photochromisme

Photochromism (French: photochromisme) is the reversible transformation of a chemical species between two forms with different absorption spectra, induced by electromagnetic radiation. The two forms may differ in color, structure, and electronic state, and interconversion is typically triggered by light and reversed by heat or light of another wavelength.

Two broad classes exist: organic photochromism and inorganic photochromism. In organic systems, compounds such as spiropyrans, spirooxazines, diarylethenes, fulgides, and azobenzenes undergo photoinduced isomerization or ring-opening/closing that changes their visible absorption. In inorganic systems, silver halides, tungsten oxides, and other metal-oxide or chalcogenide materials form color centers or undergo valence changes that alter absorption. Photochromic behavior can be driven by ultraviolet light, visible light, or both, depending on the system.

Applications are widespread: photochromic lenses that darken on UV exposure and fade in darkness; smart windows that modulate light transmission; coatings and textiles for decorative or functional purposes; security inks and data storage. Performance varies with the material, including the speed of switching, the magnitude of color change, fatigue after cycling, and temperature sensitivity.

Photochromism is distinct from related phenomena such as thermochromism and electrochromism, though some materials exhibit more than one switching mechanism. The field integrates organic synthesis, inorganic chemistry, and materials science to develop stable, reversible systems for practical use.