CHANGE OF THE LATERAL CHAIN CONFORMATION IN THE SOLID AND THE NEMATICPHASE IN LATERALLY SUBSTITUTED NEMATOGENS

A new homologous series of )-2-alkoxy-4'-(4-butoxysalicylaldimine)azob enzenes has been synthesized. These compounds contain four rings in th e main core and a lateral alkoxy branch on one of the inner rings and present enantiotropic nematic properties. The X-ray crystal structure (MoKalpha radiation, lambda = 0.7071 Angstrom, graphite monochromator) of y)-2-butoxy-4'-(4-butoxysalicylaldimine)azobenzene was investigate d (T = 293 K, R = 0.059 for the 2855 observed reflections). This compo und (C36H39N3O6) crystallizes in the monoclinic system with C2/c space group; (Z = 8); a = 28.052(7) Angstrom, b = 7.545(4)Angstrom, c = 33. 823(7)Angstrom and beta = 112.70(1)degrees. 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 roug hly linear with a 31 Angstrom length. The lateral butoxy chain is quas i-perpendicular to the long axis of the main core. The crystal cohesio n is due to numerous Van der Waals interactions. At the solid to nemat ic transition, a change in the conformation of the lateral chain occur s from an all trans-conformation to one involving a cis-conformation i n the first part of the lateral chain. This change can be monitored by the evolution of the C-13 chemical shift of the OCH2 belonging to the lateral alkoxy chain.