Novel benzoxazole-based liquid crystals with negative dielectric anisotropy and moderate optical anisotropy

ABSTRACT A series of new liquid crystal compounds with negative dielectric anisotropy was synthesised by regulating the orientation of the substituents on the benzoxazole ring, namely, 5–4-(2-(4-(alkoxy)phenyl)-2,3-difluorophenyl)benzo[d]oxazole derivatives (nPP(2,3)FO). These compounds mainly display enantiotropic nematic mesophases in the ranges of 8.62–16.09°C (heating process) and 23.98–30.32°C (cooling process), moderate birefringence (Δn) ranging from 0.25–0.32 (DFT theoretical calculation) and 0.22–0.28 (experimental measurement), as well as negative dielectric anisotropy (Δɛ) ranging from −2.77 to −2.94. These characterises are ascribed to the larger perpendicular dipole moments and orientation angles of benzoxazole. This work provides new ideas for the rational design of negative dielectric anisotropy liquid crystals. Graphical abstract


Introduction
Fringe-field switch mode liquid crystal displays (FFS-LCDs) have been extensively used for projection, directview displays, and automotive applications owing to the wide viewing angle and high resolution [1,2].In FFS-LCDs, a low driving voltage can reduce power consumption and decrease graphic display response time.In order to achieve high transmittance and lower operation voltages, the used liquid crystals need to possess negative dielectric anisotropy (∆ε=ε // -ε ⊥ <0), ensuring their fast rotation from vertical alignment to parallel alignment driven by the external electric field [3,4].Generally, the driving voltage applied by the external electric field is subject to the liquid crystal' ∆ε and proportional to the (−∆ε) 1/2 , therefore liquid crystals with highly negative −∆ε are desired [5].Over the years, many researchers have devoted themselves to developing new LC molecules with highly negative ∆ε.
Based on the Maier-Meier equation of [6,7], the Δε is primarily dependent on the dipole moment (μ) and the dipole moment orientation angle (β) between the long principle axis and the molecular frame [6].To obtain negative ∆ε, the dipole moments in the molecular lateral axis must be larger than those in the molecular long axis.This means that strong polar moieties must be placed in the lateral position of the liquid crystal molecule and are perpendicular to the molecular long axis [8].For the molecular backbones containing cyclohexane ring, some researchers introduced F or CN as side groups of the cyclohexane ring to realise a negative ∆ε.However, these lateral F groups on cyclohexane are unstable and tend to shed to form HF, and the lateral CN group usually results in high viscosity, being bad for practical application [9,10].On the other hand, placing strong electron-withdrawing groups on the lateral side of the aromatic ring of the molecule backbone is also an effective method to construct liquid crystals with negative ∆ε [11,12].Nevertheless, too many lateral groups bring the risk of mesophase disappearance due to the shortened molecular aspect ratio.However, these two methods work for the molecule skeletons containing biphenyl, triphenylene, or cyclohexane.Therefore, introducing difluoride-substituted groups at the orthoand para-position of the benzene ring is commonly used for commercial liquid crystal compounds to achieve satisfied mesomorphic behaviour and resistivity, although the negativity of ∆ε is not ideal [13,14].Apart from these mentioned groups, heterocyclic structures such as dibenzofuran and dibenzothiophene possess large polarity owing to the electron-rich characteristic of heteroatoms of N or S, and show the potential ability to construct liquid crystals with negative ∆ε, if it can effectively control their spatial configuration of molecular backbone.
In recent years, some exploratory researches have been done.For example, Volker Reiffenrath et al. constructed some novel liquid crystal molecules by introducing nitrogen heterocycles including pyridine [15] or pyridazine [16] rings, and one of these compounds showed a highly negative Δε of −6.However, such molecules had an unstable N=N group which decomposes easily under UV radiation.Gotoh et al. constructed negative liquid crystal molecules by introducing oxygen-containing heterocycles, such as tetrahydrofuran derivatives [17][18][19][20].These molecules have highly negative Δε up to −6.7, but narrow mesophase temperature range.Thomas et al. used 1,3-thiazole or 1,3,4-thiadiazole to construct the molecule's long axis, simultaneously forming a large dipole moment laterally in the molecule's short axis.The obtained liquid crystals showed a negative Δɛ of −3 [21].Unluckily, excessive π-conjugate segments in the molecular backbone caused a poor solubility in mixed liquid crystals and are not suitable for display applications.Moreover, the synthesis routes of these liquid crystals are very complex and costly.
In this work, we combined benzoxazole and fluorine groups to construct novel liquid crystal with strongly negative Δɛ, based on the electron-rich property of benzoxazole skeleton reported in our previous studies [22,23] (Figure 1), showed nematic mesophase, as well as moderate Δn and negative Δɛ.

Materials and reagents
The used solvents and reagents in this work were purchased from Aladdin-reagent Co. or Sinopharm Chemical Reagent Co., China.Before use, chloroform needed to be dehydrated by pre-dried 4 Å molecular sieves, tetrahydrofuran (THF) needed to be dried with sodium.Aryl boronic acids are difficult to obtain pure because of the susceptibility of anhydride formation and they were used without purification in the Suzuki-coupling reactions.LC host mixture (HNG-100) was purchased from Jiangsu Hecheng New Materials Co., Ltd.(China).
The phase transition temperatures and enthalpy data of liquid crystal compounds were evaluated by differential scanning calorimetry (DSC) (DSC-60, Shimadzu Corporation, Kyoto, Japan) under the nitrogen atmosphere at heating and cooling rates of 5°C/min, and the phase transitions were examined by polarising optical microscopy (POM) (Changfang XPN-300E) equipped with a heating stage and a control unit at a heating rate and cooling rate of 1°C/min.
The experimental Δn values of the compounds were measured at 25°C by Abbe refractometer (Atago NAR-4 T, Japan) at 589 nm in the host mixture 5CB with 10 wt% by a guest-host method [23,25].
Since all the target liquid crystal compounds we synthesised are solid and cannot be tested directly, we dissolved them in a commercial liquid crystal mixture of HNG-100 (Tcl = 104.8°C,Δε = −4.0)with a mass ratio of 10wt%, the Δε values of these synthesised liquid crystal compounds were measured at 25°C by an LCR measurement system (HIOKI 9262 test fixture, Japan) with a frequency of 1 kHz by a guest-host method [26].

Mesomorphic properties
Mesomorphic properties of nPP(2,3)FO compounds and the reference samples of 6PP(2,3)FP and 6PP(2,3) FB were investigated by DSC and POM.The corresponding phase transition temperatures (onset temperature of endothermic/exothermic peaks) and associated enthalpy changes were shown in Table 1. Figure 2 plots the transition temperatures of nPP(2,3)FO compounds as a function of the carbon number (n) of alkoxy chain both in the heating and cooling processes, while POM photomicrographs and DSC curves of 6PP(2,3)FO and 6PP(2,3)FB are shown in Figure 3.
The data in Table 1 shows that all of nPP(2,3)FO exhibit nematic phase and narrow mesophase temperature range of 8.62-16.09°Cand 23.98-30.32°Con heating and cooling process, respectively.The small nematic-isotropic enthalpy changes are the characteristic of nematic liquid crystal, due to its low order parameters relative to smectic phase [27][28][29].Figure 2 shows that the length of the alkoxy chain (n = 5-10) had no significant influence on the mesophase type and the mesomorphic temperature ranges for the nPP(2,3)FO series.
In order to investigate the effect of benzoxazole group on the mesophase, the text and thermal behaviour of 6PP(2,3)FO and 6PP(2,3)FB were compared.POM images (Figure 3) show that both 6PP(2,3)FO and 6PP (2,3)FB display typical nematic threaded and schlieren textures on heating and cooling processes.The DSC curves (Figure 3(a,d)) demonstrate that 6PP(2,3)FO has relative lower melting point and clear point (107.91 and 124.00°C) than that of 6PP(2,3)FB (115.33 and 136.35°C), moreover, the corresponding enthalpy changes (49.70 and 0.63 kJ mol −1 ) in these two processes are also lower than that of 6PP(2,3)FB (60.10 and 0.74 kJ mol −1 ), indicating the difference in spatial arrangement of benzoxazole group at molecular backbone has some effect on mesophase but the effect is not very significant.Further, the melting point of 6PP(2,3)FO is nearly equal to that of end group phenyl-contained 6PP(2,3)FP, but higher than that of 6PP(2,3)FB.It is well known that intermolecular force is responsible for the phase transition temperature, and weaker intermolecular force causes lower melting point [29,30].For these liquid crystals with aromatic skeleton, the π-π stacking interaction between the liquid crystal molecules is one of the most important intermolecular forces, which usually varies with molecular spatial arrangement.When dihedral angle (θ) exists in molecule, which can reduce the polarisability of π-conjugated system, weakening π-π stacking interaction and lowering the melting point.The DFT theoretical simulation results (Table 2) demonstrate that there are two θ between the two rigid planes for these three compounds; moreover, the 6PP(2,3)FO and 6PP(2,3)FP have nearly same value of θ, indicating their similar π-π stacking interaction.Therefore, their melting points are very close.However, although the first θ of 6PP(2,3)FB is almost equal to that of 6PP(2,3)FO and 6PP(2,3) FP, the second one between benzene and benzoxazole planes is close to 0°, suggesting its stronger π-π stacking interaction than that of 6PP(2,3)FO and 6PP(2,3)FP, which causes that the melting point of 6PP(2,3)FB is highest among these three compounds.Meanwhile, 6PP(2,3)FP exhibits no mesophase at all.A possible reason is that its molecular length to width ratio (L/W = 3.61) is lowest among the three analogs, being unsatisfied with the formation condition of nematic phase.

Thermal stability
The thermal stability of all nPP(2,3)FO derivatives were measured using thermal gravimetric analysis (TGA) (Figure 4).The decomposition temperatures of nPP(2,3) FO are over 260°C under N 2 atmosphere, which are higher than their clearing points in Table 1, indicating their excellent thermal stability above isotropic temperatures.Figure 4(a) shows that all these compounds completely decompose above 380°C, probably due to the instability of the C=N bond in the benzoxazole ring.Figure 4(b) shows that the T 5% values (i.e.temperature at which 5% weight loss occurred) of the nPP(2,3)FO have an increase tendency with the alkoxy chain increasing, indicating the longer alkoxy chain is beneficial to thermal stability.

Optical anisotropy
In this equation, where n e and n o are extraordinary and ordinary refractive indices, respectively.N is the molecular number density, ε 0 is the vacuum permittivity, Δα is polarisability anisotropy, α is the average polarisability.S is Saupe orientation order parameter.
In addition, the data in Table 3 also show that Δn values decrease as the alkoxy chain increases.This phenomenon may be due to the fact that the ascending of alkoxy chain length will enhance the entanglement of the alkoxy chain, which will reduce the order parameter of S, and therefore cause a small Δn according to the Vuks equation [35].

Dielectric anisotropy
The Δε value of LCs is defined as the difference between the dielectric constants parallel and perpendicular to the molecular axis (Δε = ε ∥ -ε ⊥ ), and can correlate with other parameters according to Maier-Meier equation [6,7]: There are eight required parameters (N, F, h, Δα, μ, T, β, S) for determining the Δε.Here, N, ε 0 , Δα, and S are the same in Equation 1 and Equation 2. k b is the Boltzmann constant, T is absolute temperature, μ is dipole moment, and β is the dipole moment orientation angle between the long principle axis and molecular frame.For the strongly polar LC compounds which has a very small Δα, when calculating Δε with Equation (3), Δα can be ignored and a simplified relation of Δε with μ and β, Δε / À μ 2 1 À 3cos 2 β ð Þ, is commonly used [18,36].In this relation, Δε is roughly proportional to μ and β.The benzoxazole-based LC derivatives are suitable for this simplified relation due to their strong polarity; therefore, their structure-property relationship can be discussed using it.The relation demonstrates that, when β = 54.7°,3cos 2 β is close to 1, making Δε nearly equal to zero; when β < 55°, 1−3cos 2 β is negative, corresponding to a liquid crystal with positive Δε; when β > 54.7°, 1−3cos 2 β is positive, implying a liquid crystal with negative Δε.The Δε value of liquid crystal compound can be extrapolated using concentration in weight ratio and listed in Table 4.In Table 4, all the newly synthesised nPP(2,3) FO exhibit negative Δε ranging from −2.77 to −2.94, due to their β > 54.7°.Moreover, the Δε values of nPP(2,3)FO were almost unchanged, since μ and β of the molecules do not change much as the alkoxy chain increases.
When comparing 6PP(2,3)FO with 6PP(2,3)FP and 6PP(2,3)FB, it is found that the Δε value is affected by the type and spatial configuration of terminal skeletons.First, 6PP(2,3)FP shows a lower negative dielectric anisotropy (∆ε = -2.23)than that of 6PP (2,3)FO (∆ε = -2.77),which is due to the lateral positioned benzoxazole skeleton increase the perpendicular dipole moment of the molecules.However, 6PP (2,3)FB exhibits a positive dielectric anisotropy (∆ε = 0.38) due to its β = 40.01°follows by Maier-Meier equation.This phenomenon can also be explained from the perspective of molecular structure.The large electron-absorbing benzoxazole skeleton in the longaxis direction counteracts the dipole moment generated by the 2,3-difluorine atom in the perpendicular axis, giving the small and positive ∆ε.This remarkable agreement between theoretical and experimental results further attests to the validity of our design idea of constructing negative Δε liquid crystal.

Conclusions
By precisely manipulating the orientation of substituents on the benzoxazole ring, a series of novel liquid crystal compounds with negative dielectric anisotropy have been successfully synthesised.These compounds, referred to as 5-4-(2-(4-(alkoxy)phenyl)-2,3-difluorophenyl) benzo[d]oxazole derivatives (nPP(2,3)FO), predominantly exhibit enantiotropic nematic mesophases and moderate birefringence (Δn) values ranging from 0.25 to 0.32 according to DFT theoretical calculations, and experimentally measured values between 0.22 and 0.28, as well as negative dielectric anisotropy (Δɛ) ranging from −2.77 to −2.94.These characteristics can be attributed to the larger perpendicular dipole moments and orientation angles of the benzoxazole structure.The findings of this study pave the way for the rational design of liquid crystals with negative dielectric anisotropy.

Figure 2 .
Figure 2. (Colour online) Transition behaviour of nPP(2,3)FO: dependence of the transition temperatures on carbon number(n) of the alkoxy chain on (a) heating process and (b) cooling process.

Table 3 ,
it is noted that the Δn cal and Δn exp are very close
b c Measured by experiment.

Table 4 .
Parameters from theoretical calculations and experimental dielectric anisotropy values of ∆ε in this study.
a Dipole moment (μ) determined by DFT calculations at the B3LYP/6-311++G(d,p) level of theory.b Angle between μ x and net μ t .c Measured by experiment.