Evolving Chromism

Are you familiar with the word "monochrome"? Although we now live in an age surrounded by high-definition displays with rich color tones, "monochrome" refers to a single color. Both television and photography began in monochrome. The "chrome" part of this word originates from Greek and primarily means "color." While there are indeed many kinds of colors, those with a single or very narrow range of wavelengths appearing in the visible light spectrum are called chromatic colors, corresponding to red, yellow, green, blue, and so on. Chromium, with the element symbol Cr, also shares the same origin. This is because Cr ions exhibit various colors depending on their oxidation state. The red of a ruby and the yellow in Van Gogh's sunflowers are both colors produced by Cr ions.

In our laboratory, we conduct research focusing on the color change of materials (chromism). The phenomenon where a material changes color when exposed to light is called photochromism, and when a voltage is applied, it is called electrochromism. These are used in applications such as eyeglass lenses that darken in response to ultraviolet rays and dimmable windows on airplanes flying high in the sky under strong sunlight. These phenomena are based on changes in the crystal and electronic structures of the material due to light irradiation or voltage application, which in turn alters its light absorption properties. It should be noted that the light absorbed by a material is invisible to the eye; what we see is the light that is not absorbed. In contrast, what we are currently aiming for is a change in the luminescent properties of materials. This is called fluorochromism. Unlike light absorption, luminescence involves the direct perception of the emitted color, allowing for the expectation of more sensitive chromism. Using phosphors, which are typical luminescent materials, we are creating materials that begin to glow, cease to glow, or even change their light color when placed in specific chemical environments. Figure 1 shows a green phosphor coated on quartz glass that responds to redox reactions. It demonstrates how the luminescence is quenched in the presence of an oxidizing agent and restored in the presence of a reducing agent.

Phosphors are crystalline inorganic substances, and as materials, they are aggregates of numerous crystal grains. Since chemical fluorochromism manifests as a result of chemical reactions on the surface of individual particles, it is necessary to create material microstructures with the largest possible surface area. To achieve this, we utilize various inorganic chemistry techniques—such as the sol-gel method, hydrothermal method, and thermal decomposition of metal-organic frameworks—to synthesize fluorochromic materials with individual particle sizes on the nano- to micrometer scale (Figure 2). By creating materials that are highly responsive to their chemical environment, we can, for example, realize chemical sensors that can instantly and easily detect harmful or dangerous chemical substances. From chromism that enhances human comfort to chromism that protects human safety—we continue our research with this transition in mind.