Bent-core liquid crystals based on 6-substituted 3-hydroxybenzoic acid: the role of substitution and linkage group orientation on mesomorphic properties

ABSTRACT We have prepared and studied new series of bent-core liquid crystals based on laterally substituted 3-hydroxybenzoic acid. The physical properties of the materials were tuned by the type of lateral substitution in the vicinity of the carboxylic unit. Further modification was achieved by reorientation of connecting ester linkages in the elongating side arms and by changing the length of terminal alkyl chains. The physical properties of the materials were determined by differential scanning calorimetry (DSC), polarising optical microscopy and for selected compounds by x-ray diffraction analysis. The introduction of fluorine as lateral substituent leads to the appearance of a lamellar or columnar mesophase. On contrary, methyl- or chlorine-substituted compounds are mostly crystalline. We have found a strong correlation between the size of the lateral substituent and the mesogenicity of the studied materials. GRAPHICAL ABSTRACT


Introduction
Since their discovery [1], bent-core liquid crystals have become one of the most studied subclasses of liquid crystals. Although they have limited applicability, they represent a vivid field of basic research as they possess unique properties, such as spontaneous polar order and macroscopic symmetry breaking [2][3][4][5]. Bent-core liquid crystals exhibit a variety of mesophases, most of them are still subjects of intensive research. Tilted smectic mesophases with polar packing of bent-core molecules are the most frequent. The notification SmCP has been often utilised for them, the indexes S for synclinic and A for anticlinic orientation of the tilt in neighbouring layers stand as subscript at the letter C. The subscript index F expresses ferroelectricity and A antiferroelectricity at the letter P, which stands for a polarisation state [2]. The antiferroelectric type of switching is preferentially observed in the SmCP phase. Additionally, for the bent-core mesogens, columnar mesophases were found, created from the layer fragments (molecular blocks) arranged in a twodimensional (2D) lattice. A columnar B 1 phase has been described with density modulation in the plane parallel to the polarisation vector P. In the B 1 phase, no electro-optic response and switching current were observed. When the density modulation plane is perpendicular to the P vector, the nomenclature of the B 1Rev phase was proposed [6,7]. The columnar B 1Revtype of mesophases can switch under the applied electric field and are present in different variants: with tilted or non-tilted molecules with respect to the ribbon.
The mesomorphic behaviour of bent-core liquid crystals is usually highly dependent on the molecular structure. The initial symmetrical structure of bent-core materials [1] has been widely modified by the lateral substitution of aromatic units, length of terminal alkyl chains, variation of polar spacers in elongating wings and the character of the central core itself [4,5,8,9]. Also, materials with an inherently non-symmetrical molecular structure possessing a central unit bearing two different functionalities have been studied, based, for example, on 3-hydroxybenzoic acid [5,[10][11][12][13][14], oxadiazol [15,16], 7hydroxynaphthalene-2-carboxylic acid [17][18][19][20] or hydroxybiphenylcarboxylic acids [21][22][23][24][25]. Recently, we have shown that materials based on 4-substituted 3hydroxybenzoic acid exhibit higher thermal stability of mesophases in comparison to their symmetrical resorcinol analogues [26]. Additionally, the change in the orientation of one polar ester linkage and the introduction of a terminal ester group have further stabilised mesomorphic behaviour and induced polymorphism of the studied materials [27].
In the continuation of our studies on laterally substituted bent-core materials [26,27], herein we present the synthesis and mesomorphic properties of bent-core materials possessing a lateral substituent in the position 6 of the central benzene core, that is, in the vicinity of the carboxylic unit ( Figure 1). The properties of the target compounds were tuned by the character and size of the lateral substituent (fluorine, chlorine and methyl) as well as by the orientation and number of the polar ester linkages in the molecule. With the support of quantum chemical calculations, we document both the role of the position of the lateral substituent in the central core and the ester group orientation on mesomorphic properties.
A N,N′-dicyclohexylcarbodiimide (DCC) mediated esterification of acids 1-3 with corresponding hydroxy esters 11a-d in the presence of N,Ndimethylaminopyridine (DMAP) as catalyst gave rise to intermediates 15a-d-17a-d (Scheme 2). The deprotection of the hydroxy group was performed with respect to the character of the protecting group. The silyl protecting group (PG=(CH 3 ) 3 C (CH 3 ) 2 Si) was removed by the means of tetrabutylammonium fluoride trihydrate (TBAF·3H 2 O), while the deprotection of the benzyl group (PG=PhCH 2 ) was achieved by transfer-hydrogenation using Pd/C and ammonium formate (HCOONH 4 ). Subsequently, the obtained hydroxy esters 18a-d-20a-d were acylated with acid chlorides 14a-d of the second lengthening arm in the presence of DMAP to yield the  The target compounds of series II and III (Scheme 3) have been prepared analogously. The esterification of the protected central cores 1-3 with hydroxy esters 12a-d and 13a-d, respectively, yielded the protected derivatives 21a-d-26a-d. The protecting groups were removed as above and the released hydroxy group of esters 27a-d-32a-d was acylated with acid chlorides 14-d providing the target materials of series II and III (Scheme 3). For experimental details, see Electronic Supplementary Information (ESI).

Experimental methods
Differential scanning calorimetry (DSC), namely Pyris Diamond calorimeter (Perkin Elmer, Shelton, CT, USA) was used to establish the phase transition temperatures and corresponding enthalpies. The calorimetric data were calibrated on extrapolated onset temperatures and enthalpy changes of water, indium and zinc. For each material, about 2-5 mg were hermetically closed in aluminium pans and placed into a working space filled with the gaseous nitrogen. The measurements were performed on cooling/heating runs at a rate of 5 K min −1 . The sequence of phases was determined from the textures and their changes observed under the polarising microscope Nikon Eclipse (Nikon, Tokyo, Japan). The cells for texture observation and electro-optical studies were made from glasses with ITO transparent electrodes (5 × 5 mm 2 ), separated by Mylar sheets defining the cell thickness (usually of about 3 μm). They were filled with studied compounds in the isotropic phase by capillary action. Another type of a sample can be prepared by spreading the droplet in the isotropic phase on the glass surface (one-free-surface sample). A hot stage (Linkam LTS E350) equipped with TMS 93 temperature programmer (Linkam, Tadworth, UK), which enabled temperature stabilisation within ±0.1 K, was used for the temperature control.
The polarisation current profile at a triangular electric field was detected by Tektronix DPO4034 digital oscilloscope (Tektronix, Oregon, USA). The driving voltage from an Agilent 33210A function generator (Agilent, California, USA) was amplified by a linear amplifier providing the maximum amplitude of about ±120 V. The frequency dispersion of permittivity was measured using a Schlumberger 1260 impedance analyser (Schlumberger, Houston, TX, USA) in the frequency range of 10 Hz-10 MHz, keeping the temperature of the sample stable during frequency sweeps. For this purpose, cells with gold electrodes were utilised (thickness 7 μm). The gold electrodes allowed us to shift the cut-off frequency to a megahertz region and thus detect the high frequency mode.
Bruker Nanostar and Bruker GADDS systems (Bruker, Santa Barbara, CA, USA) were utilised for x-ray diffraction studies (CuKα radiation, crosscoupled Goebel mirrors, Vantec 2000 area detector, MRI TCPU H heating stage). In both the systems, the temperature stability was 0.1 K. Powder samples (for Nanostar) were prepared in thin-walled glass capillaries (1.5 mm diameter); partially oriented samples for experiments in reflection mode (GADDS) were prepared as droplets on a heated surface.

Mesomorphic properties
Based on the number and orientation of the ester functions in the molecule, new materials are divided into three series. Compounds with four non-uniformly oriented ester units are grouped in series I. Materials having four ester groups in the molecular structure with the uniform orientation are denoted as series II, and those with five uniformly oriented ester units as series III. Within each series, three different substituents, namely fluorine-F, chlorine-Cl and the methyl group-CH 3 , were introduced to tune the properties of the new materials. Further modification of mesomorphic behaviour was achieved by the introduction of terminal alkyl chains of different length (a = octyl, b = decyl, c = dodecyl, d = tetradecyl).
First of all, the phase transitions were characterised from DSC measurements. The phase transition temperatures, associated enthalpy changes and phase identification for series I, II and III are collected in Tables 1-3, respectively. Phases were identified from the texture observation under polarising microscope and/or electrooptical behaviour under applied electric field. To support the phase identification for selected compounds, x-ray measurements were performed.
All materials of series I exhibited one mesophase (Table 1). For fluoro-substituted compounds with shorter terminal chains (Ia/F and Ib/F), a columnar phase was observed. Compounds Ic/F and Id/F with the dodecyl and tetradecyl terminal chain, respectively, Table 1. Melting point, m.p., phase transition temperatures, T tr and crystallisation temperature, T cr , in°C and corresponding enthalpy changes, ΔH in kJ mol −1 , taken on the second temperature run at a rate of 5 K min −1 (in brackets). Square brackets mark monotropic mesophase. We would like to remind that compounds of series II (Table 2) possess four uniformly oriented ester linkages in the molecular structure. Due to the different orientation of one ester linkage in the elongating side arm in comparison with compounds I, the transition temperatures increased approximately by 20°C. For fluorinated materials II/F, a columnar mesophase was observed and later identified as B 1Rev . In case of chlorosubstituted materials, only the homologue IId/Cl with the longest terminal alkyl chain exhibited a monotropic columnar phase, while the others were only crystalline. Furthermore, methyl-substituted materials (II/CH 3 ) did not exhibit mesomorphic behaviour at all. The introduction of the fifth ester unit at the terminal position of the elongating side arm for series III does not induce substantial changes in the mesomorphic behaviour in comparison with series II. For fluorosubstituted derivatives III/F, the columnar B 1Rev phase was observed (Table 3). Materials bearing the larger substituents, that is, chlorine and methyl groups, were only crystalline. Typical DSC thermographs are presented for selected mesogenic compounds in Figure 2.
Textures and their features during the temperature change and/or under applied electric field were observed under polarising microscope. In series I (for all lateral substituents), the short alkyl terminal chain induced the formation of a columnar B 1 -type of phases exhibiting typical texture with coloured domains is shown in Figure 3 for Ia/Cl. For longer terminal chains (Ic/F, Id/F and Id/Cl), a SmCP phase was observed.   The SmCP phase exhibited similar electro-optical properties for all studied materials. On cooling from isotropic phase, coloured leaf-like textures were found for studied compounds (for Ic/F, see Figure 4(a)). Under the applied electric field, a fan-shaped texture appeared. After switching off the electric field, the fans went dark, which indicates a low birefringence (see Figure 4(b)) with the extinction parallel to the crossed polarisers direction. Under the DC electric field, the birefringence increased and the extinction brushes rotated by an angle of about 45° (Figure 4(c)). After switching off the field, the dark texture with the extinction parallel to the direction of the crossed polarisers was again restored (Figure 4(b)). The virgin texture (Figure 4(a)) can also be restored by heating the sample into the isotropic phase and subsequent cooling. Under the applied AC electric field with a triangular profile, we detected two peaks per half-period, which evidences about the antiferroelectric character of the SmCP phase for all studied compounds ( Figure 5 for compound Ic/ F as an example). We can establish the observed SmCP mesophase as the SmC A P A phase, which transforms into the SmC S P F under the electric field. Microphotographs of the planar texture in the SmC A P A phase and its transformation under electric field is presented in Supplemental. We performed dielectric spectroscopy studies in the SmC A P A phase and detected one mode with the relaxation frequency of hundreds kHz (see the imaginary part of  permittivity, εʺ, versus temperature and frequency for Id/F in Figure 6). The studied mode completely disappeared in the isotropic as well as in the crystalline phase, and therefore, it can be ascribed to a collective mode characteristic for the mesophase. In series II/F and III/F, another type of a columnar phase appears. Planar fan-shaped textures are observed and the birefringence of fans reversibly changes under the application of the electric field (see Figure 7, the texture without field in Figure 7(a) and after the electric field application in Figure 7(b)). Such behaviour is characteristic for the B 1Rev phase and it will be justified later by x-ray measurements. For compound IId/Cl, the textures are different and reveal features typical for an undulated B 7 phase [9] with spherulitic and spiral domains. The planar texture is shown in Figure 8(a) and specific low-birefringent texture has been observed for the sample with one-free surface (Figure 8(b)). A schematic picture of molecular arrangement in the B 1 and B 1Rev phases is shown in Figure S5 (Supplemental info).

X-ray measurements
For selected compounds, x-ray measurements were performed and data analysed to approve the phase identification. In the columnar B 1 phase, the x-ray signal in the small-angle region shows three sharp peaks and we can calculate the lattice cell parameters. Additionally, a broad maximum, corresponding to the average distance between molecules, is observed at large scattering angles. For illustration, x-ray intensity versus the scattering angle in the B 1 phase is shown in Figure 9 for Ib/F. Miller indexes are attached to the corresponding peaks. For the lamellar SmC A P A phase, sharp peaks are commensurate and correspond to the layer spacing, d. For compound Id/F, d was estab-   lished to be about 41-41.5 Å. The d value was found temperature almost independent within the SmC A P A phase range ( Figure S1 in Supplemental information for Id/F). For all studied SmC A P A mesophases, the layer spacing is considerably lower than the approximate length of the fully stretched molecule, l. For Id/ F, l ≈ 59.9 Å corresponds to tilting of molecules with respect to the layer normal by an angle of about 46°. For Id/Cl, the d was established to be 40.5 Å just below the Iso-SmC A P A phase transition. Due to the monotropic character of the SmC A P A phase, compound Id/Cl started to crystallise and we were not able to continue measurements.
In the B 1Rev phase, the x-ray pattern is characteristic and corresponds to an oblique cell. The strongest signal indexed as (01) evidences on the presence of stripes (molecular blocks). In the B 1Rev phase of series II, the cell parameters were established slightly temperature dependent. The temperature dependences of the cell parameters are presented in Supplemental information for compounds IIb/F and IId/F. The cell parameters for selected compounds from series II and III are summarised in Table 4. Parameter c corresponds to the block (layer) thickness and we can claim that the molecules are tilted with respect to the layer normal.

Quantum chemical calculations
To assess the influence of ester unit orientation and lateral substituents on mesomorphic behaviour, we calculated conformers with minimum energy using Gaussian® 03W [31]. Initial Z-matrixes were assembled and optimised structures were visualised in GaussView 3.09. The main differences in mesomorphic behaviour with respect to the lateral substitution were observed   for compounds of series III. While the fluoro-substituted compounds exhibited B 1Rev phase, chloro-and methyl-substituted compounds were only crystalline. Therefore, we primarily focused on materials of series III with the dodecyl alkyl chain (IIIc/F, IIIc/Cl and IIIc/CH 3 ). We take into account also an analogous unsubstituted compound with the dodecyl chain, which was presented in our previous paper as IVc/H [27]. The structures of all intermediates were pre-optimised by Hartree-Fock method on a 6-31gd level (for details, see ESI). The structures of the target materials ( Figure 10) with minimum energy were further optimised by density functional theory (DFT) method using B3LYP functional at 6-31gd level. We found that there is a significant correlation between the molecular geometry and the mesomorphic behaviour of the materials. While both compounds IVc/ H and IIIc/F, exhibiting the columnar B 1Rev phase, preferred the co-planar alignment of the central unit and aromatic rings in the elongating side arm, IIIc/Cl and IIIc/CH 3 materials showed the phenyl unit close to the central ring (marked by an arrow) rotated by 66°and 62°, respectively ( Figure 10). Since the character of mesophases described for IVc/H is the same as for IIIc/F, we assume that the lateral substituents exert more steric than electronic effect leading to the change in the conformation of the side arm and thus to a different selfassembly of chloro-and methyl-substituted materials.
Similar conformational behaviour was found also for materials of series I and II (see ESI, Figures S19 and S20, respectively). For compounds of series II, the mesomorphic behaviour matched that of series III, and compounds II/Cl and II/CH 3 were, with one exception, only crystalline. Conformations with minimum energy found for compounds of series I were similar to those of series II and III, however, compounds I/Cl and I/CH 3 exhibited monotropic mesophases. We assume that the disruptive effect of the lateral substituents could be in this case compensated by the inversion of the ester linkage in the elongating side arm. Such stabilising effect of the ester group reorientation has already been described for similar materials [26].

Conclusions
First of all, we can compare studied compounds with unsubstituted materials (X=H), published previously [10,11,26,27]. The introduction of fluorine in the vicinity of the carboxylic group in derivatives I-III/F exerts only a negligible effect on mesomorphic behaviour through all series in comparison with the unsubstituted derivatives [26]. While unsubstituted compounds are only monotropic, the compounds I/F show the formation of enantiotropic phases. The differences in transition temperatures, the temperature range of phases and the type of the formed phases among the series I-III/F are induced by the number and orientation of ester linkages. The chloro-substituted materials I/Cl exhibit the same properties (B 1 phase) with the exception of the formation of SmC A P A phase for Id/Cl with the longest terminal chain in comparison with unsubstituted materials. However, the introduction of chlorine for materials II/Cl and III/Cl results in almost complete loss of mesogenicity.
While the unsubstituted materials exhibit the formation of a B 1 phase for the materials with the shorter terminal chains and SmC A P A for those with longer terminal chains [26], the introduction of a methyl group close to the carboxylic group results in the appearance of the monotropic B 1 phase in all compounds I/CH 3 . The presence of a methyl group in materials II/CH 3 and III/CH 3 has a strongly disruptive effect leading to crystallisation only. Now we compare studied 6-substituted compounds with 4-substituted materials presented previously [26,27] to establish how the position of the lateral substitution on the central core influences the mesomorphic properties. While all the 4-fluoro-substituted materials analogous to I/F exhibit nematic phases, the shift of the fluorine atom to the position 6 results in the formation of higher organised phases, namely the B 1 and SmC A P A phases. The same trend can be observed for materials of the series II/F and III/F with the formation of polar columnar phases (B 1Rev ), contrary to nematogenic 4-fluoro-substituted derivatives [26].
The 4-chloro derivatives tend to exhibit a monotropic B 1 [26] as well as the studied 6-substituted compounds I/Cl, with the exception of material with the longest terminal chain exhibiting the SmC A P A phase. Unlike studied materials II/Cl and III/Cl which lost their mesomorphic behaviour, analogous 4-substitued materials form nematic, lamellar or columnar phases [27].
The introduction of a methyl group into the position 4 of the central core results in the formation of the SmC A P A phase for materials with longer terminal chains. For the 6-substituted materials I/CH 3 studied here, a monotropic B 1 phase was detected with the exception of Id/CH 3 with the longest terminal alkyl chain (SmC A P A phase). While materials of series II/ CH 3 and III/CH 3 were only crystalline, analogous 4substituted materials exhibited SmAP and SmC S P A [27], respectively.
One can conclude that the presence of a small lateral substituent (F) in the position 6 of the central core does not influence the mesogenicity of studied compounds I. On the other hand, the introduction of a larger substituent (Cl, CH 3 ) shows a disruptive effect leading in series II and III to an almost complete loss of mesomorphic behaviour. Presumably, the presence of the larger substituent influences the conformational arrangement, and thus, supramolecular self-assembly differs from that of unsubstituted and/or small atomsubstituted analogues.