ORIENTATIONAL ORDERING OF SOME BIFORKED NEMATIC LIQUID-CRYSTALS

The orientational ordering behavior of four compounds containing a lat eral alkoxy chain has been investigated by C-13 NMR spectrometry. The main core of these compounds possesses three aromatic rings connected by imine and carboxylate linkages. The lateral chain is introduced ont o one of the outer rings in the meta position with respect to the carb oxylate linkage. Three of the compounds have a methoxy group in the pa ra position with respect to the carboxylate linkage and the fourth com pound has a hexyloxy chain in that position. By a 2D C-13 NMR techniqu e with variable angle spinning we have determined the ordering of the terminal and lateral chains as well as that of the terminal para-disub stituted ring. The temperature dependence of the order parameters was determined using the C-13 chemical shifts with slow spinning of the sa mple parallel to the magnetic field. In each compound, the first methy lene fragment in the lateral chain adopts a cis conformation so that t he lateral chain is aligned roughly parallel to the long molecular axi s. The X-ray structure of one of the compounds shows that the cis conf ormation of the fragment within the lateral chain persists in the soli d phase. It was also found that the terminal substituent near the late ral chain has a great influence on the ordering of the para-disubstitu ted aromatic ring. These results indicate that the long molecular axis can be affected to a large extent by the substitution pattern.