Radical ions photochemical methods

In this chapter we have described the photophysics and photochemistry of C6o/C70 and of fullerene derivatives. On the one hand, C6o and C70 show quite similar photophysical properties. On the other hand, fullerene derivatives show partly different photophysical properties compared to pristine C6o and C70 caused by pertuba-tion of the fullerene s TT-electron system. These properties are influenced by (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in case of multiple adducts by the addition pattern. As shown in the last part of this chapter, photochemical reactions of C60/C70 are very useful to obtain fullerene derivatives. In general, the photoinduced functionalization methods of C60/C70 are based on electron transfer activation leading to radical ions or energy transfer processes either by direct excitation of the fullerenes or the reaction partner. In the latter case, both singlet and triplet species are involved whereas most of the reactions of electronically excited fullerenes proceed via the triplet states due to their efficient intersystem crossing. 

The final contribution turns back to the more simple aspects of PET in homogeneous media. As was shown for ion pairs in one of the golden ages of physical organic chemistry, the controlled formation of various types of radical ion pairs by photochemical methods can be utilized to control the course of chemical reactions. At this point, the medium effects which govern the formation and the fate of radical ion pairs resemble the supramolecular effects of the arranged systems discussed in the first articles. 

Research on carbon-centered radical cations in solution accelerated dramatically with the development of time-resolved optical absorption and emission techniques. The research group of Th. Forster in Germany pioneered photochemical methods of production of radical cations and anions, as well as exciplexes." While the Forster group focused on structure and lifetimes, the later work of D. R. Arnold in Canada, and of H. D. Roth in the United States," reported the reactivity of photochemically generated radical cations from a mechanistic perspective. These studies of radical ion chemistry evolved into the field we now know as electron donor-acceptor interactions, arich area of science in which carbon-centered radical cations are stiU actively smdied. 

The a-aminoalkyl radicals as well as iminium ions generated as intermediates in electron-transfer reactions of amines can be used for bringing about synthetically useful transformations of amines. The synthetic applications of amine oxidation reactions brought about by thermal, electrochemical and photochemical methods as discussed below. 

Gilbert and co-workers have described the dehydrogenation of a-terpinene to p-cymene by reaction of benzophenone in the presence of cupric ions under 40 suns concentrated sunlight. The cyanovinyl radical can be obtained by irradiation of acrylonitrile. A review article has discussed the methods for formation and the behaviour of allyl radicals. The principal product from such a species is allene. However, 1,2- and 1,3-hydrogen migrations are also observed, with the formation of 1- and 2-propenyl radicals. The photochemical reactivity of... 

In summary, photochemical methods exploit the high energy of the adsorbed photon in various ways - that is, either via reaction of the electronically excited state itself, whether involving the usual addition-elimination mechanism of arenes or unimolecular fragmentations, or via electron transfer followed by fragmentation of one of the charged radical ions. 

The primary oxidant responsible for most advanced oxidation processes, i.e. the use of photochemical methods, is the hydroxyl radical, which is formed by the reduction reactions of electrophiles with water or hydroxide ions. The mechanism of the formation of the hydroxyl radical is well discussed in the literature. Heterogeneous photocatalytic oxidation of organic compounds in aqueous solution is achieved by the reactive hydroxyl radical. The photocatalytic process can remove a large number of organic hazardous compounds in water. Phenols, carboxylic acids and herbicides are among a large number of pollutants, which are destroyed in air and in water using photocatalysis. 

There is a multiplicity of pathways for thermal dediazoniations. An analogous situation is to be expected for photochemical dediazoniations. Based on the general experience that light-sensitive reactions often involve free radical intermediates, it was commonly assumed that all photolytic dediazoniations are free radical reactions. Horner and Stohr s results (1952), mentioned above, could lead to such a conclusion. More sophisticated methods of photochemistry also began to be applied to investigations on arenediazonium salts, e. g., the study of photolyses by irradiation at an absorption maximum of the diazonium ion using broad-band or monochromatic radiation. This technique was advocated by Sukigahara and Kikuchi (1967 a, 1967 b,... 

Photolytic methods are used to generate atoms, radicals, or other highly reactive molecules and ions for the purpose of studying their chemical reactivity. Along with pulse radiolysis, described in the next section, laser flash photolysis is capable of generating electronically excited molecules in an instant, although there are of course a few chemical reactions that do so at ordinary rates. To illustrate but a fraction of the capabilities, consider the following photochemical processes ... 

Carbocations have also been obtained by protonation of photochemically generated carbenes (see Eq. 17), by the fragmentation of photochemically generated cation radicals (see Eq. 18), and by the addition of one photochemically generated cation to an arene (or aUcene) to generate a second cation. As illustrated in Eq. 19, the last method has been employed to convert invisible carbocations into visible ones. Short-hved aryl cations and secondary alkyl cations are quenched by electron-rich aromatics such as mesitylene and 1,3,5-trimethoxybenzene in HEIP to give benzenium ions that can be observed by LEP in this solvent. 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

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