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

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 (MoKα 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.