Synthesis of 1,4-Bis(phenylethynyl)benzenes and Their Application as Blue Phase Liquid Crystal Composition

A number of 1,4-bis(phenylethynyl)benzene derivatives (BPEBs) and their analogues with different numbers of side-substitute fluorine atoms on benzene rings, and alkyl chains, ethoxyl groups, fluorine atoms and trifluoromethyl groups as the end groups have been synthesized. The effects of the different substituents on their properties such as thermal behavior of melting point and clearing point, the temperature of nematic phase, optical anisotropy and dielectric anisotropy have been well investigated, and it has been found that some BPEBs have a wide range of the nematic phase temperature with high optical anisotropy (Δn) and acceptable dielectric anisotropy (Δɛ), which have been applied as the crucial compositions to constitute a liquid crystal mixture having the properties of Δɛ = 29.0 and Δn = 0.283 at 25 °C. With the addition of the chiral dopant to the obtained liquid crystal mixture, blue phase liquid crystal with a blue phase temperature range of 8 °C has been achieved.


Introduction
Liquid crystals (LCs) have had a multitude of applications in the past few decades [1,2]. One of the important and unique applications is their use as the key fundamental materials to develop LC displays [3], which have actually changed people's lifestyle due to the use of mobiles, notebook computers, flat panel desktop monitors, and LCD televisions, etc. The development of excellent LC displays, which have the advantages of fast response, high contrast ratio, and low driving voltage, depends greatly on the development of new types and properties of LCs. The demand for LCDs with a fast response is one of the crucial factors to improve the quality of the displays, and blue phase liquid crystal (BPLC) is commonly considered to be one of the strongest candidates [4][5][6][7].
On the other hand, the molecular and electronic structures of 1,4-bis(phenylethynyl)-benzene derivatives (BPEBs), as well as their applications have attracted much attention recently [8][9][10][11][12]. In particular, BPEBs have been applied as the important components in LCs with the characters of high melting point (mp), clearing point (cp), and large optical anisotropy values (Δn) [13][14][15][16][17][18][19][20][21][22]. It is important and interesting to investigate the substituent effect on the properties of BPEBs. Therefore, in this paper, we describe the synthesis of a number of BPEBs with different alkyl chains, the numbers of fluorine atoms and other substituents in benzene rings as shown in Scheme 1, and the study on the substituent effects on their properties, as well as their application as blue phase liquid crystal composition.    The synthetic routes of 1-25 are outlined in Scheme 2, including the key steps of the formation of terminal and internal alkynes via Sonogashira cross-coupling reactions of aryl iodides/bromides catalyzed by palladium(0) complexes in good to high yields, and a typical synthetic procedure for the formation of 12 is described in the Experimental Section (vide infra).

Thermal Properties
The thermal properties of the melting point (mp) and clearing point (cp) are critical in the practical utilization of the synthesized BPEBs as the composition of LCs, thus the mp and cp were determined by DSC (Differential Scanning Calorimetry), and their thermal data as well as enthalpic data (ΔH) are concluded in Table 1. It was found that mp and cp greatly depended upon the molecular and electronic structures of BPEBs. BPEBs 1-4 clearly show the effect of alkyl chain length on mp and cp, and when n-C 3 H 7 and C 2 H 5 groups are used as the end groups, BPEBs 1 and 2 give the similar mp and cp (1 vs. 2). However, when the longer alkyl chains of C4 and C5 were employed, BPEBs 3 and 4 showed similar thermal properties, but both mp and cp decreased greatly (1 & 2 vs. 3 & 4) [23]. It can be concluded that BPEBs 3 and 4 bearing the longer alkyl chains have a wider range of nematic phase temperature than those of BPEBs 1 and 2, possibly due to the longer alkyl chains being more flexible than short alkyl chains. Both 3 and 4 show a nematic phase temperature range of about 140 °C , and the wide nematic phase temperature are very important to make a practical LC mixture.  By comparison of 2 and 5, it was found that decreasing the number of fluorine atoms in the end benzene ring leads to the increase of both mp and cp in a range of about 10 °C, and a similar trend of mp was also found between 4,4'-bis(phenylethynyl)biphenyls 6 and 7, which are the analogues of BPEBs 2 or 5.
The thermal properties of other BPEBs are also compared to each other, as shown in Figure 1, and two notable features of the relationship between mps and their chemical structure could be concluded as follows: (1) The introduction of the fluorine atom on the middle benzene ring shows a great effect on the change of mp. In general, one or two fluorine atom-substituted benzene result in the increase of mp, and the introduction of the second fluorine atom leads to much more significant increase of mp relative to the first fluorine atom introduction (e.g., 8→10→12; 9→11→13). (2) When the end group of OCF 3 is replaced by a fluorine atom, the mps decreased (e.g., 10 vs. 11; 12 vs. 13; 14 vs. 15), and only one exception (8 vs. 9) was observed.

Nematic Phase
Among the synthesis of BPEBs and analogues, only BPEBs 1-5, 8, 20-21 ( Figure 2, on heating run) and 9, 14-15 (on cooling run) show nematic phases under polarizing microscope with the structural character without side-substituted fluorine atoms bonded to the middle benzene ring, and the analogues of BPEBs 6 and 7 have no nematic phase either. It was found that the nematic phase temperature is quite different depending on the end groups and the numbers of fluorine atoms. As shown in Figure 3 and Table 1, BPEBs 1-5 with OC 2 H 5 as the end group possess wide nematic phase temperatures from 121.6 to 147.0 °C, and have apparently disclosed that longer alkyl chains generally result in a wider temperature of nematic phase. BPEBs 3 and 4 bearing the longer alkyl chain of n-C 4 H 9 or n-C 5 H 11 give the maximum nematic phase temperatures, while BPEBs 8, 20 and 21 with OCF 3 or F as the end groups show the relatively narrow nematic phase temperature range.

Optical Anisotropy (Δn)
The optical anisotropy or birefringence (Δn) of the BPEBs showing nematic phase in host LC was determined, and the obtained results were concluded in Table 2. As expected, a large π-conjugated structure leads to relatively high Δn value, and therefore BPEBs 1-5 have higher Δn values than 8-9, 14-15, and 20-21. It is reasonable to understand that much more side-substituted fluorine atoms in different benzene rings will decrease the integrity of π-conjugated structures to result in a decrease of optical anisotropy.

Applications of BPEBs as Blue Phase Liquid Crystal Composition
Because the synthesized some of BPEBs have high Δn and acceptable Δε as described above, we are interested in investigation of the application of them as the compositions in blue phase liquid crystals (BPLC) to possibly increase the Kerr constant, which is key parameter for practical BPLC. After detailed screening the composition and contents, we got a LC mixture containing BPEBs 1, 3-4, 8-9, 14, 15, 21 (5 wt % each) and other liquid crystal mixture (cp: 83.0 °C; Δn = 0.230, and Δε = 29.6, at 25 °C), which shows the properties of Δn = 0.283, and Δε = 29.0 at 25 °C, which is expected to have high potential applications as BPLC [24]. After adding chiral dopants (R811: 10 wt % and BDH1281: 7 wt %), we obtained a BPLC with a blue phase temperature range of 8 K (from 41 to 33 °C, on the second cooling run). Figure 4 shows a typical BP texture of the obtained BPLC at 36 °C .

General Method
All organic starting materials and catalysts are analytically pure and used without further purification. Nuclear magnetic resonance (NMR) spectra were recorded on a JEOL ECA-300 spectrometer (Tokyo, Japan) using CDCl 3 as solvent at 298 K. 1 H-NMR (300 MHz) chemical shifts (δ) were referenced to internal standard TMS (for 1H, δ = 0.00 ppm). 13 C-NMR (75 MHz) chemical shifts were referenced to internal solvent CDCl 3 (for 13 C, δ = 77.16 ppm). Mass spectra (MS) were obtained on a Shimadzu GCMS-QP2010S (Kyoto, Japan). Element analyses were obtained with a Flash EA 1112 Element Analyzer (Thermo Fisher Scientific, Waltham, MA, USA). Polarizing microscope LWT300LPT (CEWEI photoelectric technology Co. Ltd., Shanghai, China) equipped with a Weitu WT-3000 hot stage and a TCA 5.0 MP camera was used to observe and record the optical textures of liquid crystal samples. The calorimetric studies were performed on a TA Instruments DSC 2010 (TA Instruments, New Castle, DE, USA) with a heating temperature rate of 10 °C /min. A NAR-4T Abbe refractometer (ATAGO Co. Ltd., Tokyo, Japan) was used to measure optical anisotropy (Δn), and a 3522-50 LCR Hitester (HIOKI E.E. Co., Ueda, Japan) for dielectric anisotropy (Δε). All the samples for measuring the Δn and Δε were composed of BPEB and nematic host LC at a ratio of 5-10/95-90 (wt %/wt %). The nematic host LC (SLC960524) was prepared by our laboratory, which has the values of Δn = 0.1202 (589 nm) and Δε = 3.121 (1000 Hz) at 25 °C .

A Typical Experimental Procedure for Synthesis of BPEB 12 and the Characterization Data of All the BPEBs
As shown in Scheme 2, BPEBs and analogues  were synthesized by the similar synthetic route, and their structures were characterized by 1 H-NMR, 13 C-NMR (for BPEBs 6 and 7, the 13 C-NMR could not be obtained due to their very low solubility in CDCl 3 , DMSO-d 6 or DMF-d 7 ), and elemental analyses. In this section, the synthetic procedure of BPEB 12 was only described in details, and the characterization data of all the other BPEBs are given.

Conclusions
In summary, we have designed and synthesized BPEBs by convenient Sonogashira cross-coupling reactions, which have different numbers of side-substitute fluorine atoms on benzene rings, and alkyl chains, ethoxyl groups, fluorine atoms and trifluoromethyl groups as the end groups. The detailed investigation of the synthesized BPEBs properties have disclosed that the melting points, clearing points, nematic phase, optical anisotropy (Δn) and dielectric anisotropy (Δε) greatly depend on both the numbers of side-substitute fluorine atoms and structures of the end groups. On the basis of the obtained properties of the synthesized BPEBs, some of them have been expected to have high potential application as the compositions in blue phase liquid crystals. Therefore, a mixture of blue phase liquid crystal has been prepared with a relative wide blue phase temperature range of 8 °C. The obtained results have implied that the synthesized BPEBs will certainly be important in the development of new types and properties of LCs. Further study on the application of BPEBs in making other new types of LCs is underway in our laboratory.