Short chiral pitch and blue phase stability in cholesteric liquid crystal mixtures by adding achiral benzoic acid derivative

ABSTRACT Cholesteric liquid crystal (CLC) is anticipated to have applications such as blue light and ultraviolet (UV) light blocking for human skin owing to its selective wavelength reflection against incident sunlight through several hundred nanometres of a periodic helical twist structure. In this study, we investigated short chiral pitch control and enlargement of the blue phase (BP) temperature range by adding an achiral constituent of 4-n-octylbenzoic acid (8 BA) into CLC mixtures composed of three kinds of cholesteryl compounds: cholesteryl chloride, cholesteryl pelargonate and cholesteryl palmitate. Hydrogen bonding dependence of chiral pitch and BP temperature range were also investigated for achiral dopant (8 BA) and CLC mixture through temperature-dependent Fourier-transform infrared analysis. It was found that 40 wt% of 8 BA within the CLC mixture selectively reflected UV and blue light over 360–400 nm with a decrease of chiral pitch. It was also found that the BP temperature range increased by 15.6°C owing to hydrogen bond dimers composed of 8 BA molecules in the CLC mixture. GRAPHICAL ABSTRACT


Introduction
Liquid crystal (LC) is a mesophase of matter characterised by anisotropic properties between a solid crystal and an isotropic liquid phase.LC is usually classified into nematic, smectic and cholesteric phases depending on its aggregation structure.Among these LCs, cholesteric LC has attracted much attention for applications in new optical and display devices because of its selective wavelength reflection owing to its chirality.There are several ways to induce chirality of LC: 1) by directly incorporating chiral centres into LC molecules through a chemical synthetic process, 2) by adding a sufficient amount of chiral dopant into achiral nematic LC hosts and 3) chirality also can be given to LCs by bent molecules.Bent molecules can form a liquid crystalline phase with a helical structure [1].In some cases, bent molecules exist as supermolecules formed by hydrogen bonding [2].
The chirality of cholesteric liquid crystal (CLC) is usually specified by chiral pitch, the distance over which LC molecules undergo a full 360° twist.This chiral pitch (P) typically changes when the temperature is changed or when other molecules are added to the LC host.It corresponds to selective wavelength reflection against incident light based on Bragg's reflection law.For example, a CLC can approximately reflect ultraviolet (UV) light (λ = 280-380 nm) when the chiral pitch ranges from 187 to 253 CONTACT Sung-Kyu Hong hsk5457@dongguk.eduSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2023.2268041.nm, while it reflects visible light (λ = 380-780 nm) when the chiral pitch ranges from 253 to 520 nm on the basis of λ = nP (if n (refractive index) = 1.5) at a normal incident light.Therefore, controlling chiral pitch in a CLC is particularly important to determine the application field of an optical device.It can also have cosmetic applications for colour make-up [3] and UV blocking [4].
Moreover, as a special case of CLC, the blue phase (BP) can appear in a temperature range between the cholesteric phase and isotropic liquid phase under a high chirality (eq.short pitch).BP consists of a doubletwist cylinder.It exhibits blue phase I (BP I), blue phase II (BP II) and blue phase III (BP III) as functions of temperature and chirality [5][6][7].These BPs have great potential as fast light modulators due to electrically controllable Bragg diffraction of visible light [8][9][10][11][12][13][14][15].However, due to their narrow temperature ranges of less than a few degrees, their practical applications have been limited.By far, BP I stabilisation using a polymer-stabilised BP [16] or a symmetric dimer mixture with highly twisting chiral dopant [17], BP II stabilisation using hydrogen bond self-assembly between a chiral donor and an achiral acceptor [18] and BP III stabilisation using a T-shape bent-core chiral LC [19] and a nanoparticle [20] have been reported, with consequential enlargement of BP temperature ranges.In general, BP is only generated under high chirality.In other words, very short chiral pitch is accompanied by a sufficient amount of high helical twisting power (HTP) chiral dopant to host LC in a host-guest CLC system.However, it is not easy.
Therefore, decreasing the chiral pitch of a CLC is important to induce and stabilise BP in the CLC over a wider temperature range.Furthermore, decreasing chiral pitch in the CLC phase can provide new functions as a UV and blue light blocking agent.
In this study, we investigated the control of short chiral pitch and enlargement of BP stability of CLC mixtures composed of three kinds of cholesteryl compounds, cholesteryl chloride (CC), cholesteryl pelargonate (CPE) and cholesteryl palmitate (CPA), by adding achiral benzoic acid derivative of 4-n-octylbenzoic acid (8 BA) instead of high HTP chiral dopant.The relationship of the chiral pitch with BP temperature range and hydrogen bonding intermolecular interactions of achiral dopant (8 BA) with three CLC mixtures were investigated using Fourier-transform infrared (FT-IR) spectroscopy analysis at various temperatures.

Preparation of samples
Three kinds of CLC mixtures, CC, CPE and CPA, were used as host CLCs.They are non-toxic compounds available for cosmetic applications.To control the chiral pitch of the CLC mixture composed of the three CLC mixtures, 8 BA was used as an achiral constituent as shown in Figure 1.Then 4-n-octylbenzoic acid (8 BA) was added to the host LC mixture in three different concentrations as shown in Table 1.

Evaluation of phase transition behaviours of three types of CLC mixtures
Each CLC mixture was filled to the vacant space of a 10 μm gap sandwich cell without surface treatment.Phase transition temperature was evaluated based on the intrinsic optical texture observed with a polarised optical microscope (POM) equipped with a hot stage calibrated to have an accuracy of ±0.1°C using indium (LTS420, Linkam, UK) under crossed Nicols.The temperature accuracy of the device was confirmed using several compounds with known melting points.The compounds such as phenyl salicylate (melting point: 41.8 ± 0.2°C), vanillin (melting point: 81.7 ± 0.2°C) and benzoic acid (melting point: 122.4 ± 0.2°C) were purchased from Mettler Toledo.These three compounds are certified melting point reference standard substances.The cooling and heating rate of the cell were set to 1.0°C/min.The chiral pitch was indirectly characterised by reflection spectrometry based on their lattice constants of several hundred nanometres derived from Bragg reflections of circularly polarised light on cooling and heating.UV-Vis reflection spectra of sandwich cells were measured using a UV-Vis spectrometer (MSV-350, JASCO International Co., Japan) after holding the sample for 10 min at each temperature to prevent thermal hysteresis.

FT-IR analysis for three types of CLC mixtures
FT-IR analysis was carried out to investigate molecular interactions (e.g.hydrogen bonding) of the three kinds of CLC single compounds with an achiral dopant 8 BA in three types of CLC mixtures using an FT-IR spectrometer (Varian 3100 FT-IR Excalibur Series, Agilent Technologies, USA) in absorbance mode.Figure 2 shows an FT-IR setup based on specular reflection using a mirror system to use a hot stage for controlling the temperature of the CLC mixture.Two potassium bromide disks with a diameter of 23 mm and a thickness of 2 mm were laid as a sandwich type on the hot stage.Each CLC mixture was injected into the gap between the two disks using capillary force in their isotropic phase.The temperature of the CLC mixture was controlled at a cooling rate of 1°C/min.The light source was SiC, which emitted mid-IR.

Phase transition behaviours of three types of CLC mixtures
Table 2 shows phase transition temperatures of three types of CLC mixtures evaluated by POM observation of texture and UV-Vis reflective spectra upon cooling and heating at 1°C/min.
Figure 3 shows phase diagrams of three types of CLC mixtures based on phase transition temperatures listed in Table 2.
As shown in Figure 3 and Table 2, three types of CLC mixtures presented six phases: crystal (K), chiral smectic C (SmC*), chiral nematic (N*), BP I, BP III and isotropic phase (Iso).Their phase transition temperatures increased with increasing concentration of achiral constituent of 8 BA from BA0 to BA40 upon heating and cooling, in contrast to general CLC mixtures which showed decreases of phase transition temperatures with increasing concentration of chiral dopant [21].
Figure 4 shows UV-Vis spectra owing to the selective reflection of three types of CLC mixtures and their corresponding POM observation photographs under crossed Nicols upon cooling from 70°C to 30°C.
All POM observation photographs of all three CLC mixtures showed an oily streak texture in the chiral nematic phase.In BA20 and BA40 mixtures, both BP I and BP III were observed.BP I was confirmed by the      existence of platelet texture, and BP III was confirmed by the existence of texture called 'blue fog' [22].However, for the BA40 mixture, the SmC* phase was observed from 30°C to 52°C.With increasing temperature, UV-Vis spectra showed that reflection peaks of BA0 (Figure 4, left) were greatly shifted to a longer wavelength.In contrast, peaks of BA20 (Figure 4, middle) and BA40 (Figure 4, right) were slightly shifted to a shorter wavelength.Furthermore, their reflective UV-Vis spectrum peaks were shifted to a shorter wavelength with increasing concentration of achiral constituent 8 BA at each temperature.In particular, BA40 selectively reflected UV-A and blue light with wavelengths from 360 nm to 400 nm against a white incident light at a temperature range of 30°C to 70°C.
Here, we need to point out that 8 BA is not a chiral molecule because a chiral centre is not present in its molecular structure.Therefore, we investigated three hypotheses about intermolecular interactions of 8 BA molecules with three kinds of cholesteryl single compounds in CLC mixtures.The first hypothesis was that ester groups (R′-C(=O)-O-R) present in CPA and CPE molecules and carboxylic acid groups (R-COOH) present in 8 BA molecules could form hydrogen bonds with each other.The second hypothesis was that 8 BA molecules could form dimers with each other.Since 8 BA has carboxylic acid that can induce acid dimer due to hydrogen bonding among themselves, it is easy for 8 BA to induce hydrogen-bonded liquid crystals [23,24], in which two benzoic acid moieties of the dimer have a role in the rigid core, while alkyl chains of 8 BA have a role in its flexible nature.The third hypothesis was that 8 BA existed as a monomer without any hydrogen bonding.To figure out the existence of various hydrogen bonding configurations of CLC mixture in each phase, including chiral nematic phase (N*), chiral smectic C phase (SmC*), BPs and isotropic phase (Iso), temperature-dependent FT-IR analysis was carried out.

Hydrogen bonding analysis of three CLC mixtures by temperature-dependent FT-IR
Figure 5 shows FT-IR overlapped spectra of three kinds of CLC mixtures (BA0, BA20 and BA40).These spectra were collected every 1°C upon cooling at 1°C/min with a temperature range of 90°C to 30°C.FT-IR absorption spectra of each sample exhibited similar peaks at 1735 cm −1 due to C=O stretching peaks at ν(C=O) ~ 1735 cm −1 of CPA and CPE, although their intensities were different [25,26].For BA20 and BA40, if oxygen of CPA and CPE carbonyl groups forms hydrogen bonds with the hydrogen of carboxyl acid in 8 BA, peaks of the carbonyl group of CPA and CPE should be shifted to a wavenumber lower than 1735 cm −1 .However, a such phenomenon was not observed for BA20 or BA40 (Figure 5). Figure 6 shows colour contour graphs drawn to observe changes of peaks with temperature change.The red part of this graph shows a strong absorbance intensity, and the purple or blue shows a relatively weak absorbance compared to the red part.Based on the change in colour and contour line of the graph, it could be seen that the absorbance peak at around 1735 cm −1 did not move with temperature change (see dotted line box of Figures 6(d,e,f)).
On the other hand, BA20 and BA40 additionally showed absorption peaks at 1690 cm −1 and 1610 cm −1 (see Figures 5 and 6(b,c)) assigned as stretching of hydrogen-bonded C=O group caused by carboxylic acid dimers [27][28][29].From these results, it could be concluded that 8 BA molecules in BA20 and BA40 samples formed dimers by hydrogen bonding.This conclusion will be examined in more detail in Figure 7.
Figure 7 shows FT-IR spectra of three CLC mixtures at wavenumber ranging from 3700 cm −1 to 2500 cm −1 and temperature at 90°C in their isotropic phase.In the region from 3400 cm −1 to 3000 cm −1 , peak shoulders appeared in CLC mixtures BA20 and BA40 but not or very weakly in BA0.These shoulders are known as carboxyl acid dimers assigned as Fermi A-type band associated with basic stretching vibration of the O-H group in the IR spectrum [23,[30][31][32].Therefore, these shoulders in Figure 7 were considered as another evidence of hydrogen bonding dimers of 8 BA in CLC mixtures, same as the two peaks of 1690 cm −1 and 1610 cm −1 shown in Figure 5.In addition, the strength of this region increased as the amount of 8 BA increased from 20 wt% to 40 wt%.
It was found through FT-IR spectrum analysis that most of the 8 BA present in the mixture of BA20, and BA40 exists in a dimer state due to hydrogen bonding.However, it should not be overlooked that the 8 BA hydrogen bond dimers should be in a state of dynamic equilibrium.Paterson et al. [33] studied the hydrogen bond dimer of 6-(4ʹ-cyanobiphenyl-4-yl)hexyloxybenzoic acid (CB6OBA) which has a functional group, carboxylic acid.Additionally, CB6OBA has a molecular structure similar to 8 BA.According to the previous research, it was found that CB6OBA dimers were in dynamic equilibrium.CB6OBA showed a tendency to vary in the proportions of cyclic dimers, open dimers and free monomers depending on temperature and phase.The relative concentrations of the cyclic dimers, open dimers and the free monomer of CB6OBA were analysed at the semi-quantitative level.Paterson et al. deconvoluted the FT-IR spectrum of a mixture containing CB6OBA into several Gaussian peaks.Among the FT-IR absorption peaks due to carbonyl stretching, the absorption peak near 1680 cm −1 means that CB6OBA exists in a cyclic dimer state, and the absorption peak near 1700 cm −1 indicates that it exists in its open dimer state.The absorption peak near 1730 cm −1 means that it exists in a free monomer.
Similar to CB6OB, the 8 BA hydrogen bond dimer can exist in different states of dynamic equilibrium depending on temperature and LC phase.In Figure 6  (b,c), the absorption peaks of BA20 and BA40 around 1690 cm −1 gradually move to a higher wavenumber as the temperature increases and the phase changes to the low ordered phase.This suggests that as 8 BA goes to higher temperatures and lower ordered phase, the proportion of open dimer can gradually increase in dynamic equilibrium.Another possible configuration of 8 BA in the CLC mixture was that 8 BA did not form any bond with other molecules including hydrogen bonds (3rd hypothesis).If the OH group of 8 BA is a free hydroxyl group, the absorption peak would occur at ~3500 cm −1 due to the ν (OH) stretching [32,34] as shown in Figure S-1 (see supporting information).However, two CLC mixtures (BA20 and BA40) including 8 BA molecules did not have peaks around 3500 cm −1 as shown in Figure 7.It meant that these CLC mixtures did not have 8 BA as a free monomer, or just a little portion of 8 BA free monomer was included within CLC mixtures.
From the results shown above, the following two findings are considered.First, the elevation of the phase transition temperature of the CLC mixtures might be due to an increase of liquid crystalline dimer of 8 BA by hydrogen bonding when 8 BA concentration was increased in three CLC mixtures.Second, the chiral pitch of the CLC mixture was reduced by the increase of hydrogen bonding among 8 BA molecules through a decrease of selective reflection wavelength with an increase in 8 BA concentration (Figure 4).This implies that hydrogen-bonded 8 BA dimer can reduce the chiral pitch of BA20 and BA40.
Previous studies [1,2,[35][36][37] have shown that helical structures can be formed in some cases even if they are not chiral dopants.A typical example is the use of bent core molecules [35,36] (also called banana-shaped molecules).Recently, it has been reported some molecules including hydrogen-bonded dimers have twisted conformation even if they are achiral molecules (or notbent core molecules) [37].Jeong et al. [37] have demonstrated that this phenomenon can occur when two achiral molecules having carboxylic acids (4-biphenylcarboxylic acid molecules connected to alkoxyl chains and terminated with phenyl groups) generate liquid crystalline dimers for which left-handedness and right-handedness might occur racemically.8 BA, an achiral molecule used in this study, can also form a hydrogen-bonded 8 BA dimer.The hydrogenbonded part is flexible, whereas the phenolic part has a planar structure.Therefore, 8 BA dimers might make a helical structure having racemic handedness, whose priority is determined by the surrounding environment.In general, CC is known as a right-handed CLC, whose chiral pitch is greatly increased with increasing temperature [38].In contrast, CPA and CPE are known as left-handed CLCs, whose chiral pitches are decreased with increasing temperature [39,40].As shown in Figure 4, BA0 has a righthandedness characteristic, in which the chiral pitch is directly proportional to temperature, while BA20 and BA40 have an inverse relationship between temperature and pitch.Therefore, 8 BA dimer might play a role in enhancing the left-handedness nature of CPA and CPE with reducing the right-handedness of CC, because amounts of CPA and CPE were much more than those of CC in BA20 and BA40 mixtures.On the other hand, the handedness of BA0 (without 8 BA) might be dominated by CC right-handedness characteristic, because the weight ratio of CC was increased without 8 BA compared to BA20 or BA40 mixture.

BP temperature ranges of three types of CLC mixtures
Figure 8 shows UV-Vis reflection spectra owing to the selective reflection of BA20 and BA40 and their corresponding POM observation photographs under crossed Nicols in BPs upon cooling.During BP I for BA20 and BA40, a platelet structure was observed in the cooling process.The entire BP temperature range was 8.3°C for BA20 and 15.6°C for BA40 in the cooling process.On the other hand, it was 3°C for BA20 and 3.5°C for BA40 in the heating process.In particular, BP I was presented at 7.5°C for BA20 and 14°C for BA40, while BP III was presented at 0.8°C for BA20 and 1.6°C for BA40 upon cooling.These results indicated the following two facts.First, BP temperature ranges were increased with increasing concentration of achiral dopant 8 BA from 20 wt% to 40 wt% (BA0 did not show BPs).Second, the BP I temperature range was mostly enlarged with an increasing concentration of 8 BA compared to BP III upon cooling.
In general, BPs have a three-dimensional cubic structure composed of the double-twist cylinder, in which the LC director rotates spatially about any radius of a cylinder.Line defects (so called disclination) could exist to relieve elastic strain energy, where cylinder directors cannot be matched in cubic structure and stabilise BPs during a few Kelvin.In the case of BP I, disclination occurred along a straight line connecting the face centred position in the lattice (e.q.bccO 8 structure) as presented in Figure S-2 [41,42].While in the case of BP III, this phase did not show a cubic structure but an amorphous structure.Therefore, BPs became unstable when elastic strain energy was increased due to the instability of line defects with decreasing temperature upon a cooling process, resulting in phase transition to chiral nematic phase at limited temperature.Previous studies have reported that when some additives are introduced into the BP system, locations within the defect of BPs can reduce the overall free energy of an LC [16,18,43,44].
From these facts, it is considered that achiral 8 BA molecules can generate a linear dimer softly incorporated by hydrogen bonding.The linear dimer can then diffuse into the space between double twist cylinders not matched in cubic structure and cause relieving of elastic strain energy in a BP system.Therefore, for BA20 and BA40, BP might be maintained in a longer temperature range than CLC without 8 BA dimers.Furthermore, BP I is more stabilised than BP III due to its temperature range.Namely, liquid crystalline dimers of 8 BA are more effective for stabilising the disclination of BP I than disclination of BP III.Furthermore, this tendency was stronger with an increased concentration of 8 BA.In contrast, upon a heating process, soft dimers of 8 BA very slightly contributed to the reduction in elastic strain energy, since thermal fluctuation of molecules by increasing temperature could offset this effect.As a result, BP temperature ranges were increased very little compared to those upon a cooling process.

Conclusions
It was confirmed that the chiral pitch was decreased with increasing concentration of 8 BA and that a hydrogen bonding dimer was generated among carboxylic acid of achiral 8 BA constituent in CLC mixture through FT-IR analysis.It was also confirmed that BP temperature ranges enlarged to 15.6°C by adding achiral dopant 8 BA to make hydrogen-bonded liquid crystalline dimers in a CLC mixture as 8 BA concentration increased by 40 wt%.

Figure 1 .
Figure 1.Chemical structures of three kinds of host cholesteric liquid crystals and an achiral dopant were used in this study.

Figure 2 .
Figure 2. (Colour online) Schematic representation of FT-IR analysis set up for each CLC mixture.KBr: potassium bromide.
b SmC*: chiral smectic C phase.c N*: chiral nematic phase.d BP I, BP II and BP III: blue phase I, blue phase II and blue phase III.

Figure 3 .
Figure 3. Phase diagrams for three types of cholesteric liquid crystal (CLC) mixture upon heating (a) and cooling (b).

Figure 8 .
Figure 8. (Colour online) Blue phase UV-Vis reflection spectra of CLC mixtures and their corresponding polarised optical microscope observation photographs under crossed Nicols on cooling.BA20 is shown in the left and BA40 is shown in the right (BA20: 20 wt% of 4-n-octylbenzoic acid in the mixture, BA40: 40 wt% of 4-n-octylbenzoic acid in the mixture).

Table 1 .
Chemical compositions of three types of CLC mixture.

Table 2 .
Phase transition temperatures of CLC mixtures upon cooling and heating.
a K: crystal phase.