1.2.4- Oxadiazole mesogens

Oxadiazole mesogens have been significandy less investigated than their close relatives 1,3,4-oxadiazoles, but recently there has been growing interest in this moiety due to the claim of ferroelectric (FE) switching and biaxiality in the nematic phase (2009AFM2592). 

Apart from the parent compound 1 and its very simple alkyl derivatives, 1,3,4-oxadiazoles are solids. Solid oxadiazoles containing biphenyl or triphenyl substituents exhibit interesting properties upon heating. The symmetric 2,5-bisbiphenyl-4-yl-l,3,4-oxadiazole 38 melts into an isotropic phase showing small monotropic mesophase. By contrast, the asymmetric (hockey stick-shaped) mesogen 2-terphenyl-4-yl-5-phenyl-l,3,4-oxadiazole 39 exhibits a more stable enantiotropic liquid crystalline phase (a smectic phase as well as a nematic phase) <2001PCB8845>. 

Figure 13 3,5-diphenyl-1,2,4-oxadiazole-based bent-core mesogens. 

Primary-structure-based mechanisms for formation of the smectic phase also exist there are putative correlations between formation of the tilted Sc phase and the location of permanent electric dipoles on the mesogenic core. However, attempts to induce the formation of tilted smectic phases in an aromatic analog of sexiphenyl by adding large outboard dipoles failed compound 4, with 4-Debye dipoles associated with the oxadiazole rings, in fact exhibits an unusually large nematic phase spanning more than 225 °C [41] ... 

In contrast to the thiadiazole the oxadiazole 4 is neither liquid crystalline as monomer nor as polymer. The reason for the lack of mesogenic properties is that the substitution of the sulphur by oxygen introduces a bend into the molecule which prevents the formation of a LC phase. 

The next step was the synthesis of liquid crystalline polysiloxanes with oxadiazole groups in the mesogenic unit. The polymers were prepared by a polymeranalogous reaction of the monomers 10 and 11 with poly(hydrogenmethylsiloxane) 15. 

Brown GH, Shaw WG (1957) The mesomorphic state— liquid crystals. Chem Rev 57 1049-1157 Cardinaels T, Ramaekers J, Guillon D, Donnio B, Binnemans KA (2005) Propeller-like uranyl metallomesogen. J Am Chem Soc 127 17602-17603 Chai CP, Zhu XQ, Wang P, Ren MQ, Chen XF, Xu YD, Fan XH, Ye C, Chen EQ, Zhou QF (2007) Synthesis and phase structures of mesogen-jacketed liquid crystalline polymers containing 1,3,4-oxadiazole based side chains. Macromolecules 40 9361-9370 Chandrasekhar S, Sadashiva BK, Suresh KA (1977) Liquid crystals of disc-Uke molecules. Pramana J Phys 9 471 80... 

Samulski and co-workers [32] investigated two different bent-core mesogens, where the core consisted of an oxadiazole and the aliphatic chain R in the structure shown in Figure 5-19 was either an OC12- or a C7-unit. Figure 5-20 shows polarization microscopy pictures as well as conoscopic results for the OC12 sample. 

Kim, B.G., Kim, S., Park, S.Y. Synthesis of novel discotic mesogen containing electron-transportable oxadiazole moiety. Mol. Cryst. Liq. Cryst. 370,391-394 (2001)... 

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