Nematic mesophase enhanced via lateral monofluorine substitution on benzoxazole-liquid crystals

ABSTRACT Fluorine is widely used as a lateral substituent to modify the physical properties of liquid crystals. Here, laterally monofluorinated compounds, 2-(4ʹ-alkoxy-2-fluorobiphenyl-4-yl)-benzoxazole derivatives (nPPF(2)Bx) bearing different substituents (H, CH3, NO2, coded as nPPF(2)BH, nPPF(2)BM and nPPF(2)BN, respectively) at 5-position, were synthesised and characterised. It is interesting to note that these only display enantiotropic nematic mesophases with mesophase ranges of 12–28°C and 13–45°C on heating and cooling for nPPF(2)BH, 46–97°C and 62–120°C for nPPF(2)BM and 82–108°C and 87–113°C for nPPF(2)BN, which are very different from the corresponding monofluorine-substituted analogue (compounds I) with enantiotropic smectic or smectic/nematic mesophases. The enhanced nematic mesophase is attributed to the reduced π–π interaction/conjugation resulting from the twisted structure of the molecule caused by the introduction of a fluorine atom into the inter-ring of the biphenyl unit. These results suggest that modification of the monofluorine substituent position is an effective method to improve the nematic mesophase in benzoxazole-liquid crystals. GRAPHICAL ABSTRACT


Introduction
Fluorinated liquid crystals are widely used in liquid crystal displays due to their low viscosities and melting points, as well as improved dielectric properties. [1,2] The effects of lateral fluorine substituents on the physical properties of liquid crystal have been investigated in detail. [3][4][5] In general, the introduction of a fluorine atom into the p-terphenyl skeleton results in a decrease in both the melting and clearing points, and also in the appearence of a narrow nematic mesophase after a wide smetic mesophase. [6][7][8] Benzoxazole-liquid crystals are among the mesogenic heterocyclic compounds and have been widely studied in recent years, [9][10][11][12][13][14][15][16][17][18] including several kinds of benzoxazole derivatives bearing lateral fluorine atoms in the mesogenic core. However, most of these exhibit a smectic mesophase, such as our previously studied lateral monofluorinated benzoxazole derivatives (reference compounds I in Figure 1), [19] which restrict their further possible application in liquid crystal display mixtures because of their poor nematic mesophase.
To develop new nematic heterocyclic liquid crystals, and also to further investigate the effect of the monofluorine substituent on mesophase type and stability, benzoxazole derivatives bearing a lateral monofluorine atom on the inter-ring of the biphenyl unit (coded as nPPF(2)Bx in Figure 1) were synthesised and their properties investigated. Meanwhile, the twist angles of two phenyl rings in the biphenyl unit for nPPF (2)Bx and reference compounds I were calculated according to density functional theory [20] to examine the effect of twisted molecule structures resulting from monofluorine substitution on mesophase type and stability.

Synthesis and characterisation
As shown in Figure 2, nucleophilic substitution, palladium-catalysed coupling reaction, nucleophilic addition and subsequent intramolecular cyclisation were employed to prepare nPPF (2)Bx with overall yields of 15-53%, with purity greater than 98% (high-performance liquid chromatography (HPLC) or gas chromatography (GC)) being achieved for the objects and intermediates. For the compounds nPPF(2)Bx bearing different terminal substituents of H, CH 3 and NO 2 , these can be subdivided into nPPF(2)BH, nPPF(2)BM and nPPF(2)BN.
The structures of nPPF(2)Bx and the corresponding Schiff base precursors (nPPF(2)Sx, Figure 2) were confirmed by infrared (IR), proton nuclear magnetic resonance ( 1 H-NMR), gas chromatography with electron impact-mass spectrometry (GC/EI-MS), and elemental analysis (EA), as shown in Experimental and Supplementary Information. The results demonstrate that the obtained compounds have the proposed structures for the intermediates and objects.

Thermal properties
Differential scanning calorimetry (DSC) and polarising optical microscopy (POM) were employed to confirm the mesomorphic properties, where the phase transition temperatures were determined according to the onset values of the DSC curves, and are summarised in Tables 1 and 2 for nPPF(2)Sx and nPPF(2)Bx, respectively. Note that most of the Schiff base precursors nPPF(2)Sx display no mesophase on heating (Table 1), except for three compounds bearing a long terminal alkoxy chain (n = 8, 10). Compared with the Schiff base precursors of reference compounds I, the present Schiff base compounds exhibit poor mesophase stability with a narrow smectic mesophase (10-22°C).  For compounds nPPF(2)Bx, it is interesting to note that these only display enantiotropic nematic mesophases during heating and cooling, except for two nitro-substituted compounds containing a long terminal alkoxy chain with enantiotropic multiple mesophases (smectic and nematic) for 8PPF(2)BN or a  Heating process Cooling process 2PPF smectic mesophase for 10PPF(2)BN, as shown in Table 2. The terminal alkoxy chain generally influences the mesomorphic temperature range and mesophase type. As shown in Figure 3 and Table 2, with elongation of the terminal alkoxy chain the nematic mesophase ranges increase from 12 to 28°C and 13 to 45°C on heating and cooling for nPPF(2)BH, 46-97°C and 62-120°C for nPPF(2)BM and 82-108°C and 87-113°C for nPPF(2)BN (n = 2-7). Mesophases are assigned from the typical marble textures, focal conic textures and schlieren textures during both heating and cooling for typical compounds 6PPF(2)BH, 7PPF(2)BM and 8PPF(2)BN, respectively, as shown in Figure 4. In addition, the enthalpy value of the mesophase transition can also be used to confirm the mesophase type, where the enthalpy values of clearing points lower than 1 kJ mol −1 suggest the transition from nematic to liquid phase. The low enthalpy values can be interpreted in terms of the bent nature of nPPF(2)Bx molecules, which enhances their molecular biaxiality and hence reduces the entropy change at the transition. [21][22][23][24] The thermal stability of compounds nPPF(2)Bx were measured at 30-600°C under a nitrogen atmosphere, with the TGA curves shown in Figure 5. The compounds display the onset of decomposition at 278, 302 and 323°C for 8PPF(2)BH, 8PPF(2)BM and 8PPF (2)BN, respectively, which are higher than their clearing points, suggesting a stable mesophase up to their clearing points. Meanwhile, it was found that 8PPF (2) BN has the highest decomposition temperature because of its strong dipole-dipole interaction caused by the strongly polar nitro terminal substituent.

Effect of fluorine substituent on twist angle
Biphenyl is used as a rigid core to construct many mesogenic compounds, where the dihedral angle, θ, the inter-ring twist angle between the planes of two phenyl rings plays an important role in determining the physical properties of biphenyl-based materials and their molecular packing. The twist angles of biphenyl units in different molecules are calculated according to density functional theory (DFT), where full geometic optimisation was carried out without imposing any constraints using the Gaussian 09 program package, [20] and spin-restricted DFT calculations were carried out within the framework of the generalised gradient approximation (GGA) using the B3LYP exchange-correlation functional and the 6-31G (d, p) basis set. [25] For comparison, the twist angles of nPPF(2)BM and the corresponding reference analogues (compounds I and non-fluorinated compounds II) were calculated and are shown in Figure 6, where a carbon atom in the alkoxy terminal chain is fixed at 6 as an example. As expected, the position of the fluorine substituent on the biphenyl unit plays a key role in the twist angle between the planes of the two phenyl rings. The calculated value of θ for the conresponding non-fluorinated reference compounds II is about 35°, which is consistent with the reported values of biphenyl in the range 30-40°. [26][27][28] For the corresponding reference compounds I, the calculated value of θ is also about 35°, which is identical to that of the non-fluorinated compounds II, indicating that the substituent at the outer ring of the biphenyl unit has no effect on the arrangement of the two phenyl rings. It is obvious, however, that 6PPF(2)BM with the fluorine substituent at the inter-ring of the biphenyl unit has a value of θ nearly 38°, which is obviously higher than both corresponding reference compounds I and II. The large twist angle is due to the replacement of a hydrogen atom by fluorine of large atomic radius at the inter-ring of the biphenyl unit. The same phenomenon about the effect of the fluorine substituent on θ is also found for the Schiff base intermediates among nPPF(2)Sx, the Schiff base precursors of reference compounds I and II.

Effects of fluorine substituent on mesomorphic and photophysical properties
By adjusting the location of the lateral fluorine substituent, compounds nPPF(2)Bx exhibited slightly lower mesophase stability and narrower mesophase ranges than the reference compounds, which is similar to their corresponding Schiff base intermediates nPPF (2) Sx with much lower mesophase stability. The narrow mesophases are mainly attributed to their lower clearing points resulting from their reduced dipole-dipole interactions in comparison to the reference compounds I, where the polar lateral fluorine substituent produced opposite polarity direction with the polar benzoxazole unit for nPPF (2)Bx. In addition, the molecular shape and excluded volume also affect nematic-isotropic transition temperatures of liquid crystals. [29][30][31] It is interesting to note that compounds nPPF(2)Bx display enantiotropic nematic mesophases with acceptably wide mesophase ranges, especially for compounds with methyl and nitro end groups (nPPF(2)BM and nPPF(2)BN). The formation of a nematic mesophase is attributed to a strengthened twisted arrangement of the phenyl groups with large θ, which allows a better steric fit of the molecules packing together in a locking arrangment. [29] For reference compounds I and II, although both have the same ratio of length to width of molecules with nPPF(2)Bx (Table 3), they display enantiotropic multiple (smectic and nematic) mesophases. This difference in mesophase indicates that the modification of twist angle by the lateral fluorine substituent can improve the mesophase type and stability of the compoundsin other words, a lateral fluorine substituent at the inter-ring of the biphenyl unit eliminates the smectic mesophase and improves nematic mesophase stability (Table S1 in supplementary information). This is in accordance with literature results. [13] The large inter-ring twist angle of the biphenyl unit results in a decrease in π-π interaction/ conjugation between molecules, which is one of the    (Figure 7), which are slightly blue-shifted by 2-5 nm in comparison to those of reference compounds I (Table S2 in supplementary information). It is known that UV absorption is related to electronic transition originating from the π-molecular orbital, so the blue-shifted UV absorption bands suggest that nPPF(2)Bx has reduced π-conjugation because of the large twist angle resulting from the presence of a lateral fluorine substituent at the inter-ring of the biphenyl unit. Emission spectra of series nPPF(2)Bx are shown in Figure 8. The lateral fluorine substituent has the same effect on photoluminescence emission, where nPPF(2)Bx exhibit slight blue-shifted photoluminescence emission, by 2-3 nm, in comparison to those of reference compounds I (Table S2 in supplementary  information), which is also due to the reduced π-conjugation caused by the large twist angle arising from the inter-ring lateral fluorine substituent.

Conclusions
A series of nematic heterocyclic liquid crystals were prepared. These displayed enantiotropic nematic mesophases with mesophase ranges of 12-108°C and 13-120°C on heating and cooling. Compared with the reference compounds I, the compounds nPPF(2)Bx exhibited lower clearing points because of their reduced dipole-dipole interactions between molecules. It is interesting to note that compounds nPPF(2)Bx displayed only a nematic mesophase instead of multiple mesophases, suggesting that reduced π-π interaction/conjugation caused by a large twist angle is helpful in modifying the mesophase type and constructing the nematic mesophase. Meanwhile, the inter-ring lateral fluorine substituent also played an important role in photophysical properties due to the decrease in π-conjugation resulting from the increased twist angle. These results suggest that change of the twist angle for mesogenic units is an effective method to modify the mesophase type and stability of heterocyclic liquid crystals. The compounds nPPF(2)Bx have potential for use in liquid crystal display mixtures because of their acceptably wide nematic mesophase ranges.

Materials
All the reagents were purchased from Sinopharm Chemical Reagent Co. and Aladdin-reagent Co. The reagents were used as received except that anhydrous potassium carbonate was vacuum-dried at 150°C prior to use. Chloroform was dehydrated by pre-dried 4 Å molecular sieves which were activated at 350°C for 4 h prior to use.

Characterisation and measurement
The structures of the objects and the intermediates were confirmed via IR spectra (Nicolet Avatar360E spectrometer, Thermo Electron Corporation, Madison, WI, 325 USA), 1 H-NMR spectra (Bruker AV 300, Bruker Corporation, Karlsruhe, Germany),

Synthesis
Preparation procedure of 2-(4ʹ-heptyloxy-1,1ʹ-biphenyl-2-fluoro-4-yl)-5-methyl benzoxazole (7PPF(2)BM) is described below as an example.  1 mmol) and 20 mL of water were added. The reaction system was stirred at 80°C for 4 h under N 2 protection. After completion of the reaction, the mixture was diluted with water and extracted with methylene chloride three times. The combined organic phase was dried over MgSO 4 . After removal of the solvent in vacuo, the residue was purified by column chromatography on silica gel using PE/EA (50/1) as eluent to provide purity >98% for GC measurement. White crystals were obtained with a yield of 80% and m.p. 50.6-52.0°C.  The other 4-alkoxy-(1,1ʹ-biphenyl)-2ʹ-fluoro-4ʹ-carboxaldehydes (nPPF(2)CHO) were prepared using a similar procedure, and their spectroscopic data are listed in Supplementary Information. The other Schiff base compounds (nPPF(2)Sx) were prepared using a similar procedure, and their spectroscopic data are listed in Supplementary Information. To a 100 mL round-bottom flask equipped with an overhead stirrer and condenser, 0.10 g of 7PPF(2)SM (0.24 mmol), 0.07 g of DDQ (0.29 mmol) and 25 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 >98% for HPLC or GC measurements. White crystals were obtained, with yield 92% and m.p. 84.7°C.  The other lateral monofluoro-substituted compounds (nPPF(2)Bx) were prepared using a similar procedure, and their spectroscopic data are listed in Supplementary Information.