Liquid crystalline bis(N-salicylideneaniline)s: synthesis and thermal behavior of constitutional isomers

The first examples of mesogenic bis(N-salicylideneaniline)s (BSANs), wherein two lipophilic (half-disk shaped) entities are interlinked through the dihydroxydiformylbenzene core, were synthesized and characterized. In particular, three constitutional (positional) isomeric BSANs were prepared by the facile twofold condensation of 3,4,5-tris(alkoxy)anilines with 2,3-dihydroxyterephthalaldehyde, 4,6-dihydroxyisophthalaldehyde, and 2,5-dihydroxyterephthalaldehyde and their structures were established by elemental analyses, FT-IR, 1H NMR, and 13C NMR. Proton NMR experiments demonstrated their existence in enol–imine (OH) form solely. Polarizing optical microscopic, differential scanning calorimetric, and powder X-ray diffraction studies evidenced the occurrence of columnar mesomorphism in two sets of isomers.