Despite the seemingly infinite number of functional molecules that could be useful for making nanoscale devices, only a few techniques are able to manipulate molecules with wide range of properties efficiently.

Fig. 1: In liquid crystal, impurity molecules (red) can be attracted toward a region of disorder induced by irradiation with ultraviolet light (center).

Sadaki Samitsu, Yoichi Takanishi and Jun Yamamoto at Kyoto University in Japan have now developed a way to transport small molecules through liquid crystal using light.1 Although they have so far only demonstrated the technique for polymers, Samitsu believes that the method could also be used to transport other molecules, representing a significant step forward in the design of complex devices.

In ‘nematic’ liquid crystal, rod-shaped molecules all point in a common direction. Samitsu and his colleagues knew that adding small impurities to nematic liquid crystal at sufficiently high concentrations tends to affect molecular ordering. They therefore started looking for ways to use the reverse effect to move small molecules around. Their idea was to create disorder at certain spots in the liquid crystal, and that these points of disorder would either attract or repel impurity molecules (see image).

The team demonstrated this method by moving fluorescent dye molecules in a nematic liquid crystal. To control spatial variations of the disorder in the liquid crystal, they added a small amount of the molecule azobenzene — a photoisomer that changes reversibly from a linear shape to a bent form when irradiated with ultraviolet light.

When the researchers shined a spot of ultraviolet light on the liquid crystal containing azobenzene, the molecules adopted the bent morphology and disordered the area of the liquid crystal that was illuminated. Within about thirty minutes, the dye molecules, which were evenly distributed at the start of the experiment, gathered in the disordered region. With a different type of dye molecule, light was successfully used to push molecules out of the disordered region.

Samitsu expects the method to have a wide range of possible applications, including optical recording and molecular filtering. With the use of other impurities, such as carbon nanotubes or nanoparticles, this light manipulation technique could eventually be of interest for the design of nanoscale circuits.