A Comparative 13C NMR Study of Local Ordering in a Homologous Series of Bent-core Liquid Crystals

A C-13 NMR study is carried out in a number of banana-shaped molecules belonging to a homologous series. The derivatives contain chlorine or bromine substituent(s) in the center ring of 1,3-phenylene bis[4'-alkenyloxy biphenyl]-4-carboxylate (nP:BBC), and different terminal chain lengths are systematically compared in terms of their local order parameters and their ability to form an aligned nematic phase in the NMR magnet. The chemical shift anisotropy tensors measured from nPBBC by fitting the C-13 powder patterns are now extensively used on these molecules to interpret their observed C-13 chemical shifts in the nematic phase. The bend angles are estimated as a function of temperature in two members of the nPBBC series. Conformation twists in the bent-core region inferred from the local molecular biaxial paramters are discussed.