Synthesis and characterization of new liquid crystalline monomers for non-linear optics. X-ray study of re-entrant nematic behaviour with smectic-like fluctuations of C-type

The syntheses and structural properties of four liquid crystalline monomers are presented. They were prepared by two different reaction pathways. Transition temperatures and phase characterization were done by DSC, polarization microscopy and X-ray diffractometry. The dependence of phase transition temperatures on the alkyl chain lengths and the lateral substitution of the aromatic core are discussed. Two of the compounds investigated show highly ordered smectic phases. The improvement of the conjugation by means of a lateral hydroxy group disturbs the occurrence of the E and B phases. The presence of a nematic re-entrant phase with smectic-like fluctuations of the C-type was observed in a compound with a lateral hydroxy group. The existence of this re-entrant nematic phase was confirmed by X-ray measurements both on cooling and heating of the sample.