Structural Transformations in Coordination Cages

Abstract

The ability to induce structural changes in supramolecular systems is a key step in the evolution of stimuli-responsive and functional materials. This chapter highlights contributions to this area of research that involve self-assembled coordination cages. As triggers, light of selected wavelengths or chemical agents are commonly used. Stimuli of the latter type can consist of small ions, ligands, redox active compounds, acids, bases, or other reactive reagents. The results of the triggered action can be manifold and range from smaller conformational changes to more extensive structural changes such as cage-to-cage transformations and self-sorting phenomena. While conformational changes often proceed without the breaking or formation of bonds within the self-assembled structure, structural reorganization processes can be further subdivided into the interconversion between isomeric assemblies and the complete transformation between architectures of differing composition. Reversible bonding such as in coordination compounds or dynamic covalent molecules plays a major role in these processes. In self-assembled cages, a commonly sought-for goal of such triggered transformations is to achieve differences in the encapsulation state of a host–guest system. In the simplest case, this includes an on/off switching of the binding affinity for a certain guest, but more sophisticated consequences such as fine-tuning of the binding kinetics or regulation of guest specificity can be achieved.