Diarylethene

Diarylethene is the general name of a class of

geometric isomers

, E and Z.

Under the influence of light, these compounds can generally perform two kinds of reversible isomerizations:^[1]

E to Z isomerizations, most common for stilbenes (and azobenzenes). This process goes through an excited state energy minimum where the aromatic rings lie at 90° to each other. This conformation drops to the ground state and generally relaxes to trans and cis forms in a 1:1 ratio, thus the quantum yield for E-Z isomerization is very rarely greater than 0.5.
6π
aromatic character of these groups.^[2]
The quantum yield of this reaction is generally less than 0.1, and in most diarylethenes the close-ring form is thermally unstable, reverting to the cis-form in a matter of seconds or minutes under ambient conditions.

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).^[3] 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 photochromism (reversible state changes from exposure to light).^[4]^[5]^[6]

After the 6π electrocyclization of the Z form to the "close-ring" form, most unsubstituted diarylethenes are prone to

(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.^[2]

Stabilization of the closed-ring form to oxidation

One solution to the problem of oxidation is to replace the hydrogens

steric hindrance

between the substitution groups.

Dithienylethenes

Ortho-substitution of the aromatic units results in a stabilization against oxidation, but the closed-ring form still has a low thermodynamic stability in most cases (e.g. 2,3-dimesityl-2-butene has a half-life of 90 seconds at 20°C). This problem can be addressed by lowering the aromaticity of the system. The most commonly used example are the dithienylethenes, i.e. alkenes with a thiophene ring on either side.

Dithienylethene derivatives have shown different types of photochemical side reactions, e.g., oxidation or elimination reactions of the ring-closed isomer and formation of an annulated ring isomer as a byproduct of the photochromic reaction.

oxidation of the ring closed form. Also often the two free α-positions on the double bond are connected in a 5 or 6-membered ring in order to lock the double bond into the cis-form. This makes the dithienylethene undergo only open-closed ring isomerization, unconfused by E-Z isomerization. More recently, based on recent findings showing that by-product formation most likely occurs exclusively from the lowest singlet excited state,^[9]^[10] a superior fatigue resistance of dithienylethenes upon visible-light excitation has been achieved by attaching small triplet-sensitizing moieties to the diarylethene core via a

π

-conjugated linkage.^[11]

The dithienylethenes are also of interest for the fact that their isomerization requires very little change of shape. This means that their isomerization in a solid matrix can take place much more quickly than with most other

photochromic

molecules. In the case of some analogs, photochromic behavior can even be carried out in single crystals without disrupting the crystal structure.

Applications

Typically, the open-ring isomers are colorless compounds, whereas the closed-ring isomers have colors dependent on their

UV and visible light.^[12]^[13]

References

^ H. Görner, J. Kuhn, Advances in Photochemistry 19, 1-117 (1995).
^ ^a ^b J. March, Advanced Organic Chemistry, 4th ed. (1992).
J. Chem. Phys. 118:7823-7836 (2003) [1]

Angew. Chem. Int. Ed. 2000, 39, 3348 [2]
.

^ B. L. Feringa (ed.), Molecular Switches, Wiley-VCH, 2001, Weinheim.

Chem. Rev.
: Memories and Switches.

Chem. Rev.
2000, 100, 1685.

Chem. Commun. 747–750 (1999) [3]

Phys. Chem. Chem. Phys. 16:18463-18471 (2014) [4]

J. Am. Chem. Soc. 137 (7):2738–2747 (2015) [5]

Angew. Chem. Int. Ed. 55: 1208 (2016) [6]

^
Chem. Rev.
2000 , 100, 1685.

^ N. Katsonis, T. Kudernac, M. Walko, S. J. van der Molen, B. J. van Wees, B. L. Feringa, Advanced Materials 2006, 18, 1397–1400. [7]

Retrieved from "https://en.wikipedia.org/w/index.php?title=Diarylethene&oldid=1117545209"

[1] H. Görner, J. Kuhn, Advances in Photochemistry 19, 1-117 (1995).

[march-2] J. March, Advanced Organic Chemistry, 4th ed. (1992).

[3] J. Chem. Phys. 118:7823-7836 (2003) [1]

[4] Angew. Chem. Int. Ed. 2000, 39, 3348 [2]
.

[5] B. L. Feringa (ed.), Molecular Switches, Wiley-VCH, 2001, Weinheim.

[6] Chem. Rev.
: Memories and Switches.

[irie_chemrev-7] Chem. Rev.
2000, 100, 1685.

[8] Chem. Commun. 747–750 (1999) [3]

[9] Phys. Chem. Chem. Phys. 16:18463-18471 (2014) [4]

[10] J. Am. Chem. Soc. 137 (7):2738–2747 (2015) [5]

[11] Angew. Chem. Int. Ed. 55: 1208 (2016) [6]

[irie-12] 
Chem. Rev.
2000 , 100, 1685.

[13] N. Katsonis, T. Kudernac, M. Walko, S. J. van der Molen, B. J. van Wees, B. L. Feringa, Advanced Materials 2006, 18, 1397–1400. [7]

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