The Effect of Heterocyclic Nitrogen on the Mesomorphic Behavior of 4-Alkoxybenzylidene-2′-alkoxy-5′-aminopyridines

A series of Schiff bases derived from 4-alkoxybenzylidene-2′-alkoxy-5′-aminopyridines were prepared in order to investigate the effect of heterocyclic nitrogen on the mesomorphic behavior. Nematic mesomorphism was obtained when the alkoxy group in the benzaldehyde portion of the molecule was methoxy or ethoxy while the higher homologs exhibited two smectic phases. The higher and lower temperature smectic phases were identified as smectic A and B respectively on the basis of their optical properties and transition enthalpies. Plots of mesomorphic-isotropic transition temperature as a function of the number of carbon atoms in the benzaldehyde portion of these Schiff bases exhibited some unusual odd-even effects which are discussed in terms of current theory. No particularly meaningful trends were observed, however, when the transition entropies were plotted against the number of carbon atoms in the alkoxy chains. The smectogenic tendencies of this series of compounds, compared to the benzene analogs, appeared to be due to the presence of a nitrogen atom.