Single-Molecule Study Reveals Ion-Dependent Conformational Change and Nanomechanical Property of Crown Ether-Based Polymer

The host-guest recognition of crown ethers and alkali metal ions plays a key role in many functional systems. The understanding of the static conformation of the crown ether ring and guest ion is steadily growing, but little is known about the dynamic response of the complex structure when the crown ether ring is deformed. In this study, we successfully monitored the conformational changes of the host-guest complex formed by poly(dibenzo-18-crown-6) and metal ions through single-molecule force spectroscopy and constrained geometry simulated external force calculations. A distinctive shoulder-like force plateau can be observed on the force-extension curve, which depends on both the cation and anion species. Quantum chemical calculations revealed that during the conformation change, the number of coordination bonds between the oxygen atoms on the ring and K+ ions were dynamically changed from six to four with an intermediate bond number of two, and K+ was gradually squeezed out of the center of the crown ether ring. In addition, the size of the alkali metal ions was found to greatly affect its deformation pathway. This study deepened our understanding of crown ether-based host-guest interactions in a brand-new vision. And, it is useful for tuning the properties of crown ether-based functional materials because the DB18C6 unit can be easily incorporated into the network of polymer materials. Furthermore, such a method can be used to explore and tune the host-guest interactions in other macrocycle-based systems.