Synthesis and structure of lateral halogenated V-shaped liquid crystal molecules based on a 1,7-naphthalene central core and alkylthio tails

A series of small angle bent-core (V-shaped) mesogens carrying 1,7-naphthalene as a central core linked with lateral halogenated (chlorinated or fluorinated) Schiff-base side wings and alkylthio terminal tails of variable carbon number (n = 12, 16) was synthesised in order to reduce the transition temperature and improve the phase stability of bent-core liquid crystal molecules. Differential scanning calorimetry (DSC), polarising optical microscopy (POM), small-angle X-ray scattering system (SAXS) and two-dimensional X-ray diffractometer were applied to ascertain the mesomorphic structure and phase transition temperatures of the compounds. The results confirm that all the molecules show thermotropic liquid crystalline behaviour and exhibit hexagonal columnar phase (Colh) in a certain temperature range. Compared with the homologous compounds without lateral halogen, the cleaning point temperature of lateral halogenated V-shaped compounds generally decrease and the Colh phase ranges are more extensive. The influence of lateral chlorine on the cleaning point temperature is more obvious, as well as the effect of lateral fluorine on the range of Colh phase.


Introduction
Since the discovery of unusual switchable mesophases of liquid crystals based on achiral banana-shaped molecules, [1,2] extensive attention has been attracted to synthesise such bent-shaped molecules, in which the bent-shaped molecules based on 1,3-disubstituted benzene and 2,7-disubstituted naphthalene core are usually reported to show unique switchable mesophases (banana phase, B phase). [3][4][5][6][7] The noticeable ferroelectric and anti-ferroelelectric responses of these materials are now well documented. [8][9][10] Matsuzaki et al. [11,12] were the first to synthesise V-shaped molecules of about 60°based on 1,2-phenylene and 2,3-naphthalene central core, which only show conventional SmA and Nematic phase. Choi [13,14] and Ros et al. [15] have also reported some V-shaped molecules based on the 2,3-naphthalene central unit with ester type side wings, but no B phase appears for these acute-angle liquid crystal materials. In general, the introduction of obtuse angle bent-core had been thought to be essential to form the previously mentioned switchable B phases.
However, Watanabe et al. [16,17] synthesised six kinds of bent-shaped molecules based on naphthalene, the research result is stoked that V-shaped molecules based on 1,7-naphthalene display typical B4 phase. In their subsequent study, [18,19] they found a switchable banana phase (SmAP A ), but the temperature range of the switchable phases are averaged from 240°C to 200°C, which makes them very hard to be used at practical applications. On account of the above conclusions, Li et al. [20] synthesise homogeneous V-shaped molecules with alkylthio tails, exhibiting switchable hexagonal columnar phase (Col h ) phase. Under the same condition, the switchable Col h phase transition temperatures of the molecules with alkylthio tails obviously decrease, the range of which are from about 146°C to 118°C.
On the other hand, lateral substitution has an essential influence on the sterical and electronical characteristic and thereby on the intermolecular distance. Consequently, it can generate mesophase characteristics different from the related non-substituted mesogen. There are some relevant reports showing that the type and position of substituent have evidently influence on the mesogen behaviour of bent-shaped molecules. [21][22][23][24] Certainly, some research have indicated that the introduction of the halogen substituent to the position of the terminal benzene could significantly decrease the phase transition temperatures and improve phase range. [25,26] In this study, we separately prepared lateral fluorinated or chlorinated V-shaped molecules based on 1,7-dioxynaphthalene with alkylthio tails (Scheme 1) to observe their mesomorphic state and transition temperature. Four compounds uniformly show thermotropic liquid crystalline behaviour and exhibit Col h on cooling. The effect of the introduction of lateral halogenated group and alkylthio tails on the clearing points and scope of Col h phase were investigated.

Measurements
Nuclear magnetic resonance (NMR) spectra and mass spectrum were measured using a Bruker AVANCE III 400 NMR spectrometer and BIFLEX III MALDI-TOF mass spectrometer, respectively. Elemental analyses were performed with an Elementar Vario EL III analyser. Texture observations were conducted under crossed polarisation with polarising optical microscopy (POM) on a Chongqing MIC SZX-5 equipped with a temperature-controlled Weitu WT2100 hot stage. Differential scanning calorimetry (DSC) measurements were made using a Perkin-Elmer DSC 7 calorimeter in a N 2 atmosphere with a heating rate of 10°C/min and cooling rate of 2°C/min. Small-angle X-ray scattering system (SAXS) was performed using Anton paar SAXSess diffraction with Cu Kα radiation. Two-Dimensional X-ray diffraction was carried out by Bruker Nanostar U Scattering system and D8 Discover diffractometer using Cu Kα radiation. Electric field was obtained using high speed bipolar amplifier (NF, BA4825) connected to a SP F05 functional generator. Indium-tin-oxide (ITO) glass cell with 3 μm has neither polymer coating nor rubbing on the surface.

Synthesis
Scheme 2 outlines a synthetic method leading to the target compounds. The concrete synthetic routes to target compound are as follows in the case of N(1,7) SF12 (R = F, n = 12).
3a  (Figure 1), two transition peaks were observed on cooling, corresponding to two different phase. When slowly cooling the isotropic melt of N(1,7)SF16 on a hot stage between untreated glass plates (about 15 μm), a fanshaped texture appears, as is the characteristic of columnar mesophase. [25,27] We can find that some areas show optically isotropic dark texture which is regarded as column aligned perpendicular to the substrate. While further cooling to 61.8°C, all areas change to a dark blue texture which shows two domain with opposite chirality by the opposite and negative rotations of polariser from the cross position. [28][29][30] One dimensional SAXS was employed to further elucidate phase structure of the high-temperature phase, powder samples were kept in an aluminised paper to measure X-ray curve, The X-ray diffraction pattern is shown in Figure 2 At lower temperature, we use 2D X-ray diffractometer (Figure 2(b)) to get more detailed reflection information which could be unobvious in 1D SAXS. X-Ray patterns of non-oriented samples in the low-temperature phase show three reflections in the small angle region, which exhibit smectic layer spacing of d = 43.2, 38.3 and 21.6 Å. There are also several strong broadened reflections in the wide-angle region, and the broad diffuse peak at d = 4.5 Å is indicative of liquid crystal phases. The blue colour and X-ray pattern of this phase are indications for B4-like phase. [3] By applying an electric field, the texture didn't change. Therefore, a phase sequence of Iso-Col h -B4 on cooling is observed for N(1,7)SF16. For the liquid crystal molecules based on 1,7-naphthalene central core and Schiff's base linkages, B4 phase is the most common phase on low temperature which show dark blue chiral texture.
From the DSC curve of N(1,7)SF12 shown in Figure 3(a), we can find that the cooling curve shows only one obvious peak theoretical corresponding to only one phase transition. However, the change of POM texture shows the existence of another phase which differs from the higher temperature phase below specific temperature. When slowly cooling the compounds, large domains of fan-shaped texture appear as columnar phase below 138.0°C (Figure 3(b)). Figure 4(a) shows the 1D SAXS diffraction pattern of the high-temperature phase, in which inner three reflections having spacing in the ratio of 1:(1/3) 1/2 :1/2. Therefore, the high-temperature phase of N(1,7)SF12 is Col h . Further cooling to 58.0°C, the texture slowly turns into completely optical dark domain without obvious transition enthalpy. From the following 2D XRD result (Figure 4(b)), five reflection peaks in the small angle region area are found. According to the related literature, the dark texture may be cubic phase, B4 phase or Colr phase. [27,31] The reflections in the small angle region actually satisfy the feature of cubic phase. Reflection peaks were observed at d = 58.8, 48.2, 41.6, 33.9,  For N(1,7)SCl16 ( Figure 5), there are also one transition peak in DSC spectrogram, a Col h exists in the temperature region from 129.1°C to 56°C. Below the critical temperature 56°C, the whole texture of preexisting fan-shaped phase obviously changed, part area turns into blue colour texture, while the entirety texture still look like column phase. [32] Then we observe the phase texture on a liquid crystal cell with only 3 μm space. The texture of higher temperature phase is still typical fan-shaped domain. It is noteworthy that the phase transition from Col h to lower temperature phase is marked by a distinct lowering of optical birefringence, meanwhile the phase transition temperature are the same with sample observed by the lager space cells. And the phenomenon is same on a 3 μm ITO cell with no coating (Figure 6(a) and (b)). In conjunction with DSC results and POM texture, the lower temperature phase belongs to columnar phase (Col x ). By applying a rectangular-wave field, the texture of Col x changed to different fan-shaped texture with higher birefringence (Figure 6(b) and (c)), and it could recover on the field-off state. However, the 2D XRD results in the low-temperature phase show one intense reflection peak in low-angle region and a few peaks in wide angle region. The corresponding spacing are d = 39.0, 9.7, 7.1, 6.5, 4.7, 4.2 and 3.6 Å and a broad diffuse peak around 4.5 Å (Figure 7(b)). According to the number of reflections, the sample kept in an aluminised paper could be partial crystalline due to the effect of covered aluminised paper. The structure which is characteristic of N(1,7)SCl12 is similar with N(1,7) SCl16 and no more repeat (see the Appendices, available as Supplemental Content online). It is noteworthy that the ratio of the Bragg spacing of the former sharp reflections of N(1,7)SCl12 in low-temperature phase might suggest lamellar structure. [33,34] The phase transition temperatures and associated enthalpies of four compounds are summarised in Table 1 based on the results of DSC in conjunction with POM during cooling. The X-ray spacing data   Table 2.
Based on Table 1, we draw a histogram (Figure 8) to compare the transition temperatures of four compounds above with N(1,7)Sn (n = 12, 16). The cleaning point temperatures for lateral chlorinated compounds N(1,7) SCln are obviously lower than that of lateral halogen-free compounds N(1,7)Sn, to reduce the value of 21.2°C and 16.9°C corresponding to carbon number 12 and 16. The decrease value of lateral fluorinated compounds, N(1,7) SFn, is relatively less than that of lateral chlorinated molecules, N(1,7)SCln, to reduce the value of 0.4°C and 4.8°C. The result shows that steric effects play a more important role than substituent polarity for these compounds so that halogenated molecules have lower clearing point temperature owing to the decrease of molecular interaction, meanwhile N(1,7)SCln have a major influence on clearing point temperature in contrast with N (1,7)SFn due to larger steric volume. Lateral halogen compounds have a wider Col h phase extent; the Col h range of N(1,7)SFn (n = 12, 16) is the widest as 80.1°C and 79.4°C, followed by the N(1,7)SCln as 67.3°C and 73.1°C, N(1,7)Sn (n = 12, 16) have the narrowest Col h ranges as 18.9°C and 27.9°C. The compounds of fluoro substituent have the widest Col h phase range depending on the perfect combination of small size, high polarity and strength of C-F bond so as to confer the most excellent phase stability, while the phase stability of lateral chlorinated compounds slightly decrease probably owing to lager size and lower strength of C-Cl. [35] The clearing point and phase transition temperature of these V-shaped molecules will generally rise along with the increase of carbon number in alkylthio tails. The reason could be attributed to the improvement of molecular interaction along with the increase of tail chain length.

Conclusions
In this study, we prepared four V-shaped compound based on 1,7-naphthalene central core and alkylthio tails: N(1,7)SF16, N(1,7)SF12, N(1,7)SCl16 and N(1,7)SCl12. All the compounds show the Col h at high temperature on cooling from the isotropic liquid, the mesophase of low temperature are B4, Cub, unidentified Col x and Col x , respectively. The cleaning point of bent-core can be reduced by the introduction of lateral halogen, especially for the influence of Cl. Meanwhile, wider scope of Col h mesophases could be observed as a result of lateral halogen, and the compounds involving F substituent have a wider Col h range than the molecules of Cl substituent. The clearing point and phase transition temperature of these V-shaped molecules will generally rise along with the increase of carbon number in alkylthio tails. Note: (a) a is Hexagonal lattice parameter. b is Cubic lattice parameter.

Funding
(b) X-ray spacing data of Col h phase was obtained through 1D SAXS, while the X-ray data of B4, Cub, Col x were got through 2D X-ray diffractometer.