Stilbenes photochemical reactivity

The effect of association on photochemical reactivities of stilbenes and alkyl cinnamates having poor solubilities in water has been demonstrated recently (Schemes 37 and 38) [140, 205,206]. Even at concentrations of trans-stilbene in water as low as 10 6 M, dimerization occurs efficiently. The ratios of dimers were similar to those obtained in benzene when the initial stilbene concentration is high. However, in organic solvents, geometric isomerization is the only reaction observed at low stilbene concentrations. Similar behavior... 

The fluorescence Intensity of substituted stilbenes and stilbene analogues provides a useful indicator of photochemical reactivity. Virtually all of the reported bimolecular photochemical reactions of electronically excited stilbenes involve stilbenes which are fluorescent at room temperature in solution. The absence of fluorescence is indicative of a singlet lifetime too short (< 100 ps) to allow for efficient bimolecular quenching. 

Intramolecular electron transfer occurs on irradiation of the stilbene derivative (319) in methylene chloride. Irradiation populates a charge transfer state that undergoes ,Z-isomerism and when methanol is added to the system not only is isomerism, observed but also the trisilanyl group in (319) is converted into the silane (320). The photochemical reactivity of the vinyldisi-lanes (321) has been examined and direct irradiation of (321a) in cyclohexane is reported to yield two products identified as the (2+2)-dimers (322) and (323). Irradiation of (321c) in cyclohexane with added methanol afforded the methoxy addition product (324). Both these results are in agreement with the intermediacy of silenes such as (325) and (326) in these experiments and... 

Aerial oxidation of this affords anthraquinone in 37% yield. Both 1-naphthyl and 2,4-dimethylbenzene sulphonates follow the same path but yield only traces of the corresponding quinone. In the last case, 2,4-dimethylbenzenesulphonate, the quinone is accompanied by 0.8% of 2,4-dimethylphenol. The second path involves loss of sulphur trioxide to yield aryl radicals which afford the products, the arene and/or the biaryl, shown in Scheme 17212,213. Other studies have shown that anthraquinone-1-sulphonic acid (256)214,215 and anthraquinone-2-sulphonate216 are also photochemically labile. A study of the photochemical reactivity of azulene sulphonic acids has also been reported217. Photochromism has been studied with respect to the stilbene derivative 257218. 

Silver also has been demonstrated to be reactive in solution systems. Thus, silver perchlorate has been shown to influence the photochemical reactivity of stilbene in acetonitrile and methanol. The fluorescence of the stilbene is quenched on addition of the perchlorate and this is good evidence for the enhancement of the So-Ti crossing induced by the heavy ion Ag+. It seems likely that an Ag+/stilbene complex is formed. The perturbation of the system is better in methanol than in acetonitrile. However, cis.trans isomerism of the stilbene is reduced within the excited Ag+/stilbene complex since it is difficult for the geometrical isomerism to occur. Enhanced isomerism is observed with the Ag+/azobenzene system. In this complex there are steiic problems encountered in the nitrogen rehybridization process that is operative in the isomerism . Enhanced So-T crossing is also seen in the Ag+/1 1 complex with tryptophan where the fluorescence is quenched and there is a threefold increase in phosphorescence . Complexes between Ag+ and polynucleotides and DNA cause quenching of the fluorescence. Enhancement of phosphorescence and a 20-fold increase in the dimerization of thymine moieties has also been observed when silver ions are added to the reaction system . ... 

Another interesting photochemical reaction that occurs with the monolayers is dimerization. This is exempUfled by the photochemical behaviour of the SAM of 7-(10-thiodecoxy)coumarin (52) on polycrystaUine gold. Irradiation at 350 nm results in the (2 -f 2)-cycloaddition of the coumarin moieties. The photodimerization is a reversible process by irradiating at 254 nm. Better regioselectivity in the cycloaddition is obtained when the solid monolayer is irradiated rather than when it is in contact with benzene. The dimer formed is the yn-head-to-head dimer identified as 53 . Self-assembled monolayers of cis- and frani-4-cyano-4 -(10-thiodecoxy)stilbene (54) are also photochemically reactive. Irradiation of a thin film in benzene solution using A, > 350 nm results in the formation of a photostationary state with 80% of the cis-isomer present. Irradiation in the solid shows that cis.trans isomerism occurs but that trans.cis-isomerism fails. Prolonged irradiation brings about (2 - - 2)-cycloaddition of the stilbene units to afford cyclobutane adducts. Such dimerization is a well established process . The influence of irradiation at 254 nm or 350 nm of self-assembled monolayers of 10-thiodecyl 2-anthryl ether on polycrystaUine... 

P,y-Unsaturated aldehydes are a family of carbonyl compounds that have been considered ODPM unreactive for many years. This conclusion was based on literature precedents that indicated that the normal photochemical reactivity of such compounds, both on direct and sensitized irradiation, was decarbonylation to the corresponding alkene. > f> Thus, compounds 5, 6, 7, and 8 are representative examples of a series of steroidal and cyclic P,y-unsaturated aldehydes, studied by Schaffner et al. about 20 years ago, that undergo decarbonylation on irradiation. Diirr et al. have studied the photochemistry of some aryl-substituted-2-cyclopentene-l-carbaldehydes 9. In this instance, ds-stilbene-type electrocy-clic cyclization and decarbonylation were the only reactions observed. Other authors have also reported the decarbonylation of the acyclic P,y-unsaturated aldehydes 10 and 11 on direct irradiation. 

Electronic excited states are often reactive intermediates in many photochemical reactions. In a number of cases, the excited state may undergo energy relaxation. The photoisomerization reaction of fra i-stilbene provides a well-studied... 

The objective of this article is to review critically the bimolecular photochemical reactions of the stilbenes. While the emphasis will be placed upon the unsubstituted stilbenes t-1 and and c-1, comparisons with substituted stilbenes and stilbene analogues will be made in some cases. In most of the reactions that will be described, the reactive excited state is the lowest singlet of t-1, - -t. Because of the short lifetime of It, unimolecular processes (fluorescence and trans-cis isomerization) can compete with bimolecular reactions under normal reaction conditions. While competing unimolecular processes are... 

The results described in this article establish that the stilbenes are among the most versatile of organic reactants in bimolecular photochemical reactions. Only triplet cyclo-alkenones can rival the ability of It to dimerize, form [2+2] adducts with both electron-rich and electron-poor alkenes, and form acyclic adducts with amines, heterocycles, and noncon-jugated dienes. All of the known bimolecular photochemical reactions of excited stilbenes involve It as the reactive excited state. The failure of - -c, and 3C to undergo bi-... 

The photoisomerization of all types of azobenzenes is a very fast reaction on either the singlet or triplet excited-state surfaces according to the preparation of the excited state, with nearly no intersystem crossing. Bottleneck states have lifetimes on the order of 10 ps. The molecules either isomerize or return to their respective ground states with high efficiency. So photoisomerization is the predominant reactive channel, and the azobenKnes are photochemically stable. Only aminoazobenzene-type molecules and pseudo-stilbenes have small quantum yields of photodegradation. 

In conclusion, stilbenes involve in miscellaneous chemical reactions. For non-substituted stilbenes, the most chemically reactive part is double bond, which relatively easily undergoes the halogenation, epoxidation, oxidation, reduction, and addition. The chemistry of substituted stilbenes is in principle as rich as organic chemistry. Including stilbenes in dendrides, dextrins, polymers, and surfaces led to a sufficient change in their chemical, photochemical, photophysical, and mechanical properties and, therefore, establishes the basis for design of new materials. 

The effed of thienyl groups on the photoisomerization and rotamerism of symmetric and asymmetric stUbenes has been studied [73]. Stationary and pulsed fluorimetric techniques, laser flash photolysis, and conventional photochemical methods and theoretical calculations were used for investigating photochemistry of flve symmetric (bis-substituted) and asymmetric (mono-substituted) analogues of -stilbene, where one or both side aryls are 2 -thienyl or 3 -thienyl groups. It was shown that the presence of one or two thienyl groups and their positional isomerism afled the spectral behavior, the relaxation properties (radiative/reactive competition), the photoisomerization mechanism (singlet/triplet), and the ground-state rotamerism. 

Friedel-Crafts acylation catalyst, 95, 96 MA polymerization catalyst, 254 Stannous chloride, 460 Stannous oxalate, 480 Starch, MA condensations, 516 Steroids, MA Diels-Alder synthesis, 145 Steryl methacrylate, MA copolymerization, 528 Stilbene, cis and trans, 202 MA copolymerization, 307, 373, 374, 379, 394 MA reactivity ratios, 302 Stilbene oxide, MA photochemical addition, 197, 228... 

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