Japanese scientists have developed a photochromic molecular fibre that can be bent in different directions using polarised light. 

Photomechanical effects in materials are interesting from a fundamental science point of view and possible practical applications such as light-driven actuators. Previously the bending direction of these materials could only be altered by changing the position and wavelength of the light source. 

Hideyuki Nakano at Osaka University made the fibre from an azobenzene derivative that forms a glass-like material just below its melting point. They found that irradiating the fibre with polarised light results in physical changes that make it bend. When the irradiation lasts just one second, the fibre bends away from the light source and goes back to its starting position when the light is removed. The light heats up the near side of the fibre, resulting in thermal expansion, which reverses when the fibre cools, explains Nakano. 

However, when the irradiation was continued for ten minutes different effects were found for vertically and horizontally polarised light. With horizontally-polarised light, the fibre continued to bend in the same direction and did not return to its original shape afterwards. But with vertically-polarised light, the initial bending was soon reversed and the fibre ended up bending back towards the light source. Nakano says the mechanism for this effect is not clear but one possibility is that repeated changes of the double bond in the azobenzene molecule, from cis to trans, make it move in the polarisation direction causing distortion of the fibre. 

'The main benefit is that the movement of the fibre can be controlled without changing either the wavelength or the position of the light source,' says Nakano. 

'This represents a promising first step in demonstrating how the properties of light can be used to control microscale motions in shaped molecular solids,' says Christopher Bardeen, who investigates photoactive materials at the University of California, Riverside, US. 'The directional bending behaviour is surprising considering the amorphous nature of the solid,' he adds. 

Nakano now hopes to develop applications such as light-driven motors with the new fibres, as well as elucidating the relationship between molecular structure and bending behaviour. 

David Barden 

Enjoy this story? Spread the word using the 'tools' menu on the left or add a comment to the Chemistry World blog.