Directional Shuttling of a Stimuli-Responsive Cone-Like Macrocycle on a Single-State Symmetric Dumbbell Axle.

Rotaxane-based molecular shuttles are often operated using low-symmetry axles and changing the states of the binding stations. A molecular shuttle capable of directional shuttling of an acid-responsive cone-like macrocycle on a single-state symmetric dumbbell axle is now presented. The axle contains three binding stations: one symmetric di(quaternary ammonium) station and two nonsymmetric phenyl triazole stations arranged in opposite orientations. Upon addition of an acid, the protonated macrocycle shuttles from the di(quaternary ammonium) station to the phenyl triazole binding station closer to its butyl groups. This directional shuttling presumably originates from charge repulsion and an orientational binding preference between the cone-like cavity and the nonsymmetric phenyl triazole station. This mechanism for achieving directional shuttling by manipulating only the wheels instead of the tracks is new for artificial molecular machines.