Ethylenedioxythiophene as a novel central unit for bent-core liquid crystals

ABSTRACT We have discovered that ethylenedioxythiophene (EDOT) can be used as a central unit for the synthesis of bent-core liquid crystals (BC LCs). Two series of EDOT-based BC LCs are prepared via Sonogashira coupling reaction. The mesophase behaviour of all the compounds was characterised using a combination of polarising optical microscopy, differential scanning calorimetry and X-ray diffraction measurements. EDOT-based three-ring compounds were found to be nonliquid crystalline, while all the four derivatives of five-ring series, including a branched alkoxy chain derivative, display enantiotropic nematic phase over wide temperature range. The bent angle of these compounds is about 153°, which falls in between typical rod-like and banana liquid crystals. The transition temperature of branched alkoxy chain compound is lower than straight alkoxy chain compounds. The detailed XRD investigations of all the mesogens corroborate the presence of nematic phase. GRAPHICAL ABSTRACT


Introduction
Bent-core liquid crystals or banana liquid crystals (BC LCs) represent a novel class of thermotropic liquid crystals which have become important targets of extensive studies in the last decade. The design of novel thermotropic BC LCs for advanced functional materials is a challenging task for a chemist to optimise basic modification of the structural properties for various applications [1,2]. Currently, several hundred BC LCs are known; however they are derived only from a few central bent cores like, benzene, resorcinol, naphthalene, oxazole, oxadiazole, triazole, thiazole, thiadiazole, thiophene, benzothiophene, benzodithiophene, pyridine-2,6-dicarbaldehyde, pyrazabole, fluorenone etc. [3]. Therefore, it is of great importance to search new central bent-core units to understand structural property relation in these intriguing materials. Mesogenic derivatives containing heterocyclic compounds find increasing importance as core units in thermotropic liquid crystals. The presence of heteroatoms such as N, O and S causes significant effects on lateral or longitudinal dipoles. Incorporation of five-member heterocyclic compounds widens the bent angle and show extensive mesomorphic behaviour [4]. A number of 2,5-disubstituted thiophene-based liquid crystal compounds have been synthesised and investigated their structural properties [5][6][7][8][9][10][11][12][13][14][15]. The sulphur heteroatom containing central core can substantially enhance optical anisotropy, fast switching time, reduction in viscosity, elevation of a negative dielectric anisotropy and lowering melting points, which make them good candidates for technological importance [16][17][18].
Herein, we report for the first time, the synthesis and characterisation of BC LCs derivatives from EDOT central core. Initially, we designed and synthesised EDOTbased three-ring bent-core molecules as shown in Scheme 1, where the central EDOT core is flanked by alkyl or alkoxy-substituted phenyl rings. These compounds were found to be non-LC, which is not much surprising as three-ring structures are usually not conducive for BC LCs [24,25]. To enhance the length:width ratio, we added one phenyl ring each side to generate five-ring structure. The additional rings were coupled via ester bridge on both wings as shown in Scheme 1. The 2,5-disubstituted EDOT with acetylene linkage groups promotes greater stability of mesophase and provides a moderate bent angle in between 152-155°d epending on substitution. We synthesised four fivering EDOT derivatives, including one racemic mixture of branched alkoxy chain derivative, to understand structure-property relationship. All the homologues display nematic phase only. The mesophase behaviour was characterised using polarising optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray scattering measurements (XRD).

Density functional theory studies
The novel BC molecules derived from EDOT were designed as shown in Scheme 1. GAUSSIAN-09 program package was employed to carry out density functional theory (DFT) calculations at the Becke's three-parameter functional and Lee, Yang and Parr correlation functional (B3LYP) [26,27]. The 6-311G (d) basis set was used for ground-state geometry optimisation for C, H, S atoms. The internal coordinates of the system, which is used as input for the GAUSSIAN-09 program was generated by the GAUSS VIEW 4.1 program.
The absence of any imaginary frequency in the calculated vibrational frequencies ensures that the optimised geometry corresponds to a true energy minimum. From the DFT calculations, the energy and the length of fully extended three-ring compound 4b is found to be −2629.37 a.u. and 58.74 Å, respectively. These values for five-ring BC LCs 9c were −3328.47 a. u. and 59.94 Å, respectively. The analysis indicates that the bending angle is approximately 154.68°in threering compound 4b and 152.17°in five-ring compound 9c as shown in Figure 1.

Synthesis
All the compounds were synthesised following general synthetic pathway as shown in Scheme 1. Bromination of EDOT using N-bromosuccinimide gives 2,5-dibrominated EDOT 2 (DBEDOT) as the key intermediate for the synthesis of three-and five-ring derivatives. DBEDOT is highly unstable at room temperature and hence material was stored at 0°C. Sonogashira C-C bond coupling reaction of 4-alkyl phenylacetylene or 4-alkoxy phenylacetylene 3 with DBEDOT 2 in the presence of bis(triphenylphosphine)palladium(II) dichloride (PdCl 2 (PPh 3 ) 2 ) and copper(I) iodide in anhydrous triethylamine (TEA) solvent offered threemembered bent-core non-mesogenic compound 4a-4b. Here, we optimised the best condition for coupling reaction.

Mesomorphic properties
The thermotropic mesophase behaviour of all the compounds listed in Table 1 was first investigated by POM. The exact temperature of phase transition and associated enthalpy values were then determined by DSC and are summarised in Table 1. The onset temperatures are given in°C and the numbers in parentheses indicate the transition enthalpy (ΔH) in kJ mol −1 . All the homologous compounds in the 9a-9d series including one-branched alkoxy chain exhibit exclusively enantiotropic nematic phase with a relatively wide temperature range.
The mesophase texture of all the compounds is viewed under optical microscope with crossed polarisers which display characteristic defect textures with two-and four-point brushes (Figure 2), characteristics   Figure 3). Nematic droplets were observed during early growing stage upon cooling from the isotropic liquid at 216°C as shown in Figure 2 ( Figure 2(a,b), dark area are isotropic liquid). Typical schlieren texture ( Figure 2(c,d) was photographed at 202°C on cooling from isotropic phase. Similar nematic textures were observed for all other derivatives. The compound 9d having decyloxy alkyl chain with methyl group branching at 3 and 7 positions cleared to isotropic liquid phase at 191°C. Upon slow cooling, typical schlieren texture having two-and four brushes appeared at about 182°C, reflects the existence of nematic liquid crystalline phase. The compounds derived from three-ring series 4a and 4b did not show any liquid crystalline properties and was not studied further. DSC measurements were carried out to examine the phase transition temperatures. All the compounds were subjected to heating and cooling scan at 5°C min −1 . The phase transition temperature obtained in DSC data is good agreement with POM results. Compounds 9a-9d, show nematic phase exclusively over wide temperature range. These compounds 9a-9d exhibit only two phase transitions, which correspond to crystal to nematic phase and nematic to isotropic phase as shown in Table 1. Compound 9d with branched alkyl chain exhibits much lower transition temperatures compared to other derivatives. A typical DSC thermogram of 9c is dipicated in Figure 3. All the derivatives crystallise out after few hours at room temperature.   Compound 9d display the lowest melting and isotropic temperatures. This decrease in melting and clearing temperature could be due to the increased rigidity of the chain and lowering the van der Walls interactions due to methyl groups sticking out and hindering a close approach. The branching of the alkoxy chain in the novel compound does not alter the mesophase properties but has a significant effect on melting and isotropisation temperatures and stability of mesophase. The detail explanation of the effect of branched alkoxy chain on mesogenic compounds of the single component system has been well documented in the literature [28][29][30][31][32][33][34][35].

XRD studies
The mesogenic behaviour of the novel BC LCs was further characterised by X-ray scattering studies of unoriented samples filled in Lindemann capillaries. The XRD pattern observed for compounds 9b-9d was quite similar on both heating and cooling scans. XRD of compound 9a could not be taken due to its very high isotropic temperature. As a representative example, the XRD pattern of compound 9d is presented in Figure 4. The XRD taken at 150°C on cooling run in nematic phase shows a diffuse peak at wide-angle maxima at d = 4.41 Å and diffuse peak at small-angle regime which is typical for a nematic phase.

Photophysical properties
The photophysical properties of the newly synthesised compound were investigated by Perkin-Elmer UV-Vislambda 35 double-beam spectrophotometer absorption spectra. The UV absorption spectroscopic properties of compound (9a-9d) were studied in a very dilute solution (arbitrary concentration) of chloroform solvent to obtained information on absorption maxima.
All the compounds exhibit maximum absorption band at λ max = 366 nm and λ max = 267 nm which correspond to n→π* and π→π* transitions as shown in Figure 5(a), respectively. Similarly, emission spectra were recorded using FluoroMax-4(Horiba Jobin Yvon) Spectrofluorometer. The emission spectra were recorded in very dilute chloroform solution and the compounds are excited at λ ex = 340 nm. The compounds (9a-9d) exhibits strong emission maximum at 425 nm and 405 nm as shown in Figure 5(b), respectively. No appreciable changes were observed by increasing number of methylene units of alkoxy chain. The absorption and fluorescence properties could be attributed due to the presence of extended π-conjugation in all the molecules.

Raman spectroscopy
The structure of the novel bent-core liquid crystals was further studied by Raman spectroscopy. Raman spectra were recorded with Horiba Jobin Yvon T6400 Micro Raman using a He-Ne Laser operating at λ = 632.8 nm.
The laser power at 1.8 mW was kept constant throughout measurements. Spectral data were obtained at an optical resolution of 50X objective lens and accumulated at 10 s to obtain data with a sufficiently high signal-to-noise ratio. All solid samples are placed in a

Thermogravimetric analysis
The thermal stability of all the liquid crystalline compounds was checked using TGA 4000 thermogravimetric analysis instrument. All the samples (9a-9d) are subject to heat scan of 10°C min −1 under a nitrogen atmosphere. The solid samples exhibit no weight loss in the temperature range 300-330°C. All the compounds initiated weight loss at 335-350°C and whole process complete at about 520°C as shown in Figure 7. The decomposition temperature in these compounds is much higher than the isotropic temperature. It reveals that all the mesogenic derivatives possess excellent thermal stability.

General methods
All the chemicals and reagents are AR-grade quality and purchased from Sigma-Aldrich. The solvents were dried and distilled using standard protocols. The intermediate and final compounds were purified by column chromatography using Acme make silica gel (100-200) mesh and recrystallisation from the respective solvent. Spectral and elemental analysis confirmed the structural elucidation and purity of all compounds. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded using Shimadzu -8400; only major peaks are reported in cm −1. 1 H Nuclear magnetic resonance spectroscopy (NMR) and 13 (6) were prepared following the method described in literature [36,37].
A solution of tetrabutylammonium fluoride (TBAF) (12.15 mmol) in dry THF (20 mL) was added to a stirring solution of compound 7a-7d (10.13 mmol) in THF (40 mL) at room temperature under argon. The reaction mixture was kept stirring at room temperature for 8 h. At the end of reaction, ice-cold water was added and the reaction mixture was extracted with diethyl ether (3 × 50 mL). The combined organic layers were dried over Na 2 SO 4 and the solvent was evaporating under vacuum. The crude product was purified by column chromatography using silica gel (n-hexane/ ethyl acetate 9:1) to give white colour solid 8a-8d in about 75-80% yield.
DBEDOT 2 (0.22 g. 0.73 mmol) were dissolved in 30 mL of anhydrous triethylamine solvent. To this solution was added bis(triphenylphosphine) palladium dichloride (PdCl 2 (PPh 3 ) 2 ) (0.020 g, 0.02 mmol) and CuI (0.011 g, 0.05 mmol) under argon atmosphere. The reaction mixture was allowed to stir under argon for 10 min. To the above reaction mixture, 4-ethynylphenyl-4-alkoxybenzoate 8 (2.20 mmol) was added under argon flow. The resulting reaction mixture was heated to 72°C for 24 h. After cooling to room temperature, the reaction mixture was diluted with diethyl ether and filtered through celite pad. The filtrate was washed with excess of water and then extracted with diethyl ether (3 × 20 mL). The combined extracts were dried over Na 2 SO 4 and solvent was evaporated under vacuum. The residue was purified by column chromatography using silica gel (n-hexane/dichloromethane 9:1). Recrystallisation of the pure product with isopropyl alcohol affords yellow colour solid 9a-9d in about 55-60% yield. 9a

Conclusion
We have realised a new central unit for BC LCs. A number of novel BC LCs derived from EDOT were synthesised and characterised. The bent angle of these molecules is 152-155°. All the mesogenic compounds display enantiotropic nematic phase at wide temperature range. EDOT-based three-ring compounds do not exhibit any liquid crystalline phase; however, by increasing side wing with phenyl group, mesomorphism can be induced. The use of branched alkoxy chains reduces isotropic temperature significantly.