Diarylethene

In chemistry, diarylethene is the general name of a class of compounds that have aromatic groups bonded to each end of a carbon-carbon double bond.

Under the influence of light, these compounds can generally perform two kinds of reversible isomerizations:


 * E to Z isomerizations, most common for stilbenes (and azobenzenes).
 * 6π electrocyclizations of the Z form, leading to an additional bond between the two aryl functionalities and a disruption of the aromatic character of these groups.



Both processes are often applied in molecular switches.

After the 6π electrocyclization of the Z form, most unsubstituted diarylethenes are prone to oxidation, leading to a re-aromatization of the π-system. The most common example is E-stilbene, which upon irradiation undergoes an E to Z isomerization, which can be followed by a 6π electrocyclization. Reaction of the product of this reaction with molecular oxygen affords phenanthrene. Although both the E-Z isomerization and the 6π electrocyclization are reversible processes, this oxidation renders the entire sequence irreversible.

Dithienylethenes
One solution to the problem of oxidation is to replace the hydrogens ortho to the carbon-carbon double bond by groups that can not be removed during the oxidation. The most common example are the dithienylethenes, i.e. alkenes with a thiophene ring on either side. The 2-position of the thiophenes is commonly substituted with a methyl group, preventing oxidation of the ring closed form.

Following the Woodward-Hoffmann rules, the photochemical 6π cyclization takes place in a conrotatory fashion, leading to products with an anti configuration of the methyl substitutents. Because the thermal ring-opening reaction is symmetry forbidden, this type of diarylethenes is exceptionally thermally stable. As both methyl groups are attached to a stereogenic center, two enantiomers (R,R and S,S) are formed, normally as a racemic mixture.

Applications
Typically, the open-ring isomers are colorless compounds, whereas the closed-ring isomers have colors dependent on their chemical structure, due to the extended conjugation along the molecular backbone. Therefore many diarylethenes have photochromic behavior both in solution 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 indices, dielectric constants, and oxidation-reduction potentials. These properties can be readily controlled by reversible isomerization between the open- and closed-ring states using photoirradiation. 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.