Bidirectional Molecular Motors by Controlling Threading and Dethreading Pathways of a Linked Rotaxane

Artificial molecular motors have attracted significant attention as models of biological machines and molecular motors. Unlike conventional artificial molecular motors, which rely on covalent bond rotation, molecular motors with mechanically interlocked molecules (MIM) generate significant rotational motion via the dynamic shuttling of the macrocyclic components and have thus attracted considerable attention. The topology of MIM-type molecular motors is currently limited to catenane structures, which require intricate synthetic procedures that typically produce a low synthetic yield. In this study, we develop a novel class of MIM-type molecular motors based on a rotaxane-type topology. The switching of the rotaxane threading/dethreading pathway of the linked rotaxane by protection/deprotection of the bulky stopper group and changes in solvent polarity enabled unidirectional rotation of the molecular motor. Detailed spectroscopic investigations provide a quantitative evaluation of the threading/dethreading reaction rates. Repeated unidirectional rotation and switching of the direction of rotation is achieved by controlling the reaction rates. Our findings demonstrate that linked rotaxanes can serve as MIM-type molecular motors whose rotation direction can be switched by controlling the threading/dethreading reactions; these motors are expected to find applications as a component of molecular machinery.