Template-Controlled Mechanochemical Dissociation of a Rotaxane

The mechanochemical dissociation of rotaxanes has been used in the design of materials with the ability to dissipate energy and of force-controlled release systems. The former takes advantage of the dissociation by dethreading, a noncovalent process in which the macrocycle is forced over a stopper, while in the latter, the forceful contact between the macrocycle and stopper provokes the rupture of a covalent bond and release of the stoppering unit. As the dissociation pathway is dictated by the relative size of the macrocycle and the stopper, a rotaxane under tension can usually only undergo one of these two pathways. However, the ability to control the mechanochemical reactivity of a rotaxane in situ would allow the creation of multiresponsive materials. Here, we show how to selectively access unstoppering and dethreading pathways from the same rotaxane structure by respectively keeping or removing a single Pd atom to alter the size of the cavity of the macrocycle in a rotaxane built around a Pd^II template. We anticipate that these findings will have applications in the design of force-responsive materials and devices in which the mechanical response can be tuned to suit specific uses.