Synthesis and properties of biphenyl liquid crystal diluters terminated by 2,2-difluorovinyloxyl for high birefringence liquid crystals

ABSTRACT A series of liquid crystal (LC) diluters containing a rigid biphenyl core with a lateral fluorine substituent, a 2,2-difluorovinyloxyl terminal group and a flexible n-alkyl chain were designed and synthesised through classical organic synthetic reactions. Their chemical structures were characterised and confirmed by traditional methods. Meanwhile, the effects of alkyl-chain length, terminal group and lateral fluorine substituent on the melting point (Tm), birefringence (Δn), dielectric anisotropy (Δε) and rotational viscosity (γ1) of LC diluters were investigated. Density functional theory (DFT) calculations were employed to obtain the molecular polarisability, dipole moment, frontier orbitals, biphenyl dihedral angle and aspect ratio. Furthermore, their comprehensive properties in low-Δn LC mixture 002 and high-Δn LC mixture P01-F were further evaluated, and it was found that the 2,2-difluorovinyloxyl LC diluter 3FV have a significant advantage in reducing the Tm and γ1 of LC mixtures, and increasing the values of Δn and Δε. The research results provide technical support and theoretical guidance for the molecular design of new LC diluters and the development of fast response liquid crystals. GRAPHICAL ABSTRACT


Introduction
With the rapid development of liquid crystal (LC) optical devices and new display technologies, high birefringence (Δn) LC mixtures have been one of the research hotspots in a variety of electro-optical devices [1][2][3][4][5], including intelligent lubricants [6], light modulators [7,8], liquid crystal lenses [9,10], mobile communications [11,12] and so on.High-Δn liquid crystals as optical modulation materials require to meet other excellent properties such as low viscosity (γ 1 ), low melting point (T m ), wide LC phase temperature range and large dielectric anisotropy (Δε) to realise their broad applications from visible light [13], near-infrared (NIR) [14], mid infrared (MIR) [15], millimetre wave [16], terahertz [17] to microwave [18].The LC diluters [19][20][21] as one of the key components in LC mixtures have been widely applied to decrease melting point and rotational viscosity, which contributes to fast response time and widespread applications.To the best of our knowledge, however, few reports have focused on how to design LC diluter molecules for high birefringence LC mixtures.
At present, commercial LC diluter 3HHV (Figure 1), widely used in LC displays, has been proven to reduce the melting point and viscosity of LC mixtures very well, but the shortcoming is its small Δn and Δε values.LC scientists have utilised various molecular engineering strategies to design and synthesise some large Δn LC diluter molecules.For example, our group [22] employed one benzene ring instead of one cyclohexane ring as the mesomorphic unit and combined the low T m and low γ 1 of the terminal olefin structure to obtain two LC diluters with relatively large birefringence.Kelly et al. [23] developed LC diluter 3BB5 (Figure 1) with large birefringence using biphenyl structure replacing bicyclohexane rings as the mesomorphic unit.LC researchers [22,24] have further evaluated their comprehensive performance in high Δn LC mixtures and found that the addition of LC diluters can effectively lower their T m and γ 1 values, but at the same time significantly reduce their Δn and Δε values, which is not conducive to the electro-optical properties of liquid crystal optics.It is well known that the large Δε of LC materials facilitates the reduced operating voltage of liquid crystal optics, which not only saves energy but also improves the response time [25,26].Therefore, it is particularly important to develop LC diluter molecules with large Δε, low γ 1 , low T m and large Δn for high birefringence LC mixtures.We also expect that its addition will not only lower the T m and γ 1 of the LC mixture but also maintain the large Δε and high Δn.
Previous reports [27][28][29] indicated that the 2,2-difluorovinyloxyl terminal groups not only lower the melting point of LC compounds but also maintain large Δε and high Δn.Therefore, in this work, LC diluters nV and nFV (Figure 1) were designed and synthesised by introducing one 2,2-difluorovinyloxyl terminal group into LC molecules, while combining the advantages of large Δn of biphenyl molecular backbone and low T m of lateral fluorine substituent [30][31][32][33].
Their thermal properties and corresponding structureproperty relationships were studied in comparison with the reported diluter molecules.We focused on the effects of terminal group and lateral fluorine substituent on the key properties such as Δn, Δε and γ 1 of LC diluters.Furthermore, their optoelectronic properties in high birefringence LC mixture were further comprehensively evaluated.The results show that these LC diluters have excellent properties such as low melting point, low viscosity, large dielectric anisotropy and birefringence, which compensate for the existing shortcoming in commercial diluter 3HHV.

Materials
2-Fluoro-4-n-alkylbenzeneboronic acid was supplied by Xi'an Caijing Opto-Electrical Science & Technology Co. Ltd. and 4-n-alkylbenzeneboronic acid was purchased from Hebei Maison Chemical Co.Two LC mixtures P01-F and 002 as the parent LC mixtures to evaluate these diluters' properties were developed by Xi'an Modern Chemistry Research Institute.Other reagents and solvents were all obtained from Aladdin-reagent Co. or Sinopharm Chemical Reagent Co.

Characterization and measurements
The 1 H-nuclear and 13 C-nuclear magnetic resonance ( 1 H NMR and 13 C NMR) spectra were recorded on a Bruker AV 400 (400 MHz, Bruker Corporation, Karlsruhe, Germany) instrument using tetramethylsilane (TMS) as internal standard.IR spectra (Nicolet Avatar360E spectrometer, Thermo Electron Corporation, Madison, WI, USA) and mass spectra (GC/EI-MS Thermo DSQ II, Thermo Finnigan, CA, USA) were also employed to confirm the structures of all intermediates and target compounds.After drying the KBr in a vacuum oven with 120°C for 24 h, we mixed a small amount of dried sample and KBr to press into a thin slice, then tested it using the IR instrument.Their chemical purities were detected by high-performance liquid chromatograph(HPLC)and gas chromatography (GC).
Using B3LYP/6-311 G (d, p) level of theory [34,35], DFT calculations were carried out to optimise the geometric configurations of diluter molecules and obtain the corresponding molecular polarisability, dipole moment, frontier orbitals, biphenyl dihedral angle and aspect ratio.The thermal properties of compounds nV, nFV, nC and nFC were measured by differential scanning calorimeter (DSC) at a heating/cooling rate of 10°C/min.The phase transition temperatures of LC mixtures were measured by DSC in nitrogen at the heating and cooling rate of 2°C/min [7].The phase transition temperatures reported in this article were the peak values of the transition on DSC curves.The 1 mg of diluters and 5 mg of LC mixtures were used for the DSC measurement.
The Δn values were measured by an Abbe refractometer (NAR-4T, ATAGO Co., Ltd., Japan) using a 589 nm wavelength light source.The γ 1 values were measured by a Model 6254 (multi-channel liquid crystal evaluation system, TOYO Corporation, Japan).The Δε values were measured by an electrical constant instrument (EC-1, TOYO Corporation, Japan) using LC cells with a thickness of 19.8 μm.The Δn, Δε and γ 1 values of diluters were extrapolated from 15 wt% solutions in the host mixture 002 at 25°C by the guesthost method [36].

Synthesis
Target compounds nV and nFV were obtained by synthetic route as shown in Figure 2, where all target compounds and intermediate compounds were confirmed by 1 H NMR, 13 C NMR spectroscopy, mass spectroscopy, and infrared spectroscopy.These five target compounds were synthesised by four-step reactions of nucleophilic substitution, etherification, Suzuki coupling and dehydrofluorination with trifluoroethanol and 4-n-alkylbenzeneboronic acid as the raw materials.Furthermore, we will take the synthesis process and structural characterisation (see Supporting information) of target compound 3FV as an example to show the detailed information.The spectral data of compounds 2FC and nC were summarised in the Supporting information.

Synthesis of trifluoro-ethyl methane-sulphonate
The trifluoroethanol (10.00 g, 100 mmol) solution was stirred at −10°C for 10 min, then a mixture of trimethylamine (15.3 mL) and anhydrous dichloromethane (150 mL) was added to the above solution.Next, a mixture of methane-sulphonyl chloride (8.5 mL) and dried dichloromethane (20 mL) was added to the above mixture dropwise and keeping under this condition for 10 h.Then we quenched the reaction with diluted hydrochloric acid, and the reaction solution was extracted with dichloromethane, washed with sodium bicarbonate aqueous solution to neutral, dried over anhydrous magnesium sulphate, and concentrated to obtain a lightyellow liquid with 98% yield and 99.0% purity for the GC measurement.

Synthesis of 4-trifluoroethoxybromobenzene (BrOCF)
Under the N 2 atmosphere, a mixture of trifluoro-ethyl methane-sulphonate (1.78 g, 10 mmol), 4-bromophenol (1.90 g, 11 mmol), potassium iodide (16.60 mg, 0.10 mmol), potassium carbonate (2.07 g, 15 mmol) and DMF (50 mL) was stirred at 120°C for 10 h.After the mixture was cooled to room temperature, the solution was extracted three times by dichloromethane solvent, dried by anhydrous sodium sulphate and concentrated to be purified via column chromatography using petroleum ether as an eluent to obtain a white crystal with 66% yield and 99% GC purity. 1

Synthesis of 4'-((2,2-difluorovinyl) oxy)-2-fluoro-4-n-propyl-1,1'-biphenyl (3FV)
Under the N 2 atmosphere, a mixture of compound 3FC (3.12 g, 10 mmol) and dried tetrahydrofuran (60 mL) was mechanically stirred at about −75°C for 30 min, then lithium diisopropylamide (LDA, 50 mL) was added to the above mixture drop by drop and keeping under this condition for 1.5 h.Then we quenched this reaction with saturated ammonium chloride aqueous solution, and the reaction mixture returns to room temperature.Next, this reaction mixture was extracted with dichloromethane solvent, dried over anhydrous magnesium sulphate, and concentrated to be purified via column chromatography using n-hexane as an eluent to obtain a crude product, further recrystallised with n-hexane under the protection of liquid nitrogen to obtain a colourless liquid with 45% yield and above 99% GC purity. 1   (40).
Similar procedure was utilised to prepare the other target compounds, their spectroscopic data are listed as follows: 2FV:

Melting point
Low melting point is one essential characteristic of LC diluters [22,24].To investigate the melting points of target compounds and their structure-property relationships, the phase transition properties of compounds nV, nFV, nC and nFC were tested using differential scanning calorimetry (DSC, Figure S2 and S3), and the corresponding data are summarised in Figure 3.The DSC test results show that they have only one-phase transition temperature during the heating/cooling process.This means that they do not have a LC phase, which is probably related to their molecular aspect ratio of less than 4 [37].
As shown in Figure 3(a,b), the melting points of compounds nC with the trifluoro-ethoxy terminal are all greater than 100°C.Their melting points decrease gradually as the alkyl chains grow, which may be due to the inhibition of close molecular packing by the terminal alkyl chains.This phenomenon also exists for compounds nV and nFV.When the fluorine substituent is located on the inner side of the first benzene ring, the melting points of compounds nFC are reduced by more than 50°C compared to compounds nC.The fluorinated compounds nFV with the 2,2-difluorovinyloxyl terminal are liquid at room temperature, which means they can meet the requirements of LC diluters for low melting points, and further as dopants can be well mixed with LC mixture formulations uniformly.
From the view of molecular conformation for 3HHV, the linear molecular conformation (Figure 3(d)) and π-π conjugation originated from the ethylene terminal group (Figure 4) would contribute to forming the intertwined trans-cyclohexane ring, thereby exhibiting strong intermolecular interactions.In comparison, compound 3BB5 possesses a large biphenyl dihedral angle of 39° obtained by DFT calculations and a lower melting point than that of 3HHV (Figure 3(c)).Further replacing the n-pentyl group with the trifluoroethoxyl terminal group, the obtained compound 3C exhibits a melting point up to 131.2°C due to its large dipoledipole interactions.The 2,2-difluorovinyloxyl group has a lower dipole moment than the trifluoroethoxyl group, thereby compound 3V has a 22.8°C lower melting point than that of 3C, but its melting point is still greater than 100°C.The lateral fluorine substituent significantly lowers the melting point of compound 3FV to 7.2°C, which can meet the requirements of LC diluters for low melting points.For molecules 3V and 3FV in Figures 3(d) and 4, it can be found from the conformational analysis of the molecules that the lateral fluorine substituent increases the biphenyl dihedral angle, affects electronic structure and lowers molecular polarisability, which further decreases intermolecular force and melting point [38][39][40].

Optical and electrical anisotropy
The birefringence and dielectric anisotropy of compounds 3FC, 2FV, 3FV and 3V were tested using an Abbe refractometer and an electrical constants instrument, and the relevant data are summarised in Table 1.
From Table 1, the Δn values of biphenyl compounds 3FC, 2FV, 3FV and 3V exhibit a maximum of 0.149.In comparison with 3FC and 3FV, it is seen that 3FV has relatively large Δn value.The enhanced π-π conjugations formed by the tail end C=C bond and the neighbouring phenyl ring [41], cause the 2,2-difluorovinyloxyl group to increase Δn.Combing with the DFT calculation results (Figure 4), it is seen that molecule 3FV shows the highest occupied molecular orbital (HOMO) level and lowest unoccupied molecular orbital (LUMO) level mainly distributed on the biphenyl unit and 2,2-difluorovinyloxyl terminal group, while the trifluoroethoxyl terminal group in molecule 3FC contributes the p-π conjugation, which maybe support their birefringence results.From the DFT viewpoint, there is a positive correlation between molecular Δn and anisotropy Δα [36], thereby the numeric relation of Δn value in Table 1 can be effectively explicated, except compound 3V.According to the Vuks equation [42], the Δn is also related to the isotropic component α and molecular order parameter.At 25°C, compounds 3V and 3FV are the solid and isotropic liquid, respectively.Therefore, compound 3V possesses a larger molecular order parameter than that of compound 3FV.Compounds 3V and 3FV show similar isotropic component α values, we speculated that their molecular order parameters may explain why 3V exhibits a larger Δn value that of 3FV.
As can be seen from Table 1, these compounds with a 2,2-difluorovinyloxyl or trifluoroethoxyl terminal group present larger Δε values (3.7 ~ 5.7), which indicates that target compounds are conducive to meet the large Δε requirement for device applications.The lateral fluorine substituent increases the molecular dipole moment,

Rotational viscosity
Low viscosity is another essential characteristic of LC diluter, which is also a key performance to judge whether one compound is suitable to be LC diluter.In order to comparatively study the γ 1 of 2,2-difluorovinyloxyl LC diluters, we tested the γ 1 values of compounds 3FC, 2FV, 3FV, 3V and commercial diluter 3HHV using the multi-channel liquid crystal evaluation system, and the results are summarised in Table 1.
In Table 1, the rotational viscosity of commercial diluter 3HHV is 4.69 mPa•s, and other compounds have a slightly increased rotational viscosity with extrapolated values of 6-7, which means that these compounds could meet the low viscosity requirement of LC diluters.Compound 3FV is an isotropic liquid while compound 3V is a solid, it means that the lateral fluorine substituent weakens the strong intermolecular interactions of compound 3V.Therefore, compound 3FV exhibits a lower rotational viscosity than that of 3V.The substitution of 2,2-difluorovinyloxyl group with trifluoroethoxyl group as the terminal increases the dipole moment, further enhances the dipole-dipole interactions.Consequently, compound 3FC exhibits a higher rotational viscosity than that of 3FV, further confirming that the 2,2-difluorovinyloxyl terminal group is beneficial to reducing molecular rotational viscosity [27].Among them, compounds 2FV and 3FV containing lateral fluorine substituent and 2,2-difluorovinyloxyl group exhibit lower rotational viscosity, which is more favourable to reducing the rotational viscosity of high birefringence LC mixtures.

Performance in LC mixtures
A comparative study on the performance of commercial diluter 3HHV revealed that the fluorinated LC diluter terminated by 2,2-difluorovinyloxyl presents the advantages of low melting point, low viscosity, large birefringence and dielectric anisotropy, which achieves the molecular design goal of high-Δn LC diluters.In order to further investigate its effect on the performance of LC mixtures, we selected one low-Δn LC mixture 002 and one high-Δn LC mixture P01-F as the research objects to comparatively study the effects of compounds 3V, 3FV and 3HHV on their melting points, clearing points, birefringence, dielectric anisotropy and rotational viscosity, and the test results are summarised in Table 2.
From Table 2, the addition of LC diluters decreases both the melting point and the clearing point of LC mixtures.Compounds 3HHV and 3V show little effect on the T m of LC mixture 002, and compound 3FV obtained by introducing the lateral fluorine substituent, can effectively lower the T m of LC mixture 002 to −60°C while maintaining a wide LC phase temperature range.Further research indicated that LC diluter 3FV can lower the T m of high-Δn LC mixture P01-F from −16°C to −33°C while maintaining a wider liquid crystal phase temperature range, which further confirms that the function of LC diluter 3FV is to effectively lower the T m of LC mixture.The advantage of lowering the melting point of above two LC mixtures maybe come from the fact that LC diluter 3FV has a large biphenyl dihedral angle (Table 1) to reduce π-π conjugations.
Meanwhile, benefiting from its advantageous properties of low viscosity, large Δn and Δε shown in Table 1, it is found that the addition of LC diluter 3FV increases the Δn and Δε values of LC mixture 002 while significantly reducing its γ 1 .In contrast, commercial diluter 3HHV can substantially reduce the γ 1 of LC mixture 002, but it also dramatically reduces its Δn and Δε values, which is detrimental to enhancing the electrooptical performance of LC devices.Further research indicated that the doping of LC diluter 3FV in high-Δn LC mixture P01-F to obtain one new LC mixture, it could still achieve Δn of 0.26, Δε grew to 3.54, and γ 1 decreased by 61%.In summary, the above results show that LC diluter 3FV can maintain the wide LC phase temperature range of LC mixture, reduce the T m and γ 1 , increase the Δε, and maintain high Δn, which is important for the development of high-Δn LC materials.Precise molecular tailoring was applied to obtain liquid crystal diluters with excellent performance for high birefringence liquid crystal mixtures.

Conclusion
In this work, five high-Δn LC diluter molecules terminated by the 2,2-difluorovinyloxyl group were designed and synthesised, and their chemical structures were characterised and confirmed by NMR, IR and mass spectrometry instruments.Meanwhile, the effects of alkyl chain length, terminal group and lateral fluorine substituent on the thermal and electro-optical properties of these LC diluter molecules were comparatively investigated, and their comprehensive performance in two LC mixtures was further evaluated.Structure-property relationships show that the introduction of lateral fluorine substituent can lower T m and enhance Δε, while improving the low-temperature performance of LC diluter in LC mixtures.The 2,2-difluorovinyloxyl terminal group can well improve the Δn and Δε properties of LC diluter while lowering its T m and γ 1 .Compared with commercial diluter 3HHV, target diluters nFV possess comparable viscosity, 2-3 times higher Δn, lower T m and larger Δε.More importantly, the doping of these diluter molecules not only decreased the T m and γ 1 of LC mixture 002 but also improved its Δn and Δε values.In particular, LC diluter 3FV diminished the T m of LC mixture 002 to −60°C.Meanwhile, it can lower the T m of high-Δn LC mixture P01-F to below −33°C, reduce the γ 1 to 93.16 mPa•s, and increase the Δε to 3.54 while maintaining a high Δn of 0.26.This means that LC diluters nFV achieve a well balance among melting point, viscosity, ∆n and ∆ε of high-Δn LC mixture, which further provides some promising LC diluters to improve the electro-optical properties of LC devices.

Figure 1 .
Figure 1.(Colour online) The research idea and molecular structures of LC diluters 3HHV and 3BB5, target compounds nV and nFV.Herein n, ∆ε, T m denote numbers 2-5, dielectric anisotropy and melting point, respectively.
a All polarisability components and the anisotropy parameter are expressed in Bohr ^3 (with 1 Bohr = 0.52917 Å). b

Table 2 .
Physical properties of compounds 3V, 3FV and 3HHV in the LC mixtures 002 and P01-F.