Polycatenar mesogens have a shape intermediate between classical rod‐like and disc‐like liquid crystals which allows practically all the mesomorphic symmetrics to be obtained with the same molecular core by varying the temperature or the number and position of grafted chains (see Figure: R = H(CH2)nO). The synthesis, mesomorphic properties, and structures of polycatenar mesogens are reviewed, with emphasis on the structure–properties relationship.

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We have prepared two series of hexasubstituted triphenylene derivatives : hexaethers from CH3 to C13H27 and hexaesters from C3H7 to C13H27. Most of them exhibit a thermotropic mesophase with a large temperature range. We have found the existence of an additional transition within the mesophase domain of some hexaesters with long alkyl chains. The structure of this new type of liquid crystal is discussed with respect to ours results concerning miscibilities and heat of transition

Five mesomorphic phases are described in several disc-like liquid-crystals. They are hexaalkanoyloxy benzenes, triphenylenes or truxenes, hexa-alkoxy triphenylenes, hexa-benzoates of triphenylene. Several of these compounds exhibit a complex polymorphism. Informations on the structures of these phases have been obtained by means of X-Ray diffraction; except a ND namatic phase all the others one are columnar phases. At the same time a systematic study has been performed by the well known method of isomorphy. All these results allow us to propose a general classification of these phases.

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The X-ray diffraction patterns of powder samples of the fluid mesophase of the hexaalkoxybenzoates of triphenylene which is a disc-like molecule are similar to the diffraction patterns of non orientated nematic phase of rod-like molecules. Furthermore, we give evidence for a uniaxial character of this anisotropic medium with a negative diamagnetic anisotropy, the flat molecules are orientated parallel each other by a rotating magnetic field. In addition, the X-ray patterns of orientated samples of the hexyloxy compound revealed pretransitional effects in this nematic phase of disc-like molecules (ND). This short-range translational order, sort of skewed cybotactic groups originates from the existence of a low temperature columnar phase with an original tilted packing here described