Many polycyclic aromatic hydrocarbons are able to trap singlet oxygen (1)O(2). Some of the endoperoxides, thus obtained, exhibit the exceptional feature of releasing oxygen, frequently in the excited singlet state, under heating or UV irradiation. In this Account, we provide a short summary of the present knowledge on these endoperoxides: preparation and thermal and photolytic decomposition, with a special emphasis on the structural requirements to favor cycloreversion. The profitable use of this property in the development of highly reversible photochromic systems and of specific sources or traps of (1)O(2) in aqueous media is also described.

Ultraviolet A (UVA; 320±400 nm) radiation in human skin fibroblasts induces a pattern of mitogen-activated protein kinase (MAPK) activation consisting of a rapid and transient induction of p38 and c-Jun-N-terminal kinase (JNK) activity but not extracellular signal-regulated kinases (ERK). UVA activation of p38 can be inhibited by the singlet oxygen ( 1 O 2 ) quenchers azide and imidazole, but not by the hydroxyl radical scavengers mannitol or dimethylsulfoxide, pointing to the involvement of 1 O 2 . The same effect has been shown for JNK. Like UVA, 1 O 2 generated intracellularly upon photoexcitation of Rose Bengal activates p38 and JNK but not ERK. p38 and JNK activation was also elicited by chemiexcitation for the intracellular generation of 1 O 2 by the lipophilic 1,4-endoperoxide of N,N H -di(2,3-dihydroxypropyl)-1,4-naphthalene dipropionamide. In contrast, extracellular generation of 1 O 2 , by irradiation of Rose Bengal immobilized on agarose beads or by chemiexcitation employing the hydrophilic 1,4-endoperoxide of disodium 3,3 H -(1,4-naphthylidene) dipropionate, was ineffective in activating p38 or JNK. These data suggest that the activation of p38 and JNK by 1 O 2 occurs only when the electronically excited molecule is generated intracellularly.

The mono-, di-, tri-and tetraperoxomolybdate intermediates formed in the reaction between molybdate ions and hydrogen peroxide are identified by 95Mo NMR. The influences of pH and hydrogen peroxide concentration are discussed in terms of equilibrium constants. Comparison of these results with kinetic studies, performed under the same conditions, leads us to conclude that the oxotriperoxomolybdate Mo0(02)s2-is the main precursor of molecular oxygen. The decomposition rates of the different intermediates are also determined, and a mechanistic scheme is proposed.

No abstract

No abstract

Decyl-, dodecyl-, and tetradecyltrimethylammonium cations were combined with the catalytic polyoxometalate [PW(12)O(40)](3-) anion to give spherical and monodisperse nanoparticles that are able to stabilize emulsions in the presence of water and an aromatic solvent. This triphasic liquid/solid/liquid system, based on a catalytic surfactant, is particularly efficient as a reaction medium for epoxidation reactions that involve hydrogen peroxide. The reactions proceed at competitive rates with straightforward separation of the phases by centrifugation. Such catalytic "Pickering" emulsions combine the advantages of heterogeneous catalysis and biphasic catalysis without the drawbacks (e.g., catalyst leaching or separation time).

No abstract

No abstract