Nature 479, 208–211 (2011)

Credit: © 2011 NPG

Single molecules can be pushed or pulled across a surface with the tip of a scanning tunnelling microscope (STM). Depending on their structure, some molecules will simply slide over a surface without changing their relative orientation, whereas others can be made to roll if the whole molecule, or parts of it, rotate with respect to the surface. In all of these examples, however, the molecule is a passive participant and goes along for the ride without being actively involved.

Now, Tibor Kudernac and colleagues have shown that molecules with four unidirectional rotary motors as 'wheels' can be propelled across a Cu(111) surface because of changes in their conformation, rather than being pulled or pushed by an STM tip. An STM tip is still required, however, because rotation of the wheels is induced by applying a pulse with a voltage above 500 mV to the centre of the molecule. This electronic excitation causes each rotary motor to undergo a double-bond isomerization followed by a helix inversion and results in a paddlewheel-like motion of each wheel on the surface.

Each wheel is chiral, however, and can only rotate in one direction. So that they can all act in concert and rotate in the same direction, the wheels on opposite sites of the molecule must have opposite chiralities. It is only in this case that the molecule moves forward in a more-or-less linear trajectory — it is not perfectly straight because it is difficult to excite all four wheels simultaneously — and other configurations lead to random motion or no movement at all.