Photogenerated free radicals

Ma J, Jiang L (2001) Photogeneration of singlet oxygen (102) and free radicals (Sen -, 02 -) by tetra-brominated hypocrellin B derivative. Free Radic Res 35 767-777. 

Although no definitive study of scavengers in photoaffinity labeling experiments has yet been made, the present indications are that thiols are the most suitable reagents. It is expected that as strong nucleophiles they should react with most photogenerated species which are electrophiles as well as with free radicals which can abstract hydrogen atoms from the... 

Cationic cure mechanisms are an alternative approach to uv curing. This involves the photogeneration of ions, which initiate ionic polymerization. This process is not subject to oxygen inhibition, as are some of the free radical mechanisms. Cationic cure mechanisms generally also provide less shrinkage and improved adhesion. The disadvantages are that the photoinitiators are sensitive to moisture and other basic materials. The acidic species can also promote corrosion. As a result, the vast majority of uv formulations are acrylate-based and cure by a free radical mechanism. 

In this chapter, we will focus on photogeneration of free radicals of Pis and some elementary reactions of these radicals. This chapter does not aim to provide an exhaustive coverage of the subject but tried to outline the current status of this area, accomplishments and unresolved problems. For more specific information on one or another reaction or a process, the reader is referred to cited literature. 

Analysis of a CIDEP pattern with time-resolved ESR (TR ESR) spectra provides a solid conclusion to be made on the spin multiplicity of molecular precursors of polarized free radicals (a singlet or a triplet excited molecule) and the tracking of fast reactions of polarized radicals leading to secondary radicals. Thus, TR ESR is a convenient method in mechanistic photochemistry and free radical chemistry. Continuous wave TR ESR (CW TR ESR) devices are widely used for detection of photogenerated radicals. They usually consist of a pulsed ns laser with detection of transients by their ESR spectra with a X band ESR spectrometer in the direct detection mode (no hied modulation).Time-resolved Eourier transform ESR (FT ESR) has some advantages and drawbacks with respect to CW TR ESR. Rather sophisticated FT ESR devices have become available, and FT ESR studies become more common. 

Within the year a wide range of photoreactions in which an aromatic residue undergoes change in substitution has been published. As previously, the diversity of the various processes makes any classification of the reactions unrealistic, and so their order of presentation here is somewhat arbitrary. Aromatic photosubstitution reactions have been reviewed by Parkanyi although the treatment is not extensive, the processes of free radical, electrophilic, and nucleophilic photoinduced substitutions of arenes are well covered.Arene photoreactions initiated by electron transfer with electron donors or acceptors are the subject of a review by Pac and Sakurai. The requirements for the efficient photogeneration of the ion radicals are considered and the synthetic utility of the photoreactions, which include reduction, cyanation, and amination, is discussed. 

In the absence of triphenylphosphine, the photogenerated radicals undergo back electron transfer (A et) either within the solvent cage of the geminate radical pair or upon diffusional encounter (fcass) of the cage-escaped free radicals (Eq. 43). 

Given that a photogenerated free hole reacts with water or OH groups on arrival at the Ti02 surface to create, an adsorbed OH radical, a process which occurs on a small particle within a few tens of picoseconds, it was not possible under the conditions of this study to distinguish between a trapped hole (species 11 in reaction 5.15) and an adsorbed OH radical (species 1). 

S.L. Holder Sandvik, P. Bilski, J.D. Pakulski, C.F. Chignell, R.B. Coffin (2000). Photogeneration of singlet oxygen and free radicals in dissolved organic matter isolated from the Mississippi and Atchafalaya River plumes. Mar. Chem., 69, 139-152. 

Catalysis of cationic polymerization requires formation of Bronsted acid, while the product of interest in connection with free radical cures is an aryl radical. Early results [69] suggested that the photogeneration of acid, followed by an indicator technique, might be more efficient than formation of the radical products, estimated as Phi and Ph2S, for photolysis of Ph2I+ and... 

The first step in the sequence is the photogeneration of free radicals R and R (Reaction [6.20]). This is then followed by the interaction of the radicals with the monomer and the starting of the chain reaction. Reaction [6.21]. Reactions [6.20] and [6.21] together constitute the initiation process, while Reactions [6.22] and [6.23] represent the propagation of the radical function in the monomer, in other words, the growth of the polymer chain. Reaction [6.24] represents the many... 

The distinctive feature of NO is the capability of forming weak reversible complexes with some stable free radicals. The formation of such complexes is characteristic of tyrosyl radicals identified in a number of enzyme systems [16]. The interaction of tyrosyl radicals with NO has been shown to inhibit enzyme activity. The reaction between photogenerated tyrosyl radicals and NO is near diffusion-limited with the rate constant of 1-2 x 10 /M/s and gives 3-nitrosocyclohexadienone intermediate species. These species can reversibly regenerate the tyrosyl radical and NO or undergo rearrangement to form 3-nitrosotyrosine ... 

UV photografting of poly(methacrylic acid) (PMAA) on the surface of PHBHV fibers, and complexation of in-situ photogenerated silver nanoparticles (Ag NPs) (Fig. 6.10). The photografting process is conducted through a photoinduced free-radical process employing a ketone-based photoinitiator in aqueous medium [39]. 

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