Preparation and mesomorphic properties of 1-methyl-1H-benzimidazole-based compounds

ABSTRACT A series of 1-methyl-1H-benzimidazole-based compounds, 2-(4ʹ-alkoxy-1,1ʹ-biphenyl-4-yl)-1-methyl- 1H-1,3-benzimidazole derivatives (nPPMx-M) with terminal hydrogen, methyl and nitro moieties (coded as nPPMH-M, nPPMM-M and nPPMN-M, respectively), were prepared and their structures were characterised. The compounds display enantiotropic smectic mesophases for hydrogen and methyl terminated compounds (nPPMH-M and nPPMM-M), and enantiotropic nematic mesophases for nitro terminated compounds (nPPMN-M) with short alkoxy chain below than 10 carbon atoms, where the mesophase ranges are 24–72°C and 74–104°C on heating and cooling processes for nPPMH-M, 90–119°C and 110–135°C for nPPMM-M, and 102–129°C and 113–207°C for nPPMN-M, respectively. It is noted that the compounds nPPMx-M exhibit much lower melting points and much wider mesophase range both in heating and cooling than non-1-methyl substituted analogs, which are ascribed to the disruption of hydrogen bonding among the molecules caused by methyl substitution at 1-position of benzimidazole. Meanwhile, among the compounds nPPMx-M, much wider mesophase ranges are obtained for nPPMM-M and nPPMN-M, indicating a much high mesophase stability for the compounds bearing terminal moiety (CH3 and NO2). Graphical Abstract

Recently, we have reported series of benzimidazolebased mesogenic compounds, for which enantiotropic smectic liquid crystal phases and narrow mesophase ranges both in heating and cooling have been obtained [17]. The formation of smectic phase is attributed to the lateral intermolecular hydrogen bonds resulting from benzimidazole groups of the compounds. It is known to us that the introduction of a lateral or terminal fluorine atom into the mesogen is generally utilised to alter the mesophase morphology and properties in literatures [18][19][20][21][22][23]. To improve mesomorphic properties, we have developed series of monofluorinated benzimidazole-based mesogenic compounds. They display enhanced mesophase ranges and lower melting/clearing temperatures than non-fluorinated analogs, which are attributed to the disruption of the sideto-side intermolecular packing caused by the lateral fluoro substituent [24].

Preparation and characterisation
The compounds nPPMx-M, bearing alkoxy chain of 4-14 carbon atoms at one end and terminal moieties (H, CH 3 , NO 2 ) at the other end of the calamitic molecule, were prepared via reactions of nucleophilic substitution, Suzuki coupling, condensation and methyl substitution (nucleophilic substitution) in turn, as shown in Figure 1. The purities of the compounds are higher than 98% according to high performance liquid chromatography or gas chromatography (GC) results.
To confirm structures of the compounds nPPMx-M, spectra methods including infrared (IR), proton nuclear magnetic resonance ( 1 H-NMR), GC with electron impact-mass spectrometry (GC/EI-MS) and elemental analysis (EA) were utilised, and the data were listed in Experimental Part and Supplementary Materials. All the data confirm that the obtained compounds have the proposed structures of nPPMx-M.

Mesomorphic properties
Differential scanning calorimetry (DSC) and polarising optical microscopy (POM) were used to determine mesomorphic properties of the compounds nPPMx-M. Onset temperatures of DSC curves were used as phase transition temperatures of the compounds, and phase textures were confirmed via comparing with those reported ones. The repeatability of the transition temperatures and mesophase textures of the compounds nPPMx-M is good for the multiple heating/ cooling cycles. The phase transition temperatures, associate enthalpy changes and mesophase textures of the nPPMx-M are shown in Table 1 and Figure 2.
As listed in Table 1, the compounds nPPMx-M exhibit enantiotropic smectic mesophases for hydrogen and methyl terminated compounds (nPPMH-M and nPPMM-M) with alkoxy chain lengths of 4-14 carbon atoms, and enantiotropic nematic mesophases for nitro terminated compounds (nPPMN-M) with short alkoxy chain below than 10 carbon atoms. As shown in DSC  The corresponding mesophase shows a typical fanshaped focal conic smectic texture under POM observation ( Figure 2 (b)). On cooling process, the molten  8PPMH-M gives exothermic peaks at 198.2°C and 107.1°C due to the phase transitions (Figure 2 (a)), for which a typical fan-shaped focal conic smectic texture is observed (Figure 2 (c)). For methyl substituted compound 8PPMM-M, typical focal conic textures of SmC phase are observed on heating (Figure 2 (e)) or cooling (Figure 2 (f)) processes. Whereas for nitro substituted compound 8PPMN-M, typical schlieren textures of nematic phase are found under POM on heating (Figure 2 (h)) and cooling (Figure 2 (i)) processes, and very low enthalpy changes are obtained from DSC curve (Figure 2 (g)) for isotropic-nematic phase transition. As shown in Table 1, the compounds nPPMx-M give mesophase ranges of 24-129°C and 74-207°C for heating and cooling processes, respectively. Meanwhile, methyl and nitro substituted compounds (nPPMM-M and nPPMN-M) display a much larger mesophase ranges than nPPMH-M, indicating that the terminal moiety can improve the mesophase stability of the 1methyl-1H-benzimidazole-based compounds. This maybe due to the increased dipole-dipole interaction between the molecules for nPPMM-M and nPPMN-M. It is noted that the mesophase ranges of the compounds nPPMx-M are obviously larger than the ones (0-91°C) of the corresponding non-1-methyl substituted analogs [17] (Table S1 in Supplementary Materials). Especially, nPPMx-M with a short terminal alkoxy chains (4PPMx-M and 5PPMx-M) displays relative larger mesophase ranges on heating (24-107°C) and cooling (74-207°C, respectively) than the corresponding non-1-methyl substituted analogs (no mesophase at all) [17], as shown in Figure 3 and Table S1 in Supplementary Materials. It is reported that the lateral intermolecular hydrogen bonds are one of the main driving forces for the formation of smectic mesophase and high melting points for the corresponding non-1methyl substituted analogs [17]. For nPPMx-M, the improved mesophase stability is ascribed to the disruption of hydrogen-bonding interaction between the molecules via methyl substitution at 1-position of benzimidazole. In addition, nitro substituted compounds with shorter alkoxy chains of 4-8 carbon atoms give nematic phase with wide mesophase ranges on heating (102-110°C) and cooling (113-207°C), respectively.

Thermal stability
Thermal stability was characterised by thermo-gravimetric analysis (TGA) under a nitrogen atmosphere, as shown in Figure 4. Compared to the onset temperature of decomposition at 270°C for 4PPMH-M and 4PPMM-M, 4PPMN-M decomposes starting at 350°C and displays much larger weight remaining of about 68 wt.% after heating to 500°C, indicating that nitro terminal moiety is helpful to enhance the thermal stability [24]. High thermal stability of nitro substituted compound 4PPMN-M is ascribed to its enhanced strong π-π and dipole-dipole interaction between the molecules. The results prove that nPPMx-M is stable up to its clearing point, which is in accordance with DSC date in Table 1.

The dependence of liquid crystalline properties on alkoxy chain
In general, the alkoxy chain plays an important role in liquid crystalline properties including transition temperature and mesophase range. The dependence of transition temperature on carbon numbers (n) of the alkoxy chain is depicted in Figure 5    74°C to 104°C on heating and cooling for nPPMH-M ( Figure 5(a)), 90°C to 119°C and 111°C to 135°C for nPPMM-M ( Figure 5(b)), respectively. It is found that the mesophase range increases from 102°C to 129°C on heating for nPPMN-M ( Figure 5(c)), whereas the mesophase range decreases obviously from 207°C to 114°C in cooling with elongation of alkoxy chain from 4 to 7 carbons, and further elongation of alkoxy chain to 14 carbons results in an increase from 114°C to 166°C (Table 1). The compounds nPPMx-M with long terminal alkoxy chain show high mesophase stability, which is ascribed to the enhanced dipole-dipole interaction between the terminal alkoxy chains. This is in agreement with the literature results for rod-like mesogenic compounds [17,[25][26][27].
In addition, the melting temperatures decrease smoothly in ascending homologues, whereas with increase in carbon number (n), the clearing temperatures increase slightly and then decrease sharply for nPPMH-M, decrease slightly for nPPMM-M and increase gradually for nPPMN-M, respectively, as shown in Table 1 and Figure 5. It is noted that the compounds nPPMx-M exhibit much lower melting and clearing points but much wider mesophase range both in heating and cooling than non-1-methyl substituted analogs, which is ascribed to the disruption of hydrogen bonding among the benzimidazole-based molecules caused by methyl substitution at 1-position of benzimidazole moiety.

Conclusions
A series of 1-methyl-1H-benzimidazole-based compounds nPPMx-M with terminal hydrogen, methyl and nitro moieties were synthesised. They give enantiotropic liquid crystalline phases with mesophase ranges of 24-129°C and 74-207°C on heating and cooling processes. Among the compounds nPPMx-M, methyl and nitro substituted ones (nPPMM-M and nPPMN-M) display a much larger mesophase ranges than nPPMH-M, which is ascribed to the enhanced dipole-dipole interaction between the molecules. Compared to non-1-methyl substituted analogs, nPPMx-M shows much lower melting and clearing temperatures but much wider mesophase range both in heating and cooling processes, which maybe due to the disruption of hydrogen bonding among the molecules caused by methyl substitution at 1position of benzimidazole moiety.

Materials
The reagents were purchased from Sinopharm Chemical Reagent Co. and Aladdin Reagent Co. Prior to use, anhydrous potassium carbonate was dried at 150°C in vacuum and chloroform was dehydrated by pre-dried 4 Å molecular sieves, respectively.
To a 500-mL round-bottom flask equipped with an overhead stirrer, 4.00 g of 2-(4ʹ-octyloxy-1,1ʹ-biphenyl-4-yl)-1H-1,3-benzimidazole (10 mmol), 1.40 g of NaOH (35 mmol) and 100 mL of dry DMF were added. The reaction system was stirred at room temperature for 1 h. Then, 2.10 g of methyl iodide (14.8 mmol) in 100 mL of DMF was dropwised into the reaction system. The mixture was stirred at room temperature for 5 h. After completion of the reaction, the mixture was diluted with 300 mL of water, and the precipitate was filtrated to obtain white solid. The crude products were recrystallised from anhydrous ethanol for three times to give purity above 98% for GC measurement. White crystals were obtained with yield 63.5% and m.p. 136.1°C. The other compounds nPPMx-M were prepared using a similar procedure, and their spectroscopic data were listed in Supplementary Materials.