Force-Induced Ring Flipping in a Threaded Pillar[5]Arene

Pillar[n]arenes are popular macrocycles used in various supramolecular systems and materials due to their unique host-guest properties and ease of synthesis. Their pillar shape originates from the cyclic arrangement of methylene-bridged hydroquinone groups. They usually exist in one of two conformations where all the hydroquinone rings are oriented in the same direction (left or right), which is the source of their planar chirality. However, the controlled formation of pillar[n]arenes with intermediate conformations, for example, where only one ring is inverted, remains a challenge. Here we show how mechanical force can be used to reach this elusive conformation via the force-induced flipping of a single hydroquinone ring in a pillar[5]arene-based rotaxane. We show that the flipping motion can be controlled with the shape of the stopper and the axle, as well as with the nature of the pillar[5]arene substituents. This flipping behavior acts as a mechanical damper by slowing the scission of the polymer under tension. These results show how mechanical force can be used to access synthetically challenging supramolecular architectures, and we anticipate that this new dampening mechanism will prove useful in the formation of tougher materials.