Synthesis and mesomorphic properties of benzoxazole derivatives with lateral multifluoro substituents

ABSTRACT Fluorinated aromatics is generally chosen as mesogenic cores to design novel liquid crystal compounds. Here, a series of benzoxazole derivatives with laterally multifluorinated biphenyl units, 2-(3′,3-difluoro −4′-alkoxy-1,1′-biphenyl-4-yl)-benzoxazole derivatives (coded as nPF(3)PF(3)Bx), are synthesized and characterized, where methyl and nitro moieties are selected as terminal groups to investigate the effects of different polar substituents on the liquid crystal properties. The compounds nPF(3)PF(3)Bx show enantiotropic mesophases with mesophase ranges of 0–40°C and 0–63°C on heating and cooling for hydrogen-terminated derivatives (nPF(3)PF(3)BH), 43–93°C and 54–123°C for methyl-terminated ones (nPF(3)PF(3)BM), 60–108°C and 74–152°C for nitro terminated ones (nPF(3)PF(3)BN), respectively. They exhibit photoluminescence emission peaks at 390–392 nm and UV–vis absorption bands with maxima at 327–330 nm, respectively. The results reveal that lateral multifluoro substituents lead to a decrease in melting/clearing points, while electron-withdrawing terminal nitro moiety results in increases in both melting point and mesophase range. GRAPHICAL ABSTRACT


Introduction
Generally, development of new mesogenic compounds is one of the fundamental topics in the field of liquid crystal materials, in which modification of existing molecules is considered to be an effective methodology, including the alternation of the linking units, the introduction of a lateral or terminal fluorine atom into the mesogen [1][2][3][4][5][6]. Up to now, many kinds of calamitic mesogens containing heterocyclic core are developed, where benzoxazole derivatives CONTACT Xinbing Chen chenxinbing@snnu.edu.cn; Zhongwei An gmecazw@163.com Supplemental data for this article can be accessed here.
Recently, several benzoxazole-based calamitic mesogens possessing biphenyl or bithiophene units are reported, where modification of the molecular polarity via changing lateral or terminal groups can obviously alter the mesophase properties of the compounds [17][18][19][20]. It is known to us that fluorine is generally adopted as terminal or lateral moiety to improve the physical properties of the liquid crystal materials, where the position and quantity of fluorine substituent play important roles in altering liquid crystal properties [4][5][6]. To further develop new mesogens and investigate the effect of terminal units and lateral fluorine on the properties of liquid crystal compounds, a series of benzoxazole derivatives with laterally multifluorinated biphenyl units, 2-(3′,3-difluoro-4′-alkoxy-1,1′-biphenyl-4yl)-benzoxazole derivatives (coded as nPF (3)

Materials
The reagents were purchased from Sinopharm Chemical Reagent Co. or Aladdin-reagent Co., and used as received, where anhydrous potassium carbonate was dried at 150°C in vacuum, and chloroform was dehydrated by pre-dried 4Å molecular sieves prior to use.

Synthesis of 4-octyloxy
3.00 g of 2-Fluoro-4-bromophenol (15.71 mmol), 3.33 g of n-octyl bromide (17.28 mmol), 6.51 g of anhydrous potassium carbonate (47.13 mmol), 0.26 g of KI and 80 mL of DMF were added to a 250 mL, three-neck, round-bottom flask equipped with an overhead stirrer and condenser. After the mixture was stirred at 80°C for 4 h, the system was cooled to room temperature. Then under nitrogen protection, 2.72 g of 3-fluoro-4-formylphenylboronic acid (16.21 mmol), 0.36 g of Pd(PPh 3 ) 4 (0.31 mmol), 0.24 g of tetrabutylammonium bromide (0.79 mmol) and 20 mL of water were added. The reaction system was stirred at 60°C for 5 h under nitrogen protection. After completion of the reaction, the mixture was diluted with water and extracted with methylene chloride for three times. The combined organic phase was dried over magnesium sulphate. After removal of the solvent in vacuo, the residue was purified via column chromatography on silica gel using PE/EA (50/1) as eluent to give purity above 98% for GC measurement. Pale yellow liquid was obtained with yield 63%.   Similar procedure was used to prepare the other nPF (3)PF(3)CHO derivatives with different alkoxy chain (n = 2-10). The spectroscopic data for the corresponding derivatives were listed in supplementary information.

Synthesis of 2-
82 mmol) and 25 mL of anhydrous chloroform were added to a 100 mL round bottom flask equipped with an overhead stirrer and condenser. The mixture was stirred at reflux for 6 h. After completion of the reaction, the mixture was diluted with water and extracted with chloroform for three times. The combined organic phase was dried over magnesium sulphate. After removal of the solvent in vacuo, the residue was purified through recrystallization from ethanol to give purity greater than 98% for HPLC or GC measurements. White crystals were obtained with yield 47% and m.  (3)Bx with different alkoxy chain (n = 2-10). The spectroscopic data for the corresponding compounds were summarized in supplementary information.

Characterization
Spectra methods, including Infrared (IR) spectra (Nicolet Avatar360E spectrometer, Thermo Electron Corporation, Madison, WI, 325 USA), proton nuclear magnetic resonance ( 1 H-NMR) spectra (Bruker AV 400, Bruker Corporation, Karlsruhe, Germany), and gas chromatography with electron impact-mass spectrometry (GC/EI-MS) (Thermo DSQ II, Thermo Finnigan, CA, USA), as well as Elemental analysis (EA) (Elementar Vario EL III instrument, Elementar Analysensysteme GmbH, Hanau, Germany), are used to characterize the structures of the intermediates and final compounds. Shimatsu DSC-60 differential scanning calorimetry (DSC) (Shimadzu 335 Corporation, Kyoto, Japan) is chosen to investigate the phase transition temperatures of the compounds under the nitrogen condition at a heating and cooling rate of 5°C min −1 . LEICA DM2500P polarizing optical microscopy (POM) (Leica Microsystems GmbH, Wetzlar, Germany) with Linkam THMS600 hot-stage (Linkam Scientific Instruments Ltd., Tadworth, UK) are utilized to identify the mesophase textures of the compounds at a heating and cooling rate of 0.5°C min −1 . Thermal stabilities of the compounds are carried out with a TA Q50 TGA (TA Instruments, USA) in nitrogen (flow rate: 100 cm 3 min −1 ) at a heating rate of 10°C min −1 . Photophysical properties of the compounds are investigated by UV-vis absorption spectra (Hitachi U-3900UV spectrometer 345, Hitachi High-Technologies Corporation, Tokyo, Japan) and emission spectra (Hitachi F-7000 spectrometer, Hitachi High-Technologies Corporation, Tokyo, Japan).

Preparation and characterization
The benzoxazole derivatives nPF(3)PF(3)Bx, possessing alkoxy chain with carbon atoms from 2 to 10 at one end of the calamitic molecule, terminal moieties (H, CH 3 , NO 2 ) at the other end and lateral multifluoro substituents attached at 3′,3-positions of the biphenyl mesogenic core unit, were synthesized via a simple preparation method with overall yields of 22%-72%, as shown in Figure 1. They exhibit purities higher than 98% according to high performance liquid chromatography (HPLC) or gas chromatography (GC) results. The structures of intermediates and nPF (3)

Mesomorphic properties
Thermal behaviours of the compounds nPF (3) As shown in Table 1, the phase transition temperatures, the associated enthalpy changes and mesophase textures are summarized, where the compounds nPF (3) Table 1 and Figures 2(a), 3(a) and 4(a), very low enthalpy changes for nematic-isotropic phase transition are found, which is attributed to the enhanced molecular biaxiality of the bent benzoxazole derivatives nPF(3)PF (3)Bx that contribute to reduce the entropy change [21][22][23][24]. It is noted that very low enthalpy changes are found for SmC-Cr phase transition of the compounds nPF(3)PF(3)BN with n = 5-10, meanwhile, relatively high enthalpy changes for N-SmC phase transition are observed. The transitions from nematic to smectic or smectic to crystal phases are generally connected with an obviously increment of the molecular order. For compounds nPF(3)PF(3)BN, high enthalpy changes for N-SmC phase transition maybe attributed to the enhanced strong π-π interaction between the molecules resulted from nitro groups, whereas low enthalpy changes for SmC-Cr phase transition can be ascribed to small structure changes during increasing molecular order [25].

Photophysical properties
The absorption and fluorescence spectra of laterally multifluorinated compounds 7PF(3)PF(3)Bx recorded in methylene chloride are shown in Figure 6(a,b). All the compounds give broad absorption bands with λ max at 327, 328 and 330 nm for 7PF (3) (3) BH, which is attributed to the increasing σ-π and π-π conjugation resulting from the methyl and nitro terminal moieties. The results suggest that the terminal moieties play important role in electronic properties of the compounds via the corresponding σ-π and π-π conjugation. As shown in Figure 6

Structure-property relationships
The mesomorphic properties of the calamitic mesogenic compounds are affected by various kinds of factors such as molecular shape, excluded volume, mesogenic unit, linking group, lateral substituent and terminal moiety [1,2]. It is found that the laterally multifluorinated benzoxazole derivatives nPF(3)PF(3)Bx show lower melting and clearing points than non-fluorinated analogs [10], which maybe attributed to a larger free volume associated with the lateral multifluoro substituent [26]. Besides, the mesomorphic properties of nPF(3)PF(3)Bx are also affected by terminal groups, where the transition temperatures are depended on the length of the terminal alkoxy chain, as shown in Table 1 and Figure 7. It is noted that an increase in terminal length approximately leads to an enhanced mesophase ranges from 0 to 40°C and 0 to 63°C on heating and cooling for nPF In addition, it is noted that terminal moieties (H, CH 3 , NO 2 ) play an important role in mesophase range of nPF(3)PF(3)Bx, where the mesomorphic temperature domain increases with enhancing the electron -withdrawing ability, in the order NO 2 > CH 3 > H. It is known to us that polarity of the mesogenic compounds can be modiefied via terminal moiety, so the compounds nPF(2)PF(3)BN display largest mesophase range due to its strongest dipole-dipole interactions resulted from strongest electro-withdrawing ability of nitro moiety [19,27].

Disclosure statement
No potential conflict of interest was reported by the authors.