When ultraviolet light hits a certain type of colorless crystal that looks like rock sugar, the crystal immediately turns red like a ruby. Some of these crystals turn blue, yellow or other colors. However, they instantly become colorless again when they are exposed to visible light.

These mysterious crystals are diarylethenes, developed in 1988 by Prof. Irie. The compounds have an ethylene group at the center of the molecule with two aryl groups each side. When the compounds are exposed to ultraviolet light, the hexatriene at the core of the molecules changes from an open-ring to a closed-ring structure. But when they are exposed to visible light, the hexatriene returns to its original structure. The change in its structure induces a change in the color of the crystal. These crystals become red, blue, yellow or green, depending on the type of their substituents. These organic molecules, which change colors with specific wavelengths of light, are called photochromic molecules. Unlike other synthetic photochromic molecules, diarylethenes have excellent thermal stability. The half- lives of their colored state are longer than 1,000 years at 30 degree C. The color-bleaching cycles of diarylethene crystals can be repeated more than 10,000 times without degradation.

Diarylethene molecular crystals also exhibit the photomechanical effect, which involves the conversion of light energy into mechanical energy. According to x-ray structural analysis of a diarylethene molecular crystal, which become blue when their structure changes from open-ring to closed-ring, the major axis and thickness of the molecules shrink from 1.41 nm to 1.39 nm and from 0.49 nm to 0.39 nm, respectively. The crystal's photo-induced contraction and expansion of less than 1 nm has been confirmed by atomic force microscopy. The crystal could potentially be used as a photodriven nanometer-scale actuator.

The photoinduced structural changes of diarylethenes can be observed even at the single molecule level. This indicates that information could be stored in or read from individual molecules using light. The structural changes of the diarylethene molecules, which have different fluorescence intensity after the photochromic reaction, have been switched on and off experimentally by irradiating individual molecules which were embedded into a polymer film. The molecules store information in the on-state or in the off-state. Theoretically, we can store the amount of information equivalent to 1 million DVDs on a single disk using this technology.

Diarylethenes have been drawing attention from researchers all around the world. In autumn of 2002, researchers on the material got together for a conference. A Canadian group announced that it had found that diarylethenes can also be colored using an electro-chemical method. An Italian team in cooperation with astronomers also announced that diarylethenes can be used as filters to remove light noise, which can be seen when observing stars. Prof. Irie says, "I appreciate many researchers for developing various applications for diarylethenes. They encourage me to make further efforts in this area."

Kyushu University has opened its facilities to outside researchers as part of the Nanotechnology Support Project funded by MEXT to develop a base of support for the synthesis and analysis of nanomaterials. About 30 companies and 30 universities have already used the facilities.

Prof. Irie is the leader of this program at Kyushu University. His university provides supports to researchers in these companies and universities with not only its equipments but also various organic molecular materials that could be used in their research. "We want to explore the potential of nanotechnology by working with the outside researchers. We also expect to establish a basis for conducting joint research with these companies and universities in the future by extending support to them."
(Interviewer: Yu Tatsukawa, Cosmopia Inc.)

For more information,
http://www.nanonet.go.jp/english/mailmag/2004/029a.html