Fotochromie

Fotochromie, or photochromism, is a reversible change in the color or optical properties of a material triggered by light. The effect occurs when exposure to light changes the material’s electronic configuration or molecular structure, shifting its absorption spectrum so that it appears different in illuminated versus dark or low-light conditions. Photochromic changes are typically reversible, fading back when the light source is removed or altered.

Two broad mechanisms are common. In organic photochromism, light induces reversible chemical reactions within a molecule, such as isomerization or ring-opening in compounds like spiropyrans, spirooxazines, or diarylethenes. The different forms absorb different wavelengths, producing distinct colors. In inorganic or hybrid systems, photochromism can result from changes in oxidation state, charge transfer, or structural rearrangements in the solid, across materials such as certain metal oxides or doped glasses. Historically, silver halide photographic emulsions also exhibit a photochemical process that alters light absorption during exposure.

Applications of photochromic materials include eyewear that darkens under ultraviolet light and returns to a clear state indoors, photochromic glass for architectural and automotive use, as well as specialized inks, plastics, and optical devices. The performance of photochromic systems depends on factors such as speed of color change, depth of coloration, fatigue resistance after many cycles, and sensitivity to temperature and aging. Research continues into expanding the range of colors, speeds, and stability of photochromic materials.