Credit: © 2008 ACS

The motion of molecules can be controlled with chemicals, electrons, photons and other stimuli. Photons are useful because they are clean, fast and can act from long range. A molecular brake that slows down the rotation of a wheel-like molecule is one system where light could be used as a control, but previous approaches have not been effective at room temperature.

Jye-Shane Yang and co-workers1 of the National Taiwan University have now developed a light-driven molecular brake that does work at room temperature. They made a pentiptycene molecule — a four-bladed aromatic molecule that rotates freely owing to a small rotational barrier — with a substituent group that changes shape when it is exposed to light. The wheel can rotate when this group is in the trans form, but when it changes to the cis form, the rate of rotation is reduced by nine orders of magnitude.

The brake is turned on and off by applying light of varying wavelengths. The group plans to explore the effects of different substituent groups on performance.