|
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:〔H. Görner, J. Kuhn, ''Advances in Photochemistry'' 19, 1-117 (1995).〕 *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 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.〔J. March, ''Advanced Organic Chemistry'', 4th ed. (1992).〕 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).〔S. P. Kwasniewski, L. Claes, J.-P. François, M. S. Deleuze, ''J. Chem. Phys.'' 118:7823-7836 (2003) ()〕 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).〔V. Balzani, A. Credi, F. M. Raymo, J. F. Stoddart, ''Angew. Chem. Int. Ed.'' 2000, 39, 3348 ().〕〔B. L. Feringa (ed.), ''Molecular Switches'', Wiley-VCH, 2001, Weinheim.〕〔May 2000 issue of ''Chem. Rev.'': Memories and Switches.〕 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.〔 ==Stabilization of the closed-ring form to oxidation== 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. 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. As both methyl groups are attached to a stereogenic center, two enantiomers (R,R and S,S) are formed, normally as a racemic mixture.〔M. Irie, ''Chem. Rev.'' 2000, 100, 1685.〕 This approach also has the advantage that the thermal (disrotatory) ring closure can not take place because of steric hindrance between the substitution groups. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Diarylethene」の詳細全文を読む スポンサード リンク
|