Supramolecular dendrimers: Unusual mesophases of ionic liquid crystals derived from protonation of DAB dendrimers with facial amphiphilic carboxylic acids

Abstract

Supramolecular liquid crystalline (LC) dendrimers were prepared by self-assembly of first to fifth generation amino terminated DAB dendrimers with facial amphiphilic carboxylic acids. These carboxylic acids are composed of three distinct incompatible segments, a rigid rod-like terphenyl core, two terminal alkyl chains and a polar lateral carboxylate group. The COOH groups were either directly connected to the terphenyl core or via oligo(oxyethylene) chains of different lengths. Depending upon the length of the polyether chain used, the dendrimer generation, the ratio of dendrimer to carboxylic acid or the temperature, a series of six different LC phases were observed. As well as a smectic phase (SmA), two different square columnar phases (Col_squ), a mesophase combining a layer structure with a hexagonal organisation of columns (channelled layer phase, ChL_hex) and two additional mesophases (Col_hex and M) with unknown structures were found. The square columnar phases are either composed of square cylinders (plane group p4mm) or a regular arrangement of square and triangular cylinders (plane group p4gm). In these dendrimer–carboxylic acid complexes protons are transferred from the COOH groups to the amino groups of the dendrimer, which gives rise to ionic complexes (dendroelectrolyte–amphiphile complexes). This concept allows the tailoring of the mesomorphic properties in a thermodynamically controlled self assembly process. The T-shaped ternary amphiphilic structure of the acid components and the incompatibility of the ionic species formed during the self assembly process are responsible for the formation of unconventional mesophase structures.