fotokromism

Fotokromisme, also known as photochromism, is a reversible color change in a material caused by exposure to light. The change arises from light-induced transformations at the molecular or nanoscale level that alter the material’s absorption spectrum. Photocromic behavior is observed in organic dyes, inorganic compounds, and polymer matrices, and is typically reversible by exposure to light of a different wavelength, or by time and heat.

Common mechanisms involve photoisomerization or light-induced structural changes. In many organic systems such as spiropyrans and spirooxazines, light converts a colorless form into a colored, conjugated merocyanine form. Diarylethenes undergo reversible ring opening and closing, often with high fatigue resistance and fast switching. Some materials rely on changes in charge-transfer interactions or on plasmonic effects in nanocomposites.

Applications include photochromic lenses for spectacles, smart windows that adjust tint with ambient light, UV indicators, and security inks. Photocromic materials are also explored for optical data storage, sensors, and wearable displays. Key design goals are rapid response, large contrast between states, long cycle life, and operation under visible light to reduce degradation and safety concerns.

History and scope: Photocromism was observed in various materials in the 19th and 20th centuries, with major advances in the mid-to-late 20th century through the development of organic photochromic molecules and their solid-state implementations. Ongoing research seeks to expand spectral range, improve stability, and enable practical deployment in coatings, textiles, and devices.