Mesomorphic Properties of Diesters with p-Substituted-Arylazomethine Pendant Groups

Abstract

The substituent steric and electronic effects on mesophase stability and glass-forming properties of a series of 1,4-bis-(4-n-alkyloxybenzoyloxy)-2-N-(p-Y-aryl)azomethinebenzenes were evaluated. The diesters showed a typical though mild dependence of the monotropic nematic–isotropic transition temperatures on the substituent size that was attributed to the conformational flexibility of the pendant azomethine group. In contrast, the dependence of the entropy change associated with the nematic–isotropic transition showed an atypical behavior ascribed to the extreme reduction of the mesogen axial ratio. The introduction of the bulky arylazomethine group on a lateral position originated significant crystallization supercooling. The appearance of glassy mesophases was detected in some of the diesters; the OEt and CF₃ groups were the more effective substituents to promote the formation of nematic glasses.

Keywords:

• glassy mesophase
• liquid crystal
• nematic
• substituent effects

Acknowledgments

Financial support for this research was provided by Consejo Nacional de Investigaciones Científicas (CONICET), Agencia Nacional para la Promoción Cientifica y Tecnológica (ANPCyT), and Secretaría General de Ciencia y Técnica-Universidad Nacional del Sur (SGCyT).

Notes

^a T=°C, ΔH=KJ mol⁻¹, ΔS = J mol⁻¹K⁻¹.

^b T_CC.

^c Broad exotherms with the main peak displaying one or more shoulders.

^d Detected by optical microscope observations.