THE EFFECT OF A LATERAL AROMATIC BRANCH ON THE ORIENTATIONAL ORDERINGOF LATERALLY ALKOXY-SUBSTITUTED NEMATICS

Two new mesogenic series have been synthesized. They have the same mai n core which contains four aromatic rings, with two lateral substituen ts on the same side of one of the inner rings. One of the substituents is a 4-chlorobenzyloxy group and the second is an alkoxy chain. The p ositions of the lateral substituents are different in the two series. Despite the large aromatic branch, an enantiotropic nematic phase is o btained for most of the compounds. The orientational ordering behaviou r of these new compounds has been investigated by C-13 NMR, and two is omeric compounds containing a lateral dodecyloxy chain were chosen for the study. The order parameters of the chain were obtained by a 2D C- 13 NMR technique with variable angle spinning. The temperature depende nce of the order parameters was determined using the C-13 chemical shi fts with slow spinning of the sample parallel to the magnetic field. T he first methylene fragment in each lateral substituent of each compou nd has a positive C-H bond order parameter, implying that this fragmen t adopts a cis-conformation in the nematic phase. The two lateral subs tituents are folded back along the mesogenic core in opposite directio ns. The lateral chain is found to be roughly aligned along the molecul ar long axis imposed by the core, whereas the para-axis of the phenyl ring in the lateral aromatic branch makes a considerable angle with th e molecular long axis. It was also found that the different patterns o f substitution do not affect the position of the molecular long axis t o any large extent.