Molecular shuttles are core components of the molecular-machine toolbox, and they’re usually based on rotaxanes —structures containing a ring-shaped molecule threaded onto an axle, so that the ring can move back and forth.
Now, a team led by Eduardo Peris and Susana Ibáñez of the Institute of Advanced Materials at Jaume I University has invented a molecular shuttle with an entirely different architecture. It consists of a flat molecule that zips to and fro inside a narrow molecular box just 2.2 nm long and 0.7 nm wide (Angew. Chem. Int. Ed. 2024, DOI: 10.1002/anie.202318829). “This is actually paving the way to new types of molecular machines,” Peris says.
The two long faces of the box are polyaromatic molecules with iridium complexes at each end, while pyrazine molecules form the box’s short faces. Guest molecules are held inside through interactions with π-electrons in the box’s long face.
Nuclear magnetic resonance experiments showed that some guests, such as coronene, can shuttle along the length of the box, their motion slowed by bulky tert-butyl groups at the box’s halfway point. Lowering the temperature reduces the shuttling rate from 13,000 back-and-forth cycles per second at 0 °C to just 30 cycles per second at –80 °C.
The researchers now hope to modify the system so that guest molecules can be held at one end of the box, and then moved to the opposite end using a chemical or photochemical stimulus—effectively turning the shuttle into a switch.