Stilbenes isomerisation

A detailed study of the photostability of the DAST-type agent Cl Fluorescent Brightener 85 on cellophane film was carried out recently. The initial fading reaction is a photo-sensitised trans-cis isomerisation of the stilbene grouping. The subsequent oxidative attack on the molecule is concentrated on the vulnerable ethene linkage at the centre of this moiety [39]. 

Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid and accurate method of quantitative analysis. Care must be taken when interpreting the spectra of stilbene-type compounds, since turns to cis isomerisation is promoted by ultraviolet radiation. Usually, however, a control spectrum of the turns isomer can be obtained before the compound undergoes any analytically significant isomerisation. FBAs are often marketed on the basis of strength comparisons determined by ultraviolet spectroscopy. 

Affinity values of the symmetrical disazo dyes Cl Direct Yellow 12 (3.6), Red 2 (3.8 X = CH3, Y = H) and Blue 1 (3.2) on cellophane have been measured recently under hydrostatic pressures up to 600 Mpa [120]. The affinity of Yellow 12 increased slightly but values for the other two dyes decreased considerably with increasing hydrostatic pressure (Table 3.23). The sulpho groups on the central stilbene nuclei of the Yellow 12 molecule tend to inhibit aggregation, whereas Red 2 and Blue 1 aggregate much more readily. The small increase in the affinity observed with Yellow 12 may indicate that isomerisation from the cis to the more stable Linns form may occur as the hydrostatic pressure is increased. 

In this paper the author presents some of his contributions to the theory and practice of the cationic polymerisation (CP) of alkenes since 1944. The first phase of his work at the University of Manchester comprises the discovery of co-catalysis by water with TiCl4, the invention of the pseudo-Dewar reaction vessel, the use of trichloroacetic acid as co-catalyst, and the disproof of the alleged cationic isomerisation of cis-stilbene. 

Consider the photosensitised cis-trans isomerisation of stilbene The two isomers have different triplet energy levels (247kJmoT1 for the cis and 205kjmol-1 for the trans). The proportion of the cis isomer in the photostationary state varies with the energy of the photosensitiser (ET), as shown in Figure 8.5. 

The catalytic performance of the supported bimetallic nano-particles in the hydrogenation of unsaturated molecules was tested on a wide variety of unsaturated species hex-l-ene, phenyl acetylene, diphenyl acetylene, trans-stilbene, cis-cyclooctene and D-limonene. The highly efficient hydrogenation of hex-1 -ene was accompanied by the isomerisation reaction to cis-and trans-hex-2-ene, which were subsequently hydrogenated (albeit at a much slower rate) as reaction ensued. 

The stilbene carbon unit has also been peripherally bound to POPAM cores. Although it does not strictly belong to the hydrocarbon dendrimers, the formula of a G2 dendrimer of this type (Fig. 4.21) is depicted here as an example. It was obtained by alkylation of the corresponding eightfold mono-sulphonamide with 4-(bromomethyl)stilbene. Its fluorescence, E/Z isomerisation, photoisomerisation (see Section 5.2.2), and excimer formation were compared with those of non-dendritic stilbenes. The quantum yields of photoisomerisation (0.30) and fluorescence of the E isomer (0.014) of the dendrimer proved to be substantially lower [38]. 

Some simple stilbenes isolated from the roots of Polygonum cuspidatum (Polygonaceae) showed moderate inhibitory activity of the bovine thymus P56 " tyrosine kinase activity, when angiotensin 1 was used as a substrate. The most potent were resveratrol (26) and its cis form obtained by photochemical isomerisation. Both compounds showed similar potency against rat brain PKC, whereas resveratrol monoglucosides were less active [81]. 

Irradiation of the contact charge transfer complex formed between trans-stilbenes and oxygen molecules in a zeolite NaY matrix at 313 nm leads to generation of the corresponding benzaldehydes in an electron-transfer process from which stilbene cation radicals and superoxide anion radicals arise. By contrast, excitation at 254 nm induces isomerisation and phenanthrene production, but without formation of any oxygenation products. 

Stilbene formation by the Wittig reaction usually gives a mixture (here 7 3 Z E) of E- and Z-isomers 280. Irradiation of a heptane solution containing a small amount of iodine in sunlight isomerises the mixture to the -stilbene 280 which crystallises from solution in 68% yield.57... 

Up to now, photochemical studies are still confined to the reactions of simple azobenzene compounds, such as those derived by coupling a phenol or aromatic amine with a diazotised aniline. Azobenzene is isoelectronic with stilbene, its derivatives undergo facile trans cis photo isomerisation reactions in a manner analogous to that of stilbene. Azo dyes which possess a hydroxyl or amino substitutent ortho or para to the azo group... 

An experimental value for the activation barrier for the isomerisation of cw-2-butene 2.104 is 259 kJ mol-1 (62 kcal mol-1). Phenyl groups stabilise radical centres, and the barrier to rotation in stilbenes 2.105 is correspondingly reduced from that in 2-butene to 179 kJ mol-1 (43 kcal mol-1). Steric interaction between the cA-vicinal substituents raises the energy of the planar structure, and contributes to lowering the barrier to rotation. In a fairly extreme example, the bifluorenylidene 2.106 benefits from both effects, and the barrier falls to 95 kJ mol-1 (23 kcal mol-1).136... 

Fig. 4.17. Absorbance-time diagram demonstrating the change in absorbance at some selected characteristic wavelengths of the stilbene photoreaction. The wavelength 313 nm corresponds to the isomerisation of (rans-stilbene at 250 nm the formation of phenanthrene can be...

In Section 5.6.1.1 quantitative results for the partial photochemical quantum yields of the photoreaction of stilbene-1 were given. A photoisomerisation was assumed to be the mechanism as a first step. This proposal is supported by a reaction chromatogram of the photodegradation reaction of this laser dye. The two parts of the diagram (see Fig. 5.58) prove a photo-reversible isomerisation as a first step [190]. This information was used for the evaluation mentioned. 

Phenanthroquinone was induced to react with cis- and tra/ii-stilbene, separately, under irradiation (X > 390 nm), and the stereochemistry of the 1,4-dioxenes produced was examined by nmr spectroscopy after a small correction, because of some isomerisation of the stilbenes induced by quinone, the product distribution is ... 

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