Banana-shaped 1,2,4-oxadiazoles

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1 PRAMANA cfl Indian Academy of Sciences Vol. 61, No. 2 journal of August 2003 physics pp Banana-shaped 1,2,4-oxadiazoles SITORGOVA a;d, T A GEIVANDOVA a ;d, O FRANCESCANGELI b and A STRIGAZZI c;d a FSUE NIOPIK, Organic Intermediates and Dyes Institute, B. Sadovaya 1/4, Moscow , Russia b Dipartimento di Fisica e Ingegneria dei Materiali e del Territorio and INFM, Università Politecnica delle Marche, Via Brecce Bianche III-60131, Ancona, Italy c Dipartimento di Fisica and INFM, Politecnico di Torino, C. Duca degli Abruzzi 24, I Torino, Italy d Joint Laboratory of Orientationally Ordered Media (OOM-Lab), C. Duca degli Abruzzi 24, I Torino, Italy Abstract. 3,5-Disubstituted 1,2,4-oxadiazoles are a new type of liquid crystalline (LC) compounds with asymmetrical five-membered heterocycle as a central unit. They have a bent shape and are very convenient model-compounds for studying the dependence of the LC properties on the molecular design. We have also synthesized and investigated banana-shaped 1,2,4-oxadiazoles using the ester groups as the linkage units. The new compounds exhibit spontaneous polarization in the smectic phase, even if there is no chiral group in the molecules. Preliminary experimental data suggest the presence of spontaneous polarization in the nematic phase as well. In order to study the structural properties of the LC phases, X-ray diffraction (XRD) measurements on powder samples have been carried out. Based on the XRD data, a model of the structural arrangement of the bent molecules in the smectic phase is provided, which accounts for the macroscopic spontaneous polarization as well as the ferroelectric switching behavior. Keywords. Liquid crystals; banana-shaped molecules; X-ray measurements. PACS Nos i; v; Nd 1. Introduction In the last few years, bent banana-shaped mesomorphic molecules have attracted rapidly growing interest both among theoreticians and experimentalists, mainly because of their ability to originate liquid-crystalline (LC) mesophases that exhibit spontaneous polarization (P S ) in the absence of chiral groups in their molecular structure [1 4]. A very interesting review paper entitled Banana-shaped compounds A new field of liquid crystals [5] was recently published by Pelzl et al. In that paper the authors pointed out the relationship between chemical structure and mesomorphic behavior, underlining the different aspects that should be taken into account for this purpose, i.e., size of the molecule, position and magnitude of the bending, influence of substituents and linkage groups and length of the terminal chain. During recent years, several hundreds of banana-shaped molecules exhibiting mesomorphic properties [6 11] have been synthesized. 239

2 SITorgovaetal It is obvious that the molecular bending in banana-shaped compounds is a necessary condition for the formation of chiral phases, but not sufficient. For nearly all banana-shaped substances, the bent shape of the molecules is caused by the 1,3-substituted-phenylene as the central unit. Nevertheless, biaxial smectic phases were found in some compounds containing heterocyclic central units [12 14]. In the work of Semmler et al [15], 1,3,4- oxadiazole was used as heterocycle. Bis(4-hexyloxyphenyl)4,4 0 -(1,3,4-oxadiazole-2,5- diyl) dicarboxylate has a symmetric structure and a strongly non-linear shape with a bending angle of the exocyclic bonds in the 2- and 5-positions of the oxadiazole equal to 134 ffi. The molecules of 3,5-disubstituted 1,2,4-oxadiazoles, i.e., the new class of liquid crystalline compounds that we have recently synthesized and investigated, have a bent shape. The angle between the exocyclic bonds in the C-3 and C-5 position is 140 ffi : then all the derivatives have almost the classical banana-shape. But the shape alone is not enough to ensure the appearance of spontaneous polarization, it also depends on the proper molecular dipole provided by the substituents. 2. Results and discussion Before our works [16 21], 3,5-disubstituted 1,2,4-oxadiazoles had not been studied as a fragment of liquid crystalline compounds, but only for pharmaceutical purposes. On the other hand this asymmetrical 5-membered heterocyclic fragment leads to extremely interesting physico-chemical properties of the mesogens of this series (figure 1). Asymmetry of 1,2,4-oxadiazole compounds involves three distinct aspects: 1. Asymmetrical distribution of electronic density in the heterocycle due to the presence of three heteroatoms with different electro-negativity [22] (the dipole moment of this type of compounds is D). 2. A distortion of the linearity of the molecule due to the bonding angles in the 5- membered cycle [22]. 3. The different types of substituents at C-3 and C-5 atoms. Figure 1. Distribution of electronic density and values of the angles in 1,2,4-oxadiazole ring. 240 Pramana J. Phys., Vol. 61, No. 2, August 2003

3 Banana-shaped 1,2,4-oxadiazoles The different ways of synthesizing these compounds give a lot of possibilities for the molecular design [22]. Moreover, for the first time we used cross-coupling reaction for the synthesis of this class of compounds, which of course enhanced the variations of designing the molecules. where R 0, R 00 = Alk, AlkO, Alk-cyclohexyl-, Alk-Ph and so on. The transition temperatures and dielectric properties depend significantly on both the position of the substituents with respect to the heterocycle and on their donor and acceptor ability. Here we want to present only a few examples. The most evident example is the case when we have only one alkyl substituent and the other a phenylcyclohexane, or biphenyl (see figure 2). When alkyl substituent is in position 5 of the heterocycle (the first three compounds), all display only the nematic phase whereas in the reversed structures, when alkyl is in position 3, each compound exhibits either the smectic phase only or both the smectic and the nematic phases. Another illustrative example is the combination of a strong donor substituent in position 3 and a strong acceptor in position 5. In this case a big difference in the dielectric anisotropy is observed compared with that of the reversed structure (see figure 3). By means of cross-coupling reaction we had the possibility of designing the molecules with a completely different shape (see figure 4): compound 1 possesses both smectic and nematic phases, compound 2 has a bent molecular shape and exhibits only the nematic phase whereas compound 3 has no mesophase at all. It is necessary to underline that the main core of the molecule itself is bent. In [15] the 1,3,4-oxadiazole was used as a heterocyclic central unit of the banana-shaped compound. It has a symmetric structure and a strongly non-linear shape with bending of the exocyclic bonds (in positions 2 and 5 of the oxadiazole) forming an angle of 134 ffi (see figure 5, compound I). This compound exhibits biaxial smectic phases characterized by very high transition temperatures. We decided to synthesize analogue molecules, using the asymmetrical 1,2,4-oxadiazole unit [20] for the central core (see figure 5, compound II). The main differences between compounds I and II can be summarized as follows: Pramana J. Phys., Vol. 61, No. 2, August

4 SITorgovaetal ffl the reverse positions of ester groups; ffl the angle of exocyclic bonds in positions 2 and 5 is 140 ffi instead of 134 ffi ; ffl the 1,2,4-oxadiazole central unit disturbs the symmetry of the molecule. The transition temperaturesof the new compounds (II) were measured by polarizing microscopy and differential scanning calorimetry (DSC). The results of these measurements are shown in table 1. Figure 2. Transition temperatures of alkyl-substituted 1,2,4-oxadiazoles. Figure 3. Influence of the donor and acceptor feature of the substituents: combination of strong donor and strong acceptor substituents. 242 Pramana J. Phys., Vol. 61, No. 2, August 2003

5 Banana-shaped 1,2,4-oxadiazoles Figure 4. Dependence of 1,2,4-oxadiazole derivatives with transition temperatures on the geometrical factors. Figure 5. Structural formulae of the derivatives of 1,3,4- and 1,2,4-oxadiazoles. Pramana J. Phys., Vol. 61, No. 2, August

6 SITorgovaetal Table 1. Polarizing microscopy and DSC study of compounds (II) on heating. Compounds II Microscope observations DSC data Comp. Transition Temperature Transition Temperature Enthalpy No. R type ( ffi C) type ( ffi C) (J/g) IIa C 7 H 15 Cr Cr Cr 1 N Cr N N I N I IIb C 7 H 15 O Cr Sm S N N I IIc C 9 H 19 O Cr Cr Cr Cr Cr 1 Cr Cr 1 Sm Cr 2 Sm Sm N Sm N N I N I IId C 10 H 21 O Cr Sm 76.3 Sm N 90.0 N I Compounds with alkyl substituents exhibit only the nematic phase. It is necessary to underline that banana-shaped 1,2,4-oxadiazoles (IIb IId) with alkyloxy substituents exhibit not only the smectic phase, as 1,3,4-oxadiazoles do [15], but also the nematic phase extending over a wide temperature range. Moreover, compounds (II) are featured by significantly lower transition temperatures than 1,3,4-oxadiazoles. Recent measurements of repolarization currents have shown that 1,2,4-oxadiazoles exhibit spontaneous polarization (P S ) in both the smectic and nematic phase. The value of P S is nc/cm 2, depending on the temperature. However, based upon only these measurements it was not possible to conclude unambiguously that the P S measured is connected only with the polarity of the LC phases. To gain more structural information on the mesophases, X-ray diffraction measurements (XRD) were performed on compound IIc with nonyloxy-substituent. Figure 6 shows the XRD pattern of the powder (unaligned) sample IIc at two temperatures corresponding to the smectic and the nematic phases. The sharp ring of figure 6a in the small-angle region is the Bragg diffraction by the smectic planes and its position corresponds to a periodic layer spacing of d = 28:3 Å. The wide diffuse halo in the high-angle region is associated with the in-plane short-range correlations of the molecular cores and corresponds to an average molecular distance of about 4.5 Å. In order to provide a model for the structural organization of the mesophase, additional data on the molecular structure were obtained by means of computer semi-empirical calculations based on the MOPAC-PM3 molecular package. The corresponding molecular structure is shown in figure 7, where the estimated electric dipole moments along the principal molecular axes are indicated. The coincidence (within the experimental errors) of the end-to-end molecular length L obtained from these simulations, L = 28:5 Å, with the smectic layer spacing deduced from the XRD data, d = 28:3 Å, suggests an orthogonal arrangement of the molecules within 244 Pramana J. Phys., Vol. 61, No. 2, August 2003

7 Banana-shaped 1,2,4-oxadiazoles (a) (b) Figure 6. XRD on powder sample of compound IIc. (a) T = 126 ffi Cinthesmectic phase; (b) T = 150 ffi C in the nematic phase. Figure 7. Semi-empirical MOPAC-PM3 calculations of the molecular conformation. the smectic layers with the molecular plane yz normal to the smectic planes xy. Therefore, combining XRD data with the molecular structure data and considering the spontaneous polarization property of the mesophase, we can propose for the smectic phase the structural arrangement represented in figure 8, which has the symmetry of the well-known SmC PF mesophase of bent banana-shaped molecules. In this picture, the ferroelectric layering organization with synclinic layer interfaces accounts for the spontaneous polarization and the ferroelectric switching behavior observed in the mesophase. However, a definitive assignment of the structure of this smectic phase requires additional XRD measurements to be carried out on samples uniaxially aligned in an external magnetic or electric field. These Pramana J. Phys., Vol. 61, No. 2, August

8 SITorgovaetal Figure 8. Structure of the smectic phase. Figure 9. Structure formulae of the fluorinated banana-shaped oxadiazoles. measurements, which are planned for the near future, will also help to understand the peculiar features of the molecular arrangement which are responsible for the unconventional ferroelectric switching behaviour in the nematic phase. We believe that future experimental and theoretical investigations will show that the longitudinal molecular electric dipole moment P z (see figure 7) plays a fundamental role for the appearance of the spontaneous polarization in the nematic phase. Finally, we decided to enhance the polarity of banana-shaped molecules and induced the polar fluorine substituent (see figure 9). The transition temperatures of the new series are presented in table 2. According to the transition temperatures the most promising compound is IIIf. Moreover, some compounds of this series have very sharp transition from planar to homeotropic orientation, within one degree (see figure 10), and what is much more strange is that it was possible to record this transition by DSC (see figure 11). Table 2. Transition temperatures of fluorine-containing oxadiazoles. NN R 1 R 2 R 3 Mettler FP-90 ffi C IIIa F H H Cr 221 N pl 289 N hom 325 I decomp. IIIb CH 3 O F H Cr 235 N pl 280 N hom 347 I decomp. IIIc CH 3 O H F Cr 185 N 296 I decomp. IIId H F F Cr 187 N 204 I IIIe C 2 H 5 O F F Cr 178 N pl 295 N hom 309 I decomp. IIIf C 8 H 11 O F H Cr 120 Sm 186 N 261 I 246 Pramana J. Phys., Vol. 61, No. 2, August 2003

9 Banana-shaped 1,2,4-oxadiazoles A Compound IIIb - N pl -N hom : A) T = 283 ffi C; B) T = 284 ffi C Figure 10. Transition from planar to homeotropic orientation for compound IIIb (polarizing microscopy). B Figure 11. Transition from planar to homeotropic orientation for compound IIIb (DSC measurements, scanning run 5 ffi /min). The measurement of the spontaneous polarization of the fluorinated compounds will be the subject of our future studies. We are also planning to synthesize a series of such compounds with different polar substituents. It is possible to conclude that banana-shaped 1,2,4-oxadiazoles are very puzzling and promising compounds. Acknowledgements This work was partially supported by the European Community in the frame of the INCO Copernicus Concerted Action Photocom, under Contract No. IC15-CT Pramana J. Phys., Vol. 61, No. 2, August

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