Effect of alkyl chain length in the terminal ester group on mesomorphic properties of new chiral lactic acid derivatives

ABSTRACT Two series of new liquid crystalline lactic acid derivatives with a terminal ester group have been synthesised. The effect of this ester unit and the length of its alkyl chain on the mesomorphic and dielectric properties of the compounds exhibiting a broad temperature range of chiral smectic phases have been studied. We found that the mesomorphic behaviour and phase transition temperatures are strongly affected by the molecular architecture. Depending on the alkyl chain length in the terminal ester unit, the studied materials exhibited paraelectric smectic A*, ferroelectric tilted smectic C* and antiferroelectric smectic CA* phases over a broad temperature range. The physical properties of the compounds have been studied by optical polarising microscopy, differential scanning calorimetry, electro-optic measurements, small-angle X-ray scattering and dielectric spectroscopy. Furthermore, the homologues with short terminal alkyl chains showed a very small layer shrinkage at the transition from the orthogonal SmA* to the tilted SmC* phase, which is a characteristic feature of ‘de Vries-type’ behaviour. Graphical Abstract


Introduction
Chiral liquid crystals (LCs) have attracted considerable attention due to their unique self-assembling properties, which can be driven by an applied external electric field. Beyond the commonly used chiral nematics, chiral smectics possessing the polar ordering are of high interest as well due to their applicability in photonics and electro-optical devices. [1][2][3][4] Ferroelectric liquid crystals (FLCs) are capable of switching in the range of microseconds and contemporary they have found applications in high-resolution reflective microdisplays and fast electrooptical switching devices. [1,5] However, the drawback of the majority of FLCs represents the layer contraction at the transition from the orthogonal smectic A* (SmA*) to the tilted ferroelectric smectic C* (SmC*) phase. This layer contraction generates the chevron configuration of the SmC* phase and the formation of so-called 'zig-zag' defects that drastically diminish the performance of surfacestabilised LC devices. [6] Although in the early 1970s the existence of the phase transition from the orthogonal to tilted smectic phase with no layer contraction has been predicted theoretically [7] and named 'de Vries-type', only in the late 1990s an extensive search for materials with small layer contraction [8] started. Several groups have focused on the design and synthesis of 'de Vries-type' of materials. However, no rational design strategy has been proposed so far. It has been shown that materials with a polysiloxane or perfluorinated alkyl chain showed some advantages [9][10][11][12][13][14][15][16][17][18] to exhibit a low-layer shrinkage at the SmA-SmC phase transition. On the other hand, couple of examples of compounds with different structural motifs exhibiting 'de Vries-type' or nearly 'de Vries-type' behaviour have also been reported for lactic acid derivatives. [8,[19][20][21][22][23][24] During the last two decades, liquid crystalline materials derived from chiral lactic acid have been intensively studied [25][26][27][28][29][30][31][32][33][34][35][36][37] by different groups. It was proved that such type of materials exhibits several advantages in comparison with derivatives based on other chiral units. Mainly strong chirality, thermal stability and optical purity are very important. Lactic acid derivatives frequently exhibit broad variety of phases, such as the chiral nematic, the paraelectric SmA*, [34,38] the ferroelectric SmC*, [22,23,27,32] the antiferroelectric tilted smectic C A * (SmC A *) [24,[39][40][41][42] and polar hexatic phases [33] as well as frustrated ones like the twist grain boundary (TGB) phases, namely the TGB [21,25,26,37,43] or cubic SmQ* phases. [39,44] Up to now, some studies describing various structural modifications involving the number of lactate units, [33,[40][41][42] the type of the molecular core and the orientation of polar spacers have been published. [33,[45][46][47][48] More recently, a terminal keto group has been introduced into the non-chiral chain of lactic acid derivatives which resulted in the appearance of a paraelectric SmA* phase over a broad temperature range and a ferroelectric SmC* phase with large polarisation. [49] However, to our best knowledge only one paper has reported on lactic acid derivatives possessing a terminal alkyl ester connecting the non-chiral chain and the central core. [35] Thus, the objective of this work is to search for new chiral lactic acid derivatives and to establish the effect of the terminal ester unit on the mesomorphic behaviour. Furthermore, the influence of alkyl chain length in the terminal ester unit on mesomorphic properties is studied. Hence, we aim to contribute to better understanding of the molecular structure-mesomorphic property relationship of this class of liquid crystalline materials.

Synthesis
The synthetic route towards the target liquid crystalline compounds is presented in Scheme 1. Chiral (S)-2hexyloxy-and (S)-2-dodecyloxypropanoic acid (1a,1b) were synthesised by the known method. [50] N,N´-Dicyclohexylcarbodiimide (DCC)-mediated esterification of acids 1a and 1b with 4-hydroxybenzaldehyde provided aldehydes 2a-b which were oxidised with Jones reagent to provide the corresponding acids 3ab, respectively. The esters 4a-4g were prepared by Fischer esterification of 4′-hydroxybiphenyl-4-carboxylic acid with a series of alcohols C n H 2n+1 OH (a n = 1, b n = 2, c n = 3, d n = 4, e n = 6, f n = 7, g n = 12). [51] Acids 3a-b were first transformed by Scheme 1. Synthetic route and designation of compounds. means of oxalyl chloride to the corresponding acid chlorides, which subsequently acylated the hydroxy esters 4a-g in the presence of 4-N,N-dimethylaminopyridine (DMAP) to yield the target compounds of series Ia-Ig and IIa-IIg, respectively (Scheme 1).

Characterisation
All reagents and solvents were purchased from commercial sources and used without further purification. Column chromatography was carried out using Merck Kieselgel 60 (60-100 μm). The structure of intermediates and final products was confirmed by 1 H NMR spectroscopy (Varian Gemini 300 HC instrument); deuteriochloroform was used as solvent and the signals of the solvent served as an internal standard; chemical shifts are given in δ scale (ppm), J values in Hz. Elemental analyses were carried out on a Perkin-Elmer 2400 instrument. The purity of all final compounds was confirmed by high-performance liquid chromatography analysis (Luna silica 150 × 4.6 mm, 1% methanol in dichloromethane, flow rate 0.5 ml/min, room temperature) and was found ≥99.8%. The experimental part summarises procedures for the synthesis of intermediates and the target compounds of series I and II. . To a cold (0°C) solution of 2a in acetone (100 ml), a solution of Jones reagent was added drop wise. The mixture was stirred at 0°C for 20 min and at room temperature for 75 min. Then the reaction mixture was poured on an ice-water mixture (300 ml). The precipitated solid was filtered off, washed with water (3 × 20 ml) and dried. It was obtained 5.1 g (88%) of acid 3a, m. p. 65-67°C. 1 H NMR spectrum: 0.90 (t, 3 H, J = 6.7, . A mixture of acid 3a (0.30 g, 1.0 mmol), catalytic amount of dimethylformamide (0.05 ml) and oxalyl chloride (15 ml) was stirred under reflux for 1 h. The unreacted oxalyl chloride was removed at low pressure, the crude acid chloride (0.32 g; 1.0 mmol) was dissolved in dichloromethane (7 ml) and added to a solution of hydroxy ester 4a (0.20 g; 0.88 mmol) and DMAP (0.12 g; 0.98 mmol) in dichloromethane (40 ml) at room temperature. The mixture was stirred for 30 min in an argon atmosphere and decomposed with water (30 ml). Layers were separated and the aqueous layer was washed with dichloromethane (2 × 30 ml). The combined organic solution was washed with water (30 ml) and dried with anhydrous magnesium sulphate. The solvent was removed and the crude product was purified by column chromatography (hexane/ethyl acetate 10:1). 0.18 g (41%) of Ia was isolated and crystallised from an ethyl acetate/ethanol mixture. 1  Using the same synthetic pathway, materials of the series I were prepared by the reaction of acid 3a with hydroxy esters 4b-4g.

Experimental methods and set-up
Planar cells (bookshelf geometry) of 6, 12 and 25 μm thickness were observed in the polarising optical microscope (Nikon Eclipse E600POL, Nikon, Tokyo, Japan). The sequence of phases was determined from the textures and their changes. The cells for texture observation and electrooptical studies were made from glasses with ITO transparent electrodes (5 × 5 mm 2 ), separated by mylar sheets defining the cell thickness.
They were filled with studied compounds in the isotropic phase by the capillary action. The Linkam LTS E350 (Linkam, Tadworth, UK) heating/cooling stage with a TMS 93 temperature programmer was used for the temperature control, which enabled temperature stabilisation within ±0.1 K.
The phase transition temperatures were determined by differential scanning calorimetry (DSC) using Pyris Diamond Perkin-Elmer 7 calorimeter (PerkinElmer, Shelton, CT, USA). The samples of about 2-5 mg, hermetically sealed in aluminium pans, were placed into the calorimeter chamber filled with nitrogen. Temperature and enthalpy change values were calibrated on the extrapolated onset temperatures and the enthalpy changes of the melting points of water, indium and zinc. Calorimetric measurements were performed on cooling/heating runs at a rate of 5 K min −1 .
Switching studies were carried out using a driving voltage from a Phillips generator PM 5191 (Philips, Eindhoven, Netherlands), accompanied by a linear amplifier. A LeCroy 9304 memory oscilloscope (LeCroy, Heidelberg, Germany) gave the information about the switching current profile versus time. Spontaneous polarisation (P s ) was determined from the switching current detected in a triangular electric field profile at a frequency of 50 Hz and electric fields of about 10-40 V/μm. Spontaneous tilt angle, θ s , was determined optically by measuring the angular difference between the extinction positions detected at crossed polarisers of the unwound structure under opposite d.c. electric fields ±40 kV/cm.
The small angle X-ray scattering (SAXS) studies were carried out with Ni-filtered CuK α radiation (wavelength λ = 1.5418 Å). Small-angle scattering data from nonaligned samples (filled into Mark capillary tubes of 0.7 mm diameter) were obtained using a Kratky compact camera (Anton Paar) equipped with a temperature controller and a one-dimensional electronic detector (M. Braun), temperature being controlled within 0.1 K. For compounds possessing smectic phases, the layer thickness, d, was determined using Bragg's law nλ = 2d sin θ, where d was calculated from the position of the small angle (Θ = 0.2-4.5°) diffraction peaks.

Results
Based on the length of the terminal alkyl chain in the lactate unit (C m H 2m+1 ) (see Scheme 1), the materials are divided into two series according the length of the alkyl chain in the chiral lactic acid unit: series I with m = 6 (hexyl, C 6 H 13 ), and series II with m = 12 (dodecyl, C 12 H 25 ). The length of the non-chiral chain consisting of a terminal alkyl (C n H 2n+1 ) connected by ester to the molecular core, further called the ester chain, was varied from the short (n = 1-4), middle (n = 6-7) to long (n = 12).

Mesomorphic properties
For all target compounds of series I and II, DSC studies have been performed. The phase transition temperatures and associated enthalpy changes are summarised in Table 1. The representative DSC thermograms for compounds IIa, IId and IIg are shown in Figure 1(a)-(c), respectively. All studied materials exhibited at least one smectic mesophase. The type of the smectic ordering has been identified by the observation of the characteristic textures and their changes under a polarised microscope, and electrooptical behaviour under an applied electric field. The dielectric spectroscopy has been also used as a tool for the identification of the type of the smectic ordering. To support the identification of phases and to determine layer spacing values, X-ray measurements have also been performed (vide infra).
Compounds with the longest ester chain Ig and IIg formed only one mesophase, namely the SmC* phase. A typical broken fan-shaped texture of unwound ferroelectric SmC* of IIg is depicted in Figure 2(a). On shortening the ester chain, a high-temperature SmA* phase appeared above the SmC* phase on cooling. The growth of initially formed bâtonnets from an isotropic phase to a confocal texture of the SmA* phase is presented in Figure 2(b) for If, and a fan-shaped texture of a paraelectric SmA* phase of Id is shown in Figure 2(c).
It is worth noting that melting as well as crystallisation points of the compounds decreased gradually with the decreasing length of the ester chain down to propyl (compounds Ic and IIc). While for compound Ic again a SmA*-SmC* phase sequence was determined, for compound IIc a low-temperature antiferroelectric SmC A * phase was identified as well. The textures of the SmC* and SmC A * phases of compound IIc are shown in Figure 3(a,b), respectively. Further shortening of the length of the ester chain led to the increase of transition temperatures and significant changes in mesomorphic behaviour. The SmC A * phase disappeared for both methyl esters (Ia, IIa) and a new hexatic phase (Hex) was observed. For compounds Ia and Ib also, the SmC* phase disappeared, while it was preserved for compounds IIa and IIb. This behaviour can be explained by enhanced flexibility of the materials of the series II caused by the elongation of the alkyl chain in the chiral part of the molecule. The stabilising effect of the long (C 12 H 25 ) alkyl chain in the chiral moiety and the modulatory role of the ester chain is well documented for compound IIb, which exhibited a SmA*-SmC*-SmC A *-Hex phase sequence on cooling.

Spontaneous polarisation and tilt angle
Spontaneous polarisation, P s , has been determined from the switching current detected in a triangular electric field profile at a frequency of 50 Hz. The temperature dependences of the spontaneous polarisation of materials Ib-g and IIa-g are presented in Figure 5(a,b), respectively. The values of the spontaneous polarisation do not depend on the lengths of the terminal alkyl chains within series I. However, a significant difference can be seen between the series ( Figure 5(a,b)). A significant decrease of P s is observed for materials of series II in comparison to series I. Moreover, there is a tendency for saturation at lower temperatures for materials II. P s reaches the highest value of about 120 nC⋅cm -2 for IIc (Figure 4(b)), while this tendency is not as significant for compounds I. Most compounds I show higher P s in comparison with corresponding compounds II. For the materials Ic-If, the measured P s values are much higher than for the respective homologues of series II (Figure 4(a)). Only for Ib and Ig the spontaneous polarisation reach values comparable to homologues IIb and IIg, respectively. We can summarise that for series I the dependence of P s values on the ester chain length is not monotonous, reaching the maximum for n = 6.
Spontaneous tilt angle, θ s , has been determined optically by measuring the angular difference between the extinction positions detected at crossed polarisers. For compounds Ib-Ig and IIb-g, the temperature dependences of the tilt angle are presented in Figure 5(a,b), respectively. The values for θ s comprise of the actual spontaneous tilt angle (without electric field) and the field-induced tilt angle (due to the electroclinic effect). Generally, the field-induced tilt angle should be taken into consideration only close to the SmA*-SmC* phase transition. For compounds of series I, the increase of θ s at the SmA*-SmC* phase transition is non-continuous, which might be related to a weak first-order phase transition. For series I, the increase of the spontaneous tilt angle values with the length of the ester chain is observed up to n = 6 for compound Ie (hexyl). For compounds with n ≥ 6, Ie-Ig, the values of θ s do not change substantially with the length of the alkyl chain and reach 31-36°at saturation. The increase of θ s with the ester chain length is more pronounced for the materials II. The values of θ s determined for compounds IIc-IIf are slightly lower than those observed for the respective materials of series I. Generally, tilt angles are not substantially different when comparing homologues from both series.

Structural properties
The results of the SAXS measurements, namely the temperature-dependent smectic layer spacing, d(T), are shown in Figure 6 for the compounds of series I and II. Most of the studied compounds showed a slight increase of d(T) in the SmA* phase while approaching the SmA*-SmC* phase transition. This behaviour can be ascribed to the competition of two effects, namely stretching of the aliphatic molecular chains and increasing the orientational order of the molecular long axes on cooling. Due to tilting of the molecules below the SmA*-SmC* phase transition, d(T) decreases on further cooling in the ferroelectric SmC* phase. The results (see Figure 6 (a,b)) clearly show that the longer the aliphatic chain, the greater the drop in layer thickness d at the SmA*-SmC* phase transition. For the both series of compounds, a distinct drop occurs at the transition to the SmC* phase with exception for the homologues with the shortest ester chains. Very small layer shrinkage has been observed for compounds Ia-b and IIa-b.

Ab initio calculations
In order to get a deeper insight into the character of the SmA*-SmC* phase transition, we calculated the most favoured conformers with minimum energy of the studied materials (Figure 7). The ab initio calculations were performed using Gaussian 09, rev. D.01 program [52] (for details, see Supplemental data). The stepwise optimisation of the dihedral angles within the   molecules ( Figure 7) led to the most stable conformers (e.g. Ia depicted in Figure 7). The structure of the stable conformers has been used to calculate the molecular lengths, l calc , of the materials. In Figure 8, the values of l calc are presented for both series. We can compare l calc values with the layer spacing, d, measured by SAXS in the SmA* phase. For series I, the d values are smaller than the calculated molecular length l calc ; for series II, the d values approximately correspond to l calc . The discrepancy between d and l calc for compounds I n can be explained by the intercalation of molecules or by a less-extended molecular shape. We can conclude that the terminal chains, ester chain versus chiral unit chain, play different role in packing of molecules in layers.

Dielectric properties
The frequency dispersion of the complex permittivity (ε* = ε'iε'') has been measured within the temperature range of the SmA* and SmC* phases on cooling, using a Schlumberger 1260 impedance analyser in the frequency range of 1 Hz÷1 MHz keeping the temperature stable during the frequency sweep within ±0.1 K. For the present work commitments, the dielectric spectroscopy has been used mostly as a useful tool for the confirmation of the polar character of the detected phases and for distinguishing the synclinic ferroelectric and anticlinic antiferroelectric order. Representative results of the real and imaginary part of complex permittivity for two selected compounds, namely Ib and Id, are presented in Figure 9, top and bottom rows, respectively. Dielectric absorption ε'' spectra at zero bias electric field measured on the 12-μm-thick sample cells within the ferroelectric SmC* phase show a strong contribution of the Goldstone mode (a mode related to the azimuthal fluctuations of the molecules in the smectic layer) with the relaxation frequency of several kHz order. These results fully confirm the ferroelectric character of the tilted SmC* phase for compound Id. The results for compound Ib are quite different: the amplitude of the detected relaxation process is about a hundred times lower than for the Id (see Figure 9, right). Such a mode can be definitely attributed to the antiphase mode of the antiferroelectric SmC* A phase. [53,54] Before the occurrence of the phase transition to the polar phase, the soft mode behaviour (fluctuations of molecules arranged in smectic layers in the direction of the tilt angle) has been detected in the orthogonal paraelectric SmA* phase for both the selected compounds. Hence, it can be concluded that Id (top row) possesses the paraelectric SmA* and the ferroelectric SmC* phases over a broad temperature range, while Ib (bottom row) possesses the SmA* and the antiferroelectric SmC* A phases.

Discussion and conclusions
In this study, we have focused on a new class of chiral FLCs based on chiral lactic acid. The mesomorphic properties of new LC materials have been tuned by the length of terminal alkyl chains. All studied  materials exhibit at least one smectic mesophase. Comparing the series I and II, lengthening the alkyl chain in the chiral part by six carbon atoms does not substantially influence the mesomorphic behaviour within the series. While melting points remain approximately constant for both the series, clearing points decrease for materials of series II. It results in a slightly narrower mesophase range for the compounds from series II in comparison with series I. Even for homologues with short ester chains the formation of different smectic phases is supported. A variety of phases was found for materials Ia, Ib, IIa and IIb, where an additional hexagonal type of mesophase has been detected below other smectic phases at lower temperatures. Unfortunately, the temperature interval of the hexatic phase lies below the melting point, so we cannot properly characterise this phase by X-ray or other experimental technique. Mostly, the studied materials started to crystallise within the hexatic phase during measurements. No odd-even effect of ester chain lengths has been observed within the studied series.
The general structure of the presented materials seems to be favoured for the formation of smectic phases, in particular, the SmA*-SmC* phase sequence. The previously reported lactic acid-based LCs with a similar molecular structure and an alkoxy terminal chain exhibited only the SmC* phase. [34] It has been shown that a keto group affects strongly the overall dipole moment of the materials due to its electronwithdrawing nature. [33,49] The introduction of the keto group as a linkage between the biphenyl central core and a non-chiral terminal alkyl chain already led to the polymorphism and increase of polarisation for lactic acid LCs. [19,41,49] In case of the studied materials, the introduction of the ester linkage between the biphenyl central core and the terminal alkyl chain (ester chain) enhanced the overall dipole moment of the compounds. This, in consequence, led to higher P s values than in case of materials with alkoxy terminal chains. [35] A similar effect has already been reported for a different class of alkoxybiphenylcarboxylate-based ferroelectric materials with alkyl branched alkyl chains. [55] Some of the studied materials exhibit a very low layer shrinkage at the SmA*-SmC* phase transition, which evidences on 'de Vries-type' behaviour. Herein, this behaviour is found for compounds with short terminal chains. It is an encouraging result for designing a molecular structure of 'de Vries' compounds without typical features like a perfluoroalkyl or polysiloxane chain or the specific character of the central core (e.g. 2- Figure 9. 3D plots of the real, ε' (left side), and imaginary, ε'' (right side), parts of complex permittivity versus frequency and temperature for two selected compounds: Ib, top row, and Id, bottom row. The thickness of the cell was 12 μm. phenylpyrimidine core). [14] Such result is very promising for the future optimisation of new materials, leading to their application in electro-optic devices.