The Role of Substitution in the Apex Position of the Bent-Core on Mesomorphic Properties of New Series of Liquid Crystalline Materials

: We present the synthesis and mesomorphic properties of the new series of bent-core liquid crystals based on 3-hydroxybenzoic acid bearing a lateral substituent in the apex position. Four di ﬀ erent substituents of various sizes and electronic properties have been used. We have found that only compounds substituted with ﬂuorine are mesogenic and exhibit one mesophase, whose type di ﬀ ers when prolonging the terminal alkyl chain. For homologues with shorter alkyl chains (octyl, decyl), a columnar B 1 -type of a mesophase was observed, while materials with longer terminal chains (dodecyl, tetradecyl) exhibited a switchable lamellar SmC A P A phase. Calorimetric measurements, texture observations under a polarizing microscope were performed and electro-optical properties studied. Additionally, dielectric measurements were realized to characterize the molecular dynamics in the SmC A P A phase. All mesogenic compounds were further studied by X-ray measurements to conﬁrm phase identiﬁcation and obtain more information about their structural parameters.


Introduction
Bent-core materials represent a unique class of liquid crystals (LCs). The first compounds of this type were already synthesized at the beginning of the 20th century [1]. However, they were not studied more widely due to the low thermal stability of formed mesophases. They were rediscovered in the 1990s, when the polar character and macroscopic chirality of mesophases formed exclusively by bent-core materials were described [2,3]. Since then, many unique features of bent-core LCs have been discovered. Bent-core compounds can form a broad variety of mesophases ranging from low-organized nematic phases over various lamellar mesophases to structurally complex ones [4]. The nematic phase of bent-core LCs has attracted much attention due to its possible practical applications [5]. In particular, the biaxial nematic phase formed by bent molecules has been the aim of extensive research, since it could be useful for the construction of a new generation of liquid crystal displays [6,7]. The polar and chiral orders of nematic phases formed by bent-core LCs are nowadays of great interest [8][9][10] as well as their modulation with light stimulus, which can be used for the construction of diffraction devices [11]. The protected chloro acid 2 was obtained by a multistep transformation of 3,5-dinitrobenzoic acid (5). First, one of the nitro groups was substituted by the means of lithium methoxide in hexamethylphosphoric amide (HMPA) [37] to give rise to methoxy acid 6. The nitro group of 6 was then reduced by catalytic hydrogenation on Pd/C to yield the corresponding amino acid 7, see reference [38]. The amino group of 7 was subsequently diazotized and the diazonium group substituted in a standard Sandmeyer reaction to yield the chloro acid 8. Deprotection of the methoxy group was achieved with boron tribromide and the released hydroxylic group of 9 was finally reprotected by silylation with tert-butyldimethylsilyl chloride (TBSCl) to form the protected central core 2.
The intermediate methoxy nitro acid 6 served also for the synthesis of the nitro acid 4. The methoxy group of 6 was first deprotected with boron tribromide to yield the hydroxy acid 10 and the free hydroxylic group was finally reprotected with TBSCl to provide the 5-nitro protected acid 4. To introduce the lengthening arms, the known phenols 11a-d and acids 12a-d were used [20].

Synthesis of the Target Materials
The series of target materials Ia-d (X = F), IIa-d (X = Cl), IIIa-d (X = CH 3 ), and IVa-d (X = NO 2 ) (Scheme 2) were prepared by the same methodology as previously [18][19][20]. First, the acids 1−4 were coupled with the substituted phenols 11a-d in the presence of N,N'-dicyclohexylcarbodiimide (DCC) and N,N-dimethylaminopyridine (DMAP) as catalyst to yield the protected (PG = protecting group) intermediates 13a-d-16a-d. The protecting group was removed with respect to its character. While for the deprotection of the benzyl group (PG = C 6 H 5 CH 2 ) transfer-hydrogenation using Pd/C and ammonium formate was utilized, the silyl moiety (PG = (CH 3 ) 3 CSi(CH 3 ) 2 ) was removed by the means of tetrabutylammonium fluoride (Bu 4 N + F − ) in wet tetrahydrofuran [39]. In the last step, the hydroxy esters 17a-d-20a-d were acylated with acid chlorides of acids 12a-d in the presence of DMAP as a base yielding target compounds of series Ia-d (X = F), IIa-d (X = Cl), IIIa-d (X = CH 3 ), and IVa-d (X = NO 2 ), respectively.

3-Chloro-5-hydroxybenzoic acid (9)
To a solution of acid 8 (3.20 g; 17.1 mmol) in dichloromethane (40 mL) cooled to −78 • C, boron tribromide (6.1 mL; 65.2 mmol) was added drop wise in the argon atmosphere. The solution was stirred for 30 min, then the cooling bath was removed, and the stirring continued at 0 • C for 6 h. The mixture was poured on ice (300 mL) and after decomposition of the reagent extracted with ethyl acetate (3 × 150 mL). The organic solution was then washed with brine (150 mL) and dried with anhydrous magnesium sulphate. After evaporation of the solvent, the crude product was purified by column chromatography (toluene/methanol/acetic acid 32/1/1) to yield 2.67 g (90%) of 9, m.p. 213-216 • C. 1

3-Hydroxy-5-nitrobenzoic acid (10)
Boron tribromide (13.8 mL; 153 mmol) was added drop wise in an inert argon atmosphere to a solution of acid 6 (3.65 g; 18.5 mmol) in dry dichloromethane (40 mL) cooled to 78 • C. The mixture was brought slowly to 0 • C, then stirred at this temperature for 16 h, and decomposed by pouring on ice (250 mL). The product was extracted with ethyl acetate (3 × 80 mL) and the combined organic solution was washed with brine (80 mL), and dried with anhydrous magnesium sulphate. The solvent was evaporated and the crude product was purified by column chromatography (toluene/methanol/acetic acid 32/1/1) to give rise to 3.06 g (90%) of the corresponding hydroxy acid 10, m.p. 202-206 • C, m.p. 195-197 • C, see reference [37].

Synthesis of Intermediates and Target Compounds
Synthesis of intermediates 13a-d,15a-d,17a-d, and 19a-d and their characterisation was reported in the current paper [36].
By the method shown above, reaction of nitro acid 4 with phenols 11a-d yielded the corresponding intermediates 16a-d.
The 5-nitro substituted intermediates 20a-d have been prepared by deprotection of the compounds 16a-d by the same method.

Characterization
The structure of intermediates and target materials were confirmed by 1 H NMR spectroscopy (Varian Gemini 300 HC instrument, Varian, Palo Alto, CA, USA), deuteriochloroform and acetone-d 6 were used and signals of the solvents served as internal standards. Chemical shifts are given in ppm and J values in Hz. Infrared (IR) spectra were acquired on Thermo Scientific Nicolet FT-IR spectrometer in KBr discs or on Bruker ALPHA FT-IR (Bruker, Santa Barbara, CA USA) using attenuated total reflectance (ATR) technique. Elemental analyses were carried out using Elementar vario EL III instrument (Elementar Analysensysteme GmbH, Langenselbold, Germany). Chemical purity of target materials was verified by high-performance liquid chromatography analysis on a Luna Silica column (150 × 4.6 ID, 5 µ) (Phenomenex, Aschaffenburg, Germany) and found to be ≥99.8%. Column chromatography was performed using Merck Kieselgel 60 (60−100 µm) (Merck, Darmstadt, Germany). The experimental part summarizes syntheses and spectral data of the selected homologues and all target compounds of series I-IVa-d. In the case of intermediates, 1 H NMR characterization is reported for the homologue with the shortest terminal alkyl chain only. Other homologues differ only in the integral value of methylene units in the side chains.

Equipment and Set-up for Studies of Mesomorphic Property
Differential scanning calorimetry (DSC) measurements were carried out on a Perkin-Elmer 7 Pyris calorimeter (PerkinElmer, Shelton, CT, USA). Phase transition temperatures and corresponding enthalpies were determined from the second heating and cooling runs, which were taken at a rate of 10 K/min. A small amount of compound (2−5 mg) was hermetically closed in aluminum measuring pans and inserted into the calorimeter working chamber. During measurements, a nitrogen atmosphere was applied for better temperature stabilization. The calorimeter was calibrated on extrapolated onset of the melting points of water, indium and zinc.
Electro-optical properties were studied using transparent sample cells fabricated from glass with transparent ITO electrodes (5 × 5 mm 2 ). The glass plates were separated by mylar sheets to establish the cell thickness. The studied materials were heated to the isotropic phase (Iso) to fill the cells by capillary action. A Nikon Eclipse polarising optical microscope (Nikon, Tokyo, Japan) was used to observe textures and their changes with temperature. Another type of cell (one-free-surface sample) was prepared when we removed the upper glass from a cell without any surface treatment. Temperature stabilization within ±0.1 K and temperature changes were achieved by the Linkam LTS E350 heating/cooling stage (Linkam, Tadworth, UK) with temperature programmer.
Electric field was applied using driving voltage from generator Philips PM 5191 (Philips Eindhoven, Netherlands), the signal was amplified to reach the maximum amplitude of about ±120 V. A Tektronix DPO4034 digital oscilloscope (Tektronix, Beaverton, OR, USA) was utilized to obtain information about the switching current profile vs. time. Dielectric spectroscopy was measured by Solartron impedance analyser (Solartron Analytical, Farnborough, UK) to establish complex permittivity, ε*(f), in frequency range 10 Hz to 10 MHz. The real, ε', and imaginary, ε", parts of permittivity were fitted to the Cole−Cole formula: where f r is the relaxation frequency, ∆ε is the dielectric strength, α is the distribution parameter of relaxation, ε 0 is the permittivity of vacuum, ε ∞ is the high frequency permittivity and n, m, A are the parameters of fitting. We obtained f r and ∆ε values to assess dynamic properties and the relaxation process in studied compounds. X-ray diffraction measurements were performed using Bruker GADDS system (CuKα radiation, point beam collimator, Vantec 2000 area detector) working in the reflexion mode. The set-up was equipped with a modified Linkam heating stage with the temperature stability of 0.1 K. Partially oriented samples for experiments were prepared as films on a heated surface.

Mesomorphic Properties and Electro-Optical Behaviour
All compounds were studied by a calorimetric method; DSC measurements were performed at the heating and cooling runs. We analyzed the measured data and the phase transition temperatures and corresponding enthalpy values ( Table 1). The transition peaks observed in DSC plots were sharp and revealed big enthalpy values associated with phase transitions, for demonstration see Figure 1 with thermographs for two selected compounds from the series I. We observed textures under a polarizing microscope to assess mesomorphic behavior. We found that only series I showed mesogenic properties. Namely, for shorter homologues Ia and Ib a columnar B 1 -type of a mesophase was established from textural features. For illustration, Figure 2a shows the planar texture for Ia with a texture typical for a columnar B 1 phase. We observed colored domain-like textures, which did not respond to an applied electric field. For compounds Ic and Id from series I, a SmCP phase appeared with typical textures and specific electro-optical properties. For samples with one free surface, a schlieren texture was found (Figure 2b). In the planar textures without the electric field, we could distinguish a fine structure of stripes. Nevertheless, the averaged extinction without the electric field was oriented along polarizer's direction. Under the applied electric field, the color of the observed fan-shaped texture slightly changed from yellow-orange to yellow-green, so we could expect that the birefringence changes. Moreover, the extinction position rotated clockwise and anticlockwise depending on polarity of the applied field ( Figure 3). Such texture transformation is characteristic for a transition from the SmC A P A phase to the SmC S P F phase under the electric field. In the SmC S P F , all molecules are turned towards polarity of the applied field. The electro-optical behavior for the SmC A P A −SmC S P F transition has been described in literature [2,11,12], schematically, we demonstrated such a type of molecular reorientation in Figure 4. We expect that the molecules rotate on the conus and do not change the chirality. Table 1. Melting point, m.p., the phase transition temperature from the isotropic (Iso) phase to the mesophase, T iso , and temperature of crystallization, T cr , in • C, and corresponding enthalpy changes, ∆H in kJ mol −1 , detected on the second temperature runs, are in brackets.        We studied a polarization current in the SmC A P A phase. We found that there were two peaks per half-cycle in the profile of the polarization current, which was induced in an a.c. field of the triangular shape. The switching current is demonstrated in Figure 5a,b for Ic and Id, respectively. Under a sufficiently large applied electric field, higher than 10V/µm, we detected polarization, P(T), which did not change with temperature within the SmC A P A phase on cooling and P values reached about 500 nC/cm 2 and 700 nC/cm 2 for Ic and Id, respectively.

Dielectric Spectroscopy
Dielectric properties in the SmCAPA phase were studied in detail. The complex permittivity was acquired in the frequency range from 100 Hz to 10 MHz on cooling from the isotropic phase. We detected a weak high-frequency mode, which was present only in the temperature range of the SmCAPA phase. Three-dimensional (3D) graphs of the imaginary part of the permittivity, ε'', are shown in Figure 6 for compounds Ic and Id, on cooling from the isotropic phase, through the SmCAPA phase down to the crystallization. The relaxation mode was present only in the temperature range of the SmCAPA phase and it disappeared in the isotropic as well as in the crystalline phases. The mode can be attributed to the collective mode, which is often present in SmCP phases, which is probably related to the antiferroelectric character of the mesophase. We analyzed the dielectric behavior with respect to Cole−Cole formula (1) and obtained the relaxation frequency, fr, and the dielectric strength, Δε, in the SmCAPA phase. Temperature dependency of the relaxation frequency, fr, and the dielectric strength, Δε, for Ic and Id are shown in Figure 7. We found fr(T) decreased continuously on cooling and we obtained an activation energy when fitted to the Arrhenius law.

Dielectric Spectroscopy
Dielectric properties in the SmC A P A phase were studied in detail. The complex permittivity was acquired in the frequency range from 100 Hz to 10 MHz on cooling from the isotropic phase. We detected a weak high-frequency mode, which was present only in the temperature range of the SmC A P A phase. Three-dimensional (3D) graphs of the imaginary part of the permittivity, ε", are shown in Figure 6 for compounds Ic and Id, on cooling from the isotropic phase, through the SmC A P A phase down to the crystallization. The relaxation mode was present only in the temperature range of the SmC A P A phase and it disappeared in the isotropic as well as in the crystalline phases. The mode can be attributed to the collective mode, which is often present in SmCP phases, which is probably related to the antiferroelectric character of the mesophase. We analyzed the dielectric behavior with respect to Cole−Cole formula (1) and obtained the relaxation frequency, f r , and the dielectric strength, ∆ε, in the SmC A P A phase. Temperature dependency of the relaxation frequency, f r , and the dielectric strength, ∆ε, for Ic and Id are shown in Figure 7. We found f r (T) decreased continuously on cooling and we obtained an activation energy when fitted to the Arrhenius law.

X-ray Measurements
X-ray scattering measurements were performed for all mesogenic compounds from the series I. For compounds Ia and Ib in the small-angle region, the diffractograms exhibited incommensurate reflections (Figure 8a) that can be indexed assuming centred rectangular unit cells. This type of diffraction pattern is characteristic for a B 1 -type of columnar mesophases. For Ia, the unit cell at T = 114 • C was calculated and we established a = 32.5 Å and b = 41.8 Å. For Ib at T = 100 • C, a = 43.8 Å and b = 44.2 Å were found. The parameter b of the unit cell is related to the molecular length and it reflects the extension of the terminal chains, when we compare Ia and Ib. For Ic and Id in the SmC A P A phase, the XRD signal in the small-angle region revealed sharp commensurate peaks, which corresponded to the smectic layers. The layer spacing value, d, was calculated and for Ic we found d = 40.3 Å (at the temperature T = 95 • C) and for Id d = 42.2 Å (at T = 99 • C). For all studied mesogenic compounds, the diffuse high-angle maxima correspond to about 4.5 Å, which is the averaged intermolecular distance within layers. In Figure 8, we present an XRD intensity profile with respect to the scattering angle for two selected compounds. In Figure 8a, there is the intensity versus scattering angle for Ib in the B 1 phase with Miller indexes at the corresponding peaks. In Figure 8b, the intensity versus scattering angle is demonstrated for Ic. In Table 2, crystallographic parameters are summarized and compared with the length of the molecule, l, obtained from ab initio calculations. The tilt angle of molecules with respect to the layer normal, γ, can be calculated and we calculated it for both types of mesophases, the columnar B 1 phase as well as in the SmC A P A phase ( Table 2). In the SmC A P A phase, the value of the tilt angle was approaching 45 degrees, which was in agreement with observations of textures under the applied electric field, as we observed rotation of the extinction position for an angle of 42−45 degrees.

Discussion and Conclusions
In this study, we have focused on the mesomorphic behavior of bent-core liquid crystals bearing the lateral substituent in the apex position of the core. Previously, we documented for derivatives of 3-hydroxybenzoic acid with lateral substituents in position six that only materials bearing the smallest substituent (fluorine) exhibited mesomorphic behavior while other substituents caused crystallinity of the substances [34]. In this particular case, the mesomorphic behavior was

Discussion and Conclusions
In this study, we have focused on the mesomorphic behavior of bent-core liquid crystals bearing the lateral substituent in the apex position of the core. Previously, we documented for derivatives of 3-hydroxybenzoic acid with lateral substituents in position six that only materials bearing the smallest substituent (fluorine) exhibited mesomorphic behavior while other substituents caused crystallinity of the substances [34]. In this particular case, the mesomorphic behavior was highly dependent on the number and orientation of the ester linkages. Tuning the orientation of the ester groups (Figure 9a), it was possible to induce the formation of an enantiotropic columnar B 1 phase and lamellar SmC A P A phases. In the case of methyl and chlorine as the substituents in the apex position, only monotropic B 1 and an enantiotropic SmC A P A phase for the materials with longest terminal alkyl chain (C 14 H 29 ) were observed. Subsequently, this plausible orientation of ester linkages was adopted also for the materials studied here. Despite the optimum number and orientation of the ester linking units, only the fluoro-substituted materials I exhibited mesomorphic properties. It is reasonable to assume that in the case of materials of series II-IV, a negative steric effect of larger substituents hinders the formation of a mesophase. Similar behavior has already been discussed for 5-substituted resorcinol-based materials (1,3-phenylene bis[4-(4-alkyloxyphenoxycarbonyl)benzoates, (Figure 9b) for which all studied fluoro-substituted homologues exhibited a SmCP A phase, while other lateral substituents were not tolerated [29]. It should be noted that the electronic effect of the substituents is less likely to affect the physical properties of the materials. The lateral substituents are located in meta-position to the functional groups connecting the elongating side arms and, thus, cannot significantly affect their electronic state and, consequently, their conformation.
In this contribution we have synthesized central cores laterally substituted in the apex position and applied them in the synthesis of four series of novel bent-core liquid crystals. From the DSC studies and texture observation under an optical polarizing microscope, we have found that only the materials of the series I show mesomorphic behavior, while materials of series II-IV bearing bulky substituents are crystalline only. Using electro-optical investigations, dielectric spectroscopy, and X-ray measurements, we have determined the character of the mesophases. We show that the fluoro-substituted homologues with the shorter terminal alkyl chains (C 8 H 17 and C 10 H 21 ) exhibit the columnar B 1 phase while materials with longer terminal alkyl chains (C 12 H 25 and C 14 H 29 ) show the SmC A P A phase.
It can be concluded that the mesomorphic behavior of the materials substituted in the apex position strongly depends on the size of the lateral substituent with fluorine being most probably the only tolerable one. The mesomorphic properties of the fluoro-substituted materials can be tuned by the length of the terminal alkyl chains. However, the effect of the type, number, and orientation of the linking units in the side chains is yet to be studied in detail.
bis [4-(4-alkyloxyphenoxycarbonyl)benzoates, (Figure 9b) for which all studied fluoro-substituted homologues exhibited a SmCPA phase, while other lateral substituents were not tolerated [29]. It should be noted that the electronic effect of the substituents is less likely to affect the physical properties of the materials. The lateral substituents are located in meta-position to the functional groups connecting the elongating side arms and, thus, cannot significantly affect their electronic state and, consequently, their conformation. Figure 9. The structure of (a) materials based on 6-substituted 3-hydroxybenzoic acid studied in reference [34], and (b) materials derived from 5-substituted resorcinol studied in reference [29].
In this contribution we have synthesized central cores laterally substituted in the apex position and applied them in the synthesis of four series of novel bent-core liquid crystals. From the DSC studies and texture observation under an optical polarizing microscope, we have found that only the materials of the series I show mesomorphic behavior, while materials of series II-IV bearing bulky substituents are crystalline only. Using electro-optical investigations, dielectric spectroscopy, and X-ray measurements, we have determined the character of the mesophases. We show that the fluoro-substituted homologues with the shorter terminal alkyl chains (C8H17 and C10H21) exhibit the columnar B1 phase while materials with longer terminal alkyl chains (C12H25 and C14H29) show the SmCAPA phase.
It can be concluded that the mesomorphic behavior of the materials substituted in the apex position strongly depends on the size of the lateral substituent with fluorine being most probably the only tolerable one. The mesomorphic properties of the fluoro-substituted materials can be tuned by the length of the terminal alkyl chains. However, the effect of the type, number, and orientation of the linking units in the side chains is yet to be studied in detail.