Oxygen photochemical excitation

The oxygen molecule, a paramagnetic species with an unpaired electron on each atom, has already been referred to as biradical, albeit an unreactive one. The photochemical excitation of an anthracene to a biradical, or to something rather like one, has also been mentioned (p. 331) if this excitation is carried out in the absence of air or oxygen, instead of the trans-annular peroxide—(104)—a photo-dimer (130) is obtained ... 

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

Acylaminyloxides 90 were obtained in photolysis of 81 c and 89 respectively99. The mechanism of formation of these radicals is not yet known. It is suggested that formation of acylaminyloxides 90 occurs from photochemically excited oxaziridines since 90a is also formed by irradiation of oxaziridine 91. On the other hand heating of 81 c in presence of oxygen gives acylaminyloxide 90a and an aminyloxide for... 

Photooxygenation of the benzylic positions of various nitroaroinatic compounds has been achieved and interpreted in terms of the greatly enhwced C-H acidity of the benzylic methylene hydrogens on photochemical excitation,and singlet oxygen is thought to play an important role in the... 

In contrast to the laboratory experiment. Nature uses iron-protoporphyrin IX for the photochemical excitation of oxygen. 

It has been suggested that photochemically excited 53 or 54 is deprotonated by the base, resulting in the formation of the anionic species 71 or 72, respectively, which undergo electrocychc ring closure between N2 and C5 to give the stabihzed anions 73 and 74, respectively. Oxygen migration and reprotonation then provides 75 and 76, from which the observed phototransposition products 77 or 78 arise by rearomatization. 

A principal appHcation for photomedicine is the photodynamic treatment of cancer. Photochemical and clinical aspects of this topic have been reviewed (10,11). Direct irradiation of tumors coupled with adininistration of a sensitizer is used to effect necrosis of the malignancy. In this process, an excited state sensitizer interacts with dissolved in vivo to effect conversion of the oxygen from its triplet ground state to an excited singlet state, which is highly cytotoxic. In principle, excited sensitizers in either the singlet or the triplet state can effect this conversion of molecular oxygen (8). In... 

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