Study of mesomorphism dependence on molecular flexibility of an azoester series containing a napthyl unit

A novel azoester homologous series of liquid crystalline (LC) compounds: RO?C₆H₄-COO?C₁₀H₆-N:N-C₆H₄?OC₄H₉(n) without lateral substitution has been synthesized and studied with a view to understanding and establishing the effects of molecular structure on thermotropic LC substances with reference to tailed-end group. The novel homologous series consists of 13 homologs (C₁ to C₁₈) whose nematogenic and smectogenic mesomorphism commences enantiotropically from C₆ and C₁₂ members of the series, respectively. The C₁₂–C₁₈ homologs are smectogenic and C₆–C₁₈ are nematogenic, of which C₁₂–C₁₈ homologs are smectogenic plus nematogenic. The C₁–C₅ homologs are nonmesogenic. Transition temperatures and the textures of the homologs were determined and identified by an optical polarizing microscope (POM) equipped with a heating stage. Textures of a nematic phase are threaded or Schlieren and that of the smectic phase are of the type A or C. Transition curves Cr-M/I, Sm-N and N-I of a phase diagram behaved in normal manner except N-I transition temperature of C₁₀ homolog which deviated by 9°C–10°C from normal behavior. N-I transition curve exhibited odd-even effect. Analytical, spectral, and thermal data confirms the molecular structures of homologs. Thermal stability for smectic and nematic are 115.5°C and 138.5°C, respectively whose corresponding mesophaselengths are varied from 10.0°C to 16.0°C and 13.0°C to 24.0°C, respectively. Group efficiency order for smectic and nematic are derived from comparative study of structurally similar analogous series; as smectic: ?OC₄H₉ (n) > ?CH₃ > ?H; Nematic: ?H > ?OC₄H₉ (n) > ?CH₃