Azopyridine-based hydrogen-bonded liquid crystals with thioether tail

ABSTRACT The first examples of azopyridine-based hydrogen-bonded liquid crystals (HBLCs) with thioether tail are reported. For this purpose, new HBLCs were designed and synthesised using 4-octylthiobenzoic acid as the proton donor and two different types of 4-alkoxyazopyridines as the proton acceptors. The first group of HBLCs has no lateral substitution, whereas the second has a lateral fluorine atom next to the alkoxy chain attached to the azopyridine derivative. Therefore, it was possible to investigate not only the effect of thioalkyl tail but also the influence of the aromatic core fluorination on the phase behaviour of the newly synthesised HBLCs. Hydrogen bond formation between the proton donor and the proton acceptors was confirmed using FTIR spectroscopy and X-ray diffraction (XRD), while the liquid crystalline behaviour of the HBLCs was investigated using differential scanning calorimetry (DSC) and polarised light microscope (PLM). It was found that all the reported HBLCs are mesomorphic exhibiting smectic A (SmA) and/or smectic C (SmC) phases depending on the length of the alkoxy chain as well as the presence or absence of the lateral fluorine atom. The possibility of photo-switching of the reported HBLCs under UV light irradiation was demonstrated for some selected examples in solution as well as in the bulk state. GRAPHICAL ABSTRACT


Introduction
Hydrogen bonding between complementary components is an interesting non-covalent interaction promoting molecular self-assembly [1] and has been used as a versatile tool to drive liquid crystalline (LC) phases from non-mesomorphic components [2][3][4].It is known that 4-alkoxybenzoic acids are LCs owing to their spontaneous dimerisation through intermolecular hydrogen bond formation between the free carboxylic groups of their individual components.Kato et al. used this concept to report the first examples of hydrogenbonded liquid crystals (HBLCs) formed between heterogenous components.Therefore, he reported HBLCs constructed using pyridine-derivatives as the proton acceptors and benzoic acids as proton donors [5,6].This work opened a new era of LC research based on supramolecular chemistry and led not only to new LCs but also to less time-consuming and inexpensive synthesis of LC materials.Later, different classes of HBLCs were designed including calamitic [7,8], bent-shaped [9][10][11][12][13], liquid crystal polymers [14], dimers displaying the heliconical twist-bend nematic (N TB ) phase [15] and polycatenars exhibiting mirror-symmetry breaking in their isotropic liquids and LC networks [16][17][18].It was also recently used to produce materials displaying the interesting ferroelectric nematic (N F ) phase [19].
Most of the proton donors used to design HBLCs are 4-alkoxybenzoic or 4-alkylbenzoic acids and their derivatives.On the other hand, 4-alkylthiobenzoic acids are barely used.To the best of our knowledge, there are only five reports about HBLCs derived from 4-alkylthiobenzoic acids (Figure 1).Three of these reports describe the phase behaviour of the dimeric form of the individual acid molecules (Figure 1(a)) [20][21][22] and the other two deal with HBLCs formed between 4-alkylthiobenzoic acids and 4-phenylpyridine (Figure 1(b)) [23] or 4,4'bipyridine (Figure 1(c)) [24].The inclusion of sulphur during the design of low molar mass liquid crystals is a demanding challenge for better understanding of structure-property relationships [25][26][27].Sulphurcontaining materials exhibit high birefringence due to the high polarisability of the sulphur atom [28][29][30].These LCs with high birefringence are of special interest for several technological applications such as fast thirdorder non-linear switching [31], liquid crystal displays [32,33], liquid crystal lenses [34,35] and laser applications [36].
Recently, we have reported the LC behaviour of fluorinated HBLCs formed between azopyridine derivatives and 4-octylbenzoic acid or 4-octyloxybenzoic acids (HFn, OFn, Figure 2) [59], and the results were compared with related nonfluorinated analogues (Hn, Figure 2) [56] or (On, Figure 2) [60].The results revealed that both types and ranges of LC phases are greatly affected by the type of the terminal chain at the acid side, ie either octyl-or octyloxy-chain as well as by aromatic core fluorination.In this context, it is interesting to check the effect of using an octylthio-chain in such hydrogenbonded supramolecules instead of the octyl-or octyloxy-chains on their phase behaviour.
Herein we report the first examples of azopyridine-based HBLCs derived from 4-octylthiobenzoic acid as the proton donor and azopyridines as proton acceptors (HSn and S3Fn Figure 2).The azopyridines used in this report are nonfluorinated or with one fluorine atom at ortho position next to the terminal alkoxy chain.Additionally, two selected examples were modified by changing the position of the fluorine atom to be in meta position with respect to the terminal alkoxy chain (S2F8, Figure 2) or by using double fluorine substitution (S23F8, Figure 2).Moreover, the phase behaviour of the new HBLCs is compared to that of the reported alkyl-and alkoxy analogs.This study is important not only to offer more insights about thioalkylderived HBLCs regarding the few numbers of related HBLCs known up to date [20][21][22][23][24] but also to provide new photo-switchable materials using easily accessible synthetic tools.[56], On [57] and the newly synthesised HSn, S3Fn, S2F8 and S23F8.

Synthesis of 3F, 2F and 23F
General procedure.To the corresponding fluoro substituted phenol (30 mmol, 1.0 eq.), sodium nitrite (2.2 g, 33 mmol, 1.1 eq.) dissolved in 12 ml water and 25 ml of a 10% potassium hydroxide aqueous solution were added, and the solution was cooled to −20°C using acetone/dry ice mixture.To the previous solution, a cooled solution of 4-aminopyridine (3.3 g, 36 mmol, 1.2 eq.) dissolved in 10 ml of water and 16 ml of concentrated HCl was added drop wisely under a vigorous stirring over 90 min.The temperature of the reaction was kept all the time around −15°C.After the complete addition the reaction mixture was left under stirring for additional 1 h followed by addition of sodium bicarbonate till no effervescence is observed.The resulting solid material was filtered-off and washed with deionised water, dried under vacuum and used without further purification for the next step.

The HBLCs (HSn, S3Fn, S2F8 and S23F8)
The new HBLCs (HSn, S3Fn, S2F8 and S23F8) were prepared by mixing equimolar amounts of each of the azopyridine derivatives (Azo-n, Azo-3Fn, Azo-2F8 or Azo-23F8) and the acid (SC8) in a DSC pan and melting them together over ~1 min with stirring to give a homogeneous mixture.The obtained blend was then cooled to room temperature to give an orange solid material.Homogenous melting and reproducible phase transition temperatures were observed in DSC investigations for all complexes indicating the formation of stable hydrogen-bonded supramolecules (Table 1).

Characterization
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 different spectral data.The structure characterisation of the prepared materials was assessed by 1 H-NMR, 13 C-NMR and 19 F-NMR (Varian Unity 400 spectrometers, in CDCl 3 solution, with tetramethylsilane as internal standard).
Infrared absorption spectra were measured with Perkin Elmer FT-IR spectrometer 'Spectrum Two' with an ATR (UATR)-unit using attenuated total reflection method.The build-in measurement-crystal is a diamond, and the total number of reflections is 1.The detector is a normal MIR-Detector (LiTaO 3 ).
The mesophase behaviour and transition temperatures of the hydrogen-bonded complexes 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 DSCthermograms which were recorded on a Perkin-Elmer DSC-7 at a heating and cooling rate of 10 K min −1 .
The trans-to-cis-to-trans photoisomerisation in solution was performed at room temperature in a quartz cuvette (1 cm) using Ocean Optics HR-2000 spectrophotometer at a wavelength of 365 nm, while in the bulk state, it was performed using a UV laser pointer (405 nm, 5 mW/mm 2 ).

FTIR investigations
To confirm the hydrogen bond formation between the 4-octylthiobenzoic acid SC8 and each of the azopyridine derivatives, FTIR investigations were performed for two selected examples.FTIR is well known as an effective tool to study intermolecular interactions such as hydrogen or halogen bonding [63][64][65][66][67][68].Therefore, FTIR spectroscopy was performed in the crystalline state at room temperature for HS8 and S3F8 as representative examples for the nonfluorinated HSn and the fluorinated S3Fn complexes, respectively.
The results are shown in Figure 3 and compared to their complementary components, ie the azopyridines (Azo-8 and Azo-3F8) and the 4-octylthiobenzoic acid (SC8).As evident from Figure 3, the FTIR bands between the complementary components and the dimeric SC8 acid are quite different to one another.For the FTIR spectra of the pure acid (blue curve, Figure 3a,c), there is a broad band at 2500-3300 cm −1 , which is absent for both HS8 and F3S8 (black curve, Figure 3(a,c)).This indicates that the intermolecular hydrogen bonding between the acid molecules is replaced by another H-bonding between the individual SC8 molecules and the azopyridine derivatives Azo-8 and Azo-3F8.Also, two newly broadened bands centred at approximately 2448 and 1899 cm −1 for HS8 (black curve, Figure 3(a)) and 2452 and 1881 cm −1 for F3S8 (black curve, Figure 3(c)) that are caused by a Fermi resonance effect from the overtone of δ O-H and γ O-H , respectively, clearly indicated the H-bond formation [23,24,56,[69][70][71]. Furthermore, the band resulted from ν C=O at 1678 cm −1 for the acid SC8 shifts to a higher wavenumber of ~1698 cm −1 for both complexes HS8 and F3S8 and much less intensity compared to that of the pure acid SC8 (Figure 3b,d).These FTIR results are in good agreement with those previously reported supramolecules derived from 4-octylbenzoic acid and azopyridine derivatives (Hn and HFn) [56], which further confirm the successful formation of the hydrogen bonding between SC8 and each of the azopyridine derivatives Azo-n, Azo-3Fn, Azo-2F8 and Azo-23F8.

Mesomorphic properties of HSn and S3Fn
The phase transition temperatures and mesomorphism of the prepared HBLCs were revealed based on DSC measurements and textural observations under PLM, for which the obtained data for HSn and S3Fn are summarised in Table 1 and represented graphically on Figure 4(a,b).For comparison, the data for the previously reported supramolecules derived from 4-octyloxybenzoic acid (On and OFn) or from 4-octylbenzoic acid (Hn and HFn) are given in Figure 4(c-f).
Before discussing the phase behaviour of the new supramolecules, it should be noted that neither the nonfluorinated azopyridine derivatives Azo-n nor the fluorinated ones are liquid crystalline.Instead, they are all crystalline materials and melt directly into isotropic liquids [16,56].On the other hand, the proton donor, ie 4-octylthiobenzoic acid (SC8), is an LC material exhibiting the so-called cybotactic nematic phase with smectic C clusters (N CybC ) in a relatively short temperature range with the following transition temperatures on heating: Cr 106°C N 110°C Iso [20].As can be seen from Table 1, all of the prepared HBLCs are LCs exhibiting different types of mesophases depending on the length of the terminal alkoxy chain attached to the azopyridine segment as well as on the degree of the aromatic core fluorination, as will be discussed in the following section.Interestingly, the N CybC phase of the pure acid SC8 is not observed in any of the formed HBLCs; instead, different mesophases are induced, which further confirm the formation of H-bonding between the complementary components.
As can be seen from Table 1, the first group of HBLCs without lateral fluorine group (HSn) exhibits two different types of LC phases depending on the alkoxy chain length.For the shortest complex HS8, an enantiotropic mesophase is observed on heating the sample under and Azo-3F8 (red) in the crystalline state at room temperature: (a, c) enlarged area between 1770 cm −1 and 3500 cm −1 ; (b, d) enlarged area between 1605 cm −1 and 1800 cm −1 .The complete spectra are given in the supporting information (Figures S1 and S2).
crossed polarisers from the birefringent crystalline state at ~92°C and remains over ~ 27 K as the only formed LC phase till the transition to the isotropic liquid on further heating.This agrees with the DSC investigations (Figure 5(a)), where the transition for both Cr-LC and LC-Iso states could be detected as first-order transitions with enthalpy values of ~ 44.6 and 20.6 J/g, respectively (Table 1).This LC phase is characterised by its isotropic appearance in the homeotropic region and by fanshaped textures with the dark extinctions parallel to the direction of the polarisers in a planar alignment (Figure 6(a,c)).
These are typical observations for the uniaxial smectic A (SmA) phase, therefore the LC phase exhibited by HS8 is designated as SmA phase.This SmA phase was further confirmed by XRD investigations (see section 4.4).The SmA phase is observed for all following HBLCs of HSn series with n ≥10 as the only mesophase on heating, whereas on cooling the longer two homologues with n ≥12, ie HS12 and HS14, an additional monotropic LC phase is observed below the SmA phase (Figure 6(b,d)).At the transition from the SmA phase to this metastable phase, birefringence suddenly increased in the homeotropic area in line with increasing molecular tilt with the observation of four brush disclinations.In the planar cell, the fan-shaped texture observed in the SmA (Figure 6(c)) becomes broken (Figure 6(d)).This indicates the presence of a synclinic tilted smectic phase (SmC s , Figure 6(c)).The presence of the SmC phase was further confirmed by XRD investigations (Section 4.4).Therefore, the lowertemperature LC phase is assigned as SmC phase in all cases.For additional textures for the complex HS12 in the SmA and SmC phases, see Figure S3a, b in the SI.[57], (d) Bn [56], (e) B3Fn [56] and (f) B2Fn [56].For each complex, the left column represents the data obtained on heating (H), while the right one represents the data obtained on cooling (c) as shown in (a) for HS8.

Mesomorphic properties of S3Fn, S2F8 and S23F8
From Table 1 and comparing Figure 4(a) with 4(b), we can observe that S3Fn complexes form exclusively the same LC phases observed for their related nonfluorinated analogues HSn.However, due to the steric effect of the F group, the melting temperatures for S3Fn are lower compared to those of HSn (see Figure 5(a,b) as examples).As can be seen from Figure 4(b), the SmA phase range of S3Fn is reduced compared to HSn complexes, and with chain elongation, the SmC phase starts to appear for S3F10 with n = 10 as a monotropic phase, which becomes an enantiotropic one for the longest complex S3F14 with n = 14 (see Fig. S3c in SI for optical textures).Therefore, core fluorination could be used in this type of HBLCs to stabilise SmC phases as they were observed as only metastable phases in case of HSn complexes (compare Figure 4  (a,b)).
To investigate the effect of changing the position of core fluorination and the degree of fluorination, additional two examples were designed and synthesised (S2F8 and S23F8, see Scheme 1 and Table 1).In these two HBLCs, the terminal chains at both ends were kept fixed with m = n = 8, but the F atom was shifted to inner position to be in meta position with respect to the alkoxy chain at the azopyridine side in the case of S2F8, while double core fluorination were used at ortho and meta positions in the case of S23F8.As can be seen from Table 1 and Figure 7, due to the increased steric effect of F atom at this inner position, S2F8 forms solely SmA but with much lower-phase stability compared to either HS8 or S3F8.Therefore, the SmA LC phase for S2F8 is a monotropic one.
In the case of the double fluorinated complex S23F8, there is an increased steric effect which leads to lower melting point of ~62°C compared to that of HS8 (~92°C) and those of the single fluorinated HBLCs (F3S8 ~83°C and F2S8 ~91°C).This leads to the appearance of SmC and SmA as enantiotropic LC phases in reasonable range (Figure 7(b)).Therefore, using double core fluorination further stabilise the SmC phase formation.Based on these results, we can reach the conclusion that aromatic core fluorination could be used as a very effective tool to modify the phase behaviour of the reported HBLCs.

X-ray diffraction (XRD)
To further investigate the different types of LC phases observed in the reported HBLCs, the three complexes having n = m = 8 without fluorine substitution (HS8) or bearing one F (SF38) or two F atoms (S23F8) were selected for XRD measurements.In all cases surfacealigned samples were examined in different LC phases.For the complex HS8, a diffuse wide-angle scattering (WAXS) at d = 0.45 nm is observed confirming the presence of LC phase.It has maxima perpendicular to the direction of the layer reflection, confirming a non-tilted organisation in the LC phase, ie SmA phase.In the small-angle region, one sharp reflection corresponding to d ~3.94 nm could be found (Figure 8), which is larger than the molecular lengths of the individual components, ie for the 4-octylthiobenzoic acid (L mol = 1.81 nm), its dimer (L mol = 3.53 nm) and for the azopyridine derivative (L mol = 2.13 nm) calculated with Materials Studio, confirming the formation of the hydrogen bonding between the two components.
The experimental d-value ~3.89 nm agrees with the length of a single linear complex (L mol = 3.94 nm) for all-trans stretched alkyl chains Figure 8(b-d).This further confirm the presence of SmA phase in HS8.Similar results were also obtained in the SmA phase formed by the monofluorinated complex S3F8 or by the double fluorinated S23F8 (Table 2).The d-value measured for the three complexes in the SmA phase has the following order: HS8 > S3F8 > S23F8 (Table 2) as the degree of aromatic core fluorination increases.At the transition from the higher LC phase to the lower-temperature LC in the case of S23F8, the d-value decreases from 3.68 nm to 3.37 nm in line with increasing tilt of the molecules in the lower-temperature LC phase and the transition from random to uniform tilt, ie SmA-SmC transition, which is in agreement with the onset of phase biaxiality (Figure 6(b) as an example).It appears that strengthening of the core-core πstacking interactions with increasing core-fluorination leads to a staggered core packing and the resulting tilt reduces the d-spacing.

Comparison with related alkoxy-and alkyl-derived HBLCs
As shown in Figure 4, the phase behaviour of HBLCs with thioalkyl tail reported in the present study (HSn and F3Sn, Figure 4(a,b)) are compared with those reported earlier with alkoxy (On and OFn, Figure 4(c,d)) or alkyl (Hn and HFn, Figure 4(e,f)) tails, which were collected from the literature [56,57].All data were recorded for the second heating and cooling scans.Concerning the nonfluorinated HBLCs, it is obvious that those terminated with alkoxy chain (On, Figure 4(c)) exhibit SmC regardless the chain length.For the HBLCs bearing alkyl or thioalkyl chain, the shortest homologues in both cases display only SmA phases, and as chain increases, SmC phase starts to appear below the SmA as enantiotropic one for Hn complexes (with n ≥ 10, Figure 4 (e)) and as a metastable LC phase for HSn (with n ≥ 12, Figure 4(a)).Additional monotropic unknown SmX phase is also observed for the longest homologue with alkyl chain (H14).
On the other hand, core fluorination modifies the LC phase type in all cases, where an additional nematic phase is induced as enantiotropic one above the SmC phase for HBLCs terminated with alkoxy chain (OFn, Figure 4(d)) and the SmA phases are totally removed in case of HBLCs ended with alkyl chains (HFn, Figure 4(f)).Whereas, in case of HBLCs bearing thioalkyl chain, aromatic core fluorination retains the SmA and SmC phases, but the SmC phase is more stabilised and becomes enantiotropic for the longest chain (S3Fn, Figure 4(b)).
The melting temperatures (T m ), ie Cr-LC transition temperatures, are comparable in all cases irrespective of the type of the terminal chain and vary slightly depending on the chain length in each homologues series.[20][21][22][23][24]72].This order could be explained based on the steric factors resulted from the rotation barriers and bending angles of the terminal chain.The rotation barrier for C-S-C in the alkylthio chain is lower than that of both C-O-C in the alkoxy chain [73] and C-C-C in the alkyl one [74].From previous single crystal structures investigation, the bonding angles of C-S-C in alkylthio group, of C-C-C in alkyl group and of C-O-C in alkoxy group attached to the phenyl ring are as follows: 105° [75], 110° [76] and 118° [20], respectively.This means highest bending value for the thioalkyl chain and leads to the following order of the bending angle: alkylthio > alkyl > alkoxy chains.This results in expanding the molecular rotation radii in the same order.Combining these two effects, one can conclude that thioalkyl chainterminated HBLCs should show the highest degree of disorder in the mesophase, thus in turn, leads to lower T C temperatures compared to alkyl-or alkoxycontaining HBLCs, which is typically the case for HSn compared to On and Hn supramolecules.
On the other hand, the T C trend for the fluorinated HBLCs is slightly different, where the highest values for T C are exhibited by alkoxy HBLCs and those of the alkyl and thioalkyl are comparable resulting in the following order: alkoxy > alkyl = thioalkyl.This indicates that the  core-core interaction between the deficit aromatic rings because of fluorination is more effective in controlling the transition temperatures than the type of the terminal chain [56].

Photo-switching investigations
The synthesised HBLCs were designed to exhibit photoswitchable properties due to the expected trans-to-cis photo isomerisation of the azopyridine moiety.The HBLC S3F10 was selected as a representative example to investigate such interesting phenomenon both in solution and in the bulk state.
The complex S3F10 was dissolved in chloroform solution, and its UV-vis absorption spectra were measured at three different conditions (Figure 9).The maximum absorption observed at ~368 nm for the fresh solution is due to the π-π* transition of the chromophore (Figure 9, green curve), confirming the presence of the HBLC in the most stable trans isomer.After 1 h irradiation of the solution with 365 nm light, a new broad band at ~452 nm appears, while the intensity of that at ~368 nm greatly decreased (Figure 9, blue curve).This confirms a photoisomerisation to the cis isomer from the trans-isomer upon photo irradiation.To check if this process is a reversible one, the solution was stored overnight in dark and measured again.As can be seen from Figure 9, the data obtained for the stored and fresh solution (black and green curves, respectively) are almost the same, meaning that photoisomerisation process in the investigated HBLC is a reversible one.
To study the photoisomerisation in the bulk state, a UV laser pointer (405 nm,5 mW/mm 2 ) was used.Figure 10 shows the photo investigations of the complex S3F10 in a homeotropic cell under crossed polarisers.Before UV irradiation (Figure 10(a)), the sample exhibits the characteristic birefringent texture of the SmC phase at T = 85°C.Keeping the temperature fixed and starting of UV illumination, the birefringent texture starts to convert to a dark texture (Figure 10(b)), and within 3 s, the texture becomes mostly dark (Figure 10 (c)), which indicates the transition to the isotropic liquid phase.On switching off the light source, the Iso phase relaxes back quickly to the SmC again.
The UV investigation was also performed for the same complex in the SmA phase at T = 95°C and similar results were obtained for SmA-isotropic transition under the effect of light irradiation.This attributed to the less order induced in the LC phase by the bent cis isomers formed under light irradiation, which leads to isotropisation of the LC phase.
These observations indicate a fast and reversible isothermal photo-switching process as a result of trans-to-cis photoisomerisation of the azopyridine segment [56,65,77].

Conclusion
In summary, we have reported herein the design and synthesis of the first examples of azopyridine-based hydrogen-bonded liquid crystals bearing a thioether tail.The HBLCs are derived from 4-octylthiobenzoic acid as the proton donor and different azopyridine derivatives as the proton acceptors (HSn).The formation of the intermolecular hydrogen bonding was confirmed via FTIR and XRD, while the liquid crystalline properties were investigated using DSC and PLM.The effect of core fluorination on the phase behaviour of the HBLCs was also investigated by synthesising related HBLCs using the same 4-octylthiobenzoic acid and fluorinated azopyridines  (F3Sn).For two selected examples, the effect of changing the position of core fluorination or using double fluorine substitutions was investigated (F2S8 and F23S8).
It was found that all HBLCs exhibit LC phases (SmA and SmC) depending on the terminal chain length.Aromatic core fluorination was proved to stabilise the SmC phases in such HBLCs, where the SmC phase was observed as an enantiotropic phase for the longest fluorinated complex S3F14.More interesting, despite its steric effect, double-core fluorination in the case of S23F8 widens the LC phase range and reduces the melting temperature indicating that fluorination is a very effective tool to modify the LC behaviour of the reported HBLCs.
A comprehensive comparison between the previously reported supramolecules with either alkoxy chain (On and OFn) or alkyl chain (Hn and HFn) indicates that the type of LC phase depends strongly on the type of the terminal chain, which also applies for the clearing temperatures.
Finally, the trans-to-cis photoisomerisation of the reported HBLCs was investigated both in solution and in the bulk state for a selected example, where isothermal fast and reversible photo-switching between SmC or SmA phase and the isotropic liquid (Iso) phase under UV irradiation was observed.

Figure 4 .
Figure 4. (Colour online) Phase diagrams of the new HBLCs (a) Hn and (b) S3Fn and the previously reported HBLCs: (c) On[57], (d) Bn[56], (e) B3Fn[56] and (f) B2Fn[56].For each complex, the left column represents the data obtained on heating (H), while the right one represents the data obtained on cooling (c) as shown in (a) for HS8.

Figure 5 .
Figure 5. (Colour online) DSC of (a) HS8 and (b) S3F8 with heating and cooling rates of 10 K min −1 .

Figure 6 .
Figure 6.(Colour online) Optical textures observed on cooling for HS14 in (a,c) the SmA phase at 115°C and in (b,d) the SmC phase at 105°C.(a,b) in a homeotropic cell without any treatment; (c,d) in a 10 μm ITO planar cell.The direction of the polarizers is shown in (a).

Figure 4 (
Figure4(c,d), while the thioalkyl-terminated complexes display the lowest values resulting in the following order for T C : alkoxy > alkyl > thioalkyl HBLCs.These results are in agreement with other alkylthio-containing rodlike mesogens[20][21][22][23][24]72].This order could be explained based on the steric factors resulted from the rotation barriers and bending angles of the terminal chain.The rotation barrier for C-S-C in the alkylthio chain is lower than that of both C-O-C in the alkoxy chain[73] and C-C-C in the alkyl one[74].From previous single crystal structures investigation, the bonding angles of C-S-C in alkylthio group, of C-C-C in alkyl group and of C-O-C in alkoxy group attached to the phenyl ring are as follows: 105°[75], 110°[76] and 118°[20], respectively.This means highest bending value for the thioalkyl chain and leads to the following order of the bending angle: alkylthio > alkyl > alkoxy chains.This results in expanding the molecular rotation radii in the same order.Combining these two effects, one can conclude that thioalkyl chainterminated HBLCs should show the highest degree of disorder in the mesophase, thus in turn, leads to lower T C temperatures compared to alkyl-or alkoxycontaining HBLCs, which is typically the case for HSn compared to On and Hn supramolecules.On the other hand, the T C trend for the fluorinated HBLCs is slightly different, where the highest values for T C are exhibited by alkoxy HBLCs and those of the alkyl and thioalkyl are comparable resulting in the following order: alkoxy > alkyl = thioalkyl.This indicates that the

Figure 9 .
Figure 9. (Colour online) UV-vis spectra (absorbance vs. wavelength) of S3F10 dissolved in chloroform at room temperature: freshly prepared sample (green curve); after irradiation for 1 h with 365 nm light (blue curve) and after keeping the sample in dark overnight (black curve).

Table 1 .
Phase transition temperatures (T/°C), mesophase types and transition enthalpies [ΔH (J/g)] of the HBLCs (HSn, S3Fn, S2F8 and S23F8).a a Peak temperature as determined from second heating (H) and second cooling (C) DSC scans with rate 10 K min −1 ;abbreviations: Cr = crystalline solid; SmC = smectic C phase; SmA = smectic A phase; Cr = crystalline solid; Iso = isotropic liquid.b The enthalpy value could not be detected with DSC investigations.

Table 2 .
d-Values of the complexes HS8, S3F8 and S23F8 in different LC phases and the corresponding d/L mol values.