ORIENTATIONAL ORDERING IN SOME NEMATOGENS DEVIATING FROM THE CLASSICAL ROD-SHAPE

Several new laterally substituted liquid crystalline compounds have be en synthesized. They have the same main core which contains four rings (two aromatic, two alicyclic) with two lateral substituents introduce d on the same side of one of the inner rings. One of the substituents is a 4-X-benzyloxy group (X = CH3, CN, Cl) and the other is a hexyloxy chain. The presence of the lateral aromatic substituent makes these c ompounds deviate markedly from the classical rod-shape. However, a wid e enantiotropic nematic phase is present for all the compounds. The or der parameters of the chain and the para-disubstituted aromatic rings were obtained by using a 2D C-13 NMR technique with variable angle spi nning. The temperature dependence of the order parameters was estimate d using C-13 chemical shifts with slow spinning of the sample parallel to the magnetic held. The results indicate that the two lateral subst ituents are more or less folded back along the mesogenic core. Thus, t he flexible lateral chain is found to be roughly aligned with the mole cular long axis, whereas the para-axis of the less flexible aromatic b ranch makes a considerable angle with the molecular long axis imposed by the core, substantially increasing the mean width of the molecule. The core ordering does not seem to be influenced by the type and posit ion of the substituents. The folding back of the lateral chain and the substantial tilt of the lateral aromatic branch with respect to the c ore main axis are confirmed by the X-ray structure of a parent compoun d.