Dithienylethene

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. The simplest example is stilbene, which has two geometric isomers, E and Z.

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

Thermal isomerization is also possible. In E-Z isomerization, the thermal equilibrium lies well towards the trans-form because of its lower energy (~15 kJ mol⁻¹ in stilbene). The activation energy for thermal E-Z isomerization is 150–190 kJ mol⁻¹ for stilbene, meaning that temperatures above 200°C are required to isomerize stilbene at a reasonable rate, but most derivatives have lower energy barriers (e.g. 65 kJ mol⁻¹ for 4-aminostilbene). The activation energy of the electrocyclization is 73 kJ mol⁻¹ for stilbene.

Both processes are often applied in molecular switches and for (reversible state changes from exposure to light).

After the 6π electrocyclization of the Z form to the "close-ring" 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, and it has been suggested by some studies that dehydrogenation may even occur spontaneously. The dihydrophenanthrene intermediate has never been isolated, but it has been detected spectroscopically in pump-probe experiments by virtue of its long wavelength optical absorption band. Although both the E-Z isomerization and the 6π electrocyclization are reversible processes, this oxidation renders the entire sequence irreversible.

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