Non‐Equilibrium Kinetic States of a [2]Rotaxane‐Based Molecular Shuttle Controlled by Acid Concentrations

A [2]rotaxane‐based molecular shuttle with an acid‐responsive asymmetric macrocycle on a symmetric dumbbell axle is reported. Upon adding TFA, the macrocycle, namely the amine naphthotube, is protonated and translocates from the di(quaternary ammonium) station to the triazole stations because of electrostatic repulsion and weakened binding. The shuttling kinetics are slow due to the steric hindrance caused by the ethyl group on the quaternary ammonium center and can be followed by 1H NMR spectroscopy. Interestingly, it was found that the shuttling kinetics depends on the concentration of TFA. A kinetic intermediate was detected and can even be captured in the presence of a high concentration of TFA. Extensive control experiments revealed that the shuttling kinetics and the capture of the kinetic intermediate are related to the different protonation states of the rotaxanes. A [2]rotaxane‐based molecular shuttle with an acid‐responsive asymmetric macrocycle on a symmetric dumbbell axle has been synthesized. The shuttling kinetics can be controlled by the acid concentration and the kinetic intermediate can even be captured at a high concentration of acid. This was explained by invoking different protonation states of the rotaxane at different concentrations of acid.