An ExBox [2]catenane

A donor-acceptor [2]catenane, in which an extended tetracationic cyclophane is mechanically interlocked by a porphyrin-containing macrocycle, was synthesised using a template-directed protocol and alkene metathesis as the ring-closing step. In the ground state of this [2]catenane, the porphyrin ring resides inside the cavity of the cyclophane on account of favourable charge-transfer interactions between the electron-rich porphyrin and the electron-deficient cyclophane. The [2]catenane can act as a push-button molecular switch where the co-conformations of the [2]catenane can be controlled either chemically or electrochemically. Addition of acid protonates the porphyrin ring and a relative circumrotational motion of the macrocycle positions the charged porphyrin ring outside the cavity of the cyclophane. The switch can be reset to its ground-state co-conformation by the addition of base. Electrochemical reduction of the extended bipyridinium units of the cyclophane decreases the strength of the donor-acceptor interactions in the [2]catenane, leading to a loss of recognition between the mechanically interlocked rings. The chemical and electrochemical switching mechanisms are both reversible.