Synthesis , characterization and structure-property relationships of 1 , 3-bis ( 5-substituted-1 , 3 , 4-oxadiazol-2-yl ) benzenes

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Introduction
Liquid-crystalline (LC) materials have been known for over a century [1].It is clear that architecture and functionalization are essential aspects in molecular engineering of liquid crystals [2].Introduction of fluorine atom in the molecular structure presents a successful strategy to control the liquid crystal proprieties.Fluorine element presents the highest electronegativity, the lowest polarizability and a small radius.When bonded to carbon, it forms the strongest single bond in organic chemistry [3].The C-F bond is highly polarized and this polarity inhibits the lone pair donation from fluorine, making this element a weak coordinator.These properties are the basis for the unique properties of perfluoroalkylated compounds such as high viscosity, high density, high chemical stability, low surface tension, low dielectric constants and low refractive index [4].The usefulness of these properties makes of fluorine an element of choose for the enhancement of promising properties, remained inaccessible otherwise.However, the combination of small size and high polarity of fluorine atom leads to a subtle modification of properties such as melting point, mesophase morphology, transition temperatures, optical anisotropy, dielectric anisotropy, and viscoelasticity [5][6][7][8][9][10].Therefore, many fluorinated liquid crystals have been prepared, and the fluoro-substitution effect has been well studied, especially in the fluoroaromatic derivatives [11][12][13].
Additionally, the mesomorphic properties of liquid crystals depend strongly on the nature of the terminal chains that are present.Terminal perfluorocarbon chain present in a LC molecule causes stiffening, which generates a lamellar packing and thus contributes to smectic phase stability [14].It was reported that even simple n-alkanes containing a fluorocarbon block produces smectic phases [15][16][17][18][19].Some molecules having only a single aromatic ring and fluorinated tail show smectic phases, while their hydrocarbon counterparts are nonmesomorphic [20].

Synthesis
When benzene-1,3-dicarbohydrazide [21] 1 was allowed to react with hydro-or perfluorocarboxylic acids in the presence of phosphorus oxychloride according to standard methods [22], oxadiazole derivatives 2 were obtained (Scheme 1).On the other hand, we converted compound 1 into sulfanyloxadiazole derivatives 4 by treatment with carbon disulfide and subsequent alkylation of the obtained intermediate 3 (Scheme 1).
Table 1 shows the yields and melting points of the synthesized compounds 2 and 4. The tautomeric equilibrium of compound 3 is illustrated in Scheme 2. On the basis of FT-IR data, it has been concluded that in solution, the equilibrium is shifted to the thione form 3a rather than the thiol one 3b.The observed IR absorptions at 3387 cm -1 (νN-H) and 1263 cm -1 (νC=S) and the absence of absorptions in the 2600-2550 cm -1 region (νS-H) support the preference for the thione form in solution.This latter is obviously more solvated than thiol.

Liquid Crystal properties
The structure of compounds 2 and 4 is constituted by a rigid core (three aromatic rings) to which are attached the terminal chains.Based on this structure, some liquid crystalline mesophases were expected.Differential Scanning Calorimetry (DSC), Polarized Optical Microscopy (POM) and X-ray diffraction patterns analysis were used to investigate this behaviour.

DSC measurements
Phase transition temperature (Tt), melting temperatures (Ti) and enthalpy changes (∆Ht) of compounds 2 and 4 are summarized in Table 2.
Based on data given in Table 2, we found that fluorinated compounds present supplementary transition temperatures compared with the hydrocarbonated analogues.DSC thermograms of fluorinated compounds 2b, 4a and 4b are shown in Figure 1.
Thermogram of derivative 2b presents only one peak between the crystalline and the isotropic phase in cooling.However, it exhibits two peaks in heating which indicates the presence of monotropic intermediate phase between T=115.8°C and T=125.1°C[23].As it can be seen in the thermogram of compound 4a, the monotropic liquid crystal mesophase is observed in cooling between T= 91.6°C and 98.9°C.

Polarized Optical Microscopy (POM)
In order to achieve a further illustration of the liquid crystal behaviour, POM observations were realized in cooling and heating cycles for compounds 2b, 4a and 4b (Figure 2).POM technique has illustrated smectic phases for all compounds.Identification of the phase textures was accomplished by comparing with those reported in literature. 24  The texture of 2b (Figure 2

X-ray patterns analysis
In order to correlate the obtained results from POM and DSC, we have investigated the X-ray diffraction at the mesophases in cooling and heating cycle of 2b, 4a and 4b. Figure 3 illustrates typical diffraction spectra for compound 2b.A typical X-ray pattern recorded for the SmA phase is obtained, it showed two small reflections at 23.69 Å and 15.71 Å at low angle region, and a broad reflection at 5.64 Å.These features are characteristic of SmA phase and are close to data described in the literature [25].Figure 4 shows a typical X-ray pattern for the hexagonal SmB of 4a (anyway, we must emphasize that both compounds 4a and 4b exhibit identical X-ray pattern).As we can observe, three peaks of diffraction are recorded.
The diffraction pattern shows oriented reflections in the small angle region.Bragg peaks at 33.04 Å, and approximately its second-and third order multiples at d = 16.09Å and 10.69 Å, indicate a highly condensed layered structure [26].
However, in the high 2θ-region of Figure 4, slightly different values of the d-spacings calculated from these peaks suggest a weakly distorted hexagonal lattice [26].3.In Figure 7 we plotted the calculated dipole moments.To quantify the effect of terminal chains, we calculated dipole moment µ0 of 1,3-bis(1,3,4-oxadiazol-2-yl)benzene (RC) as it contains no terminal tails and can serves as standard.Oxadiazole moieties are thought to be responsible for the majority of the dipole moment in the prepared compounds.Due to the electron-donating effect of alkyl groups in combination with the electron accepting oxadiazole [27], the prepared hydrocarbon compounds are expected to perform dipole moment magnitudes higher than µ0.In the fluorinated counterparts, the electron-withdrawing effect of perfluoroalkyl groups would produce the reverse effect leading to a dipole lower than µ0. Figure 7 shows an inverse result: the orange plot corresponding to the fluorocarbon derivatives is entirely in the upper side compared to the horizontal green line (standard µ0), the blue plot of the hydrocarbon derivatives being in the bottom side.
On the other hand, we can note from Figure 5 that the conformation adopted by the hydrocarbon compounds is different from that of the fluorinated homologues, moreover these latter do not adopt the same conformation, according to whether they carry a sulphur atom or not.
Obviously, the inductive effects alone are inconsistent with the observed results.Hence, we considered the rigid-core (RC) and the terminal chains separately in order to identify a possible interaction between them.

Rigid-core
With three electronegative heteroatoms and only two carbons, the 1,3,4-oxadiazole core has a great electron deficient character.Nevertheless, when incorporated in the rigid-core, the two oxadiazole rings exhibit a slight difference in their electron deficiency.
We depicted in Table 4 the electric charge of heteroatoms in the obtained compounds, as well as those of RC.As shown in Table 4, with the exception of O13, all the other heteroatoms have almost the same electrical charge.The mean value of electric charge for O13 is -0.27.
Hence, O13 is highly charged relative to the other heteroatoms, suggesting that Oxd 1 is more polar than Oxd 2. Such a difference in polarity was corroborated by the direction of the dipole moment of RC (Table 5).Therefore, it is not surprising to find the molecular electron-deficient center so far from the axis of the molecule in the prepared compounds.

Terminal chains
The intrinsic difference between fluorocarbon and hydrocarbon chains has to be taken into account to elucidate the conformational arrangements of the prepared compounds.
Because of electrostatic repulsions of fluorine atoms in the relative 1,3-positions in the crystalline state, perfluorocarbon chain adopt a helical conformation of the carbon backbone [28].The cylinder-like structure of the segment (CF2)m resembles a stiff rod in which the carbon skeleton is covered by fluorine atoms.In the hydrocarbon counterpart the segment (CH2)n adopts the typical in-plane zigzag conformation [28].
Fluorine is the most electronegative element of the periodic table.This high electronegativity confers to CF bond a large dipole moment of 1.39 D while that of CH bond is only 0.40 D [29].Owing to the all-trans conformation, the local dipole moments C -H + of hydrocarbon chain are mutually neutralized.In (CF2)m segment, Hasegawa et al noted that the local dipoles C + F -cannot be cancelled out and the surface of fluorocarbon chain remains polar [30].

Interaction rigid core-terminal chains
The typical model for aromatic electron Donor-Acceptor (D-A) interactions was established by Hunter and Sanders in 1990 [31].According to this model, benzene and hexafluorobenzene form a complex where benzene is the donor (electron-rich) and hexafluorobenzene is the acceptor (electron-deficient). Experimental evidence for this complex was first reported in 1960 [32].The electron D-A concept may be regarded as Lewis base-Lewis acid type or charge-transfer.On the other hand, direct linkage of perfluoroalky group to the rigid-core makes the system more rigid.As shown in Figure 5(c), the conformation C adopted by compound 2b is quite different from conformation B described above.While RF2 (RF linked to Oxd 2) folds to interact with the molecular electron-deficient center according to an electron D-A interaction, RF1 (RF linked to Oxd 1) remains straight.The dipole moment of compound 2b (Figure 6) originates close Oxd 1.With the presumption that the molecular electron-deficient center is near the origin of the vector, we can attribute the absence of folding on the Oxd 1 side to the huge sprain that RF1 must undergo to interact with a center that is so close to it.

Conclusion
In this paper we have described the preparation and characterization of two new series of 2,2'-

Supporting Information
Experimental details, compound characterization data, copies of 1 H, 13 C NMR and IR spectra.
(a)) seems to be a variant of focal conic texture with unusually narrow ellipses of a SmA phase.Compound 4a presents a mosaic SmB phase where the molecules are organized in a hexagonal network as shown in Figure 2(b).The low average of enthalpy value (1.89 J/g) given in Table2is due to the first order phase transition marked by the coexistence of the crystalline phase and the liquid crystal phase at least 5°C as it is shown in Figure2(c).Figure2(d)shows the POM observation for compound 4b in heating cycles at 7 126°C.The texture in cooling is very similar to that in heating; this indicates a remarkable thermodynamic stability of the compound.Based on the texture in Figure2(d), we can note that the mesophase is hexagonal SmB with strongly double refracting lancets and regions[24].

Figure 2 :
Figure 2: Optical texture (x10) of liquid crystal phase for fluorinated compounds, (a): SmA phase observed in heating at 120°C for compound 2b; (b): Coexistence between cristalline phase and liquid crystal phase observed in cooling at 91°C for compound 4a; (c): Mosaic SmB phase observed at 96°C for compound 4a and (d): SmB phase observed in heating cycle at 126°C for 4b compound.

Figure 5 :Figure 6 :
Figure 5: conformer of lowest energy (side-view) of compounds (a): 4c, conformation A; (b) 4b, conformation B and (c): 2b, conformation C. Carbon atoms are shown in gray, hydrogen atoms in white, sulfur atoms in yellow, nitrogen atoms in blue, oxygen atoms in red and fluorine atoms in orange.
Based upon the above considerations, we could attribute the close proximity of fluorinated chains in conformation B to a throw space electron D-A intramolecular interaction between the perfluoroalkyl chains (electron-rich moieties) and the electron-deficient center of the molecule.The two fluorocarbon chains are symmetrically arranged with respect to the origin of the vector of dipole moment (Figure6, compound 4b).In compound 4b, which adopts the conformation B (Figure5(b)), the fluorinated terminal chains resemble a twin[33].The molecule is linear in the meaning that the rigid-core is in one side and the two arms in the other.Thus arranged, the molecule is polar and performs liquid crystal phases.The presence of the linking group C2H4S in compounds 4a and 4b has the desired effect of increasing conformational flexibility, bringing the two fluoroalkyl chains closer.As a result, 4a and 4b adopt the conformation B (Figure5(b)).However, although the non-fluorinated analogues 4c and 4d also carry the segment C2H4S, the electron D-A interaction was not observed in these compounds, obviously because of the fundamental difference between fluoro-and hydrocarbon chains mentioned above.The conformation A adopted by these compounds (Figure5(a)) lowers the dipole moments and thereby precludes mesophases formation.

( 1 , 3 -
phenylene) bis(1,3,4-oxadiazole) derivatives bearing different hydro-and fluorocarbonated chains.Structures of the obtained compounds were established by usual spectroscopic techniques.DSC, POM and X-ray diffraction investigations evidence the existence of the liquid crystal mesophase in the perfluorinated derivatives whereas the hydrocarbonated counterparts just present a thermotropic character.The dipole momentmolecular conformation relationship was scrutinized in order to elucidate the role of the molecular conformation on the dipole moment magnitude.Since some of the studied compounds show mesomorphic properties, it is important to lead on a deep study in order to investigate other parameters such as response time, viscosity and dielectric anisotropy.

Table 2 :
DSC thermograms data of compounds 2 and 4

Table 3 :
Calculated dipole moments of 2 and 4

Table 4 :
Electric charge of heteroatoms in compounds 2 and 4

Table 5 :
Dipole moment of RC