Diarylethenes compounds

Molecular hyperpolarizabilities of the two isomers of different photochromes are presented in Tables 10,1 and 10.2. As Disperse Red 1 (DRl) is probably the most studied photochrome due to its very interesting NLO properties in polymers, its nonlinear coefficients have also been tabulated. The spiropy-ran/photomerocyanine group has also been investigated in detail in polymers and in solutions as well, and the NLO properties have been described already.More recently, Atassi et al. have shown that it was possible to observe NLO response with a furylfulgide system," " " and Lehn et al. have shown that some diarylethene compounds can photochemically switch between a low and a high level of NLO response. The first two systems are... 

Diarylethenes, 1,1-diarylallylalcohols and aryl vinyl ethers were succesfully hydroformylated in water/toluene or water/cyclohexane biphasic mixtures with a catalyst prepared in situ from[ RhCl(COD) 2] and TPPTS (Scheme 4.15). Yields of the desired linear aldehyde product were around 80%. This method was applied for the synthesis of the neuroleptics Fluspirilen and Penfluridol (Scheme 4.16) and for other pharmaceutically active compounds containing the 4,4-bis(p-fluorophenyl)butyl group [153]. 

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. 

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). 

There are five main classes of compounds which can approach these ideal requirements spiropyrans, specifically spiroindolinobenzopyrans, spironaphthoxazines, naphthopyrans, fulgides and diarylethenes. 

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],... 

Benzylic electrophiles bearing electron-withdrawing groups at the arene do not always yield the expected products of nucleophilic substitution on treatment with a nucleophile. One important side reaction is the dimerization of these compounds to yield 1,2-diarylethenes (stilbenes). This dimerization does not require such highly activated systems as the example sketched in Scheme 4.28, but can even occur with, for example, 2- or 4-nitrobenzyl chloride [120, 121]. The latter compounds are converted into the corresponding stilbenes by treatment with KOH in ethanol [120]. Di-arylmethyl halides behave similarly and can yield tetraarylethenes on treatment with a base. These reactions presumably proceed via the mechanism sketched in Scheme 4.27, in which the amphiphilic character of the nitro group plays a decisive role (metalated nitroalkanes or 4-nitrobenzyl derivatives can act as nucleophiles and as electrophiles). 

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. 

With conventional protocols requiring low reaction temperatures, typically —78 °C, to prevent side reactions from occurring, scaling the reaction for industrial production of such compounds has proved difficult. As such, the authors evaluated the process under continuous flow, proposing that the effective temperature control and accurate residence times attainable within miniaturized flow reactors would enable the synthesis of diarylethenes at temperatures above — 78 °C and thus facilitate the large-scale synthesis of such compounds. 

To realize the above-mentioned systems, we carefully chose suitable switching units and radical moieties. As an initial attempt, we employed l,2-bis(2-methyl-l-benzothiophen-3-yl)perfluorocyclopentene (9a) as a photochromic spin coupler (Scheme 9.2). Compound 9a is one of the most fatigue-resistant diarylethenes [21]. Nitronyl nitroxide was chosen for the spin source, because this radical is jr-conjugative. Thus, we designed molecule 10a, which is an embodiment of the simplified model 8a [37, 62]. 

Compound 24(00) was synthesized according to Scheme 9.10. The key step was the Suzuki coupling of iodo-substituted diarylethene and p-phenylenedi-boronic add. 

Scheme 9.12 shows the synthetic route. [4-Methyl-2,5-bis(trimethylsilylethy-nyl)-3-thienyl]heptafluorocyclopentene (31) was prepared from 3-bromo-4-methylthiophene (28) in three steps. 2 -methoxy derivative 33 was synthesized by the coupling of 31 with lithiated 32. After desilylation with KOH, Sonogashira coupling with bromoformyl compounds gave bisformylated diarylethenes 34. Formyl derivative 34 was converted into nitroxide radical 27a. 

The photochemical reactions of naphthoquinones and quinolinediones with 1,1-diarylethenes have led to the synthesis of many new polycyclic aromatic and heteroaromatic compounds. A review has appeared16 in which a number of these results are tabulated. 

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

The mono- and disubstituted perfluorocyclopentenes were selectively prepared by controlling the ratio of perfluorocyclopentene and aryllithium. The monosub-stituted compound can be used for the synthesis of nonsymmetric diarylethenes. 

When we apply diarylethenes to full color display, it is indispensable to synthesize yellow-developing compounds. We found that the color of the closed-... 

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. 

Irie M. Diarylethenes with Heterocyclic Aryl Groups In Crano JC, Guglielmetti RJ, Eds. Organic Photochromic and Thermochromic Compounds. Main Photochromic Families. Vol. Vol. 1. New York Plenum Press, 1999 207-222. 

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