Liquid Crystalline Octopus: An Alternative Class of Mesomorphic Dendrimers

The synthesis and the mesomorphic properties of a new series of liquid crystalline dendrimers, referred thereafter as to octopus dendrimers owing to their eight sidearms, are reported. In these dendritic systems, the arborescence is constituted of mesogenic segments and aliphatic spacers, and as such they differ quite substantially from the classical end-functionalized dendrimers whose dentritic core is more flexible. Such a molecular design was adopted in order for the internal mesogenic segmentsconstituting the skeleton of the dendrimerto participate at every level of the molecular organization within the mesophases. The dendritic branches, prepared by a mixed convergent/divergent method, were attached to a tetrapodant central core to yield second generation dendrimers via amido linkage. X-ray diffraction studies showed that the acidic dendron precursors and the final dendrimers exhibit a hexagonal columnar mesophase, whose mesophase stability was found to depend on the number and position of the terminal alkoxy chains. The high density of aliphatic chains imposes curved interfaces at all hierarchical levels of the dendrimers, i.e., between the pro-mesogenic units, the aliphatic spacers, and the terminal chains, forcing the molecules to adopt a wedgelike conformation, promoting the self-assembling toward a supramolecular columnar organization. Because of their original molecular structure, the internal columnar “hard” core of the columns is compatible with an onion-type structure, i.e., made up of successive, concentric crowns occupied by aromatic and aliphatic constituents, as deduced from the analysis of the X-ray diffraction data.