Electrochemically controlled supramolecular systems

Large and complex molecular structures can be assembled by supramolecular chemistry and self-organization. For practical purposes it is required that the assembly and disassembly of supramolecular complexes and materials can be directed and controlled by external stimuli in order to build, for instance, molecular machines or components for molecular electronics. In this respect, redox-responsive molecules or supramolecular complexes are a logical choice since they are directly compatible with state-of-the-art semiconductor-based microelectronics. Here, redox-switchable supramolecular assemblies, host–guest complexes and materials are reviewed. Ferrocenes, viologens, quinones, tetrathiafulvalenes, and transition metals, e.g. Ni and Cu, have two or three stable redox states and their electrochemical interconversion is completely reversible. Therefore, these redox centers are frequently used as active components in supramolecular assembly. The electrochemical action of the redox-active component can be divided into two main categories: (i) conformational changes within a molecule and/or the motion of one component with respect to another in interlocked molecules, and (ii) the assembly and/or disassembly of supramolecular complexes. The first part of this review deals with the conformational changes within molecules, and with electrochemically induced movement of one component with respect to the other in interlocked molecules. The second part describes the electrochemically controlled formation of supramolecular complexes. A wide range of non-covalent interactions, i.e. hydrophobic interactions, Van der Waals interactions, hydrogen bonds, electrostatic interactions, and π–π stacking can be electrochemically controlled to direct the association or dissociation of supramolecular complexes and materials.