New liquid crystals derived from thiophene connected to the 1,2,4-oxadiazole heterocycle

ABSTRACT In this study, eight new compounds derived from thiophene were synthesised and characterised. The four symmetrical compounds contain two heterocyclic 1,2,4- oxadiazole on each side of the 2,5-disubstituted thiophene, and the four non-symmetrical compounds contain alkyne groups as a spacer replacing one of these heterocycles. Some of the compounds presented liquid crystalline properties with smectic and nematic mesophases. The non-symmetrical compounds presented low emission of blue light. GRAPHICAL ABSTRACT


Introduction
Over the past few decades liquid crystals (LCs) have received special attention due to the possibility of reorientation with electric and magnetic fields [1,2], which allows control of their self-organising ability [3][4][5]. Thus, these materials have become good candidates for technologic application in electro-optic displays, in solar cells [6], as sensors [7], for high-density memory storage [8] and in light-emitting diodes (OLEDs) [9].
The design of new molecules is important to gaining a better understanding of the mesophase formation and the influence on the LC properties. Heterocycles with five-ring members have displayed non-linearity and less mesogenic stability [10][11][12][13][14][15]. However, these heterocycles generally have heteroatoms (N, O, S) in their structure, giving the molecules a longitudinal dipole moment, which can induce interesting liquid crystal properties [10,16]. There are many reports in the literatures on 1,2,4-oxadiazole heterocycle derivatives with good thermal stability and emission in the blue region of the visible electromagnetic spectrum [17][18][19][20]. Liquid crystalline compounds can present interesting optical and mesomorphic properties influenced by the curvature [21][22][23][24][25][26].
Studies on the 1,2,4-oxadiazole heterocycle connected to thiophene, which presents liquid crystals properties, appear to be absent from the literature. Thus, eight new compounds of the 1,2,4-oxadiazole heterocycle derivatives were synthesised and characterised. Four symmetrical molecules were designed by changing the number of chains (Scheme 1) and to the four asymmetrical molecules a triple bond was added (Scheme 2). Some of the compounds presented liquid crystalline properties, with smectic C (SmC) and nematic (N) mesophases.

Measurements and characterisation
1 H and 13 C NMR spectra were obtained with a Varian Mercury Plus 400 MHz instrument using tetramethylsilane (TMS) as the internal standard. Infrared spectra were recorded on a Perkin-Elmer model 283 spectrometer using KBr discs or films. Mass spectra were recorded on a Bruker micrOTOF-Q II APCI-Qq-TOF mass spectrometer.
angle) with the sample in the mesophase, obtained by cooling from the isotropic state. The absorption spectra in solution were obtained with an HP UV-vis model 8453 spectrometer. The fluorescence spectra were recorded in solution on a Hitachi-F-4500. The relative fluorescence quantum yields (Φ F ) were determined according to a published method [27].

Synthesis
The synthesis procedures of the symmetrical compounds 8a-d are illustrated in Scheme 1. First, the thiophene 1 was carefully iodinated using molecular I 2 . The dinitrile 3 was obtained through a nucleophilic substitution using copper cyanide. The intermediate 4 was then synthesised by reacting dinitrile 3 with NH 2 OH.HCl, NaOH and ethanol. The alkylated acids 6a-d were obtained using the same synthetic route. The commercial carboxylate acids 5a-d were esterified via the Fischer reaction, using ethanol as the solvent and the reactant in the presence of sulfuric acid as a catalyser. The esters were then O-alkylated by Williamson etherification, using the 1-bromodecane, K 2 CO 3 and butanone as the solvent. The catalyst TBAB (tetrabutylammonium bromide) was used for the alkylation of the compounds with two or more hydroxyl groups. The esters were saponified affording the carboxylic acids 6a-d. The acid chlorides 7a-d were obtained using SOCl 2 and reacted with dioxime 4 in pyridine to form the final compounds 8a-d.
The synthetic route used to obtain the asymmetrical compounds 13a-d is illustrated in Scheme 2. Compound 9 was previously synthesised according to the literature [28] and then reacted with the intermediate 2 through Sonogashira coupling, affording compound 10. The nitrile 10 was obtained through a nucleophilic substitution using copper cyanide, the nitrile group being converted to the oxime group 12. The oxime 12 was reacted with the acid chlorides 7a-d to form the oxadiazoles 13a-d.
All compounds were characterised by IR and 1 H and 13 C NMR spectroscopy (with the exception of compound 8a, which was not characterised by 13 C NMR due to its low solubility in deuterated solvents) and mass spectroscopy.

Thermal behaviour
The thermal properties of the final compounds were investigated by polarising optical microscopy (POM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The results are reported in Table 1.
All of the compounds presented good thermal stability, with decomposition temperatures above 300°C. Only compounds 8a and 13a presented LC behaviour. Figure 1 shows the thermograms for compound 13a, where three reversible transitions are clearly seen. On heating, the first endotherm peak at 96.6°C (37.5 kJ/ mol) with high enthalpy indicates a transition from solid state to an LC mesophase. The second and third endothermic transitions at 127.7°C (1.8 kJ/mol) and 163.6°C (2.3 kJ/mol) presented lower associated enthalpy, which indicates transitions between distinct mesophases and to the isotropic state. The DSC thermograms for compound 8a are available in the supporting information.
The thermal behaviour obtained by DSC is consistent with the data observed by POM, where it was possible to identify the LC character of the mesophases. The observation by POM was performed with slight heating to the isotropic temperature and subsequent cooling to the crystallisation temperature at a rate of 5°C/min. This procedure was adopted for all compounds. Compound 8a displayed a droplet texture, which coalesced into a schlieren texture with four-fold defect domains. Further cooling resulted in the appearance of a marble texture. The schlieren, droplet and marble textures clearly indicate a nematic mesophase. Figure 2(c) shows, for compound 13a, the moment when the transition from an isotropic liquid to a fluid mesophase occurs, which is characteristic of nematics. As the sample is cooled it evolves a schlieren texture, which is also characteristic of SmC (Figure 2(d)). Based on the above described studies on the triazole derivatives, it was possible to establish that the chemical structure strongly influences the existence of mesomorphism. Replacing a heterocycle with C ≡ C bonds caused a lowering of the melting point and the appearance of the mesophase SmC, along with an increase in the mesomorphic range. The compounds with only a para-substituent, with a total of two alkoxy chains, presented liquid crystal properties. Compounds with more than two alkoxy chains did not show mesomorphic behaviour. The absence of mesomorphism is probably related to the increased number of alkoxy chains, which hinders the free rotation of molecules in the layer caused by an increased volume, this being unfavourable for the appearance of the liquid-crystalline properties [29][30][31].

X-ray diffraction studies
XRD experiments were carried out on compound 13a to investigate the smectic structure exhibited by the mesophase. The XRD pattern for compound 13a ( Figure 3) shows two reflection peaks (d 001 = 30.4 Å and d 002 = 15.3 Å) at the low-angle region with a ratio of d 001 /d 002~2 , which confirms a smectic organisation [30,32]. The diffuse peak observed in the region of 2θ = 4.4 degrees is assigned to the lateral distance between the neighbouring molecules within the layers [33,34]. Comparing the first diffraction peak d 001 , which corresponds to the interlayer spacing, to the molecular length L = 36.2 Å (estimated by ChemBio3D Ultra Software, version 11.0.1.), one can infer that the aliphatic chains are folded or the molecules are tilted within the layers. Considering the molecules in the most extended form, the tilt angle was calculated using the relationship (cos θ = d 100 /L) and a value of θ ≈ 33 degrees was obtained, which is consistent with the values for the SmC phases [35].

Optical properties
The symmetrical compounds 8a-d do not display fluorescence. However, after modification of the 1,2,4oxadiazole heterocycle with the triple bond, the  compounds 13a-d became luminescent. This substitution probably provided a higher conjugation of the molecules.
The absorption and emission studies of the asymmetrical 13a-c compounds were carried out in chloroform solution at 10 −5 M (Figure 4). The absorption spectra are very similar, with a maximum centred at around 332 nm. The molar absorptivity of these compounds was in the range of 21,000-26,000 L mol −1 cm −1 , attributed to π-π* transitions. These compounds present fluorescence in the blue region of the visible spectrum with maxima at around 400 nm. However, they are poor fluorophores with fluorescence quantum yields (Φ F ) in the range of 0.19-0.24. The Stokes shift remained between 59-76 nm.  Through the optical studies it was possible to observe that the variation in the number of alkyl chains did not result in significant changes in the properties of the asymmetrical compounds 13a-c. These results are summarised in Table 2.

Conclusions
A new series of compounds derivative from thiophene connected to the 1,2,4-oxadiazole heterocycle were synthesised. Liquid crystal behaviour (smectic and nematic) was observed for two of the compounds obtained. On increasing of number of chains in the symmetrical and non-symmetrical compounds the mesomorphic behaviour is not favoured, probably due to an increase in the volume. The insertion of the triple bond as a spacer, replacing one of the oxadiazole heterocycles, in the case of the non-symmetrical compounds, allows the lowering of the melting point and the appearance of the more organised SmC mesophase. The symmetrical compounds do not present fluorescence properties, however the nonsymmetrical compounds with triple bonds show fluorescence in the blue region of the visible spectrum.

Experimental
Compounds 2, 6a-d and 10 were synthesised according to procedures described in the literature [31,36].
Thiophene-2,5-dicarbonitrile (3). In the first step, 5.0 g (15 mmol) of 2,5-diiodothiophene 2 in 30 mL of dimethylformamide were placed into a 120 mL flask equipped with a condenser. The solution was heated to 100°C and 3.4 g (37 mmol) of copper cyanide were added. The suspension was stirred and refluxed for over 8 h. Next, 2.0 g (8 mmol) of FeCl 3 .6H 2 O in 50 mL of a solution hydrochloric acid (1.7 mol/L) were added at room temperature and the mixture was stirred for 30 min. The suspension was filtered off and the solid was washed with 30 mL of CH 2 Cl 2 . The solution was then extracted with CH 2 Cl 2 (3 × 50 mL) and washed with 30 mL of a saturated solution of NaH 2 SO 3 . The organic phase was dried using NaSO 4 and the solvent was removed by rotatory evaporation. The final compounds (1,2,4-oxadiazoles derivatives 8b-d) were synthesised according to the procedure described for compound 8a.      5-(4-(decyloxy)phenyl)-3-(5-((4-(decyloxy)phenyl) ethynyl)thiophen-2-yl)-1,2,4-oxadiazole (13a). Firstly, 0.24 g (0.88 mmol) of compound 6a and 5 mL of thionyl chloride were placed into a 50 mL flask equipped with a condenser and a tube with drying agent (CaCl 2 ). The solution was stirred for 5 h at 70°C. The excess thionyl chloride was removed in the rotatory evaporator and 0.25 g (0.63 mmol) of amidoxime 12 and 20 mL of dry pyridine were then added. The mixture was stirred for 24 h under reflux and then poured into 100 mL of water/ice. The solid was filtered off and purified by column chromatography on silica, with hexane/ethyl acetate (95/5) as the eluent, affording 0.