Synthesis and characterisation of chiral liquid crystal compounds containing cholesterol and D(+)-camphoric acid

ABSTRACT Two series of chiral liquid crystal compounds were synthesised from benzoic acid containing cholesterol with different carbon-chain lengths and bisphenol monoester containing camphoric acid with different carbon-chain lengths, named BCP-C and DCP-C. The structure of the monomers is characterised by IR, 1H NMR and 13C NMR, which indicate that the synthesised molecule conforms to the molecular design. The textures of monomers were characterised by POM, and their selective reflection was studied. It was found that the synthesised liquid crystal compounds exhibited good liquid crystal and optical properties. The phase transition behaviour was characterised using DSC, and the results were consistent with those observed by POM. TGA showed that they had good thermal stability. GRAPHICAL ABSTRACT


Introduction
Liquid crystal (LC) is a substance that exhibits anisotropy in physical properties and has both crystal and liquid properties.The molecular structure of chiral liquid crystal (CLC) forms a helically twisted structure due to the inclusion of asymmetric carbon atoms.Due to the special helical twisted structure, CLC exhibits special optical properties, such as selective reflection [1][2][3], circular dichroism, optical rotation, thermochromism and ferroelectricity, making it widely used in various fields [4][5][6][7][8].Among them, selective reflection is the ability to selectively reflect different wavelengths of visible light to generate structural colours, so it can present different colours to the naked eye.Molecules are arranged in layers with the long axes of the molecules parallel to the plane of the layers, and each plane is rotated at an angle relative to its adjacent plane [9].The direction vector of the liquid crystal molecules in the layer direction returns to the initial orientation state after rotating 360°, and this periodic interlayer spacing is called the pitch (P) [10,11].The pitch (P) can vary with light, temperature [12][13][14], electric field [15], stress [16], pH [17] and chemical environment, etc.According to the Bragg reflection law λ = np (n is the average refractive index of the liquid crystal matrix), the chiral liquid crystal can selectively reflect visible light by adjusting the helical pitch of the liquid crystal.In recent years, the selective reflection of CLC has become a research hotspot.CLC with wavelengths reflected in the visible light region can be applied to temperature indication, luminescent film of liquid crystal display panels, anti-counterfeiting trademarks, chiral dopants [18][19][20][21][22][23][24], etc.The liquid crystal type of chiral liquid crystal is generally the cholesteric phase or chiral smectic phase due to the existence of a chiral centre.Therefore, researchers have attracted great interest in chiral liquid crystals due to their unique self-assembled helical structures and selective reflection properties.
There are many ways to obtain chiral liquid crystals, which can be obtained by adding chiral molecules to nematic liquid crystals, or by liquid crystal molecules with chiral groups.The latter can avoid problems caused by non-uniform doping.Reports of chiral liquid crystals obtained by introducing chiral groups into liquid crystal molecules are common.Until now, selecting or preparing novel optical active chiral entities for new CLC construction has attracted much attention from researchers.Cholesterol ([a] D 20 = −36°) is one of the most widely used chiral reagents for the construction of CLC.As a chiral cyclopentane derivative, D (+)-camphoric acid is not only easy to extract from natural D-camphor but also difficult to racemise during the synthesis process, and retains good chiral properties.As far as we know, there are few reports of introducing two chiral groups into liquid crystal molecules at the same time.
In this study, two series of dichiral liquid crystals, namely BCP-C and DCP-C, were synthesised by linking two chiral groups of cholesterol and D (+)-camphoric acid using biphenyl liquid crystal cores.The chemical structures and mesogenic properties of liquid crystal compounds were characterised by common detection methods.By changing the length of the alkoxy carbon chain at the end of camphoric acid and the length of the aliphatic carbon chain at the end of cholesterol, the effects on liquid crystallinity, thermal properties and selective reflection were specifically studied.
Fourier transform infrared spectroscopy (FT-IR) was conducted by a PerkinElmer Spectrum One FT-IR spectrometer (PerkinElmer Instruments, USA).NMR spectra were performed on Bruker ARX 600 MHz or 300 MHz (Bruker, Karlsruhe, Germany) NMR spectrometer, and tetramethylsilane (TMS) was used as an internal standard to report chemical shifts in ppm.Phase transition temperatures and thermal performance data were determined by using a Netzsch DSC 204 (Netzsch, Germany) with a liquid nitrogen cooling system (the heating and cooling rate is 20°C/min).The thermal behaviours of the liquid crystal monomers were analysed using a NETZSCH TGA 209 C thermogravimetric analyser.POM observation of liquid-crystal transitions and optical textures was made using a Leica DMRX (Leica, Wetzlar, Germany) microscope with crossed polarisers and equipped with a Linkam THMSE-600 (Linkam, Surrey, England) hot stage.

Synthesis of liquid crystal compounds
The chemical structures and synthesis routes of the intermediate compounds were described in Scheme 1.The synthesis process of eight liquid crystal compounds in BCP-C and DCP-C series is similar.Here, we choose BCP-C 1 and DCP-C 4 as examples for detailed description.
The first solution was mixed with 2.76 g (0.02 mol) of 4-hydroxybenzoic acid, 8 mL of pyridine, 30 mL of CH 2 Cl 2 and 5 mL of THF.The prepared 4-cholsterooxy-4-oxybutanyl chloride (N 1 ) was dissolved in 30 ml CH 2 Cl 2 , slowly dropped into the first solution and refluxed for 24 h at 65°C.After the reaction was completed, the solvent was removed under reduced pressure.The crude product was washed with hot water and then 4-((4-cholestoxy-4-oxobutyryl) oxy) benzoic acid (BCP) was obtained after recrystallisation from hot ethanol (yield 58%, mp 186°C).
The first solution was prepared by adding 10 g (0.05 mol) of decanedioyl dichloride (1) and 60 mL THF.A second solution was mixed with 7.72 g (0.02 mol) of cholesterol, 5 mL of pyridine and 30 mL of THF.This solution added dropwise to the first solution at 40°C and refluxed for 8 h.After the reaction was completed, the solvent was removed by rotary evaporation under reduced pressure, and the residue was washed with water, dried and filtered.The crude product was washed with hot water and purified by recrystallisation from ethanol to obtain a white powder product (DC) (yield 73%, mp 94°C).

Analysis of the chemical structure of monomers
Two series of novel di-chiral liquid crystal compounds based on D (+)-camphoric acid and cholesterol, namely BCP-C and DCP-C, were designed and synthesised.All di-chiral liquid crystal compounds were characterised by FT-IR, 1 H NMR and 13 C NMR, which were consistent with the expected results.In infrared spectrum analysis, the infrared spectra of representative intermediate C 1 and liquid crystal compound BCP-C 1 are shown in Figure 1.BCP-C 1 was obtained by esterification of C 1 with BCP, and it was found that the hydroxyl stretching absorption band of intermediate C 1 disappeared.At the same time, the ester group stretching band moved from 1726 cm −1 to about 1736 cm −1 as shown in Figure 1 The 1 H NMR spectrum of the representative liquid crystal compound BCP-C 1 and DCP-C 4 is shown in Figures 2 and 3. From the figures, we can see the chemical shifts of protons of two series of monomers, so we take BCP-C 1 as an example.In the 1 H NMR spectra, all proton signals were well assigned to the proposed structure.In particular, the protons of the aryl skeleton showed multiple peaks at 8.41-7.11ppm,indicating that it was the mesomorphic core of BCP-C 1 .Additionally, the signal 5.38ppm corresponds to olefin protons in cholesterol and 4.97-4.13ppmcorresponds to methylene (methylene) protons in the alkoxy part.In addition, 3.30-2.53,2.42-1.30and 1.30-0.80ppmcorrespond to methylene (methylene) protons in the ester group, methylene (methylene) protons and methyl protons in cholesterol and D (+)-camphoric acid.All the characteristic data were by the proposed structures of the compound BCP-C 1 .The position of each peak was consistent with the expected results, which proved that the synthesised monomers accord with the molecular design.

Texture analysis
Liquid crystals were discovered many years ago, and since that time texture analysis by polarising microscope (POM) has been employed as a primary tool for the characterisation of different liquid crystalline phases [26].Throughout the entire observation process, a lot of information can be observed such as melting point,  clearing point, crystallisation point, texture of different LC phases and change of texture and so on.
During the process of observation of the texture, it was found that the synthesised monomers exhibited a stage of no obvious texture before complete flow.This is due to the large molecular weight of the synthesised molecules and poor fluidity and is also due to the existence of dichiral centres, which is not conducive to the molecular arrangement, so we will uniformly set this segment as the S X phase, and we only study the change of liquid crystal phase when monomer full flow.

Analysis of BCP-C series
When BCP-C 1 was heated to 101°C, a characteristic step particle texture of the S A phase gradually appeared (as shown in Figure S2(a)), and heated to 167°C, the sample began to flow quickly, a characteristic focal conic texture of the N* phase appeared, and then turned into an oily streaks texture when pressed (as shown in Figure 4(a)), which is due to changes in molecular arrangement caused by external forces, a typical change process of liquid crystals texture for cholesteric-phase monomers.As the temperature increased gradually, the oily streaks texture gradually disappeared into the pseudo-isotropic phase (as shown in Figure S2(b)), but the cholesteric Grandjean texture appeared soon (as shown in Figure 4(b)), whose background colour changed from red-green-blue before the clearing point.During the cooling process, the focal conic texture first appeared, and then soon came into the changing process for the Grand-jean texture from bluegreen-red, which can be concluded as the cholesteric phase (as shown in Figure S2(c-e)).A characteristic short stick mosaic texture of the S A * phase appeared when the temperature was cooled to 146°C (as shown in Figure 4(c)).Then, there was a gradual transformation into the characteristic broken focal conic texture of the S C * phase (as shown in Figure 4(d)).It can be concluded that BCP-C 1 turned out smectic and cholesteric phases on the heating and cooling cycle.
When BCP-C 2 was heated to 70°C, the sample began to flow and gradually presented a field of view where the blue focal conic and the oily streaks texture coexisted.And the colour gradually turned green with the increase in temperature (as shown in Figure 4(e)) and gradually disappeared at 242°C.The focal conic texture appeared first during cooling, and then the oily streaks texture appeared when pressed (as shown in Figure 4(f)), which can be concluded as cholesteric phase.Then as the temperature decreased, the oily streaks texture and the focal conic texture were maintained and could be transformed into each other.From the texture analysis, it could be seen that BCP-C 2 was in the cholesteric(N*) phase during the heating and cooling process.
When BCP-C 3 was heated, the texture which is unclear as S X phase turned out at the beginning and then the characteristic fine focal conic texture of S A * phase began to appear (as shown in Figure 4(i)) at 150°C and gradually evolved to the striped texture which still belongs to S A * phase (as shown in Figure 4(j)).When the temperature reached 239°C, the oily streaks texture of the cholesteric phase appeared at instant and disappeared quickly.During the cooling process, the characteristic cholesteric focal conic texture first appeared, and after pressing, the oily streaks texture of cholestericphase characteristics appeared (as shown in Figure 4(k)).When the temperature dropped to 201°C, the focal conic texture of the S A * phase appeared, and then gradually turned into the fan-shaped texture (as shown in Figure 4(l)), and striped texture appeared after pressing (as shown in Figure S2(f-g)), which showed that this stage was S A * phase.It can be concluded that BCP-C 3 turned out cholesteric and S A *phase in the heating and cooling cycle.
When the sample BCP-C 4 was heated to 120°C, it gradually softened and spread out to form a characteristic band texture of S A * phase, and then transformed into a fanshaped texture (as shown in Figure 4(g)) until to clearing point.When the temperature dropped to 205°C, the characteristic rod-shaped focal conic texture of S A * phase appeared immediately, and then gradually turned into a fan-shaped texture (as shown in Figure 4(h)).From the above analysis, it could be seen that BCP-C 4 was S A * during the heating and cooling process.
Through the above texture analysis, we can see that with the increase of carbon chain in camphoric acid, the liquid crystals phase of the BCP-C series is prone to appear smectic phase, which is due to the increased intermolecular force and the enhanced ability to arrange into layers after the increase of carbon chain.

Analysis of DCP-C series
Similarly, we study the texture of the DCP-C series carefully.
When the temperature of DCP-C 1 increased, the colourful texture started to appear which is unclear as S X phase, and heated to 70°C, the oily streaks texture with red background gradually turned out, and the coexistence of focal conic and oily streaks texture began to appear as the sample flows.During cooling, a focal conic texture appeared first and then changed into an oily streaks texture when applied pressure to the sample.Further cooling, the colour change of oil silk texture was quite abundant, which was red-orange-yellow-blue and kept until room temperature and without obvious crystallisation process.Throughout the analysis, it can be concluded that DCP-C 1 was cholesteric phases (N*) in heating and cooling processes.The characteristic texture diagram is shown in Figure 5(a,b).
When DCP-C 2 was heated to flow, the oil streaks texture gradually appeared with the increase of temperature and the characteristic cholesteric focal conic texture and oily streak texture can be seen (as shown in Figure 5  (c-d)), at the same time and the background colour of texture changed to blue-yellow-orange-red-green.During the cooling process, a focal conic texture appeared first and gradually transformed into an oily streaks texture and remained unchanged all the time and without an obvious crystallisation process.The background colour change order of oil streaks texture was green-red-green-blue.It can be concluded that DCP-C 2 belongs to the cholesteric phase.The characteristic texture diagram is shown in Figure S3(a-d).
For sample DCP-C 3 , during the heating process, although there were two texture changes (at temperatures of 90°C and 130°C), the texture was not obvious, and it was temporarily named as S X , S X 1, S X 2 phases.When the temperature rose to 152°C, the liquid crystal texture gradually transformed into an oil streak texture, and the background colour showed a blue shift with increasing temperature (as shown in Figure S3(d-f).During cooling down, a small focal cone texture immediately appears, which then transforms into an oil silk texture (as shown in Figure 5(e)), causing a red shift in the background colour.When the temperature dropped to 150°C, the texture gradually transformed into a fan-shaped texture (S A *) and finally into a banded texture (as shown in Figure 5(f)) without an obvious crystallisation process, indicating that DCP-C 3 is a cholesteric-smectic phase.
The texture changes of DCP-C 4 are similar to those of DCP-C 3 in the heating cycle, which have S X , S X 1 and S X 2 phases at the beginning.Continuing to heat up to 162°C, the sample started to appear focal conic and oily streaks texture of the cholesteric phase (as shown in Figure 5(g)).During cooling, the rare blue-phase texture appeared at 180°C (as shown in Figure 5(h)) and then the oil steak texture of the cholesteric phase gradually appeared.When cooling down to 174°C, the coloured pattern texture appeared, which turned into an oil streaks texture with pressure applied to the sample.Upon cooling to 153°C, a fan-shaped focal conic and the coloured band texture of the S A * phase were formed (as shown in Figure S3(g-h)) and remained unchanged without an obvious crystallisation process.
During the heating process, there are many changes in phase texture due to the entanglement and different arrangement of molecules after the tail carbon chain reaches a certain length, resulting in more changes in phase texture.After heating up to the clear point, the molecules are well arranged, so when cooling down, only the cholesteric phase and chiral S A phase appear at the corresponding temperature.
Comparing the BCP-C with the DCP-C series, it can be found that the DCP-C series was more prone to forming cholesteric liquid crystals.Attributing to the reduced steric hindrance caused by the lengthening of flexible chains in the middle part of the molecule, the chiral groups at the two ends affect each other less and are more conducive to the arrangement of the molecules.
In the study, it was found that the characteristic texture could not form at lower temperatures during the heating process, and the characteristic texture only turned out after the temperature increased and flowed, which was mainly due to the large molecular weight and the large steric hindrance between chiral groups at both ends, the molecules are not arranged fast enough at the beginning.In the cooling process, the characteristic texture was formed rapidly and distinctly due to the molecules being better oriented and arranged through the melting state, but no obvious crystallisation process could be observed.

Thermal analysis
The thermal stability of a compound is very important in evaluating its working temperature limit.We investigated the thermal stability of the liquid crystalline compounds by thermogravimetric methods.Representative TG curves are shown in Figure 6, the temperatures of BCP-C 2 and DCP-C 4 at 5% weight loss were 298°C and 314°C, respectively, which were much higher than the clear transition temperature in their respective series.The first derivative curves showed that representative samples had maximum decomposition rate temperatures between 343°C and 358°C.Therefore, all liquid crystal monomers maintained good thermal stability before being in an isotropic state.
The thermal properties of the novel liquid crystal monomers were examined by differential scanning calorimetry (DSC).Phase transition temperatures of compounds were measured at a rate of 10°C min −1 in the first heating and cooling cycle, representative liquid crystalline compound DSC curves were shown in Figure S4.The DSC curves of each series were relatively similar, we selected two of each series for detailed analysis.The characteristic curves of the BCP-C series are shown in Figure S4(a,b) and the characteristic curves of the DCP-C series are shown in Figure S4(c,d).The newly synthesised liquid crystal monomers are larger in molecular weight, and T g peaks appear.In addition, the chiral groups introduced to both ends of the molecule make the steric hindrance increase, and the molecules are not arranged close enough so that the crystallinity is poor, the crystalline peaks are not obvious.In order to express the process of phase change more clearly, we expressed it in Table 1, which summarises the phase transition temperatures of the liquid crystal monomers obtained in the first heating and cooling cycle.At the same time, we created a trend chart of the data to better reflect its changing process, as shown in Figure 7.
The following analysis results can be obtained from the characteristic DSC curve and Table 1.
For the BCP-C series, BCP-C 1 showed T g was at 47°C and T i was at 240°C, and at the same time there was a transformation of S A phase to N* at 166.7°C and a pseudo isotropic transition at the N* phase interval on the heating cycle.On the cooling curve, isotropic-N* transition at 223°C, N*-S A * phase transition at 146°C, S A *-S C * phase transition at 130°C and unnoticeable crystallisation transition at 40°C.BCP-C 3 showed that T g was at 38°C and T i was at 235°C and at the same time there was transformation from S X -S A * phase occurred at 158°C and from S A *-N* phase occurred at 225°C.On the cooling curve, isotropic-N* phase transition at 220°C, N*-S A * transition at 201°C and unnoticeable crystallisation peak at 47°C.
For the DCP-C series, the results of DCP-C 1 showed that during the first heating cycle, T g was at 25°C and T i was at 174°C.The S X -N* transition occurred at 70°C.On the cooling cycle, isotropic-N* transition at 168°C and unnoticeable crystallisation transition at 40°C.The results of DCP-C 3 showed that during the first heating cycle, T g was at 23°C and T i was at 177°C.At the same time, there was a transformation from S X , S X 1, S X 2, S A * phases that occurred at 97°C, 128°C, 146°C and 174°C   separately.The cooling curve showed isotropic-N* phase transition at 174°C, N*-S A * transition at 150°C and unnoticeable crystallisation peak at 47°C.
As shown in Table 1, the T g values of BCP-C are slightly higher than the DCP-C series and the T i values of BCP-C are much higher than that of the DCP-C series.The main factors influencing the thermal properties of liquid crystals are ascribed to molecular structure, molecular weight and interaction between molecules.The size of the T g value reflects the ability of molecules to move.Comparing BCP-C with DCP-C, the BCP-C molecular chain is slightly hard, so T g values are slightly higher, but the difference in the mobility of the two series of molecules is not significant, so the T g values are not much different.The size of T i values depends on the molecular orientation ability, the molecular chain of BCP is harder than that of DCP, so the molecules are more easily oriented to maintain their liquid crystalline state and the T i values of BCP-C are much higher than that of DCP-C series.

Analysis of selective reflectance performance
The cholesteric phase, due to its helical arrangement of molecules, exhibits some unique optical properties.The most striking is the so-called selective reflection, where circularly polarised light of a specific handedness and wavelength is reflected.This phenomenon can readily be observed by the naked eye as an iridescent colour play when the pitch of the cholesteric structure is in the range of the visible spectrum [26].The wavelength of selective reflection of light λ obeyed the Bragg condition: where n is the refractive index, θ is the angle of incidence of the beam, and P is the pitch length of the CLC, which is defined as the distance of a complete 2π rotation of the director of the CLC.When the pitch P changes with different external conditions, such as temperature, electromagnetic field and force field, the change of the colour of the selective reflection can be observed, so the CLC can be used as an optical detector for external stimulation.As the temperature rises, the helical structure of CLCPs distorts, altering the pitch and resulting in a shift of wavelength.This shift corresponds to a visible colour change, either red or blue.
The selective reflection of the BCP-C and DCP-C series are shown in Figure 8. BCP-C 1 was placed in the middle of two glass sheets (named liquid crystal glass) and heated to observe its colour change.After heating to 180°C, the liquid crystal glass exhibited bright and abundant colour change, and the change order was red-yellow-green-blue-purple, which changed from long wave to short wave, i.e. blue shift.During the cooling process, the abundant colour change order was blue-green-yellow-red-green-blue, which changed from short wave to long wave, i.e. red shift.When cooling to around 140°C the colour changes from red to blue, which conforms to the pitch variation law of the chiral S C phase, on the other hand, this is consistent with the phase transition conclusion of polarised texture analysis.
Compound BCP-C 2 exhibited visible selective reflection when heated to 140°C.The heating and cooling cycle causes a colour change from red to blue and blue to red.These colour changes correspond to observed pitch changes, specifically a decrease in pitch with increasing temperature and vice versa.
For both BCP-C 3 and BCP-C 4 , no selective reflection colour was observed, due to BCP-C 3 and BCP-C 4 were dominated in the smectic phase.
When DCP-C 1 was heated to 90°C, only redyellow selective reflection was observed, and only yellow-green appeared during the cooling process and remained at room temperature.DCP-C 2 , during both heating and cooling, abundant and vivid colour changes were observed, and the change order was yellow-green-blue-purple in heating and greenyellow-red in cooling, and the colour was maintained until room temperature.DCP-C 3 was observed to change from yellow-green to blue-purple during heating up and from blue-green to red-yellow during cooling.Only blue-purple selective reflections were observed in DCP-C 4 during heating, and the colour changes were rich during cooling, with the order of change being blue-green-yellow-green-orange.
Through the above analysis, we can see that the performance of selective reflection is completely consistent with the phase results of polarised texture analysis, indicating that our analysis results are in line with the actual situation.

Conclusion
In this paper, we synthesised two series of di-chiral liquid crystal compounds (BCP-C and DCP-C) with cholesterol and camphoric acid as chiral centres and studied their liquid crystal and optical properties at different lengths of flexible chains.The chemical structures of all the compounds were characterised by IR, 1 H NMR and 13 C NMR, all conforming to the expected molecular design.The results of the thermogravimetric analysis showed that the temperature of 5% weight loss of the compounds was around 300°C, indicating that the synthesised compounds had good thermal stability.In general, compounds BCP-C (n = 2, 3) and DCP-C mainly showed an oil streak texture in both heating and cooling cycles, however, compound BCP-C (n = 5, 7) mainly showed a smectic focal cone texture in both heating and cooling cycles.In the BCP-C series, with the increase of the length of the alkoxy group at the end of D (+)-camphoric acid, the corresponding temperature of T i decreased and the N* range decreased.In the DCP-C series, the corresponding T i raised gradually and the N* range decreased.BCP-C (n = 2,3) and DCP-C in the N* phase revealed thermochromism due to selective reflection of visible light.However, no thermochromism was observed in BCP-C (n = 5,7).In addition, the S C * phase in BCP-C 1 turned out thermochromism.Cholesterol and D (+)-camphoric acid with high chirality and high torque can be simultaneously introduced into the molecule to prepare cholesteric liquid crystals.From the above studies, we can see that the introduction of the di-chiral centres makes the liquid crystal state and liquid crystal texture more colourful, and the optical properties are also excellent.The chiral liquid crystal monomers are expected to have potential application value in anti-counterfeiting and optical devices.
(a).Similarly, DCP-C 4 was obtained by esterification between C 4 and DCP.As shown in Figure 1(b), the hydroxyl stretching absorption band of intermediate C 4 disappeared, and the ester group stretching absorption band moved from 1726 cm −1 to 1736 cm −1 .

Figure 4 .
Figure 4. (Colour online) Optical texture of BCPC series: (a) oily streak texture of cholesteric phase.(b) Grandjean green texture of cholesteric phase.(c) rod-shaped texture of S A * phase.(d) broken focal conic texture of S C * phase.(e) tiny oily streak texture of cholesteric phase.(f) oily streak texture of cholesteric phase.(g) graded texture of S A *phase.(h) striped texture of S A * phase.(i) oily streak texture of cholesteric phase.(j) fan-shaped texture of S A *phase.(k) fan-shaped texture of S A * phase.(l) focal conic texture of S A * phase.

Figure 7 .
Figure 7. (Colour online) Phase diagram of BCP-C and DCP-C series.

Scheme 1 .
Synthesis of the intermediates and liquid crystal compounds.

Table 1 .
Phase transitions and thermodynamic data for liquid crystal compounds.