photochrome

Photochrome refers to a substance or material that reversibly changes color in response to light. The change arises from a reversible photochemical or photophysical process that converts one molecular form into another with different light absorption properties. In a typical scenario, exposure to light of a specific wavelength induces a colored form, while the reverse transformation can occur either thermally in the dark or upon irradiation with light of another wavelength.

Common photochromic systems include spiropyrans, spirooxazines, diarylethenes, and azobenzenes. In spiropyrans, light triggers a ring opening to form a conjugated merocyanine that absorbs visible light, producing color; the ring can recluse and the material returns to the original state. Diarylethenes exhibit reversible ring-opening and ring-closing with often high fatigue resistance, enabling many switching cycles. Azobenzenes switch between trans and cis geometries with distinct absorption profiles, while spiro compounds rely on changes in connectivity that alter conjugation. The specific wavelengths and speeds of switching depend on molecular structure and environment.

Applications of photochromic materials include eyewear that darkens in bright light, smart windows that regulate indoor light and heat, and optical data storage or signaling devices. They are also explored for sensors, displays, and reversible adhesives. Limitations include partial fading, fatigue over repeated cycles, and dependence on temperature and ambient conditions. Advanced families aim to improve fatigue resistance, thermal stability, and visible-light activation to broaden practical use.