Improved nematic mesophase stability of benzoxazole-liquid crystals via modification of inter-ring twist angle of biphenyl unit

ABSTRACT A series of nematic heterocyclic liquid crystals, 2-(2ʹ-fluoro-4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-benzoxazole derivatives (coded as nPF(2)PBx) bearing hydrogen (nPF(2)PBH), methyl (nPF(2)PBM) and nitro (nPF(2)PBN) terminal groups, respectively, are developped and investigated. The results show that the nematic mesophase stability is obviously improved via a slightly increase in the inter-ring twist angle between the planes of two phenyl rings in the biphenyl unit, where modification of the twist angle can be achieved by adjusting the position of the lateral monofluorine substituent from the outer to the inter ring of the biphenyl mesogenic core unit. Meanwhile, the inter-ring lateral fluorine substituent results in a decrease in both melting and clearing points but only a nematic mesophase with an acceptably wide mesophase range for nPF(2)PBx, and thus a potential application in liquid crystal display mixtures is expected, especially for the compounds nPF(2)PBM and nPF(2)PBN. The reduced π–π interaction/conjugation caused by the increased twist angle is considered as one of the possible reasons for the formation of a nematic mesophase. GRAPHICAL ABSTRACT


Introduction
Many kinds of mesogenic heterocyclic compounds have been developed for fundamental research and potential application due to their optical and photochemical properties, where benzoxazole derivatives have been widely studied in recent years through being among the mesogenic heterocyclic compounds. [1][2][3][4][5][6][7][8][9][10] It is commonly believed that molecular order in liquid crystal phases depends largely on the mesogenic core structure, linking units as well as lateral and terminal groups, so altering these structures is regarded as an effective method to bring significant changes to mesomorphic properties. [11,12] The fluorine atom is widely used as a lateral or terminal substituent to modify the physical properties of liquid crystals. [13][14][15][16] In our previous reports, [5,10,17,18] several fluorinated biphenyl units were employed to build a mesogenic heterocyclic core with a benzoxazole group, where the liquid crystal properties of benzoxazole derivatives, such as melting and clearing points, mesophase type and range, as well as mesophase stability, vary with change in the position and quantity of the fluorine substituent. It was found that two lateral fluorine substituents led to an obvious decrease in melting point because of the disruption of the side-to-side intermolecular packing, [5,10] while the lateral monofluorine substituent ortho to benzoxazole unit resulted in high mesophase stability with a wide mesophase range due to the enhanced dipole-dipole interactions caused by increased dipole moments. [18] It was noted that benzoxazole-liquid crystals containing the 3ʹ-fluoro-1,1ʹ-biphenyl unit (reference compounds I, Figure 1) display multiple (smectic C and nematic) mesophases with a narrow nematic mesophase range. [17] However, 2,3-difluorophenyl unit-based fluorinated benzoxazole-liquid crystals (reference compounds II, Figure 1) exhibit enantiotropic nematic mesophases, [5] which indicate that these compounds are attractive from the practical application point of view, because few studies on nematic heterocyclic liquid crystals are reported in the literature. [5,19,20] The various liquid crystal properties are attributed to the position and quantity of fluorine substituent for benzoxazole derivatives bearing the 2,3difluoro phenyl and 3ʹ-fluoro-1,1ʹ-biphenyl units, where the fluorine substituent results in a change in the interring twist angle (θ) between the planes of two phenyl rings in the biphenyl unit, thus resulting in a change in molecule packing.

Synthesis and characterisation
A simple and straightforward synthetic methodology for the synthesis of nPF(2)PBx was adopted, as shown in Figure 2. Four-step reactions, including nucleophilic substitution, palladium-catalysed coupling reaction, nucleophilic addition and subsequent intramolecular cyclisation, were employed to prepare nPF(2)PBx of high purity (>98%) using high-performance liquid chromatography (HPLC) or gas chromatography (GC)). The compounds nPF(2)PBx possess a lateral fluorine substituent linked at the inter-ring of the biphenyl mesogenic core unit, an alkoxy chain attached at one end of the calamitic molecule, and terminal groups (H, CH 3 , NO 2 ) linked at the other end. The lateral fluorine substituent and nitro group in the molecule contribute to the large dipole moment, which has been confirmed. [18] The structures of the objects and intermediates were confirmed via 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 the Experimental and Supplementary information. The results demonstrate that the compounds thus obtained have the proposed structures for nPF (2)PBx and the corresponding Schiff base precursors nPF(2)PSx ( Figure 2).

Thermal properties
The mesomorphic properties of nPF(2)PBx and nPF (2) PSx were confirmed by differential scanning calorimetry (DSC) and polarising optical microscopy (POM), where the onset temperature of the transition on DSC curves was employed as the phase transition temperature. The phase transition temperature, associated enthalpy changes and mesophase textures of nPF(2)PSx and nPF(2)PBx are summarised in Tables 1 and 2, respectively, where the nematic-isotropic enthalpy changes are very low for both nPF(2)PBx and nPF (2)PSx. This can be interpreted in terms of the bent nature of these molecules, which enhanced their molecular biaxiality and hence reduced the entropy change at the transition. [21][22][23][24] It is noted that the nitro-terminated Schiff base intermediate nPF (2)PSN exhibit no mesophase on both heating and cooling, while nPF(2)PSH and nPF(2) PSM display a narrow smectic mesophase only for those bearing a long terminal alkoxy chain (n > 6) ( Table 1).
Compared to the Schiff base intermediates of the reference compounds I, [17] nPF(2)PSx have much lower mesophase stability with a much narrower mesophase range, which may be related to the twisted molecular shapes resulting from the inter-ring fluorine substituent on the biphenyl unit.
It is interesting to note that most nPF(2)PBx compounds show enantiotropic nematic mesophases during heating and cooling for terminal alkoxy chain lengths of 2-8 carbons, whereas nitro-terminated compounds nPF(2)PBN with a long terminal chain display multiple (smectic C and nematic) mesophases (n = 7-8) and smectic C mesophase (n = 10) during heating or cooling ( Table 2). It is known that mesomorphic properties can be affected by the mesogenic core structure, C Figure 2. Synthesis of nPF(2)PSx and nPF(2)PBx.Conditions: i: C n H 2n+1 Br, K 2 CO 3 , KI, DMF; ii: Pd/C, K 2 CO 3 , TBAB, DMF; iii: CH 2 Cl 2 , reflux; iv: CHCl 3 , reflux.  linking units as well as lateral and terminal groups. The dependence of transition temperature and mesophase type on the number (n) of methylene units in the terminal alkoxy chain is shown in Table 2 and Figure 3. The formation of the smectic C mesophase for nPF(2)PBN with a long terminal chain is attributed to the enhanced induced dipole-induced dipole interaction between the terminal chains. Meanwhile, it was found that an increase in terminal length resulted in enhanced nematic mesophase ranges from 0 to 24 K and 19 to 40 K on heating and cooling for nPF(2)PBH, 70-86 K and 99-128 K for nPF(2)PBM, and 69-129 K and 76-155 K for nPF(2)PBN (n = 2-6). The mesophases of typical compounds 6PF(2)PSH, 8PF(2)PBM and 7PF(2)PBN are assigned based on their typical marble, focal conic and schlieren textures during both heating and cooling, as shown in Figure 4. The compounds nPF(2)PBx have high thermal stability ( Figure 5), where the onset temperatures of decomposition are 273, 301 and 298°C for 8PF(2) PBH, 8PF(2)PBM and 8PF(2)PBN, respectively, which indicates that they display stable mesophase up to their clearing points. Meanwhile, among the compounds nPF(2)PBx, although nitro-substituted compound 8PF (2)PBN displays a slightly lower onset temperature of decomposition than 8PF(2)PBM, the former exhibits the highest thermal stability with weight remaining >40% at nearly 500°C. This is attributed to its strong dipole-dipole interactions caused by the strongly polar nitro terminal substituent at the 5-position of the benzoxazole unit.

Dipole moment and inter-ring twist angle of biphenyl unit
The dipole moments of compounds nPF(2)PBx and reference compounds I are calculated according to density functional theory (DFT) using the Gaussian 09 programme package, [25] where 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. [26] The results are listed in Table 3. Similar to results from the literature, [18] compounds with the nitro terminal group exhibit large dipole moments for both nPF(2)PBx and reference compounds I, which is due to the strong electro-withdrawing ability of the nitro group among the present terminal substituents (H, CH 3 and NO 2 ). Compared with reference compounds I, nPF(2)PBx display larger dipole moments by about 2D (Table 3) because the polar lateral fluorine substituent produces the same polarity direction as the polar benzoxazole unit.
The inter-ring twist angle, θ, plays an important role in the molecular packing of biphenyl-based materials and thus affects their physical properties. Here, θ is calculated using DFT. [25] As shown in Figure 6, different types of fluorinated biphenyl have different θ values. It is obvious that biphenyls bearing a fluorine atom at the inter-ring have a higher twist angle (about 40°) than other fluorinated biphenyl structures such as 2-fluoro-1,1ʹ-biphenyl and 2,3-difluoro-1,1ʹ-biphenyl, while biphenyl containing a fluorine atom at the  (2) outer ring has almost the same twist angle as nonfluorinated biphenyl (37-38°). For comparison, the θ value of compounds nPF(2) PBx and a series of benzoxazole analogues containing different fluorinated biphenyl units is calculated and shown in Figure 7. As expected, those compounds with a fluorine substituent at the inter-ring of the biphenyl unit, such as nPF(2)PBx and reference compounds II, show a higher value of θ (nearly 38°) than the other analogues (about 35°), which is attributed to the steric hindrance of the lateral inter-ring fluorine substituent, which has a greater atomic radius than hydrogen. It is revealed that high values of θ can be obtained via transforming the position of the fluorine substituent from the outer to the inner ring (either 2-position or 2ʹ-position). For the Schiff base precursors of these benzoxazole analogues, the same result was found. The calculated θ values of compounds nPF(2)PBx and the series of benzoxazole analogues are consistent with the values of biphenyl (30-40°) in the literature. [27][28][29] 2.4. The effects of twist angle and dipole moment on mesomorphic and photophysical properties According to the θ values shown in Figure 7, the lateral fluorine substituent has a key role in the twist angle. By adjusting the location of the lateral fluorine substituent, most compounds nPF(2)PBx display lower melting and clearing points than reference compounds I, this being related to the increased twist angles derived from the inter-ring fluorine substituent, because these have nearly the same aspect ratio (molecular length to breadth) and molecular shape. However, compared with the reference compounds I, nitro-terminated compounds nPF(2)PBN exhibit improved mesophase stability with a wider mesophase range, due to their enhanced dipole-dipole interactions resulting from their much greater dipole moments (Table 3). In general, molecular shape and excluded volume also affect nematic-isotropic transition temperatures of liquid crystals. [30][31][32] Compared with non-fluorinated compounds, [4] fluorinated analogues, both nPF(2)PBx and reference compounds I, have much lower clearing points, which may be attributed to a larger free volume associated with the lateral fluorine substituent. [30] As expected, compounds nPF(2)PBx display enantiotropic nematic mesophases with acceptably wide mesophase ranges, especially for those compounds with methyl and nitro end groups (nPF(2)PBM and nPF(2)PBN). The formation of a nematic mesophase is attributed to a strengthened twisted arrangement of the phenyl groups resulting from the increased θ, which allows a better steric fit of the molecules packing together in a locking arrangment. [30] This is consistent with our recent results, [33] suggesting that modification of the twist angle by the lateral fluorine substituent is an efficient methodology to improve the mesophase type and stability of mesogenic heterocyclic compounds. Where θ increases by shifting the fluorine substituent from the 3-position to the 2-position of the biphenyl unit, which leads to a decrease in π-π interaction/conjugation (one of the main driving forces for the formation of mesophase) between molecules, this eliminates the smectic mesophase and improves nematic mesophase stability (Table S1 in supplementary information).
The decrease in π-π interaction/conjugation derived from the increased θ can be verified by comparison of the photophysical properties of nPF(2)PBx and reference compounds I. As shown in Figure 8, nPF(2)PBx display broad UV absorption bands with maxima at 319-323 nm, which are slightly blue-shifted by 3-5 nm in comparison to those of reference compounds I (Table S2 in   suggest that nPF(2)PBx have reduced π-conjugation because UV absorption is related to the electronic transition originating from the π-molecular orbital, where the decrease in π-π interaction/conjugation is attributed to the increase in θ . From Figure 9, it is seen that nPF(2) PBx exhibit slightly blue-shifted photoluminescence emission, by 4-5 nm more than reference compounds I (Table S2 in supplementary information), which also indicates a decrease in π-π interaction/conjugation caused by the increase in θ for nPF(2)PBx.

Conclusions
This study has highlighted the importance of the paramter θ in determining the type and stability of the mesophase, where the twist angle can be modified by transforming the position of the fluorine substituent on the biphenyl unit. In summary, a series of nematic benzoxazole-based liquid crystals were prepared and investigated. They displayed enantiotropic nematic mesophases with mesophase ranges of 0-129 and 19-155 K on heating and cooling, respectively, which is attributed to a decrease in π-π interaction/conjugation between molecules caused by an increase in θ. Compared with the reference compounds I, the compounds nPF(2)PBx exhibited lower melting and clearing points, as well as a blue-shifted UV absorption band and photoluminescence emission, because of their reduced π-π interaction/conjugation. These results suggest that modification of the twist angle for the mesogenic unit is an effective method  to improve the mesophase type and stability of heterocyclic liquid crystals. The compounds nPF(2)PBx have potential for use in liquid crystal display mixtures.

Materials
All 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 intermediates were confirmed by IR spectra (Nicolet Avatar360E spectrometer, Thermo  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.

Synthesis of 2-(2'-fluoro-4'-heptyloxy-1,1'biphenyl-4-yl)-5-methylbenzoxazole (7PF(2)PBM)
To a 100 mL round-bottom flask equipped with an overhead stirrer and condenser, 0.50 g of 7PF(2)PSM (1.19 mmol), 0.33 g of DDQ (1.43 mmol) and 25 mL of anhydrous chloroform were added. 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 three times. The combined organic phase was dried over MgSO 4 . After removal of the solvent in vacuo, the residue was purified by recrystallisation from ethanol to give purity >98% for HPLC or GC measurement. White crystals were obtained with yield 81% and m.p. 97.4°C.  The other lateral monofluoro-substituted compounds (nPF(2)PBx) were prepared using a similar procedure, and their spectroscopic data are listed in Supplementary Information.