Preparation and properties of lateral monofluoro-substituted benzoxazole-based mesogenic compounds

A series of 2-(3ʹ-fluoro-4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-benzoxazole liquid crystals (coded as nPF(3)PBx) were prepared, where a lateral fluorine substituent, as well as methyl, chlorine and nitro terminal groups, was introduced into the molecules to investigate the effects of different polar substituents on the liquid crystal properties. The mesomorphic and photophysical properties were investigated. The results show that compounds nPF(3)PBx have enantiotropic mesophases; meanwhile, they exhibit UV–vis absorption bands with maxima at 323–326 nm and photoluminescence emission peaks at 389–395 nm, respectively. It is noted that nPF(3)PBx with terminal polar groups or electron-withdrawing groups (NO2, Cl) display higher clearing temperatures and wider mesophase range than those of the corresponding homologues with terminal non-polar groups or electron-donating groups (CH3, H). Meanwhile, compared with two lateral fluorine-substituted analogues containing 3,5-difluorophenyl unit, lateral monofluoro-substituted nPF(3)PBx display enhanced mesophase range both in heating and cooling except for terminal methyl-substituted compounds, as well as show obvious red-shifted UV–vis absorption bands and photoluminescence emission, which are attributed to the enhanced dipole–dipole interaction caused by increased dipole moment.


Introduction
To improve mesomorphic properties of the classical mesogenic compounds, altering the molecular polarity and geometry of the mesogenic core structures is generally regarded as an effective method. [1] Benzoxazole derivatives have been developed as one kind of mesogenic heterocyclic compounds, including benzoxazole-based mesogenic [2][3][4][5][6][7] and metallomesogenic materials. [8][9][10] Since fluorine is a relatively small atom with a large electronegativity, fluorine atom has been widely introduced into the mesogenic core structure to alter the molecular polarity, and furthermore, to modify mesophase morphology and the physical properties of liquid crystalline materials. [11][12][13] Recently, we prepared a series of two lateral fluorine-substituted mesogenic benzoxazole compounds, [14] 2-(3ʹ,5ʹ-difluoro-4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-benzoxazole derivatives (compounds I in Figure 1), whereas the compounds display a narrow mesophase range, which is attributed to the enhanced molecular width resulting from the two lateral fluorines located at both sides of the molecule and to the reduced molecular polarity caused by anti-parallel dipole ordering of two lateral fluorines compared with the benzoxazole group. It is known that dipole-dipole interaction among molecules is one of the key factors for the formation of the mesophase, and the interaction can be enhanced via increasing the molecular polarity. Therefore, to further investigate the effects of different polar substituents on the liquid crystal properties, a series of new 2-(3ʹfluoro-4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-benzoxazole liquid crystals (coded as nPF(3)PBx, Figure 1) were prepared with lateral monofluorine substituents incorporated into the molecule, where the compounds can be subdivided into nPF(3)PBH, nPF(3)PBM, nPF(3)PBC and nPF(3)PBN according to the different terminal substituents of H, CH 3 , Cl and NO 2 , respectively. Based on the calculated dipole moments by density functional theory, [15] it is demonstrated that compounds nPF(3)PBx have much larger dipole moments than the corresponding two lateral fluorine-substituted analogues (Table 1); hence, an enlarged mesophase range is expected for the present lateral monofluorosubstituted compounds nPF(3)PBx because of their enhanced dipole-dipole interactions among molecules.

Synthesis and characterisation
A series of nPF(3)PBx were prepared via four-step reactions, as shown in Figure 2 from 4 to 10 were obtained in purities greater than 98% (high-performance liquid chromatography (HPLC) or gas chromatography (GC)).
Infrared (IR), proton nuclear magnetic resonance ( 1 H-NMR), gas chromatography with electron impact-mass spectrometry (GC/EI-MS) and elemental analysis (EA) were employed to confirm the structures of nPF(3)PBx and nPF (3)PSx. Here, 8PF(3)PBM is chosen as an example to analyse. For IR spectrum, the peaks at 1606, 1571 and 1496 cm −1 are attributed to vibrations of the aromatic ring skeleton, and the strong peak at 1057 cm −1 is attributed to vibration of C-F bond. It is noted that the absorption band of C=N in the benzoxazole moiety overlaps with the vibrations of the aromatic ring skeleton, and a similar phenomenon is found with compounds nPF(3)PBH, nPF(3) PBM and nPF(3)PBC. In the 1 H-NMR spectrum, the protons of phenyl rings in both benzoxazole and biphenyl groups overlap with each other to some extent, but the quantities of the aromatic protons are consistent with 8PF(3)PBM molecules. The terminal methyl protons in benzoxazole moiety appear at 2.49 ppm. The protons in the CH 2 group adjacent to the oxygen atom are assigned at 4.10 ppm, and the protons in the alkoxyl group appear at 0.80-2.00 ppm. For the GC/EI-MS spectrum, the peak of the positively charged molecular ion appears at m/z 431.27 with a relative intensity of 12%, which is consistent with the theoretical value (431.54) of 8PF (3)

Mesomorphic properties
Differential scanning calorimetry (DSC) and polarising optical microscopy (POM) are employed to determine the mesomorphic properties of nPF ( Figure 1 in supplementary information. A series of nPF(3)PBx with alkoxy chain lengths of 4-10 carbons exhibit enantiotropic smectic/nematic mesophases (Table 3), where small nematic-isotropic enthalpy changes can be interpreted in terms of the bent nature of these benzoxazole-based molecules, which enhanced their molecular biaxiality and hence reduced the entropy change at the transition. [16][17][18] The liquid crystal phases are assigned from the typical marble textures, focal conic textures and Schlieren textures during heating and cooling for 7PF(3)PBH, 8PF(3)PBM, 6PF(3)PBC and 4PF(3)PBN, respectively, as shown in Figure 3. It is found that nPF(3) PBH and nPF(3)PBC exhibit only smectic mesophase, nPF(3)PBN show smectic mesophase for a long terminal alkoxy chain (n > 8) and nematic mesophase for a short terminal chain (n ≦ 8), whereas nPF(3)PBM display multiple mesophases (smectic C and nematic) during heating and cooling.
Compared with two lateral fluorine-substituted analogues (compounds I in Figure 1), [14] lateral monofluoro-substituted compounds nPF(3)PBx have approximately the same type of mesophases for the Table 2. Types of phase transition, temperatures and corresponding enthalpies obtained by POM and DSC methods for compounds nPF(3)PSx.  Table 1 in supplementary information), which are attributed to their bigger length-width ratio and larger dipole-dipole interaction resulting from the enhanced dipole moments (Table 1). Moreover, it is noted that lateral monofluoro-substituted compounds nPF(3)PBx (except for nPF (3) PBM) exhibit much wider mesophase range (during heating or cooling) than the corresponding two lateral fluorine-substituted analogues because of the above same reason, which indicate that the lateral monofluoro substituent appears to be much more effective to increase mesophase stability than the two lateral fluorines lying at both sides of the molecule.

Thermal stability
Thermogravimetric analysis (TGA) is used to measure the thermal stability of nPF (3)

The effects of alkoxy chain and terminal substituents on liquid crystalline properties
The effects of alkoxy chain on liquid crystalline properties are investigated, where the dependence of transition temperatures during heating on the number of methylenic units (n) in the alkoxy chain is shown in Figure 5. It is obvious that the length of the alkoxy chain plays an important role on both the mesophase type and the mesomorphic temperature range. It is noted that compounds nPF(3)PBN start to display smectic C mesophase from nematic mesophase when the length of the terminal alkoxy chain increases above 8 carbon atoms, which is due to the enhanced interaction between the terminal chains. On the other hand, the elongation of the terminal alkoxy chain generally produces an increase in the mesomorphic temperature ranges 39-52°C and 51-71°C for nPF (3) PBH, 76-94°C and 111-124°C for nPF(3)PBM,  (3)   indicating that terminal substituents are useful to enhance the mesophase stability. It is noted that compounds nPF(3)PBx with chloro substituent exhibit widest mesophase ranges among the corresponding homologues, where the terminal substituents enhance the mesophase ranges in the order Cl > NO 2 > CH 3 > H, which is not completely consistent with the polarity of these groups (NO 2 > Cl > CH 3 > H). These indicate that calamitic mesogenic compounds substituted with polar terminal groups generally lead to wider mesophase range, but too high dipole moment will hamper to increase the mesophase stability.

Photophysical properties
Compounds with π-conjugated fused rings generally have good photophysical properties such as intense luminescence. [19,20] Here, UV-vis and fluorescence spectra are used to characterise the photophysical properties of nPF(3)PBx, respectively, where the spectra are recorded in methylene chloride, as shown in Figures 7 and 8. From Figure 7, it is found that 7PF (3) PBx exhibit broad absorption bands with maxima at 323-326 nm, which is attributed to the electronic transition originating from the π-molecular orbitals. It is noted that, compared with unsubstituted homologues   (3)PBN) exhibit slightly red-shifted absorption bands, which is due to the impact of substituents on electronic properties resulted from σ-π, p-π and π-π conjugation, respectively. Nitro terminal-substituted compound 7F(3)PBN shows a very weak emission (Figure 8), which is a common phenomenon in nitro-substituted fluorophores. Except for 7PF(3)PBN, the other compounds exhibit intense photoluminescence emission bands at 389 nm (7PF(3)PBH), 390 nm (7PF(3)PBM) and 395 nm (7PF(3)PBC), respectively. It is noted that, compared with above UV-vis absorption spectra, the terminal substituents have same effects on photoluminescence emission, where methyl, chloro and nitro substituents enhance the photoluminescence emission because of σ-π, p-π and π-π conjugation, respectively.
Besides, compared with the photophysical properties of the two lateral fluorine-substituted analogues (compounds I in Figure 1), [14] compounds nPF (3) PBx show obviously red-shifted UV-vis absorption bands by 2-10 nm and red-shifted photoluminescence emission by 7-10 nm (see Table 2 in supplementary information), respectively, which suggest that π-π interaction between molecules is reinforced by the enhanced dipole-dipole interaction caused by increased dipole moments.

Conclusions
A series of nPF(3)PBx were prepared and their properties investigated. They show enantiotropic mesophases with the mesophase ranges of 39-163°C and 51-192°C during heating and cooling. Moreover, they exhibit UV-vis absorption bands with maxima at 323-326 nm and photoluminescence emission peaks at 389-395 nm, respectively. Compared with two lateral fluorine-substituted analogues (compounds I in Figure 1), it is found that lateral monofluoro-substituted nPF(3)PBx display enhanced mesophase ranges both in heating and cooling except for terminal methyl-substituted compounds, which are attributed to the enhanced dipole-dipole interactions caused by increased dipole moments. Meanwhile, compounds nPF(3)PBx show obviously red-shifted UV-vis absorption bands and photoluminescence emission than two lateral fluorine-substituted analogues, which suggest that π-π interaction between molecules is reinforced by the enhanced dipole-dipole interaction caused by increased dipole moment. These results indicate that the quantity and position of fluorine atom in molecule play important roles in mesomorphic and photophysical properties.

Characterisation and measurements
IR spectra (Nicolet Avatar360E spectrometer, Thermo Electron Corporation, Madison, WI, USA), 1H-NMR spectra (Bruker AV 300, Bruker Corporation, Karlsruhe, Germany), mass spectra (GC/EI-MS Thermo DSQ II, Thermo Finnigan, CA, USA) and Elemental analysis (Elementar Vario EL III instrument, Elementar Analysensysteme GmbH, Hanau, Germany) were employed to confirm the structures of the intermediates and final products. The phase transition temperatures were measured by DSC (Shimatsu DSC-60, Shimadzu Corporation, Kyoto, Japan) in nitrogen at a heating and cooling rate of 5°C min −1 , and POM (LEICA DM2500P, Leica Microsystems GmbH, Wetzlar, Germany) with Linkam THMS600 hot-stage (Linkam Scientific Instruments Ltd., Tadworth, UK) and control 340 unit at a heating and cooling rate of 0.5°C min −1 . TGA was carried out with a TA Q50 (TA Instruments, USA) in nitrogen (flow rate: 100 cm 3 min −1 ) at a heating rate of 10°C min −1 . 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) were used to investigate the photophysical properties of the products.
Then under nitrogen protection, 1.65 g of 4-formylphenylboronic acid (11.0 mmol), 0.1 g tetrabutyl ammonium bromide, 0.58 g of tetrakis (triphenylphosphine)palladium (0.5 mmol) and 10 mL of water were added. Then the reaction system was stirred at 100°C for 3 h under N 2 protection. After completion of the reaction, the mixture was diluted with water and extracted with ethyl acetate for three times. The combined organic phase was dried over MgSO 4 . After removal of the solvent in vacuo, the residue was purified via column chromatography on silica gel using PE/EA (30/1) as eluent to give purity above 98% for GC measurement. White crystals were obtained with yield 89%, mp 44.0-44.9°C.

Synthesis of 2-(((3ʹ-fluoro-4ʹ-octyloxy-1,1ʹbiphenyl-4-yl)methylene)amino)-4-methylphenol (8PF(3)PSM)
To a 100 mL, three-neck, round-bottom flask equipped with an overhead stirrer and condenser, 0.72 g of 3-fluoro-4-octyloxy-(1,1ʹ-biphenyl)-4ʹ-carboxaldehyde (2.2 mmol), 0.33 g of 2-amino-4methyl-phenol (2.64 mmol) and 25 mL of ethanol were added. The reaction system was stirred at reflux for 6 h. After the mixture was cooled to room temperature, it was filtrated through Celite. The solid was washed with 15 mL of ethanol for several times to give purity above 98%, suitable for HPLC or GC, yellow crystals, yield 91%; mp 97°C. To a 100 mL round-bottom flask equipped with an overhead stirrer and condenser, 0.46 g of 8PF(3)PSM (1.07 mmol), 0.30 g of DDQ (1.32 mmol) and 50 mL of anhydrous chloroform. The reaction system 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 MgSO 4 . After removal of the solvent in vacuo, the residue was purified through recrystallisation from ethanol to give purity greater than 98% for HPLC or GC measurements. Pale white crystals were obtained with yield 75% and mp 105°C.