Light-Induced Rotation of a Molecular Motor in the Confined Space of a Metal-Organic Nanocage.

Molecular motors have been operated in a myriad of environments since their inception more than two decades ago. Of particular interest are systems in which a structural frame of reference for the motor's rotary motion is established. Examples include motors adsorbed on surfaces, embedded in framework materials, used as dopants in liquid crystals, or incorporated into polymer matrices. Embedding the molecular motor as a guest in a supramolecular cage assembly, however, remains an unexplored strategy. Such systems are particularly attractive, as they would allow the motor's rotation to drive mechanical motion from within the assembly, provided the motion does not lead to the guest motor's release. Here, the first motor-nanocage host-guest system in which a thermal and light-driven full 360° unidirectional rotational cycle occurs within the confined space of the nanocage is reported. We identified key structural elements that enable the formation of a host-molecular motor complex with unprecedented stability, governed by a noncovalent interaction between the motor's alkyl-COOH moiety and a carbonyl residue on the cage. This strategy allows the formation of stable host-guest complexes without relying on a size-induced fit, as is commonly observed in other inclusion complexes. This enables rotation to occur within the cage cavity despite dramatic geometric changes. We envision this strategy as a valuable tool for developing a new generation of molecular motors operating in confined spaces.