EFFECT OF A DIMETHYL SUBSTITUTION ON THE ORDERING OF 4 ISOMERIC NEMATOGENS

The different orientational ordering of four laterally dimethylated co mpounds has been investigated by C-13 NMR. The main core of these comp ounds contains two aromatic rings connected by a diazo linkage and an alicyclic ring connected by a carboxylate linkage. The two methyl grou ps are introduced on the inner aromatic ring, forming four structural isomers. The order parameters of the terminal aromatic ring were obtai ned by a 2D C-13 NMR technique with variable angle spinning. The tempe rature dependence of the order parameters was deter mined using the C- 13 chemical shifts with slow spinning of the sample parallel to the ma gnetic field. It was found that the ordering of this fragment is profo undly affected by the pattern of disubstitution in the neighboring rin g. The results indicate that in the nematic phase the rigidness of an aromatic fragment is related to the extended conjugation of the ring i n a straightforward way. On the other hand, flexible fragments like th e terminal chains are not markedly affected by the disubstitution of t he inner ring.