Four ring achiral ferroelectric liquid crystals of 1,2,4-oxadiazoles: synthesis and characterisation

ABSTRACT A novel series of four-ring achiral ferroelectric liquid crystals containing 1,2,4-oxadiazole cores with unsymmetrical substitutions at C-3 and C-5 positions are synthesised and characterised. A fluoro substituted biphenyl moiety is prepared by Suzuki coupling reaction and is directly attached to the oxadiazole core at the C-5 position for the first time in the literature. An octyl benzoate is attached to the oxadiazole core at the C-3 position of it. All the compounds exhibit polar smectic (B2) mesophases with ferroelectric switching along with the orthogonal smectic-A mesophases. These compounds possess high mesomorphic thermal ranges of polar smectic phases and are towards the ambient temperatures. The influence of a more electronegative fluorine substituent on the electron rich biphenyl moiety (at the C-5 position) of the oxadiazole core is analysed for the prevalence and abundance of polar smectic (ferroelectric) mesophases. GRAPHICAL ABSTRACT


Introduction
The functional materials of the recent origin [1][2][3][4] evince their applications in addressing the societal technical issues. Due to rich electro-optic (EO) response, the liquid crystalline (LC) materials have been exploited for various technical [5,6] and biological [7,8] applications. The various classes of LC materials include calamitic [9], discotic [10], sanidic and phosmidic [11], bent [12] (banana), supramolecular hydrogen bonded [13,14] (HB), organometallic [15] etc. represent the shape imparted performance materials in EO devices. Owing to the device savvy nature of ferroelectric (FE) phase structures, LC research initially is directed to involve the chiral ingredients to realise [16] FE phase structures. As chiral synthesis has a natural rustication of yield economy imbroglio, LC researchers switched over to design new LC systems with bent cores which are achiral but capable of exhibiting [12,17,18] polar smectic phase structures.
Although non-zero spontaneous polarisation (Ps) is realised in bent core LCs (BLCs), realisation of their FE phase structure (polar smectic) abundance in the vicinity of ambient temperatures remains a challenging task. An overview of the literature [17,18] in the field of design of BLCs seems to hint for a tailoring approach involving proper selection of central and lateral moieties, flexible end chains and or polar end groups etc.
The researchers have put considerable efforts to realise the structure-property relationships in achiral bent/ banana shape molecules with different central cores viz., resorcinol [19], 2,7-naphthalenediol [20], isophthalaldehyde [21], 3-amino-benzoic acid [22], isophthalic acid [23], oxadiazole [24][25][26][27], oxazole biphenol [28] etc., and different rod-like side fragments (mesogenic/non-mesogenic) covalently bonded with the central core. The effect of the number of aromatic rings [29] in the molecular frame on the realisation of polar smectic (FE) phase structures in these achiral systems is reported. The achiral compounds [30,31] with less number (three or four) of aromatic rings in the molecular skeleton have been synthesised by various authors in anticipation of the FE phase structures at ambient temperatures. However, the attempts were not fruitful. Also, the five membered heterocyclic oxadiazoles are reported as multifunctional materials due to their wide spectrum of biological [32][33][34] and technical applications [35][36][37][38] such as organic light emitting diodes (OLEDs), electroluminescence and LCs. The oxadiazoles contain two nitrogens and oxygen in its ring structure. Depending on the relative positions of the three atoms (N, N and O) in its structure, four different isomers [24] are possible. The 2,4-substituted 1,3,5-oxadiazole derivatives are reported [25] with the conventional smectic phases. The 2,5-substituted 1,3,4-oxadiazole derivatives are reported [39] with the nematic phase. The 3,5-substitued 1,2,4-oxadiazole derivatives are of great interest [40,41] at the momentum due to their biaxial nematic phase structure and the FE like switching in external electrical stimuli due to cybotactic cluster formation.
An extensive survey and the implications involved with respect to the number of aromatic rings, nature of substitutions and length of the flexible end chain in realising polar smectic phase structures in achiral bent core systems is presented [37] in our earlier report. The literature reveals that the non-symmetric (about the central core) molecular frames favour the polar smectic mesomorphism. A novel series of achiral non-symmetric LCs with the 1,2,4-oxadiazole central core exhibiting the polar smectic phase structures with wide thermal ranges are synthesised [37] and reported. In our other reports on LCs formed through HB, the more electronegative fluoro substituent is found to influence [42,43] the melting and clearing temperatures as well as the mesomorphic thermal ranges appreciably. By keeping in mind, the low clearing and melting temperatures due to the fluoro substituent and a wide thermal range of polar smectic mesophases in non-symmetric 1,2,4-oxadiazole derivatives, a humble attempt is presently made to synthesise and study the influence of a lateral fluoro substituent (on biphenyl fragment of the 1,2,4-oxadiazole) on mesomorphism. The lateral fluoro group on the biphenyl moiety is assumed to enhance the non-symmetric nature of the molecular frame due to higher C-F bond length than the corresponding C-H bond length in unsubstituted compounds. The biphenyl moiety with the fluoro substituent is prepared by Suzuki coupling reaction and is attached to the oxadiazole core at the C-5 position of it.

Experimental methods
The required chemicals for the synthesis viz., 4-cyanobenzoic acid, 1-octanol, 4-bromo-2-fluoro phenol, 1-bromoalkanes (octyl, decyl, dodecyl, pentadecyl and hexadecyl) are procured from Sigma-Aldrich and are used without further purification. Catalysts like Pd(dppf) 2 Cl 2 and Pd(pph 3 ) 2 Cl 2 are procured from Hindustan Platinum, India. AR grade solvents like dimethylformamide (DMF), 1,4-dioxane, 1,2-dimethoxy ethane, acetonitrile and ethanol are procured from Spectrochem. The intermediate products and the final products during the synthesis are purified by column chromatography. The intermediate products are characterised by TLC. All the final products are characterised by 1 H NMR, 13 C NMR, FTIR spectroscopy techniques and elemental analysis. The optical textures exhibited by the mesophases are characterised using a polarising optical microscope (POM) equipped with a hot stage. The phase transition temperatures and the enthalpy changes across the transitions are determined using a Shimadzu (DSC-60 model) differential scanning calorimeter (DSC). The spontaneous polarisation of the ferroelectric mesophases in these compounds is determined by the field reversal [44,45] method.
The 4-bromo-2-fluoro-phenol (2) is alkylated by Williamson's process with the corresponding 1-bromoalkane to give the alkylated bromofluoro compounds (3a-3e). To get a biphenyl moiety through Suzuki coupling, either of the bromo compounds viz., 1 or (3a-3e), is converted into the corresponding boronic esters, and then treated with the other bromo compound. In the present case, the alkylated bromofluoro compounds (3a-3e) are converted into the respective boronic esters (4a-4e) by treating it with Pin 2 B 2 in presence of Palladium catalyst for convenience. The boronic esters are reacted with the 1,2,4-oxadiazole derivative (1) under Suzuki coupling [47,48] conditions, yielded the final products (5a-5e), C 8 .Ox. FBP.C n . The compounds are purified by recrystallisation. The synthetic route for the preparation of this series of compounds is presented in the following scheme 1. The 1 H and 13 C NMR spectra of important intermediates and the products are given in supplementary information file.
General procedure for synthesis of 4-bromo-1-alkyloxy-2-fluorobenzene (3a-3e): To a stirred solution of 4-bromo-2-fluoro-phenol 6 (10.4 mmol) in dry acetonitrile (20 mL), K 2 CO 3 (15.6 mmol) and the respective n-alkyl bromide (11.5 mmol) are added and the reaction mixture is refluxed at 80°C for 8 h. After completing the reaction, the solvent is distilled off and the residue is extracted with EtOAc. The extractant is washed with water followed by brine. The products obtained (3a-3e) are colourless oils. They are dried over anhydrous sodium sulphate.

Synthesis of boronate esters (4a-4e):
The compounds obtained in the previous step, 4bromo-1-alkyloxy-2-fluorobenzenes (3a-3e) are converted into the corresponding boronate esters by following the reported [37] procedure. The products are purified by column chromatography using silica gel as the stationary phase and the ethylacetate:hexane (2:98) mixture as the solvent system.

Phase characterisation by POM, DSC and electro-optical studies
The mesophases exhibited by this series, C 8 .Ox.FBP.C n (n = 8, 10, 12, 15 and 16) of compounds are characterised using a POM attached with a hot stage. On cooling the sample, batonnets are formed and they coalesce to form a focal conic fan texture. Simultaneously, pseudo isotropic texture is noticed in homeotropic regions. These textural changes are similar to the textural changes reported [49] for smectic-A (SmA) of a calamitic LC viz., N-(tridecyloxybenzylidene)-4-methylaniline. These observations confirm [50] that the phase is an orthogonal smectic-A (SmA) mesophase. Similar observations are noticed in all other compounds of this series. The characteristic texture of SmA exhibited by 5a is given in Figure 1 as a representative of the series. On further cooling, the focal conic fan texture is changed into a paramorphotic broken focal conic fan texture. A schlieren texture is noticed in the homeotropic regions simultaneously on shearing. This infers [50] that the phase is a tilted phase. This texture is similar to the texture reported [37] for B 2 phase of octyl-4-(5-(4′-(decyloxy)-biphenyl-4-yl)-1,2,4-oxadiazol-3-yl)benzoate (C 8 C 10 ) compound of achiral FE compounds. All the compounds of this series are found to exhibit bistable switching by application of an electric field, which is observed clearly in the polarising microscope. The switching is observed both in heating and cooling cycles in applied electric fields. A single polarisation current peak per half period of triangular wave voltage is found in this phase. Since there is no chiral centre in the molecule and showing a bistable switching, the phase is confirmed as a ferroelectric smectic i.e., B 2 phase. All the compounds in the series are found to exhibit similar textural changes. A characteristic texture of B 2 phase exhibited by 9a is given in Figure 2 as a representative of the series. The phase transition temperatures observed in the cooling cycle of POM studies are tabulated in Table 1 along with the DSC data.
The heating or cooling rate applied for studies are 10°C/min. It is noticed that the 1-D orthogonal SmA mesophase is an enantiotropic in all the compounds of the present series, whereas the polar smectic B 2 phase is a monotropic in 5a and 5b compounds and is an enantiotropic in higher homologues of the series viz., 5c, 5d and 5e. The DSC thermograms of the compounds 5b and 5d are given in Figure 3 as representatives of the series. The phase transition viz., Iso.-SmA, is obtained as a prominent peak for all the compounds in both heating and cooling cycles. But the SmA-B 2 phase transition is not well resolved in 5a and 5b compounds but the same phase transition is well resolved for other compounds (5c, 5d and 5e) of the series. This may be due to the weakly first order or second order nature of the orthogonal SmA to tilted B 2 phase transition, which are also reflected in their enthalpy change values. An attempt is made to resolve these phase transitions by recording the thermograms at slow scan rate, 5°C/min. Even at slow scan rate, the SmA-B 2 phase transition is not well resolved in 5a and 5b compounds but is resolved in the other compounds of the series. The DSC thermograms of 5c recorded at 5°C/min. is given in supplementary information file as a representative case along with the enlarged graph in the region of SmA-B 2 phase transition. The enthalpy changes (ΔH in kJ/mol) across the phase transitions are computed and are given in parenthesis in Table 1    diagram is constructed by using the DSC cooling cycle data and is given in Figure 4. The ferroelectric switching behaviour of these compounds is studied by using a modified [44] Sawyer-Tower bridge and a 10 Vpp of 100 Hz triangular wave is applied. An ITO coated buffed cells (Instec. Inc., USA) of 5.5 μm thickness are used for studies. The bridge was calibrated with a standard ferroelectric material (DOBAMBC) of known P s and the P s of the materials of the present study is computed by comparing the areas under the peaks obtained for each sample and at each temp. The P s profile of 5b at 72.0°C is given in Figure 5 as a representative of the series. In achiral systems, the supramolecular macroscopic chiral domains are forming which orient in the direction of external stimuli and results the P s . The temp. variable P s is studied in all the compounds. The P s is found to increase with decreasing the temp. and attained a max. (saturated) value. The temp. dependent P s for 5a compound of the series is given in Figure 6 as a    representative of the series. The same trend of temp. variable P s is noticed in all other compounds of the series. The observed maximum values of P s for 5a-5e compounds are about 100, 85, 72, 68 and 65 nC/cm 2 respectively. Recently, we have reported [37,46] the synthesis and characterisation of 3,5-disubtituted 1,2,4-oxadiazoles with and without chiral centre, exhibiting the conventional and FE smectic phases. The general molecular structures of the different series of compounds are given below for convenient comparative studies.
All these series (I-IV) contain the same 1,2,4-oxadiazole central core with different substitutions at C-3 and C-5 positions. It is observed that the end moieties on either sides influence the phase variant, mesomorphic thermal ranges and the phase transition temperatures. One common feature of all the series is the exhibition of SmA phase. In series-I viz., octyl-4-(5-(4′-(alkoxy)-biphenyl-4-yl)-1,2,4oxadiazol-3-yl)benzoate (C8C n , n = 8, 9, 10, 12, 15 and 16), the end moieties are flexible chains without a lateral substitution on the biphenyl moiety. It is found that the compounds of this series are exhibiting orthogonal smectic phases along with the polar smectic phases. In series-II, the molecular frame and the end groups are similar to that of the series-I, except a lateral 'F' substituent on the biphenyl moiety (at C-5 of the oxadiazole core), which are found to exhibit SmA and B 2 mesophases. The compounds of series-III and series-IV possess the same central core, but the end groups are different. The series-III compounds are found [46] to exhibit SmA mesophase whereas the series-IV compounds are found to exhibit both SmA and SmC* (FE) mesophases. Series-I and series-II compounds contain an ester and an ether linkage whereas the series-III and series-IV contain three ester groups. Except the series-III compounds, the remaining three series of compounds are exhibiting the ferroelectric mesophases. It infers that the molecular structure including the substituents and the linking (functional) groups influence the stability of the mesophase structures.
The mesomorphic thermal ranges of various phases exhibited by series-II compounds are computed from their DSC heating cycle data and are given in Table 2. It is noticed that the thermal range of SmA mesophase of 5c-5e compounds are almost the same in both heating and cooling cycles. But in case of 5a and 5b, the B 2 -SmA transition is not obtained in the heating cycle while it is noticed as an unresolved peak in the cooling cycle. It is observed that the thermal range of SmA phase in 5a and 5b compounds in POM studies during heating are in concurrence with their DSC cooling cycle data (given in Table 2). The thermal range of B 2 mesophase is more in cooling cycle due to supercooling of about 20-25°C below to their respective melting temperatures (Cryst.-B 2 /SmA).
The lower members of the series (5a, 5b and 5c) are found to possess almost the same mesomorphic thermal range (104.5 ± 0.6°C) and may be attributed to the high dipolar interactions between the molecules with an optimum rigidity and flexibility. The other two higher members of the series (5d and 5e) are found to possess low ΔT LC values and may be attributed to the dilution [51] of the core in comparison with the other members of the series. ΔT SmA of the compounds (except 5a) in the series (5b-5e) increases upon increasing the alkyl chain length. The FE B 2 Series -IV ( SmA and SmC*) [46] phase abundance (ranging from 57-97°C) changed appreciably in this series of compounds. The compound 5b contains ten carbons in the alkyl chain possessing the highest ΔT FE value in the series. This chain length (C 10 ) may be the optimum for stabilising the polar smectic mesophase in this series. It is well established that the mesomorphic thermal ranges are influenced [42,43] by the polarity, polarisability and intermolecular interactions of the molecules. The fluorine atom is more electronegative and the C-F bond length (135 pm) is more than the C-H bond length (109 pm). The electronegative fluorine atom influence [52] the delocalised π-electron density of aromatic rings which further influence the π-π stacking and π . . .C-H electrostatic interactions between the aromatic rings and the overall polarisability of the conjugated system. The fluorine atom also influences [53] the conformation of the groups/substituents present in its vicinity (i.e., neighbouring molecules) and consequently the net intermolecular interactions. As a result of the net interactions, the mesophase thermal range may be stabilised (induced) or destabilised or eliminated completely. Torgova et. al., have studied the mesomorphism [24,54,55] in various compounds of the 1,2,4-oxadiazole core with unsymmetrical substitutions at C-3 and C-5 positions. It was reported that the compounds with five aromatic rings were exhibiting nematic and smectic phases with switching in both the phases due to the longitudinal molecular dipole moment. However, the derivatives of 1,2,4-oxadiazoles with four aromatic rings are reported [24] to exhibit the conventional smectic phases. It is believed that if the exocyclic bond angle between the rod-like moieties at the central core is greater than 140°, the molecules may prefer to exhibit conventional calamitic smectic phases and if it is close to 120°as in the case of 1,3-substituted phenylene systems may exhibit the polar smectic phases. The compounds of 2,5-disubstitued 1,3,4-oxadiazoles with an exocyclic bond angle of about 134°were reported [24] with conventional smectic mesophases (even though the exocyclic bond angle is 134°i.e., <140°), but are not exhibiting the polar smectic phases. But in the present series of 1,2,4-oxadiazole compounds consisting four aromatic rings i.e., a benzoate moiety at C-3 and a fluoro substituted biphenyl moiety at C-5 (exocyclic bond angle [41] is 140°) are found to exhibit an orthogonal SmA and polar smectic (B 2 ) phase with FE switching. It infers that the substituents at C-3 as well as at C-5 are crucial to realise the supramolecular FE phase structures in oxadiazole systems. The wide range polar smectic phase in the present series of compounds may be due to the presence of a fluoro substituted non-coplanar biphenyl moiety at the C-5 position of the oxadiazole core, in which long range intermolecular interactions due to the conformation of molecules in adjacent layers are expected.
The lower homologues of the series-II (5a, 5b and 5c) are found to have the isotropic temperatures of about 180 ± 2°C, whereas the higher members of the series-II (5d and 5e) are found to have the isotropic temperatures of about 168 ± 2°C. The isotropic temperatures of the structurally similar compounds of series-I are about 20°C higher than the corresponding fluoro substituted compounds of series-II. The lowering of the isotropic temperatures in the present investigations of series-II can be argued due to the presence of 'F' substitution at the ortho position to the alkoxy chain. The crystallisation temperatures (cooling cycle) of the compounds 5a, 5b, 5c, 5d and 5e are 50.2, 52.9, 51.5, 65.2 and 60.7°C, respectively. Interestingly, the clearing temperatures of series-III are still lower than series-II. In series-III, the end moiety at C-3 of oxadiazole is a bulky phenyl and is a flexible long alkyl chain in the other three series (I, II and IV) of compounds. It may be argued due to the bulky phenyl moiety, which increase the interlayer distances and may possess lower London dispersions in comparison with the other series of compounds. The crystallisation temperatures (cooling cycle) of series-II are found to be lesser than that of the series-III and series-IV due to supercooling.

Summary
A new series of achiral FE LC materials of four aromatic rings with 1,2,4-oxadiazole as a central core are synthesised and characterised. A lateral fluoro substituent is introduced in the molecular structure and the mesomorphism is studied by POM, DSC and electro-optical studies. The low clearing and crystallisation temperatures are achieved with the fluorine substitution in the present series of compounds. The FE phase stability is found to vary with the flexible alkoxy end chain length of the compounds in this series. The polar smectic (FE) mesomorphism is observed over a wide range of temperatures (about 57-97°C) in the present series of compounds containing four aromatic rings with fluorine substitution in comparison with a similar central core with five or more aromatic rings. The fluoro substituted non-coplanar biphenyl moiety at C-5 of the oxadiazole core is found to play a crucial role in stabilising the FE mesomorphism in 1,2,4-oxadiazole achiral systems.
Kakinada, for infrastructural facilities. Also, the Doctoral Advisory Committee (DAC) members of Mahabaleshwara for their valuable suggestions is acknowledged.

Disclosure statement
No potential conflict of interest was reported by the authors.