A Dynamic Tetracationic Macrocycle Exhibiting Photoswitchable Molecular Encapsulation

Designing macrocycles with appropriate molecular recognition features that allow for the integration of suitable external stimuli to control host-guest processes is a challenging endeavor which enables molecular containers to solubilize, stabilize, and separate chemical entities in an externally controllable manner. Herein, we introduce photo- and thermal-responsive elements into a semi-rigid tetracationic cyclophane, OPVEx ² Box ⁴⁺ , that is composed of oligo(p-phenylenevinylene) pyridinium units and the biphenylene-bridged 4,4-bipyridinium extended viologens and adopts a rectangle-like geometry. It transpires that when the photoactive oligo(p-phenylenevinylene) pyridinium unit is incorporated in a macrocyclic scaffold, its reversibility is dramatically improved, and the configurations of the cyclophane can go back and forth between (EE)- and (EZ)-isomers upon alternating blue light irradiation and heating. When the macrocycle is found in its (EE)-configuration, it is capable of binding various π-electron-rich guests - e.g., anthracene and perylene - as well as π-electron-deficient guests - e.g., 9,10-anthraquinone and 5,12-tetracenequinone - through charge-transfer and van der Waals interactions. When irradiated with blue light, the (EE)-isomer of the cyclophane can be transformed successfully to the (EZ)-isomer, resulting in the switching off of the binding affinity for guest molecules, which are bound once again upon heating. The use of light and heat as external stimuli to control host-guest interactions involving a multi-responsive host and various guests provides us with a new opportunity to design and construct more-advanced molecular switches and machines.