Photochromism is the phenomena that of reversible transformation between two forms that have different absorption spectra by photo-irradiation. The history of photochromism dates back to the era of the Alexander the Great. Inspired by Fischer and co-workers who first reported photochromism in spiropyran analogues in 1952, intense studies on photochromic compounds have continued up to the present. The mechanism for photochromism of spiropyran analogues is shown below.

Today, research for the application of photochromism is being achieved in a variety of fields including light modulation materials, optical recording materials, optical switches, and photochromic ink. Some of them are being put into practical use. Furthermore, an energetic attempt to introduce photochromic compounds into functional materials for photochemical control is being studied. Especially, diarylethenes 1 and 2 developed by Irie and co-workers are stable at high temperature and have been shown high repeated durability. In addition, various functional molecules can easily be introduced to the aromatic ring of 1 and 2. The solution of 1 changes to red upon irradiation of UV light, but rapidly converts back to olorless by visible light. In this reversible reaction, 1 has been shown a repeated durability over 10,000 times. The crystal of 2 changes to blue upon irradiation of UV light, and returns to colorless by visible light. These changes are rapidly finished within ten pico second. Moreover, the photocyclization quantum yields is limitlessly near to one. Therefore 2 shows extremely effective photochromic reaction. 1 and 2 are ideal optical device material and are expected as one of the material supporting the photonics age in the 21th century.