fotokromik

Fotokromik refers to materials that change color or opacity in response to light. This behavior results from reversible chemical or physical changes that alter how the material absorbs light. In most photochromic systems, ultraviolet radiation triggers the colored form, while removal of the light source or exposure to visible light or heat gradually reverts the material to its initial state.

Two broad classes are organic and inorganic photochromic materials. Organic photochromism typically relies on reversible isomerization of molecular structures, such as spiropyrans, spirooxazines, and naphthopyrans, or ring-closing reactions in diarylethenes. Inorganic systems include materials that change crystal structure or electronic state under illumination, such as certain metal oxides or silver halides. Many organic systems are highly tunable through chemical modification, allowing control of the color change, speed, and fatigue resistance. Diarylethenes, for example, are noted for high cycling stability.

The most familiar use is photochromic eyewear, where lenses darken outdoors and clear indoors. Other applications include smart windows that adjust transmittance, color-changing textiles, indicators in security inks, optical data storage, and sensors that detect light exposure or UV dose.

Advantages and limitations: The reversible change requires little or no energy input beyond light; the response is fast, and the color change is often strong. Limitations include fading back to the original state over time, dependence on temperature, fatigue after many cycles, and potential incompatibility with indoor lighting or certain substrates.

History and status: Photochromism has been studied since the 19th century, with commercial progress in the late 20th century leading to widespread photochromic sunglasses and specialized coatings. Research continues to expand material classes, improve durability, and tailor spectral response for specific applications.