Preliminary communication: A novel class of banana-shaped azo compounds exhibiting antiferroelectric switching behaviour

pounds. Later, antiferroelectri c properties were also found dimension to this  eld, namely photochromism. in chiral materials [2]. Theoretical work has however The synthesis of these banana-shaped azo compounds shown that to exhibit these properties the molecules need was achieved by following the synthetic routes shown in not be chiral [3, 4]. Indeed, in 1996, Niori et al. [5] the scheme. Details of the synthesis will be reported in reported ferroelectric behaviour in a smectic phase formed a forthcoming paper. The spectral data for all the by ‘banana-shaped ’ non-chiral molecules. Observation intermediate compounds were satisfactory. Selected data of antiferroelectric switching properties in such materials for compound 6 : max /cm Õ 1 (KBr pellet): 2937, 2870, followed almost immediately [6–8]. Since then, this new 1731 and 1600; dH (200 MHz, CDCl3 , SiMe4 ): 0.92 liquid crystal  eld has attracted the interest of a number (t, J 5 6.0 Hz, 6H, CH3 ), 1.35–1.87 (m, 16H, CH2 ), 4.06 of research groups [9–14]. As the structure of the (t, J 5 6.4 Hz, 4H, Ar OCH2 ), 7.03 (d, J 5 8.0 Hz, 4H, mesophases exhibited by these banana-shaped molecules ArH), 7.3 (d, J 5 8.0 Hz, 4H, ArH), 7.42 (m, 2H, ArH), is not completely understood, a tentative classi cation 7.57 (m, 1H, ArH), 7.79–8.01 (m, 9H, ArH), 8.33 scheme has been proposed in which the phases are (d, J 5 8.8 Hz, 2H, ArH), 8.38 (d, J 5 8.8 Hz, 2H, ArH), labelled with the code letters B1 . B7 according to the 8.52 (s, 1H, CH N ); m/z (FAB): 829 [M]+ (calcd. for sequence of their discovery [12]. Among the physical 515165 ). properties of the ‘banana mesophases’, the electro-optical The transition temperatures for the azo compounds behaviour is of particular interest and gives some hints were determined using a Mettler FP82HT hot stage and about the structure of these phases. Among the seven central processor in conjunction with a Leitz DMRXP phases, electro-optical switching [12] has been observed polarizing microscope. The enthalpies of transitions were only in the B2 , B3 and B5 phases. determined from thermograms recorded on a diŒerential From the view point of chemical structure, molecules scanning calorimeter (DSC7 Perkin-Elmer). The heating with  ve, six and seven aromatic rings have been found and cooling rate was 10 ß C min Õ 1. X-ray measurements to form this type of mesophase. However, in all the were carried out using a MAC Science image plate banana-shaped mesogens reported so far, the bent-shape set-up [15]. The lowest homologue (with n 5 4, n being of the molecules is achieved by either the same type of the number of carbon atoms in the alkyl chains, R) linkage ( COO ) or two diŒerent types of linkages showed only one mesophase while the compound with ( COO and CH N or COS or C C ) [12]. n 5 6 exhibited two mesophases, both of which are In this communication we report the  rst example of enantiotropic. In contrast the two higher homologues, azo compounds exhibiting banana-mesophases . This with n 5 8 and 10, showed one enantiotropic and one monotropic mesophase. Hereafter, we refer to these