Cybotactic nematic phases of photoisomerisable hockey-stick liquid crystals

ABSTRACT New five-ring hockey-stick liquid crystalline materials with 4-bromoresorcinol as the central core unit and an azobenzene-based side arm were synthesised and their mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and under a triangular wave electric field. Additional structural modification was done by introducing a lateral fluorine atom in the terminal ring of one of the side arms. It is found that regardless of the alkyl chain length or the lateral fluorine substitution, all of the prepared materials are liquid crystalline exhibiting nematic phases composed of cybotactic clusters of the SmC-type (NCybC) in addition to a monotropic SmC phase for the longest homologue. GRAPHICAL ABSTRACT

Therefore, we report herein a new series of azobenzene-containing HSLCs involving the 4-bromoresorcinol core as the bent unit (compounds An) with n = 8, 12, 20. In addition, the core fluorinated compound AF12 was synthesised in order to study the effect of introducing lateral fluorine on the outer ring of these hockey-stick molecules (see Scheme 1). The LC phase behaviour of all these new materials has been investigated by polarising optical microscopy and differential scanning calorimetry (DSC). The longest homologue A20 was also investigated by X-ray diffraction (XRD) and with respect to switching under an applied electric field.

Synthesis
The synthesis of compounds An and AF12 is outlined in Scheme 1. These compounds were obtained by esterification of the azobenzene-derived benzoic acids 3/n and 3F12 with 4-bromoresorcinol monobenzoate 6. The 4-(4ʹ-n-alkyloxyphenylazo)benzoic acids 1/n and 4-(4ʹ-n-alkyloxy-3ʹ-fluorophenylazo) benzoic acid 1F12 synthesised according to the methods described previously [41] were esterified with 4-hydroxybenzaldehyde to give the corresponding aldehydes 2/n and 2F12. The desired acids (3/n and 3F12) were obtained by oxidising the aldehydes 2/n and 2F12 using similar oxidation procedure to that reported elsewhere [42]. Purification of the final compounds was performed by column chromatography with dichloromethane as an eluent followed by recrystallisation from chloroform/ethanol mixture. The synthesis of the compounds, together with the experimental details for each step with the analytical data for the final hockey-stick compounds, are given in the Supplemental data.

Charactersiation
The mesophase behaviour and transition temperatures of the prepared HSLCs were measured using a Mettler FP-82 HT hot stage and control unit in conjunction with a Nikon Optiphot-2 polarising microscope. The associated enthalpies were obtained from DSC-thermograms which were recorded on a Perkin-Elmer DSC-7, with heating and cooling rate 10 K min −1 . The electro-optical switching characteristics were examined using the triangular-wave method [43] using 6 μm polyimide-coated indium tin oxide (ITO) cells, EHC, Japan.
The X-ray diffraction patterns were recorded with a 2D detector (Vantec 500, Bruker). Ni filtered and pin hole collimated CuK α radiation was used. For the wideangle X-ray diffraction measurements, the exposure time was 15 min and the sample to detector distance was 9.0 cm. Uniform orientation was achieved by alignment in a magnetic field (B ≈ 1 T) using thin capillaries. The samples were held on a temperaturecontrolled heating stage.

Mesomorphic properties
The transition temperatures (°C) and the associated enthalpies (kJ mol −1 ) obtained from DSC thermograms of compounds An and AF12 are given in Table 1 and represented graphically in Figure 1. The DSC thermograms obtained for compounds A8 and A20 as examples are shown in Figure 2. All compounds are thermally stable as confirmed by the reproducibility of thermograms on several heating and cooling cycles.
On heating the shortest derivative A8 with n = 8, a direct transition from the crystalline state to the isotropic liquid takes place at T = 114°C. On cooling the isotropic liquid state of A8, a monotropic nematic phase is observed below~114°C (see Figure 3(a)) which crystallises at 87°C. On chain elongation, the melting temperatures decrease and the monotropic nematic phase is converted to enantiotropic liquid crystalline phase. For example, compound A12 with n = 12 exhibits an enantiotropic nematic phase that exists over a wider temperature range (32 K on cooling) compared to that of A8 (27 K). Further chain elongation (compound A20) induces an additional LC phase below the nematic phase. Based on textural observations between crossed polarisers, it is a biaxial smectic phase, most probably being a SmC phase (see Figure 3(b)). The absence of in-plane order (SmI, SmF) is confirmed by the small enthalpy of the N-SmC transition (ΔH = 0.5 kJ·mol −1 ) [44,45]. This monotropic SmC phase between the nematic and the crystalline phases narrows the nematic phase range compared to the shorter homologues.
None of the compound with very distinct chain lengths shows an indication of symmetry breaking or HNC phase formation in their LC phases, not even in the SmC phase [46,47]. As core fluorination is known to affect the formation of DC and HNC phases considerably [25][26][27][28], compound AF12 with a lateral fluorine atom at the outer ring of the bent core molecule in the ortho position with respect to the alkyl chain (see Scheme 1) was synthesised. The melting point and the crystallisation temperature are not significantly influenced by the additional fluorine substituent (compare A12 and AF12 in Figure 1). Also the type of the mesophase is not affected by the peripheral  fluorination of the aromatic core as AF12 shows only a nematic phase similar to its related analogue A12. However, the clearing temperature as well as the range of the nematic phase of AF12 is reduced compared to A12.
In switching experiments using a triangular wave voltage, no current peak could be observed in the nematic phases of all compounds An and AF12 up to a voltage of 200 V pp in a 6 µm ITO cell, indicating the nonpolar nature of these nematic phases [48,49]. Also the SmC phase of compound A20 was found to be nonpolar as no current peak could be detected. There is also no optical response on the applied fields, except some electro-convections occurring in the nematic phases at high voltage [48].
X-ray diffraction measurements were performed with compound A20 as an example. The X-ray diffraction pattern of a magnetically aligned sample (Figure 4 (a)) shows a diffuse scattering in the wide angle region with a maximum at d = 0.45 nm positioned on the equator, and a weak diffuse scattering in the small angle region with a maximum at d = 5.02 nm. The diffuse small angle scattering has clear maxima beside the meridian (dumbbell shape) with relatively high intensity compared to the wide angle scattering, indicating the presence of cybotactic clusters of the SmC type and thus confirming a skewed cybotactic nematic phase (N CybC phase) [50][51][52]. The tilt β in the cluster is about 34°which is in line with the maximum possible value β = 44°as calculated with cosβ = d/L mol (L mol = 7.0 nm determined between the ends of the alkyl chains in a conformation with 120°bend of the aromatic core and stretched alkyl chains (see Figure 5)). Formation of a cybotactic nematic phase is in agreement with the low transition enthalpy value observed for the N-SmC transition (ΔH = 0.5 kJ·mol −1 ) being smaller than the N-Iso transition enthalpy (ΔH = 0.7 kJ·mol −1 ). This kind of cybotactic nematic phases is a typical feature of BCLCs [50,52] and other compounds with extended rigid cores [53].  Unfortunately, the investigation of the monotropic SmC phase exhibited by A20 was not possible due to rapid crystallisation during the time of exposure.

Comparison with related compounds
Compounds Bn (see Scheme 2), reported by Monika et al. [37], have the same aromatic core structure as well as the same sequence and types of linkage units as in compounds An, only without the bromine at the bent resorcinol core. Comparing A12 with B12 with the same alkyl chain length at the end of the azobenzene-containing side arm indicates that the bromine atom at the apex of the bent core molecule suppresses the smectic phases (SmC a and SmX) of B12 and induces a nematic phase. Moreover, the melting, clearing and crystallisation temperatures are reduced by the lateral bromine substituent. These differences in the mesophase types and in the transitions temperatures between both compounds may be attributed to the weaker core-core interactions present in compound A12 compared to B12 as a result of the bromine substitution. There are two major effects of bromine: one is the effect on the molecular conformation by favouring twisted conformations [26] and the other is the direct distortion of the packing of the aromatic cores due to the bulkiness of this lateral substituent.
Comparison with the series of related bent-core molecules Cn (see Scheme 2) with the same orientation of the COO linking groups and the same number of benzene rings, only differing in the position of the bent 4-bromoresorcinol core, indicates that the HNC phases of compounds Cn are completely removed, though the LC-Iso transition temperatures of both series are in the same range. Obviously the reduced molecular symmetry, which requires a mixed packing of the short and long wings, leads to a reduced packing density which is unfavourable for chirality synchronisation [54] as required for the formation of HNC and DC phases.

Photosensitivity
To study the trans-cis photoisomerisation of the prepared hockey-stick molecules, UV-vis absorption spectroscopy was performed on compound A20. Figure 6 shows the effects of UV irradiation on the UV-vis spectra of A20 in chloroform solution at three different conditions: (a) freshly prepared, (b) exposed to 365 nm light and (c) after storing the sample in dark overnight. A maximum absorption at 367 nm is observed for the freshly prepared sample as a result of the π-π*  transition of the chromophore in the molecule indicating the presence of the more stable trans-isomer. After irradiation with 365 nm light for 20 mins, the absorption at 367 nm decreases greatly and another absorption band at 465 nm starts to appear confirming the transformation from the trans-isomer to the less stable cis-isomer. Measuring the same solution after storing in dark overnight gives nearly an identical spectrum to that observed for the freshly prepared solution indicating the relaxing back of the cis-isomer to the transisomer. These results are very similar to that reported for the hockey-stick molecules Bn [37] as well as for other azobenzene-containing BCLCs [29,46,47,55].

Summary and conclusions
In summary, we reported new halogen-substituted azobenzene-containing hockey-stick molecules exhibiting mainly nematic phases (compounds An and

AF12).
A monotropic nematic phase is observed for the shortest homologue (A8) which converts to an enantiotropic phase on chain elongation (A12 and A20) in addition to the formation of a monotropic SmC phase for the longest homologue (A20). The mesogen with additional peripheral fluorine substitution (AF12) exhibits also a nematic phase similar to its non-fluorinated analogue A12. X-ray diffraction carried out for the longest homologue (A20) proved that the nematic phases are composed of cybotactic clusters of the SmC type (N CybC ) similar to that formed by BCLCs. Comparing the prepared materials with related hockey-stick compounds without bromine substitution indicates that halogen substitution at the apex of the bent-core structure reduces the formation of smectic phases and favours nematic phases as a result of the reduced core-core interactions. Finally, the trans-cis photoisomerisation in solution was studied by performing UV-vis absorption spectroscopy for one selected example.