Change of the lateral chain conformation in the solid and the nematic phase in laterally substituted nematogens

Abstract

A new homologous series of 4-(4′-ethoxybenzoyloxy)-2-alkoxy-4′-(4-butoxysalicylaldimine)-azobenzenes has been synthesized. These compounds contain four rings in the main core and a lateral alkoxy branch on one of the inner rings and present enantiotropic nematic properties. The X-ray crystal structure (MoK_α radiation, λ = 0.7071 Å, graphite monochromator) of 4-(4′-ethoxybenzoyloxy)-2-butoxy-4′-(4-butoxysalicylaldimine)azobenzene was investigated (T = 293K, R = 0.059 for the 2855 observed reflections). This compound (C₃₆H₃₉N₃O₆) crystallizes in the monoclinic system with C2/c space group; (Z = 8); a = 28.052(7) A, b = 7.545(4) Å, c = 33.823(7) Å and β = 112.70(1)°. There is a strong chelated hydrogen bond between the OH group and the neighbouring nitrogen atom engaged in the imine bridge. In the solid state, the molecule is roughly linear with a 31 Å length. The lateral butoxy chain is quasi-perpendicular to the long axis of the main core. The crystal cohesion is due to numerous Van der Waals interactions. At the solid to nematic transition, a change in the conformation of the lateral chain occurs from an all trans-conformation to one involving a cis-conformation in the first part of the lateral chain. This change can be monitored by the evolution of the ¹³C chemical shift of the OCH₂ belonging to the lateral alkoxy chain.