Stilbene cis-trans isomerization

Many other examples can be found in the literature. Most familiar isomerization reaction is that of trans-stilbene to cis-stilbene, shown above. It was observed that the quantum yield of stilbene cis-trans isomerization decreased with an increase in viscosity of the medium [89]. In addition, it was also fotmd that in a polymeric matrix, the photo-isomerization is not inhibited, provided that it occurs above the glass transition temperature of the polymer. An example of a fragmentation of a molecule is the decomposition of disulfides upon irradiation with ultraviolet light of the appropriate wavelength ... 

The cis-trans isomerization of stilbenes is technically another photochromic reaction (18). Although the absorption bands of the stilbene isomers, occur at nearly identical wavelengths, the extinction coefficient of the lowest energy band of cis-stilbene [645-49-8] is generally less than that of stilbene [103-30-0],... 

Most of the olefins shown so far, for which reversibility of the bromonium ion formation had been demonstrated, are particular olefins, in which either steric bulk impedes the product forming step, or ring strain in the dibromide product retards this step. In order to check the general occurrence of the reversibility during the bromination reaction, a further approach, based on the cis-trans isomerization of stilbene derivatives during the bromination of the cis isomers, was devised. 

The occurrence of the same cis-trans isomerization observed in chlorinated hydrocarbons was finally tested for substituted c/i-stilbenes in two protic solvents, acetic acid and methanol, where the counteranion is a Br instead of a Br3". 

The investigated cw-stilbene derivatives, 4-methoxy, 4,4 -dimethyl, unsubstituted, and 4,4 -bis(trifluoromethyl)stilbenes, had k2 values spanning 6-7 powers of ten both in methanol and in acetic acid. Products 2, 4, 5 and 6 were formed. Table 8 reports the results of the cis-trans isomerization test in acetic acid (ref. 29). No acid catalyzed or free radical process was found to be responsible for these isomerizations. 

Category 4. Photoisomerization. The most common reaction in this category is photochemical cis-trans isomerization." For example, cw-stilbene can be... 

Stilbenes that are used as fluorescent whitening agents are photolytically degraded by reactions involving cis-trans isomerization followed by hydration of the double bond, or oxidative fission of the double bond to yield aldehydes (Kramer et al. 1996). 

The mechanism for the direct photochemical cis-trans isomerization of stilbene has been a highly controversial subject. However, a recent review by Saltiel and co-workers greatly helps to clarify this area of research by painting a detailed and beautifully consistent picture. We will make extensive reference to this review.U)... 

The previous sections have shown that cis-trans isomerization of stilbene can take place via the lowest triplet state of stilbene. The question to be considered now is whether the isomerization upon direct photolysis takes place via the singlet state, the triplet state, or a vibrationally excited ground state.a 7 81 50)... 

In addition,<1,48,48) it was noted that whereas the azulene effect on the sensitized reaction is sensitive to changes in solvent viscosity, the azulene effect on the direct photoreaction was independent of solvent viscosity, as would be predicted for Forster-type energy transfer. The inescapable conclusion is that cis-trans isomerization upon direct irradiation of stilbene takes place in the singlet manifold. 

Stilbene is known to undergo cis-trans isomerization in its triplet state to yield a photostationary state which is 60% cis and 40% irons,... 

Evidence that eliminates the triplet mechanism as the mode for the cis-trans isomerization of stilbene upon direct photolysis has been provided by azulene quenching studies.(48) Using the experimentally determined decay ratio a/(l — a) and the triplet mechanism, it is possible to calculate what the effect of azulene is upon the pss. The predicted and observed azulene effects on the direct photoisomerization are shown in Figure 9.6. The failure of the triplet mechanism in predicting the very small changes observed in the pss provides a crucial test that is the basis for rejecting the triplet mechanism. 

If fluorescence and cis-trans isomerization (9.26)-(9.29) are the main competing reactions upon direct excitation, then inhibition of rotation about the central bond should produce an increase in the fluorescence quantum yield. The rigid systems (3) and (4) both have fluorescence quantum yields of 1.0 at room temperature.<44,52) While the fluorescence of /rmy-stilbene is a... 

The effects of nitro substituents on the cis-trans isomerization of stilbenes has been reviewed70 (equation 63). The trans-to-cis isomerization occurs from a triplet excited state, whereas the reverse cis-to-trans isomerization occurs through a main route which bypasses the triplet state. A nitro substituent usually causes a significant enhancement of the quantum yield of the intersystem crossing. Nitro substituent effects on the photoisomerization of trans-styrylnaphthalene71 (equation 64), trans-azobenzenes72 and 4-nitrodiphenylazomethines73 (equation 65) have been studied for their mechanisms. 

Nitro substituted stilbenes have been used as models in stUbene cis trans-isomerization studies. It had been reported previously, that cis -v benzene solution, and a photostationary state of 99.5% trans isomer had been reached 132) ... 

In simple olefins of the stilbene type, cis —> trans isomerization in the cation-radical state is known to proceed readily because the exocyclic double bond is weakened. In the case of the 1,2-di(aryloxy)cylobutane cation-radical, cis —> trans isomerization also takes place (see Evans et al. 1975), but the reaction inevitably should include the open form of the distonic type, ArOCH CH2— CH2CH+OAr. Free rotation in this distonic cation-radical around CH2—CH2 bond permits ensuing ring closure with the formation of the more stable trans isomer. 

In the absence of oxygen, photoirradiation of stilbene with the same sensitizer (DCNA) provokes cis —> trans isomerization of the olefine. The reaction is initiated with a 365 nm light and proceeds at 25°C (Lewis et al. 1985) ... 

The enormous amount of overactivation in photochemistry is not always required for solid-state cis-trans isomerizations. There are also some thermal E/Z isomerizations of crystalline olefins that are catalyzed by iodine. For example, crystalline czs-stilbenes 91 can be isomerized to give frans-stilbenes 92 without intervening liquid phases (Scheme 8). The isomerizations follow first-order kinetics with various rate constants for 4-MeO, two modifications of 2-MeO, 2-EtO, 2-n-PrO, and 2-i-PrO substitution. The activation energies vary from 20 to 32 kcal mol but could not be interpreted [54]. Similarly, cfs-l,2-diben-... 

Furthermore, the conclusion that the formation of the meso dibromide upon gas-solid addition of bromine to czs-stilbene is due to cis-trans isomerization prior to or during addition [54,56] cannot be accepted without further proof, as there is also the possibility for cis addition [58-61]. 

Crowns with Stilbene Fluorophores. Stilbenes undergo cis-trans isomerism on irradiation and this feature is exploited in (3.82). In the case of the frany-isomer, large ionic radius ions such as K+, and Cs, give fluorescent complexes whilst the smaller Lb and Na " cause quenching. UV irradiation to the dy-isomer causes a considerable change in the conformation of the crown ether substructure and increases its ability to extract alkali metals, including Lb and Na+, from water into benzene. 

A particularly interesting case of transfer from T2 has been noted238 in the 9,10-dichloroanthracene (DCA) sensitized isomerization of stilbene. As shown below, the DCA ground state which results from quenching of T2 by stilbene (St) can act as a quencher of stilbene triplets and thereby alter the stilbene cis. trans photostationary state. 

Certain azo compounds and substituted ethylenes, such as stilbenes, can undergo photo-induced cis-trans isomerization. The trans isomer is invariably the thermodynamically more stable one and absorbs at longer wavelengths than the cis isomer. 

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