INFLUENCE OF LARGE LATERAL SUBSTITUENTS ON THE NEMATIC LIQUID-CRYSTALLINE PROPERTIES STUDIED BY C-13 NMR

It has been shown that two nearby large lateral substituents can be in troduced on the same side of a mesogenic core without destroying the n ematic range, even if the pattern of substitution is not favourable. T he newly synthesized compounds contain four rings in the main core, a lateral hexyloxy chain and a lateral aromatic branch. This aromatic ri ng itself is modified by different substituents at the meta or para po sition with respect to the core linkage. Two isomeric compounds contai ning a dodecyloxy chain in these positions have been studied by C-13 N MR spectroscopy in the liquid crystalline phase. On entering the nemat ic phase, the C-13 chemical shift showed a positive jump for the oxome thylenes attached to the con, but a negative jump for the oxomethylene on the aromatic lateral branch. It is found that the two lateral subs tituents are folded back along the core, implying a gauche conformatio n for the first fragment. However, in the dodecyloxy chain, the first and the subsequent methylene carbons have negative order parameters, s howing a classical all-trans mean chain conformation. The values of th e order parameters were derived from the transient oscillations observ ed during variable contact-time H-1-C-13 cross-polarization experiment s. The order parameter values are not very dependent on the substituti on position of the dodecyloxy chain. As a consequence, the dodecyloxy chain aligned in the nematic field affects the average molecular shape and the mean orientation of the whole molecule.