Photostimulated electron

The cleavage of C—S bonds in C—SO2R anion radicals plays an important role in SrnI tyP processes ". Kornblum and coworkers described a photostimulated electron transfer chain substitution at a saturated carbon where the leaving group is PhSOj ... 

Using alkene radical cations generated under photostimulated electron-transfer conditions, Arnold and coworkers showed that the addition of an-... 

Kochi s group obtained similar results for the reaction between ArH and C(N02)4 (Sankararaman et al. 1987, Masnovi et al. 1985,1989). After photostimulated electron transfer, the related triad was formed... 

Fig. 5. Energy above the valence band of levels reported in the literature for GaP. Arrangement and notations are the same as in Fig. 4. Abbreviations for experimental methods not defined in Fig. 4. are temperature dependence of resistivity (RT), temperature dependence of minority-carrier lifetime (LT), Hall effect (H), and photostimulated electron paramagnetic resonance (PEPR).

Fig. 15 Upper plot Photostimulated electron transfer activation induced by irradiation of DHA 19a in acetonitrile (c = 1(T3 mol dirT3) at working potential -1050 mV vs. Fc/Fc+.

Fig. 16 Schematic representation of electronic changes due to photostimulated electron transfer.

J. O M. Bockris and M. Schuaib, Trans. Adv. Electrochem. Sci. Tech. 13 4 (1978). Photostimulated electron transfer to and from photosystem 1 and photosystem 2 from ions in aqueous solutions. (First evidence for a photoelectrochemical mechanism in photosynthesis.)... 

The mechanism of the reduction process (formation of benzene) in the photostimulated reaction of benzeneselenate ion with iodobenzene was found to be photostimulated electron transfer from the nucleophile to the substitution product (equations 187-189)705. 

Shkrob lA, Sauer MC. (2005) Photostimulated electron detrapping and two-state model for electron transport in nonpolar liquids. / Chem Phys 122 134503. 

A few further examples of nucleophilic displacement of halogen from halogenothiophenes by a radical chain SrN, mechanism have been reported. The photostimulated reaction of 2-halo or 3-halothiophene with excess of tetrabutylammonium benzenethiolate in MeCN takes place by an SrN mechanism <87JOC5382>. The 2- or 3-(phenylthio)thiophene is formed in modest yields. The initiation and propagation steps are shown in Scheme 119. The first step may be the photostimulated electron transfer from thiolate anion to the halothiophene (ThX). The reaction does not take place in the dark. 

Photonucleophilic aromatic substitution reactions of phenyl selenide and telluride with haloarenes have also been proven to involve the S jlAr mechanism, with the formation of anion radical intermediates. Another photonucleophihc substitution, cyanomethylation, proves the presence of radical cations in the reaction mechanism. Liu and Weiss have reported that hydroxy and cyano substitution competes with photo substitution of fluorinated anisoles in aqueous solutions, where cation and anion radical intermediates have been shown to be the key factors for the nucleophilic substitution type. Rossi et al. have proposed the S j lAr mechanism for photonucleophihc substitution of carbanions and naphthox-ides to halo anisoles and l-iodonaphthalene. > An anion radical intermediate photonucleophilic substitution mechanism has been shown for the reactions of triphenyl(methyl)stannyl anion with halo arenes in liquid ammonia. Trimethylstannyl anion has been found to be more reactive than triphenylstannyl anion in the photostimulated electron- transfer initiation step. 

The flexible large area screen with photostimulable phosphor called the imaging plate, which temporarily stores the image induced by the ionizing radiation, e g. by X-rays, electrons or other charged particles ... 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

I. Willner and B. Willner, Electronic transduction of photostimulated binding interactions at photoisomerizable monolayer electrodes novel approaches for optobioelectronic systems and reversible immunosensor devices. Biotechnol. Prog. 15, 991-1002 (1999). 

Radical inhibitors and electron acceptors prevent the formation of the snbstituted product. Oxygen drastically retards the snbstitntion bnt facilitates the formation of p-nitrocumyl peroxide. Photoirradiation somewhat accelerates the reaction. Conseqnently, the reaction is of the ion-radical type. The nucleophile (PhS ) is sufficiently active as an electron donor and a radical interceptor. The reaction does not need photostimulation. 

The arylation of organyl tellurolates, restricted first to aryl halides activated by electron-withdrawing groups,2 or requiring special conditions such as heating in HMPA in the presence of Cul, or photostimulation in liquid ammonia,ijggjj achieved successfully with non-activated aryl halides under normal conditions. ... 

In Chap. 2 and Chap. 3, Sect. 1.2, the appropriate boundary and initial conditions for reactions between statistically independent pairs of reactants were formulated to model a homogeneous reaction. In these cases, if there is no inter-reactant force, all that is required is one or other reactant to be in vast excess on the other. Since the excited donor or the electron donor has to be produced in situ by photostimulation or high-energy radiation, it is natural to choose [D ] < [A], though there are exceptions. Locating the donor at the origin in a sea of acceptor molecules distributed randomly leads to the initial condition, as before... 

This one-dimensional escape probability can be compared with the three-dimensional expression (168). The observed escape probability for a distribution of initial distances between electrode and electron is the average of that distribution over all escape probabilities given by eqn. (178). Under steady-state photostimulation of the cathode, an electric current flows between cathode and anode and the current is proportional to the escape probability of these electrons from their image potential. As in the three-dimensional (Onsager) case, the field dependence of the electric current may be used to estimate the range of photoejected electrons from the cathode. However, these photoejected electrons have... 

An example of transfer of electron from nucleophile to substrate is seen in the formation of the radical anions (observable by ESR) of 5-halo-2A/,3W-benzotf>]thiophene-2,3-diones on treatment with nucleophiles.20 It has been proposed in some cases, that this single-electron transfer step takes place through a charge transfer complex between the nucleophile and the aromatic substrate.21-22 Some reactions occur spontaneously, i.e. without any catalysts or reagents other than the substrate and the nucleophile, but the initiation process is usually, although not invariably, photostimulated (near-ultraviolet radiation, 300-... 

The number of solvents that have been used in SrnI reactions is somewhat limited in scope, but this causes no practical difficulties. Characteristics that are required of a solvent for use in SrnI reactions are that it should dissolve both the organic substrate and the ionic alkali metal salt (M+Nu ), not have hydrogen atoms that can be readily abstracted by aryl radicals (c/. equation 13), not have protons which can be ionized by the bases (e.g. Nth- or Bu O" ions), or the basic nucleophiles (Nu ) and radical ions (RX -or RNu- ) involved in the reaction, and not undergo electron transfer reactions with the various intermediates in the reaction. In addition to these characteristics, the solvent should not absorb significantly in the wavelength range normally used in photostimulated processes (300-400 nm), should not react with solvated electrons and/or alkali metals in reactions stimulated by these species, and should not undergo reduction at the potentials employed in electrochemically promoted reactions, but should be sufficiently polar to facilitate electron transfer processes. 

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