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A relatively large number of mesomorphic compounds containing heterocyclic units have been synthesized and interest in such structures constantly grows [l, 21. This is not only because of the greater possibilities with heterocyclic rings for the design of new mesogenic molecules, but also because the introduction of heteroatoms can cause considerable changes in polarity, polarizability and sometimes the geometry of a molecule in relation to the geometry with phenyl rings [3, 41. These can greatly influence the types of mesophases which occur, the phase transition temperatures, and the dielectric and other properties of the mesogens [5, 61. Continuing our investigations of compounds with heterocyclic rings [7, 81 we have synthesized new Schiff's bases mesogenic derivatives of 5-(p-n-alkoxy)phenyl-2-amino-l,3,4-thiadiazole. SynthesisSchiff's bases 5a-5f were synthesized according to Scheme I. 1-4 were synthesized through the detailed procedures described in reference [9].The structures for all these compounds were confirmed by IR (Perkin-Elmer 577), 'H NMR, 13C NMR (Bruker AC-250 MHz), using tetramethylsilane as the standard, and elementary analyses ( Table 1).

1-(n-Hexil)-3-metil-2-pirazolin-5-ona foi acilada com cloretos de acidos e a condensação com aminas primárias forneceu uma série de enaminas. De acordo com os dados de RMN de 1 H e 13 C, os derivados acilas têm principamente uma estrutura 4-acilpirazol-5-ol com ligação de hidrogênio intramolecular e os derivados 4-aminometilenos existem predominantemente na forma de enamina estabilizada também por este tipo de interação. 1-(n-Hexyl)-3-methyl-2-pyrazolin-5-one was acylated with acid chlorides. Condensation of acyl derivatives with primary amines afforded enamines. According to the 1 H and 13 C NMR data, the acyl derivatives have mainly a 4-acylpyrazol-5-ol structure with intramolecular hydrogen bond, and the 4-aminomethylene derivatives exist predominantly in the enamine form stabilized by the same type of interaction.

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The synthesis and mesomorphic behaviour of new series of liquid crystals containing 1,3,4- thiadiazole and thiophene rings with azo central bond are reported (series 5a and 5b). All compounds of series 5a exhibit enantiotropic nematic mesophase and the higher homologues (n = 9 - 10) also show a monotropic smectic C phase. Series 5b show dimorphism SC - N (for n = 5 - 7 the SC is monotropic). These series are compared with the Schiff’s bases analogues, the imine bond gives rise to similar liquid crystals phase but larger mesomorphic range.