Researchers in the UK have shown that the rotation of large molecules called porphyrins can be reversibly switched on and off. Controlling the motion of molecules is central to the operation of many proposed nanoscale devices.

Richard Lambert and co-workers at Cambridge University attach four bulky ‘legs’ to the porphyrins — flat molecules with, in this case, a zinc atom at their centre — and then place them on a silver surface. STM images reveal four bright spots for each molecule, one for every leg. However, after exposure to 4-methoxypyridine (4MP) — small molecules that link the zinc and silver atoms — a series of bright rings can be seen in the STM images, which suggests that the porphyrin molecules are rotating. Further analysis confirms that the porphyrins are lifted off the surface, which allows them to rotate about the 4MP ‘axles’. On heating, 4MP desorbs from the surface and the rotation stops.

The switching process is reversible and is controlled by the addition or removal of 4MP. Moreover, by modifying the structure of the porphyrin or the axle, or changing the surface characteristics, it should be possible to tune the temperature at which the rotation can be turned on and off.