Light-induced reactions

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

The light-induced reaction of pentacarbonyliron with oxepin or 2,7-dimethyloxepin results in the formation of small quantities (3-5 %) of a tricarbonyliron complex of the seven-mem-bered heterocycle.253,251 The main products are benzene (o-xylene) and phenol (2,6-dimethyl-phenol). When 1-benzoxepin is treated with pentacarbonyliron, the tricarbonyliron complex is obtained in 22% yield.254... 

Ring expansions of vinylaziridines with carbon monoxide are a useful transformation with which to produce lactams. I11 1974, a light-induced reaction between vinylaziridine 256 and Fe(CO)5, forming the 7t-allyltricarbonyliron lactam complex... 

Alicyclic ew-dialkyldiazenes are very thermolabile when compared to the corresponding tram-isomers, often having only transient existence under typical reaction conditions. It has been proposed49 that the main light-induced reaction of the dialkyldiazenes is tram-cis isomerization. Dissociation to radicals and nitrogen is then a thermal reaction of the cis-isomer (Scheme 3.19),... 

Various light-induced reactions including hydrogen atom abstraction, electron transfer and (3-scission occur under the influence of UV light. Certain... 

The mechanism of the peroxide- or light-induced reaction seems to be as follows (in the presence of thiols) ... 

In conclusion, all these types of light-induced reactions involving polyphosphazenes readily account for the great importance assumed by this topic in the phosphazene domain and for the remarkable application potentials of especially designed phosphazene materials. 

Although thermodynamically favorable, reductive dissolution of Fe(III)(hydr)oxides by some metastable ligands (even those, such as oxalate, that can form surface complexes) does not occur in the absence of light. The photochemical pathway is depicted in Fig. 9.3e. In the presence of light, surface complex formation is followed by electron transfer via an excited state (indicated by ) either of the iron oxide bulk phase or of the surface complex. (Light-induced reactions will be discussed in Chapter 10.)... 

Again, polyesters based on naphthalene-2,6-dicarboxylic acid can meet the desired properties, plus have other additional advantages. Food components containing double bonds or conjugated double bonds are extremely sensitive to light-induced reactions, such as photo-oxidation. The addition of UV stabilizers is therefore necessary to protect the content of PET bottles against UV irradiation. PET, in contrast to PEN, does not show intrinsic UV protection properties. 

C. Other Light-Induced Reactions of Aromatic Nitro Compounds. 78... 

Although many light-induced reactions of nitro compounds are known, attempts to elucidate the multiplicity of the reacting excited state have been made in only a fraction of cases. Conclusive information is not easily obtained since in general nitroaromatics suffer from the disadvantage that they show very weak or even no fluorescence and have very short triplet lifetimes. 

Nucleophilic aromatic photosubstitution in general is a rapidly growing field 88-89) which originated from research on light-induced reactions of aromatic nitro compoimds but is by no means restricted to this class of compounds. 

There are many reports on other intriguing light-induced reactions of aromatic nitro compounds, such as ... 

There are also many photoreactions, in which rapid, efficient charge transfer between electron donor and acceptor moieties occurs, yielding a radical ion pair that subsequently dissociates because the solvent is not oxidized or reduced in these light-induced reactions, these reactions belong to photo- rather than radiation chemistry. Of particular interest, in the context of radiation chemistry studies, are photoreactions in which the solvent is... 

The effect of frany-piperylene upon the rate of the light-induced reaction between uracil and methanol suggests that only part of this reaction proceeds via a triplet excited state of uracil and that the lifetime of this triplet uracil in methanol is about 10 7 sec.141... 

Light-induced reaction of bicyclo[2.1.1]hexane-l,4-dicarboxylic acid with rcr/-butyl hypoiodite gave l,4-diiodobicyclo[2.1.1]hexane (4) in 60% yield.23... 

McGinnis, V. D. "Ultraviolet Light Induced Reactions in Polymers" Labana, S. S., Eds. ACS SYMPOSIUM SERIES No. 25, American Chemical Society Washington, D.C., 1976. 

IT. Light-Induced Reactions of Sulfur Organic Compounds... 

In the light-induced reactions with diazoalkanes, methylene, the photolytic fragment from diazomethane, may be interposed between the halogen atom in the organic halide and the carbon atom to which it is attached,289 290 or may add to the double bonds in the aromatic ring.58 69... 

The light-induced reaction of o-quinone diazides finds occasional use in the synthesis of a large number of compounds which are used in photography.273,277 Photolysis of diazoindauoues (CCVIII and CCX) provides a new synthetic route to cyclobutene carboxylic acid derivatives CCIX and CCXI, respectively.117 A good demonstration that ketenes are actually involved in the photodecomposition of o-quinone... 

Direct Repair Several types of damage are repaired without removing a base or nucleotide. The best-characterized example is direct photoreactivation of cyclobutane pyrimidine dimers, a reaction promoted by DNA photolyases. Pyrimidine dimers result from an ultraviolet light-induced reaction, and photolyases use energy derived from absorbed light to reverse the dam-... 

This provides an illustration of the way in which the chemical outcome of a light-induced reaction can depend on the nature of the reactive excited state triplet states lead to long-lived primary products which can take part in further bimolecular processes, whereas singlet states lead to fast uni-... 

In terms of tons of chemicals per year, this is by far the most important industrial process in which a light-induced reaction plays a key role. The photochemical reaction itself is extremely simple, being the homolytic dissociation of molecular chlorine, Cl2, into Cl atoms,... 

The sequence of dark reactions which follow the photochemical processes during the night are not shown here. The light-induced reactions are then stopped, and much human activity also declines sharply. The graph in Figure 6.29 shows that great care must be taken when comparisons of pollutant concentrations are made without proper reference to the time of day and time of year. 

The aim of this book is to provide a concise introductory overview of the various light-induced processes in physics, chemistry, biology, as well as, in medicine and industry. It is largely based on the course of photophysics and photochemistry given at the University of Fribourg, and every effort has been made to keep it up to date with the latest developments in this field, in particular the probing of the fastest light-induced reactions on picosecond and femtosecond time scales. 

A chapter on photochemistry provides a discussion of photophysical processes needed as background for this increasingly important area of chemistry and treats the main categories of light-induced reactions. 

Mirbach (6) and others (7 -9), we have suggested the following terms be used for the description of the different light-induced reactions leading to the catalysed conversion of diverse substrates ... 

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