Diarylethenes, photochromic compounds

Recently, the synthesis of photochromic compounds containing two or more diarylethene fragments bound either directly or through linkers has been performed. Scheme 32 illustrates one approach to 96 containing two hexafluorocyclopentene-bridged dihetarylethene fragments (05JACS8922). 

Such thermally irreversible photochromic chromophores represent the other class, classified as P-type (photochemically reversible type). Although many photochromic compounds have been so far reported, P-type chromophores are very rare. Only two families, furylfulgide derivatives and diarylethene derivatives, exhibit this reactivity.19 101 The photogenerated isomers of these derivatives are thermally stable and never revert to their initial isomers even at elevated temperatures (-100 °C). The thermally stable photochromic compounds offer potential for various applications in photoswitching and memory devices. 

A crucial point that must be addressed concerns the thermal stability and the fatigue phenomenon observed in the chromophores. It is a fact that many photo-chromic compounds are irreversibly degraded upon long exposure to light, thus limiting their use for various applications. Major advances in the preparation and performance of photochromic materials have been made in the past five years. Irie et al.11271 have recently developed new photochromic compounds, 1,2-diarylethenes, which display photochromic behavior with unchanged intensity even after 104 coloration decoloration cycles. 

Chapter 5 (Diarylethenes with heteroaryl groups). The bis-heteroaryl ethenes (for example 5, Scheme 5) are photochromic compounds similar to the fulgides in... 

Consequently, the main class of photochromic compounds that shows change in chirality by photochromic transformation is that based on electrocyclization. Fulgides [11-13], diarylethenes [14-16], and spiropyrans [17,18] are included in this class. It may be somewhat strange to put spiropyrans into this category because the colored merocyanine forms. ure known to take zwitterionic structures, which do not cyclize through the electrocyclization mechanism. However, in order to simplify the classification, we consider spiropyrans to cyclize from the neutral dienone structures as the merocyanine forms. 

The Dithienylethene System and Related Compounds. The area of diaryl-ethenes has been the subjeet of a review.Calculations have been carried out dealing with the photochromism of such compounds.A description of the preparation of diarylethene nanoparticles has been published, and a decrease in photocyclization was attributed to aggregation. Some novel photochromic diarylethenes have been synthesized. The photochemistry exhibited by these compounds suggests that they could be of use as optical switches.Tian and Yang have reviewed the area of diarylethene photochromic switches. 1,2-Bis(2-methylbenzo[b]thiophen-3-yl)hexafluorocyclopentene is at the basis of a new photochromic liquid crystal system. Yokoyama and his co-workers have patented details for such photochromic materials." ... 

Photochromic materials have been widely used in textiles. " °°Typical organic photochromic compounds include azobenzenes, spiropyrans, sprroooxaztnes, viologens, fulgides, 1,4-dihydroxy anthraquinone and diarylethenes.They are based on different photochromisms, that is, isomerization of molecules, ionization of molecules and redox reaction of molecules. Azobenzenes, spiropyrans and viologens have been most widely used in textiles. 

Diarylethenes with heterocyclic aryl groups are newcomers to the photochromic field. They belong to the thermally irreversible (P-type) photochromic compounds. The most striking feature of the compounds is their fatigue resistance. The coloration/decoloration cycle can be repeated more than 10 times while retaining the photochromic performance. Both properties, thermal stability of both isomers and fatigue resistance, are indispensable for application to optoelectronic devices, such as devices for memory and switches. In this chapter, recent research on diarylethene derivatives will be described. 

The terms diarylethenes, dithienylethenes, and dihetarylethenes, as applied to the structures of photochromic products II, of course, do not strictly meet the nomenclature requirements. However, these terms have gained wide acceptance in the special literature. The classification of compounds containing heterocycles as bridges is also rather arbitrary. 

A number of photochromic systems have been extensively investigated that undergo cis-trans isomerization (indigos, azo compounds) cleavage (spiropyrans), electrocyclic processes (fulgides, 1,2-diarylethenes) [8.229, 8.244, 8.245], For instance, cis-trans isomerization of a thio-indigo derivative allows the reading of pyrene excimer or monomer fluorescence [8.246]. The 1,2-dithienylethene system presents particularly attractive interconversion properties by photoreversible cyc-litation [8.245],... 

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