Mesomorphism in Relation to Laterally Substituted Homologous Series with Azo and Azomethine Linkages

(E)-N-(4-Alkyloxybenzylidene)-4-((E)-phenyldiazenyl)aniline (9a–9e) and (E)-N-(4-alkyloxy-3-methoxybenzylidene)-4-((E)-phenyldiazenyl)aniline (10a–10e), containing –N=N– and –CH=N– linkages, were synthesized and studied for their mesogenic behavior. All synthesized compounds exhibited enantiotropic smectic behavior and followed a typical transition curve. Thermal phase transitions were recorded using a polarizing optical microscope equipped with a heating stage. The structures of the synthesized compounds were confirmed based on ¹H NMR and IR spectral analyses. The introduction of a methoxy (–OCH₃) substitution at the ortho position in a phenyl ring with a terminal alkyl chain resulted in a lower Cr–Sm phase transition temperature and decreased thermal stability. The average onset of the smectic phase for 9a–9e was 55.6°C, whereas for 10a–10e, it was 84.2°C. Comparatively, the average thermal stability for the unsubstituted homologous series was 130.2°C, whereas for the methoxy-substituted homologous series, it was 83.4°C.