Copper-metallomesogen structures obtained by ionic self-assembly (ISA): Molecular electromechanical switching driven by cooperativity

In a stepwise noncovalent multiple-interaction strategy, copper(II) salts were complexed with the sodium salts of bathophenanthrolinedisulfonic acid (BPS) and bathocuproinedisulfonic acid (BCS), and organized into nanostructured materials by the addition of ammonium surfactants by means of the ionic self-assembly (ISA) route. In the case of the methyl-substituted BCS complexes, a slow color change from green to brick red was observed. UV and EPR investigations showed that the color change was due to a change in oxidation state, the resulting brick red color is typical for Cut species. It is concluded that steric interactions and mechanical packing into a supramolecular structure drive this electronic transition at the metal center. When complexation is performed with double-tail ammonium surfactants, these metallo-mesogenic materials exhibit thermotropic liquid-crystalline phase behavior, as investigated by polarized light microscopy, differential scanning calorimetry (DSC), and temperature-dependent wide-angle and small-angle X-ray analyses. The complexity of the observed phases increased with increasing tail length of the surfactants. Complexation with double-tail C-18 surfactants yielded highly organized materials for both the BPS and BCS ligands.