Photogenerated reactive species

Photolabile Ligand Photogenerated Reactive Species o Scavenger ... 

In aerated waters, which are essentially all sunlight illuminated waters, the reactive species present in the highest concentration is O2 [1], with a concentration at saturation of about 2.5 x 10mol 1 Hence, O2 is the dominant scavenger of the photogenerated reactive species. The overall photoprocesses will thus be mediated by reactions with DOM itself and the reactions of the three primary photo transients with ground-state oxygen, Oj [13] ... 

CATEGORY I Nucleophilic Addition to Photogenerated Reactive Species... 

An important question is what happens in the absence of a suitable substrate that can trap the photogenerated intermediate ) Are reactive species such as [Mo(dppe)2] and [Mo(PPh2 ... 

We conclude that oxidative transformations of organic substrates can be readily understood as emanating from either photogenerated surface adsorbed radical cations or from radicals formed by activated oxygen radicals (surface oxides or adsorbed hydroxy, hydroperoxy or peroxy radicals). Photoelectrochemical methods not only generate the reactive species by interfacial electron transfer, but also control the subsequent activity of the surface adsorbed intermediate. 

While it has been assumed (Galardy et al., 1973) that the reactive species is the mr excited state this may not always be the case. For acetophenones the nature of the photogenerated triplet is highly solvent dependent (Turro, 1979). In more polar environments the species formed is the jot state which is less reactive than the mr state and in certain cases it may prove useful to substitute the molecule with an electron withdrawing group (e.g. CFr) on the aromatic ring to ensure formation of the desired T, (mt ) intermediate. 

Figure 18.2 Schematic diagram illustrating origin and fate of reactive species photogenerated at the surface of photosensitized Ti02 irradiated with visible or UV light...

C Roffey. Photogeneration of Reactive Species for Ultraviolet Curing. Wiley, Chichester, 1997. 

The formation of intermolecular cross-links, i.e. covalent bonds between different polymer chains, causes an increase in the average molar mass and eventually combines all of the macromolecules into a three-dimensional insoluble network. Cross-linking can be accomplished in various ways. Several methods rely on reactions of electronically excited pendant groups on the polymer chains, others on reactions of various kinds of reactive species in the ground state that are photogenerated in polymeric systems. Typical of the former reaction type are [2-f-2] cycloadditions that occur in the case of linear polymers bear-... 

From the cycloaddition reaction standpoint, the photo-generated reactive species suitable for both 1,3-dipolar cycloaddition reactions and Diels-Alder reactions have been studied in the literature. In this review, we will focus our discussion on four cycloaddition reactions (1) 1,3-dipolar cycloaddition between aUcenes and photogenerated nitrile imines (2) 1,3-dipolar cycloaddition between aUcenes and photogenerated nitrile ylides (3) photoinduced hetero Diels-Alder reactions and (4) photoinduced strain promoted azide-alkyne cycloaddition (Scheme 3). It is... 

Roffey, C., Radiation-curable adhesives, in Photogeneration of Reactive Species for UV-curing (pp. 631-641), John Wiley and Sons, New York, 1997. 

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

Only a limited number of reliable prediction tools are currently available for photoinduced toxicity. This is not surprising since establishing phototoxic potential is a complex task. Phototoxicity can be the consequence of various mechanisms such as photogeneration of reactive oxygen species, production of toxic photoproducts or sensitization of DNA damage by energy transfer. In addition, so far, there are no available universal descriptors (indicators) to predict the photodynamic potency of chemicals. 

In early works, the reactivity associated widi TiO2 photocatalysis was assigned to free OH radicals, but more recently die oxidative species has been identified as bound HO radicals and photogenerated holes. Reaction of the substrate with these species would require either die direct adsorption of organic compounds to the semiconductor interface or Fickian diffusion of the substrate to the semiconductor surface under depletion conditions. However, these two pathways imply different mechanisms, and in some cases different reaction products or intermediates. 

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