and in solid state. Moreover, these two isomers differ from one another not only in their absorption spectra but also in various physical and chemical properties, such as their refractive index, dielectric constant, and oxidation-reduction potential. These properties can be readily controlled by reversible isomerization between the open- and closed-ring states using photoirradiation, and thus they have been suggested for use in optical data storage and 3D optical data storage in particular. The closed form has a conjugated path from one end of the molecule to the other, whereas the open form has not. This allows for the electronic communication between functional groups attached to the far ends of the diarylethene to be switched on and off using UV and visible light. References Aromatic compounds Photochemistry