Synthesis and liquid crystalline property of H-shaped 1,3,4-thiadiazole dimers

Two H-shaped liquid crystalline 1,3,4-thiadiazole dimers D1 and D2 and the corresponding monomers M1 and M2 were synthesised and characterised by 1H/13C nuclear magnetic resonance (NMR) and high-resolution mass spectrometry. The thermal properties of the dimers and monomers were investigated by polarised optical microscopy, differential scanning calorimetry and thermogravimetric analysis. All of these compounds exhibited liquid crystalline behaviours with excellent thermal stabilities. The dimers displayed an enantiotropic or a monotropic nematic phase, while the monomers showed nematic and/or smectic A phases enantiotropically. The nature of the mesophases was determined by the molecular shapes and the terminal groups. Notably, the H-shaped dimers exhibited much lower clearing temperatures than the corresponding rod-like monomers.


Introduction
Over the past decade, 1,3,4-thiadiazole-based liquid crystals have drawn a continuing interest in display materials and electronic devices due to their excellent thermal stability, electron-deficient nature and photoluminescent properties. [1] Many researchers, for example, Parra, [2,3] Gallardo, [4,5] Sato, [6] Lehmann, [7] Tschierske, [8] Lemieux, [9] Li, [10] Kelly, [11] Tandel, [12] and our research group [13][14][15] have recently reported various mesogenic 1,3,4thiadiazole derivatives, and the relationship between the structures and liquid crystalline properties has been investigated intensively. Compared to the bending angle 134°of analogous 1,3,4-oxadiazoles, [16] the 1,3,4-thiadiazole moiety has a larger bending angle of 168°, [17] which makes the molecular shape more linear. In addition, the lateral dipole and polarisability of the sulphur atom are larger than those of the oxygen atom. All of these facts facilitate to form stable thermotropic mesophases with very high melting and clearing points, [17,18] which often bring about some technical difficulty in practical application as well as characterisation of the nature of the mesophases.
Liquid crystalline dimers are referred to the molecules composed of two mesogenic units linked by a flexible spacer. These kinds of materials often exhibited different liquid crystalline behaviour from that of the corresponding monomers and thus attracted intense attention in recent years. [19][20][21][22] Among many types of liquid crystalline dimers, the H-shaped ones are special because they can exhibit interesting photoluminescent, [23] electro-optic, [24] photochromism, [25,26] and second-order non-linear properties. [27] In addition, such kinds of liquid crystals usually display lower melting and clearing temperature points owing to the linking spacer. [28,29] In order to develop 1,3,4thiadiazole-based liquid crystals with low temperature range, two H-shaped dimers D 1 and D 2 and the respective monomers M 1 and M 2 were synthesised, and the molecular structures and the reaction conditions are shown in Scheme 1. The thermal behaviours have been investigated with an emphasis on the relationship between the molecular structures and liquid crystalline properties.

Results and discussion
The mesomorphic properties of H-shaped dimers D 1 and D 2 and their corresponding monomers M 1 and M 2 were studied by polarising optical microscopy (POM) equipped with a hot stage. The mesophases were identified according to the textures observed under POM. All the H-shaped dimers and the monomers displayed enantiotropic or monotropic nematic phases, which were assigned by the typical schlieren textures. For the monomer M 2 , it further exhibited an enantiotropical smectic A phase with rarely observed oily streak texture. [30] The selected texture pictures are shown in Figure 1. The liquid crystalline properties were also investigated by differential scanning calorimetry (DSC), which are well consistent with the POM observation results. The phase transition temperature and associated enthalpy changes derived from DSC measurements are listed in Table 1 and in the supplementary information (Figure S1-4).
As seen in Table 1, both of the rod-like monomers M 1 and M 2 exhibited liquid crystalline behaviours in wide temperature ranges with very high clearing points. In sharp contrast, the corresponding H-shaped dimers D 1 and D 2 have much lower clearing points but narrower mesomorphic temperature ranges. Compared to the rod-like monomers, the distances among the H-shaped molecules become larger and weaken the intermolecular interactions, which may greatly decrease the clearing points as well as the ability to form stable mesophases. It is well known that the conventional low molar mass mesogens with lateral chains often show relatively low clearing enthalpies. [31] The H-shaped dimers can be regarded as monomers with lateral chains, and it is easy to understand the lower clearing enthalpies of the Hdimers. [32] In addition, the H-shaped mesogens exhibit high molecular biaxiality, which may reduce the orientational order at the nematic-isotropic transition and decrease the melting and clearing points greatly. [33,34] The liquid crystalline behaviours were also affected by the nature of the terminal groups. For example, the monomer M 1 exhibited a nematic mesophase, probably because the dynamic nature of the freely rotating methoxy group might coherently destroy the positional orders and result in smaller dipole molecular moment. In the case of M 2 with a terminal fluoro substituent, it displayed an additional smectic A mesophase besides the nematic mesophase. The secondary weak interactions such as nonbonded F … F interactions and noncovalent C-H … F hydrogen bonds are believed to stabilise the packing of molecules and lead to the appearance of higherorder smectic A mesophase. The effect of the terminal groups on liquid crystalline properties has also been Scheme 1. Synthetic route of the H-shaped dimers D 1 and D 2 and the rod-like monomers M 1 and M 2. discussed in literature. [17] The effect of polarisability and shape on thermal behaviours of liquid crystal dimers has also been reported by Yeap et al. recently.
[35] Figure 2 shows thermogravimetric analysis (TGA) curves of compounds D 1 , D 2 , M 1 and M 2 , which show that all these solid samples exhibited no apparent weight loss in the temperature range 25-150°C, indicative of the absence of occluded solvent. All of the solid samples started to lose weight above 320°C in a single step with different onset decomposition temperatures (342°C for D 1 , 321°C for D 2 , 343°C for M 1 , 316°C for M 2 ), which are consistent with the results observed under POM. The thermal stability of this class of compounds is mainly affected by the conjugated core and the terminal groups, while the effect of the molecular shapes on the thermal stability is very slight.

Experimental section 3.1 General methods
4-Decyloxybenzoic acid 9 was prepared according to the reported procedure. [36] All other chemicals were commercially available and used as received. Nuclear magnetic resonance (NMR) spectra were obtained using a Bruker AVANCE-400 MHz DRX FT-NMR spectrometer (Bruker, Switzerland) with chemical shifts (in ppm) relative to tetramethylsilane. High-resolution electron ionisation mass spectra (HRMS) were recorded by a Finnigan MAT 95 mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA).
The phase transition temperatures and enthalpy changes were measured on a Perkin-Elmer DSC-7a (NETZSCH, Bavaria, Germany) differential scanning calorimeter with a heating rate of 10°C/min and precalibrated with a pure indium sample. The optical textures were observed on an optical polarised microscopy OLYMPUS BX51 (OLYMPUS, Tokyo, Japan) equipped with a heating stage. TGAs were performed on a Perkin-Elmer TGA-7 (NETZSCH, Bavaria, Germany) with a heating rate of 10°C/min under a stream of flowing nitrogen.

Synthesis of 2a
To a round-bottom flask were added 4-acyloxybenzoic acid 1a (5.0 g, 27.8 mmol) and thionyl chloride   (10 mL). The reaction mixture was refluxed for 5 h and the excessive thionyl chloride was removed by vacuum distillation to give the 4-acyloxybenzoyl chloride. Then, the solution of 4-methoxybenzoic hydrazide (4.6 g, 27.8 mmol) in dry pyridine (15 mL) was added dropwise to the as-formed 4acyloxybenzoyl chloride. The reaction mixture was stirred overnight at room temperature. Crude solid was precipitated by pouring the reaction mixture into distilled water (50 mL). The solid was washed with distilled water and recrystallised from ethanol to afford 6.6 g of 2a as a white solid. Yield: 72%. 1

Synthesis of 4a
To a round-bottom flask were added the ester 3a (2.0 g, 6.0 mmol), KOH (2.2 g), water (4 mL) and ethanol (30 mL), and the reaction solution was refluxed for 6 h. Then, the solution was cooled to room temperature and poured into ice-cooled water (100 mL). The resulting mixture was acidified with diluted HCl to precipitate the crude product, which was collected by filtration, dried and recrystallised from petroleum ether/dichloromethane to yield 1.6 g of 4a as a yellow solid. Yield, 92%. 1

Synthesis of 4b
To a round-bottom flask with a solution of 3b (1.6 g, 5.6 mmol) in dry chloroform (20 mL), BBr 3 (1.8 g, 7.2 mmol) was added dropwise at −78°C. The reaction mixture was stirred overnight at room temperature. The solvent was removed under vacuum, and the crude product was recrystallised from dichloromethane to afford 1.4 g of 4b as a yellow solid. Yield, 90%. 1

Synthesis of 8
To a round-bottom flask were added a suspension of 7 (2.0 g, 2.9 mmol) in ethanol (30 mL) and a solution of potassium hydroxide (1.6 g, 29 mmol) in water (10 mL), and the reaction mixture was refluxed for 8 h. After cooling to room temperature, the mixture was poured into ice water (50 mL) and neutralised with hydrochloric acid. The crude solids were collected and recrystallised from petroleum ether/ethanol to yield the product 8 (1.8 g, 95%) as white solids. 1  3.1.8 General procedure for synthesis of D 1 , D 2 , M 1 and M 2 These four compounds were prepared according to the same procedure. As an example, the synthesis of D 1 was described as follows. To a round-bottom flask were added the acid 8 (335 mg, 0.5 mmol) and thionyl chloride (10 mL). The mixture was refluxed for 4 h and the excessive SOCl 2 was removed by vacuum distillation, and the residue was dissolved in dry pyridine (10 mL), which was added dropwise to the solution of 4a (312 mg, 1.1 mmol) in dry pyridine (10 mL). The reaction mixture was stirred overnight at room temperature. After cooling, the reaction solution was poured into distilled water (50 mL) to precipitate the crude white solid, which was purified by silica gel column chromatography using ethyl acetate/dichloromethane (v/v = 1:10) as an eluent.