Reaction with electronically excited singlet states

A. Reaction with Electronically Excited Singlet States. 264... 

Many reactions of photosensibilized oxidation run with oxygen, taking part in it, and being in electron-excited singlet state. Singlet oxygen may form products of oxidation with the yield being close to theoretical [241],... 

In the oxygen-independent Type III reactions the excited/sensi-tized psoralen donates its excitation energy directly to, or reacts with, the target compound. This occurs if the substrate and the target compound (e.g., DNA) are already in close proximity or intercalated. The reactions will proceed very rapidly via the excited singlet state, and are, typically, cyclization reactions or electron-transfer between the sensitizer and the target. In addition, the psoralen can be ionized, either directly or via the excited state, and react with the target compound in the form of a radical cation. Furocoumarins are also employed in treatment of cutaneous T-cell lymphoma and some infections connected with AIDS, by so-called photopheresis processes [71, 74-76]. In this case, peripheral blood is exposed to, e.g., photoactivated (sensitized) 8-methoxypsoralen (8-MOP) in an extracorporeal flow system. This... 

Most of the reactions of triplet carbenes discussed in this chapter will deal with reactions in solution, but some reactions in the gas phase will also be included. Triplet carbenes may be expected to show a radical-like behaviour, since their reactions usually involve only one of their two electrons. In this, triplet carbenes differ from singlet carbenes, which resemble both carbenium ions (electron sextet) and carbanions (free electron pair). Radical like behaviour may, also be expected in the first excited singlet state Sr e.g. the state in CH2) since here, too, two unpaired electrons are present in the reactive intermediate. These Sj-carbenes are magnetically inert, i.e., should not show ESR activity. Since in a number of studies ESR spectra could be taken of the triplet carbene, the reactions most probably involved the Ti-carbene state. However, this question should be studied in more detail. 

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