Single-crystalline photochromism of diarylethenes

Diarylethene single crystals which exhibit thermally irreversible and fatigue-resistant photochromic reactions have been developed. Upon irradiation with ultraviolet light, the colorless diarylethene crystals turn yellow, red, blue, or green, depending on the substituents of the aryl groups. Dichroism of the photogenerated absorption spectra and in situ X-ray crystallographic analysis confirmed that the diarylethene molecules underwent photochromism in the single-crystalline phase. The photocyclization quantum yields were found to depend on the distance between the reactive carbon atoms in the crystal; the yields were close to unity when the distance is less than 4.2 Angstrom. Multi-color photochromism was demonstrated for the crystals composed of two types of diarylethenes. The diarylethene single crystals showed reversible surface morphology changes upon photoirradiation.