases in pure compounds. In other words, compounds based on (S)-(+)-3-octanol possess only one tilted chiral phase (SmC* or SmC A *) rather than phase sequences SmC A *-SmC*. This opens new ways for developing materials with direct SmC A *-Iso transition, which are very interesting because they enable us to obtain the layer with the smallest number of defects in the electrooptical cell [9,12]. The ethyl group in the chiral carbon atom also causes a decrease in helical pitch while maintaining its temperature dependence and helix handedness. All new compounds can be used as potential ingredients in new liquid crystalline multi-component mixtures for different applications like displays. We can also modified the properties of the known mixtures using these compounds to formulate mixtures with improved electrooptical properties and long helical pitch.
Acknowledgments I want to thank Mr. Karol Ofiara for the help with synthesis of some esters.

Disclosure statement
No potential conflict of interest was reported by the authors.