Conformational behaviour of laterally dialkoxy branched mesogens. Part one

The crystal structures of two mesogenic compounds having two neighbouring l ateral alkoxy chains in the central part of the molecules have been solved. In both structures, the common four ring central core is extended and quit e rigid with a total length close to 28.5 A. All the molecules are strictly parallel because of the P<(1) over bar> space group; the molecular arrange ment is characteristic of a nematogenic system. In these structures, the la teral alkoxy chains are folded back to the core, they exhibit slightly diff erent conformations and are quite disordered. The C-13 chemical shift of th e first -OCH2- within the lateral chains can probe the mean conformation of the chain in the nematic phase. This chemical shift is independent of the compound. Nevertheless, in the solid phase, this chemical shift is dependen t on small geometrical changes due to the influence of the oxygen in the ne ighbouring lateral chain. In addition, temperature cycling of the sample le ads to different crystalline solid forms as evidenced by DSC and NMR studie s.