photochromism

Photochromism is a reversible change in the color or optical properties of a material induced by electromagnetic radiation. In a photochromic substance, photons trigger a structural transformation in a molecular component, producing a form that absorbs differently in the visible region. The process is typically reversible: exposure to certain wavelengths converts the colored form to a less colored or differently colored form, and subsequent irradiation or thermal relaxation returns the material to its original state. The practical effect is a material that darkens under light and bleaches in darkness or under a different light stimulus.

Mechanisms of photochromism often involve photoisomerization or tautomerization, where a molecule changes shape or bonding arrangement upon absorption of light. Common classes include spiropyrans and spirooxazines, which undergo ring-opening to more conjugated forms, diarylethenes that switch between closed and open ring structures, and azobenzenes that reversibly switch between trans and cis configurations. The absorption spectrum shifts as conjugation and geometry change, giving rise to different colors. Materials are engineered to respond to specific wavelength ranges and to operate under particular conditions.

Back reactions can be thermally driven, photochemically driven, or a combination of both, affecting how long the colored state persists. Fatigue resistance and photostability are important for practical use; some systems cycle rapidly without degradation, while others gradually lose efficiency.

Applications include photochromic eyewear that darkens in sunlight, smart windows that modulate light transmission, and security inks or data storage systems. Notable photochromic systems include spiropyrans, spirooxazines, diarylethenes, and azobenzenes, which are actively studied in chemistry and materials science for their tunable response to light.