Non-symmetric ether-linked liquid crystalline dimers with a highly polar end group

ABSTRACT A new class of non-symmetric dimeric compounds derived from 4-cyano-4′-hydroxybiphenyl in which two rigid parts are connected via flexible spacers have been designed and synthesised. These materials possess trialkoxy chains attached at one end of the molecule, while the other end consists of a biphenyl moiety terminated with the highly polar cyano group. The molecular structures of these dimers have been confirmed by elemental analysis and spectroscopic data and their phase behaviour has been characterised by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Almost all of the synthesised materials exhibit liquid crystalline properties depending on the number of carbon atoms in the terminal chains, where all short chains derivatives form nematic phases and depending on the length of the internal spacer long terminal chains homologues display crystalline or unidentified smectic phase. GRAPHICAL ABSTRACT


Introduction
Since 1980s numerous non-conventional molecular structures were reported to display mesomorphic properties. [1,2] The liquid crystal (LC) dimers containing two mesogenic units connected by a flexible spacer represent one class of these molecular architectures. Dimeric LCs can be classified into two main classes depending on the type of the two mesogenic units: symmetrical and non-symmetrical dimers. In the first class of materials the two mesogenic units are identical, while in the second they are not. In most cases the phase behaviour of dimers are different from their building blocks and more complex. The subject of LCs dimers is well reviewed in the literature. [3][4][5][6] In recent years, there is a growing interest in these materials as they proved to be a rich source for interesting new mesophases. They are able to display wide temperature range blue phases, [7,8] flexoelectric behaviour, [9][10][11] helical-nanofilament phases (B4 phases) [12,13] and more interestingly some dimeric materials incorporating cyanobiphenyl exhibit a lower temperature nematic mesophase, the so-called twist-bend nematic phase (N TB ). [14][15][16][17][18][19][20] On the other hand, there is a significant progress in the field of mirror symmetry breaking in liquid crystalline phases formed by bent-core molecules [21][22][23][24][25][26][27][28][29] and bent-shaped mesogenic dimers. [12,13,20,30,31] Spontaneous formation of chiral domains was also observed in bicontinuous cubic phases [32,33] and in isotropic liquids formed by some achiral polycatenar (multi chain-terminated) molecules. [32,34,35] Up to date, there is only one report about non-symmetric LCs dimers involving polycatenar building blocks, which were found to display three-dimensional (3D) mesophases with cubic symmetry. [36] Therefore, the question arises if the combination of polycatenar and cyanobiphenyl building units in the same molecule could lead to N TB , [37] chiral 3D phases [36] or chiral isotropic liquids. [32][33][34][35] In this article, we report about a new class of etherlinked non-symmetric liquid crystalline dimers (An/m, see Table 1) consisting of cyanobiphenyl group attached to one terminus of a polycatenar molecule which is known to induce the formation of chiral cubic phases and chiral isotropic liquids. [32,33,35] The number of carbon atoms in the terminal alkyl chains (n) was varied between 4, 6 and 10, while those in the aliphatic spacer (m) were odd numbers and changed between 5 and 7 as it is well known from previous studies that the odd-parity spacers favour the formation of N TB phases. [15,18,19,37] The phase behaviour of these new dimers has been investigated by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC).

Characterisation
Thin-layer chromatography (TLC) was performed on aluminium sheet precoated with silica gel. Analytical quality chemicals were obtained from commercial sources and used as obtained. The solvents were dried using the standard methods when required. The purity and the chemical structures of all synthesised materials were confirmed by the spectral data. The structure characterisation of the prepared materials is based on 1 H-NMR and 13 C-NMR (Varian Unity 500 and Varian Unity 400 spectrometers, in CDCl 3 solutions, with tetramethylsilane as internal standard). Microanalyses were performed using a Leco CHNS-932 elemental analyser.
The mesophase behaviour and transition temperatures of the dimeric molecules were measured using a Mettler FP-82 HT hot stage and control unit in conjunction with a Nikon Optiphot-2 polarizing microscope. The associated enthalpies were obtained from DSC-thermograms which were recorded on a Perkin-Elmer DSC-7, heating and cooling rate: 10 K min −1 .

Mesomorphic properties
The transition temperatures (°C) and the associated enthalpies (kJ mol −1 ) obtained from DSC thermograms of the newly synthesised dimers An/m are given in Table 1 and represented graphically in Figure 1. The DSC thermograms obtained for compounds A6/5 and A10/5 as representative examples are shown in Figure 2. All compounds are thermally stable as confirmed by the reproducibility of thermograms on several heating and cooling cycles.
With the exception of A10/7, all synthesised compounds exhibit liquid crystalline phases. However, the type of the mesophases depends on the number of the carbon atoms in the aliphatic spacer and/or in the terminal alkyl chains.
Let us first consider compounds incorporating the shorter aliphatic spacer (m = 5). On heating the shortest homologue A4/5 a direct transition from the crystalline state to the isotropic liquid takes place at T = 156°C. On cooling A4/5 from the isotropic liquid a nematic phase is observed at T = 144°C with a value of transition enthalpy ΔH = 0.5 kJ.mol −1 (see Figure 3(a)) which crystallizes on further cooling at T = 128°C. Elongation of the terminal alkyl chains from n = 4 to 6 (compound A6/5) leads to a decrease of the melting temperature and also the formation of a monotropic nematic phase (see Figure 3(b)). However, the range of this nematic phase is slightly wider than that observed for A4/5 (see Figure 1). Further elongation of the terminal chains results in further lowering of the melting temperature as would be expected and completely removes the nematic phase (compound A10/5), where a monotropic smectic phase is formed as indicated from the observed optical texture (see Figure 3(c)) and the change of the value of transition enthalpy (ΔH = 1.1 kJ.mol −1 ). After shearing this unidentified texture a birefringent schlieren texture could be observed (see Figure 3(d)) in a similar manner to that observed for B2 phase exhibited by bent-core liquid crystals. [38] Moreover, uniform planar or homeotropic alignment was not achieved between glass plates, as typical for B2 phase which usually cannot be aligned. [22] Therefore, the smectic phase of A10/5 it is not an ordinary SmA or SmC phases. Though textural features would support a B2 phase, the Iso-SmX transition enthalpy is unusual small for a B2 phase. Because further investigation/phase confirmation of this phase with X-ray diffraction (XRD) studies or electro optical measurements was not possible due to rapid crystallization during the time of measurement it is assigned as SmX phase.
Increasing the length of the internal spacer from m = 5 to m = 7 results in the formation of the dimeric molecules A4/7, A6/7 and A10/7. These materials exhibit similar phase behaviour to that observed in their related analogues with m = 5, where the melting temperatures for all materials decrease with elongation of the terminal alkyl chains and the nematic phases are observed as monotropic phases for the short terminal chains derivatives A4/7 and A6/7. Moreover, the nematic phase range is also increasing with chain elongation typically as observed for A4/5 and A6/5. However, the nematic phase range of A6/7 is much wider compared to its analogue A6/5 (see Figure 1). Another observation is the disappearance of any liquid crystalline phases on further chain elongation, where compound A10/7 is only crystalline with a melting point T = 131°C.

Investigation of mixtures with 5-CB
Aiming to stabilise the nematic phases exhibited by the synthesised dimers we investigated the 1:1 mixtures (mole:mole) of the nematic host liquid crystalline material 5-CB (4′-n-pentyl-4-cyanobiphenyl) with all An/m compounds (see Table 2).
Remarkably in all cases the Cr-Iso transition temperatures were greatly reduced in comparison with the pure compounds (29-58 K), however the formation of the nematic phase in these mixtures depends strongly on the length of the aliphatic spacer.
For the mixtures of the dimeric molecules with m = 5, the nematic phase was completely removed for the shortest homologue A4/5 and formed as monotropic phases for the next homologue A6/5. Moreover, the nematic phase range for the mixture formed between A6/5 and 5-CB is 35 K which is much wider compared to the pure compound (19 K). On the other hand, the SmX phase observed in the pure A10/5 is replaced by a monotropic nematic phase which crystallises at T = 28°C near to room temperature in the mixed system.  Mixing the other types of dimers with longer spacer (m = 7) with 5-CB results in slightly different phase behaviour, where the nematic phases formed by the pure compounds A4/7 and A6/7 are completely removed and only crystalline materials are formed in their mixtures. An interesting observation is the formation of a nematic phase for the mixture formed between 5-CB and the longest homologue A10/7 which is crystalline. Therefore, mixing these dimers with 5-CB removes or induces nematic phases depending on the length of the spacer and/or the terminal alkyl chains.

Conclusion
In summary, we reported a new class of dimeric mesogens incorporating the highly polar cyano group at one end of the molecule and trialkyl chains at the other end (An/m), where n represents the number of carbon atoms in the terminal alkyl chains and m is the number of carbon atoms in the aliphatic spacer. It was found that these dimeric materials do not form any cubic 3D mesophases or chiral isotropic liquid phases. Moreover, there is no indication of nematic-nematic phase transitions in any of the synthesised dimers but instead the dimeric molecules with m = 5 show monotropic nematic phases for the short chains homologues and SmX phase for the longest homologue with n = 10. On the other hand, the dimeric molecules with longer internal spacer, m = 7, form monotropic nematic phases for the homologues with n = 4 or 6 which is totally removed for the longest derivative with n = 10, where only crystalline material is formed. Additionally, we investigated the 1:1 mixtures formed between these new dimers and 5-CB, where the melting points for all dimers are greatly reduced in their mixed systems and depending on m or n nematic phases or crystalline materials are formed by these mixtures.

Synthesis
The synthesis of the non-symmetric dimers under discussion is shown in Scheme 1. As an example the synthesis of the final compound (A6/5) is given  below. The analytical data for An/m compounds as well as their 1 H-NMR spectra and the synthesis details for all intermediates are reported in the electronic supporting information file (ESI).

A6/5
The acid 6 (0.40 g, 1.0 mmol) was heated under reflux with excess thionyl chloride (3 mL) and a catalytic amount of DMF for 1 hour. The excess of thionyl chloride was removed by distillation under reduced pressure. The resulting acid chloride was then dissolved in dry dichloromethane (20 mL) followed by addition of the phenolic compound 3 (0.59 g, 1.0 mmol) previously dissolved in DCM, triethylamine (0.17 mL, 1.2 mmol) and a catalytic amount of pyridine and the reaction mixture was then refluxed for 6 hours under argon atmosphere. The reaction progress was checked with TLC and at the end of the reaction the solution was cooled to room temperature, washed with 10% HCl (2 × 50 mL) and three times with cold water followed by extraction with dichloromethane (3 × 50 mL) and finally dried over anhydrous sodium sulphate. The crude material was chromatographed on silica gel using DCM followed by recrystallisation from chloroform/ethanol mixture affording 0.65 g (65%) of