Yasuo Norikane (Researcher) of the Molecular Thin Films Group (Leader: Reiko Azumi), the Photonics Research Institute (Director: Masanobu Watanabe) of the National Institute of Advanced Industrial Science and Technology (AIST; President: Tamotsu Nomakuchi), and Masaru Yoshida (Leader) of the Smart Materials Group, the Nanosystem Research Institute (Director: Kiyoshi Yase) of AIST, and others have developed organic materials that melt from solid to liquid (i.e., undergo phase transition) with only the irradiation of light, without heating, and can furthermore be restored to their original solid state.

Unlike irreversible photosensitive resins, which cannot be restored to their original state after their state have changed once, the developed organic materials are characterized by the fact that they undergo state transitions by photoisomerization, and so can be restored to their original state (i.e., the transitions are reversible). Many organic compounds are already known that undergo a change in molecular structure (shape) by photoisomerization, but while that isomerization takes place in solution, it is thought to rarely occur in crystals. The newly synthesized organic compounds have molecular weight of about 1,100-1,700 and undergo photoisomerization even in crystals, so that they change from solid state to liquid state by light. This is an important discovery concerning the basic concept of the melting phenomenon of matter. The next objectives are to establish techniques for the large-scale synthesis of these organic materials and to explore the possibilities for various applications, including photolithography.

The research results will be published online on December 2, 2010 in Chemical Communications, a British scientific journal.

Figure 1 Phase transition in two organic compounds (upper and lower photos). The liquid phase appears as a black image.

It is a fundamental characteristic of all matter that it undergoes state transitions, as exhibited by water: solid (ice), liquid (water), and gas (steam). This phenomenon usually happens as a result of heat transfer (change in temperature). Photosensitive resins are the materials whose states change by light irradiation. These materials change, for example from liquid to solid or solid to liquid, when they are exposed to light. The materials have been widely applied in platemaking for printing, micro processing in the field of electronics, and so on, and they have played a very important role in the development of industry. However, most of photosensitive resins use photopolymerization or photolysis, so they cannot be restored to their original state if they have been used once, and they cannot be reused. In terms of building a sustainable society, the development of reusable photo-responsive materials is an important issue of green innovation to save energy and resources.

AIST has advanced the development of new reusable photo-responsive materials, and focusing on the photoisomerization, which is a reversible photochemical reaction, has conducted a number of investigations from the perspective of synthetic chemistry. New molecular designs were applied to this research on the photoisomerization in crystals, an area little known until now.

This research and development was supported by a “Grant-in-Aid for Young Scientists (B) (FY2009-10)” for scientific research (KAKENHI) from the Japan Society for the Promotion of Science and an “FY2009 Research Incentive Grant” from the Association for the Progress of New Chemistry.

AIST has focused on azobenzene, a typical molecule exhibiting photoisomerization, in relation to the development of reusable photo-responsive materials with reversibility. When azobenzene is irradiated with ultraviolet light, its structure changes from a trans isomer to a cis isomer, and conversely, when it is irradiated with visible light or heated, the cis isomer reverts to a trans isomer (Fig. 2). In general, this phenomenon occurs only in solution and not in crystals. This is because the motion of molecules is blocked by the surrounding molecules in crystals and thus structural changes (isomerization) are interfered. Recently, crystals that change their form by light irradiation have been reported as some of the few examples of photoisomerization occurring in crystals. However, there have been no reports of the phenomenon of a solid changing reversibly to a liquid, and it has not been clear whether such a phenomenon is possible even in principle.

Figure 2 Photoisomerization of azobenzene. Azobenzene undergoes a reversible photoisomerization between a trans isomer and cis isomer.

Considering this status, two organic compounds were newly synthesized first (Fig. 3) in order to study light-induced phase transition from a liquid crystal state (which could be considered as an intermediate state between solid and liquid) to a liquid state. The structures of these organic compounds have azobenzene units joined in ring forms and moderately flexible side chains. As multiple azobenzene units within the molecule undergo photoisomerization, the form of the entire molecule changes largely. A light-induced phase transition from a crystal state to a liquid state and a phase transition from a liquid crystal state to a liquid state were confirmed with these compounds. Polarizing optical micrographs of the phase transitions are shown in Fig. 4. Melting these substances with heat requires temperatures of more than 100 °C, but it was found that only the parts irradiated with light melted at room temperature. (The birefringence specific to crystals vanishes in the melted region, which becomes isotropic, so this part appears as a black image in the polarizing optical microscope).

Figure 3 Chemical structures of the newly developed two organic compounds (top) and schematic diagram of the phase transitions of the compounds (bottom). Phase transitions between solid state (left) and liquid state (right) are induced by ultraviolet light or heat.

Figure 4 Polarizing optical photomicrographs of organic compound 1 (top) and 2 (bottom) at 25°C (thickness 5 µm). Exposed to ultraviolet light, the compounds isomerize to cis isomers and the phase become liquid (observed as a black image). Exposed to heat, they change to trans isomers and return to the solid state.

Additionally, by making a thin film of microcrystalline powder with high photoreactivity, melting of crystals was successfully observed by an optical microscope. This change in state can be induced repeatedly by controlling two conditions, namely the irradiated light and the temperature. These results indicate that with appropriate molecular design, it is possible to achieve the photoisomerization in crystals, and that this reaction causes the order of the molecular arrangement in crystals to break down, leading to melting, or in other words a phase transformation to a liquid state. This is the first report of the phase transition from solid to liquid, which usually can only be induced by heating, occurs as a result of photoisomerization.

Aiming to elucidate the mechanism of the melting of matter by light, a phenomenon of great interest to material science, we are studying the relationship between molecular structure and properties through the detailed analyses of the reaction and the investigation of similar organic compounds.

We will also conduct research to establish a large-scale synthesis method for the compounds, in order to provide samples to outside parties. In parallel with this, we plan to study the application of the melting by light in reusable photolithographic materials and in gluing technologies, e.g. materials that can be peeled off easily by light irradiation.