Topics that shape the future
- Closer look at research results -

Photochromic molecules are molecules that change their shape, or structure, when exposed to light energy, mainly ultraviolet light, and consequently change their color, or more precisely, the way they absorb light, in a reversible manner. A familiar example is photochromic lenses, or so-called photochromic sunglasses, which darken when exposed to ultraviolet light. This is because the surface of the lens is coated with a film containing photochromic molecules, causing the color to change when exposed to ultraviolet light outdoors.

Photochromic molecules not only change color, but their shape, or structure, also changes in response to light. This property allows molecules to be switched between two different states. For example, by associating each state with the digital data "0" and "1," they can be used as optical recording materials that can write and erase information with light. Research is also progressing in various cutting-edge fields, including molecular machines and photomechanical materials that operate in response to light, nanostructures whose shape and properties change in response to light, and catalysts whose functions can be switched by light. Furthermore, applications in the medical and life science fields are expected, such as technologies for observing the fine structures within cells with high resolution and technologies for controlling biological functions with light.

Photochromism, the phenomenon in which the structure and color of molecules reversibly change in response to light, has been known for a long time. However, many conventional photochromic molecules take several minutes to return to their original state after changing color, and the slow reaction rate has been a major challenge for applications.

Our laboratory was quick to address this challenge, and in 2005, we developed the world's first high-speed photochromic molecule that returns to its original colorless state in an extremely short time of approximately 180 milliseconds at room temperature. This is a groundbreaking molecule whose color disappears in less time than a blink. Furthermore, in 2008, we succeeded in developing a second-generation high-speed photochromic molecule with an even faster reaction rate.

These results represent a world-first achievement in enabling high-speed responses of photochromic molecules and were published in the prestigious academic journal "Journal of the American Chemical Society (JACS)." Furthermore, due to its novelty and high applicability, it was widely featured as a groundbreaking technology in various media outlets.

The key point is that we can now control the state of molecules, such as their color and shape, at high speed, in milliseconds. For example, consider photochromic lenses. When driving a car, if you enter a tunnel with colored lenses on, your vision suddenly becomes dark. However, by using photochromic molecules that quickly return to their original colorless state, the lenses can quickly return to colorless the moment you enter a tunnel, maintaining a bright and clear field of vision. Furthermore, the ability to control color changes at high speed opens up applications in video and display technologies. For example, in 2011, our technology was used in the artwork of media artist Daito Manabe.

A hologram is created by interfering an object wave, which contains the spatial information of an object, with a reference light, and recording the resulting interference fringes on a recording material. However, ordinary holograms are displayed as fixed "still images." On the other hand, by using high-speed photochromic molecules, it becomes possible to rapidly rewrite the interference fringes. In fact, we have succeeded in realizing the world's first hologram video using photochromic film.

Thus, the ability to rapidly control the colored and colorless states has greatly expanded the potential applications of photochromic molecules. We believe this is one of the reasons why this research has attracted so much attention.

The paper summarizing these results, like the one on the development of the high-speed photochromic molecule mentioned earlier, was published in JACS and attracted attention from many researchers. Currently, research based on the molecule we developed is spreading, mainly among researchers in Europe, and related papers are being actively published.

Conventional photochromic molecules have the property of changing color when exposed to light, meaning that the irradiated light is easily absorbed near the surface of the material, resulting in a problem where the light does not penetrate sufficiently into the interior of the object. Therefore, photochromic reactions mainly occurred near the surface.

On the other hand, high-speed inverse photochromic molecules have the property of losing their color when exposed to light, allowing light to penetrate more easily into deeper parts of an object. Furthermore, it is possible to eliminate color using near-infrared light, which is invisible to the human eye, making it possible to achieve the mysterious phenomenon of manipulating color at will, almost like magic.

By utilizing this property, for example, high-speed inverse photochromic molecules can be incorporated into liquid crystal materials, and by changing the shape of the molecules in response to light, the arrangement of liquid crystal molecules and the pitch of the helical structure can be changed in conjunction. As a result, the wavelength of light reflected by the liquid crystal, that is, the colors visible to our eyes, can be freely controlled. This mechanism has the potential to develop into a new display technology that creates colors and images using light signals, unlike conventional displays that control display using electrical signals. In the future, it is expected to be applied to next-generation displays with significantly reduced power consumption and light-controlled display materials.