Photosensitized reaction definition

Schenck and Steinmetz [26] have found that the addition of maleic anhydride to benzene can be photosensitized by benzophenone. The photosensitized reaction also works with toluene, o-xylene, and chlorobenzene. Hardham and Hammond [27,28] have also studied the photosensitized addition and they obtained definitive evidence that this reaction proceeds via the triplet state. They also proposed an ortho adduct as intermediate but were unable to isolate it. Bryce-Smith and Vickery [29] found that phenanthrene reacts with maleic anhydride at the 9,10-positions to give an adduct that does not further react with a molecule of maleic anhydride. This reaction can also be sensitized by benzophenone. 

The nonequivalence of the rates of photosensitized reactions in heterogeneous nanophases of glassy polymers is proved in experiments with naphthalene phosphorescence decay. For example, it is shown [13] that in aerated PMMA films fluorescence of singlet-excited naphthalene molecules N can be decayed by tinuvin P, but this does not affect the rate of naphthalene dissociation. The latter is consumed in the process, the rate of which is not defined by the concentration of particles responsible for fluorescence. Under such conditions, according to definition by the authors [13], primary chemical acts are inevitable. However, in the absence of oxygen tinuvin P slows the photochemical process down in accordance with a decrease of singlet-excited naphthalene molecule concentration. 

Ru(CN)jNO reactions with OH , SH and SOj" resemble those of the nitroprusside ion, with attack at the coordinated nitrosyl to give analogous transients and similar second-order rate constants. Ruthenium(II) complexes of the general type Ru(N2), Nj = biden-tate hgands, are important reactants. The relative inertness of Ru(NH3) + and Ru(diimine)f+ towards substitution makes these complexes definite, although weak, outer-sphere reductants (Tables 5.4, 5.5, 5.6 and 5.1). Ruthenium(ll) complexes of the general type Ru(diimine)f +, and particularly the complex Ru(bpy)j+, have unique excited state properties. They can be used as photosensitizers in the photochemical conversion of solar energy. Scheme 8.1 ... 

Substrates containing an electron-rich double bond, such as enol ethers and enol acetates, are easily oxidized by means of PET to electron-deficient aromatic compounds, such as dicyanoanthracene (DCA) or dicyanonaphthalene (DCN), which act as photosensitizers. Cyclization reactions of the initially formed silyloxy radical cation in cyclic silyl enol ethers tethered to an olefinic or an electron-rich aromatic ring, can produce bicyclic and tricyclic ketones with definite stereochemistry (Scheme 9.14) [20, 21]. 

Figure . Proposed reaction scheme for BTD-photosensitized reduction of AQDS by MES in CTAB micelles (adapted from [73]). BTD is either BTDB or BTDE (see text for definitions).

Sensitization was another word requiring definition and correct application. Thus, a stable molecule could be made photosensitive by the additirai of a small amount of another molecule, but this might involve different phenomena, as illustrated by Berthoud [62]. These ranged from the photochemical formation of a reactive compound to that of a catalyst. Atoms could be generated and transfer their energy to other species, as demonstrated for atoms of mercury or of oxygen [63]. Noyes pointed out to the reaction of molecules, e.g., NO2, at an irradiated mercury surface, and on the differences and similarity between photochemistry and photo-ionization [64]. 

A substance can be thought to be a catalyst when it accelerates a chemical reaction without being consumed as a reactant that is to say, it appears in the rate expression describing a thermal reaction without appearing in the stoichiometric equation [49], A catalyst is a compound that lowers the free activation enthalpy of the reaction. Then, photocatalysis can be defined as the acceleration of a photoreaction by the presence of a catalyst [38, pp. 362-375], This definition, as pointed out in [29, pp. 1-8], includes photosensitization, a process by which a photochemical alteration occurs in one molecular entity as a result of initial absorption of radiation by another molecular entity called the photosensitizer [13], but it excludes the photoacceleration of a stoichiometric thermal reaction irrespective of whether it occurs in homogeneous solution or at the surface of an illuminated electrode. Otherwise, any photoreaction would be catalytic [29, pp. 1-8]. Depending on the specific photoreaction, the catalyst may accelerate the photoreaction by interaction with the substrate in its ground or excited state and/or with a primary photoproduct. 

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