Novel amides and Schiff's bases derived from 1,3,4-oxadiazole derivatives: synthesis and mesomorphic behaviour

The synthesis and liquid crystalline properties of novel achiral amides (Ia-g, IIa-g and IVa,b), achiral Schiff's bases (IIIa-g and Va-g), chiral amides (VI, VII) and chiral Schiff's bases (VIII-XI) incorporating a 1,3,4-oxadiazole ring are reported. All amides of the series I and II display an enantiotropic smectic A phase. The amide IVa,b did not show mesomorphic properties. Amides of the series Ia-g and IIa-g contain a flexible n-tetradecylthio chain, the other terminal substituent is an n-alkoxy chain and n-alkyl chain, respectively (n = 4-10) and the 1,3,4-oxadiazole is in the terminal rigid core. Amides Ia-g have broader mesomorphic range and higher thermal stability than the corresponding amides IIa-g. Amides IVa,d contain the 1,3,4-oxadiazole ring in the centre of the rigid core and two flexible alkoxy chains as flexible terminal substituents. Thus, the mesomorphic properties are favoured if 1,3,4-oxadiazole is shifted to a terminal position of the rigid core. Schiff's bases IIIa-g display an enantiotropic dimorphism smecticC-smectic A. Schiff's bases IIIa-g have a broader mesomorphic range than the analogous amides Ia-g. Schiff's bases Va-g exhibit a dimorphism smectic A-nematic, and in contrast to this the analogous amide IVa,b did not show mesomorphism. The chiral amides VI and VII and chiral Schiff's bases X and XI did not show mesomorphic properties and only the chiral Schiff's bases VIII and IX display a chiral smectic C phase in a short mesomorphic range. A density functional theory theoretical study at the B3LYP/6-311++G(d,p) level was performed in order to analyse the structural features that must be related with the mesomorphic behaviour of the reported compounds.