The shape of nanocrystalline molecules can by controlled by light, report scientists in the Netherlands and Japan. The molecules have the potential to act as nanosized valves in molecular devices.

Photochromism is the reversible light-induced transformation of a molecule between two states. Ben Feringa, at the University of Groningen, the Netherlands, and colleagues synthesised crystals of photochromic compounds called diarylethenes. When they shone UV light on the diarylethenes, the rod-like crystals bent. When they switched to visible light, the crystals returned to their original shape.

They also observed for the first time that flat crystals roll up when irradiated with UV light and unroll under visible light. They found that the direction of the light and the packing of the crystals affected the direction of bending and rolling. 

'The most important and highly significant step forward is the control of bending in a concave or convex way with respect to the incident light beam,' said Feringa. 'It is most remarkable that this change in behaviour can be achieved by small structural variations at the molecular level.' 

A major challenge for Feringa is to understand the mechanism of motion at the molecular level. He hopes that this will provide crucial information to design future generations of adaptable systems and materials. 

Joanne Thomson