Yasuo Norikane (Senior Researcher) and others of Molecular Assembly Group (Leader: Reiko Azumi), the Electronics and Photonics Research Institute (Director: Masahiko Mori) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Ryoji Chubachi), have discovered a phenomenon in which a crystal of organic substances having the simple structure of azobenzene moves on a glass plate while changing form as a result of photoirradiation. In addition, the researchers observed that the crystals rise in the vertical direction when placed on a vertically positioned glass plate.

The discovered phenomenon was that a crystal of an azobenzene (derivative) placed on a commercially available glass plate moves when it is simultaneously irradiated from different directions with ultraviolet light which liquefies the crystal and visible light which solidifies the crystal. Because the crystal moves away from the ultraviolet light source, it is possible to control the direction of crystal movement. Also, this phenomenon does not require a special surface treatment for the glass plate or a special light source, such as a laser. The phenomenon in which the crystal transforms and moves on a solid substrate by photoirradiation has not been reported previously. In the future, applications to valves and the transport of substances and objects on a microscopic level are anticipated.

The details of this research were published in a British scientific journal, Nature Communications, on June 18, 2015.

Schematic illustration (left) and micrographs (right) of the phenomenon in which light causes crystals to move

In recent years, miniaturization of equipment has been pursued in various industrial fields for the purpose of saving energy and natural resources and reducing costs. Along these lines, ultra-miniaturized micromachines and microfactories, etc. have been proposed. In order to achieve them, not only must existing parts and devices be miniaturized, but also new technologies and mechanisms must be developed. In particular, the development of technologies for moving objects and substances is important. For bending, stretching/contracting, and vibrating objects, actuators, which use MEMS, polymers, and photochromic compounds, are known. On the other hand, there are many issues involved in the transport of objects and substances. In the past, although light irradiation has been used to move liquid drops and solids on a solid surface, the methods to move them were problematic because they required a unique surface treatment of the substrate, precise control of the photoirradiation, and a laser as the light source.

AIST has developed organic materials that change from solid to liquid merely by being irradiated with light (AIST press release on December 2, 2010) and materials that can be repeatedly liquefied and solidified by photoirradiation (AIST press release on April 6, 2012). Azobenzene is a typical compound that photoisomerizes, but while the photoisomerization in solution is well known, the photoisomerization in crystalline solid state remains obscure. Since developing organic compounds in which the phenomenon of liquefaction and solidification (crystallization) by photoirradiation occurs utilizing the photoisomerization of azobenzene, AIST has researched azobenzene derivatives with various molecular structures in order to reveal the mechanism of this phenomenon and to develop application technologies, and the result of that research led to the present discovery.

This research was partially supported by the “Development of planographic printing method utilizing photochromic reactions and its application in organic electronics (23760680)” Grant-in-Aid for Scientific Research of the Japan Society for the Promotion of Science, and the “The Creation of Industrial Infrastructure grant” program of the Canon Foundation.

This research mainly focused on 3,3’-dimethylazobenzene (DMAB), an azobenzene derivative with a simple molecular structure. This compound is a crystal at room temperature (melting point of trans form: 53 °C), but it liquefies in conjunction with photoisomerization from the trans form to the cis form that occurs when it is irradiated with ultraviolet light (wavelength 365 nm). On the other hand, photoisomerization from the cis form to the trans form occurs as a result of visible light (wavelength: 465 nm) irradiation and DMAB crystallizes. When DMAB crystals (size: several tens µm; thickness: several µm) on a glass plate are irradiated with inclined beams of ultraviolet and visible light from opposite directions, the crystals moved away from the ultraviolet light (Fig. 1). The speed the crystal movement varied depending on the intensity and angle of the light. For example, when ultraviolet and visible light were irradiated at intensities of 200 mW cm^-2 and 50 mW cm^-2 respectively, and an angle of 45 degrees, the mean movement speed of the crystals was 1.8 µm/min. In the experiments, an LED and high-pressure mercury lamp used in general photochemical experiments were used as the light sources, and commercially available cover glass was used as the glass plate. Neither a special light source, such as a laser, nor a surface-treated substrate was required. Also, the light was irradiated onto the entire crystal; it was not necessary to control the light so that it irradiated only a portion of the crystal, etc.

Figure 1: The phenomenon in which light causes crystals to move Schematic illustration (left) and micrographs (right) of the phenomenon

The discovered movement phenomenon occurs only when two beams of light are irradiated simultaneously onto a crystal. Specifically, even when a completely liquefied liquid drop was irradiated with light, it was not observed to move. Also, a crystal irradiated with only ultraviolet light or only visible light did not move. In addition, the balance of the light intensities was important (Fig. 2).

When an attempt was made to move the crystal on a vertically positioned glass substrate, the crystal was also able to climb the wall surface (move vertically) (Fig. 3).

Future study is needed to fully reveal the detailed mechanism of the crystal movement phenomenon, but the crystal is believed to move as a result of liquefaction occurring in the rear and solidification occurring in the front of the crystal, with respect to the direction of movement. In order to investigate the generality of the phenomenon, the researchers studied the movement phenomenon in an azobenzene crystal, which has a simpler molecular structure. Azobenzene differs from DMAB in that ultraviolet photoirradiation causes liquefaction not at room temperature, but at 50 °C. When the temperature was maintained at 50 °C and azobenzene was irradiated simultaneously with ultraviolet and visible light, the azobenzene crystals moved. This result suggests that the movement phenomenon can occur as long as light causes the compound to liquefy and crystalize.

Figure 2: Conditions for light causing crystal movement Schematic illustration showing the conditions of crystal movement (top) and plot of the light intensity at which the crystals move (bottom)

Figure 3: Crystals moving on a vertically positioned glass substrate Schematic illustration (left) and micrographs (right) of the crystal movement phenomenon

The researchers aim to reveal the detailed mechanism of the crystal movement phenomenon induced by light. Also, research is planned to study compounds that move more quickly, as well as those that are moved by less-intense light, in order to freely control the movement of substances and objects and to develop application technologies such as valves.