Electron transfer photosensitized

Photosensitized electron transfer cyclization of compound 173 to pyridazinooxadiazine 160 by UV irradiation in the presence of 1,4-dicyanonaphthalene (DCN) and methyl viologen (MV) was carried out. The cyclized product was obtained as a diastereomeric mixture with a 7 1 diastereomeric ratio m-stereochemistry between H-2 and H-9 was established for the major product (Equation 26) <1997TL9073>. 

The photosensitized electron transfer by 9,10-dicyanoanthracene (DCA) has been shown to initiate the addition of the a-silyl amine 44 to 4,4-dimethylcyclohexenone47 (equation 14). Intramolecular addition of a-silyl amine 45 was also shown to be feasible45,46 (equation 15). The primary step is electron transfer to give the aminium... 

D Souza, F. Ito, 0., Photosensitized electron transfer processes of nanocarbons applicable to solar cells. Chem. Soc. Rev. 2012,41 86-96. 

The kinetically controlled Cope rearrangement of 2,5-bis(4-methoxyphen-yl)hexa-l,5-dienes induced by photosensitized electron transfer to DCA was examined by Miyashi and co-workers [101-103]. Remarkable in this context was the temperature-dependent change of the photostationary ratio of this rearrangement, yielding the thermodynamically less stable compound at — 80°C in 96%. A radical cation-cyclization diradical cleavage mechanism (RCCY-DRCL) is... 

Figure 19.1 Mechanisms involved in sunlight-induced phototoxicity of drugs. Type 1 photosensitization (electron transfer) mainly generates singlet oxygen Oj", whereas type 2 reaction (energy transfer) leads to adduct formation or singlet oxygen...

Surfactant vesicles constitute a very flexible medium for the support of semiconductors. Semiconductor particles can be localized at the outer, the inner, or at both surfaces of single-bilayer vesicles (Fig. 102). Each of these arrangements has certain advantages. Semiconductor particles on outer vesicle surfaces are more accessible to reagents and can, therefore, undergo photosensitized electron transfer more rapidly. Smaller and more monodispersed CdS particles can be prepared and maintained for longer periods of time in the interior of vesicles than in any other arrangement... 

Another mode for catalyzed cycloaddition involves the generation of radical cations from electron-rich alkenes with single-electron oxidants such as tris(4-bromophenyl)amminium hexachloroantimonate (TBAH). An equivalent reaction involves the photosensitized electron transfer (PET) process (see Section 1.3.2.3.). These processes have been recently reviewed,9 and are limited to electron-rich alkenes capable of producing radical cations. Furthermore, some of the cyclobutanes themselves undergo secondary isomerization under the oxidative conditions, e.g. formation of 31-35.10-12... 

The Mg2+ ion also shows an acceleration effect on the photoreduction of dimethylfumarate and some other related olefins by 1-benzyl-1,4-dihydro-nicotinamide (BNAH) used as an NADH model compound via photosensitized electron transfer from BNAH to [Ru(bpy)3]2+ (Scheme 16) [160]. In this case, however, the complex formation of BNAH with Mg2+ results in an increase in the one-electron oxidation potential of BNAH [87]... 

Cobalt(III) cage complexes can also perform as electron transfer agents in the photoreduction of water.180181 Because of the kinetic inertness of the encapsulated cobalt(II) ion, the cobalt(II)/co-balt(III) redox couple can be repeatedly cycled without decomposition. Thus these complexes are potentially, useful electron transfer agents, e.g, in the photochemical reduction of water, in energy transfer and as relays in photosensitized electron transfer reactions.180,181 The problem of the short excited-state lifetimes of these complexes can be circumvented by the formation of Co(sep)3+ ion pairs, so that the complexes can be used as photosensitizers for cyclic redox processes.182 183... 

Photosensitized Electron-Transfer Reactions in Organized Systems... 

Charged colloids and water-in-oil microemulsions provide organized environments that control photosensitized electron transfer reactions. Effective charge separation of the primary encounter cage complex, and subsequent stabilization of the photoproducts against back electron transfer reactions is achieved by means of electrostatic and hydrophobic interactions of the photoproducts and the organized media. 

The photosensitized electron transfer process involves two successive steps (eq. 5) In the primary event an encounter cage complex of the photoproducts is formed. This can either recombine to yield the original reactants or dissociate into separated photoproducts. The separated photoproducts can then recombine by a diffusional back electron transfer reaction to form the original reactants. We have introduced two conceptional approaches as a means for assisting the separation of the encounter cage complex and for the stabilization... 

Figure 1. Conversion of light energy to chemical potential by means of photosensitized electron transfer reactions.

Figure 3. Functions of the Si02 colloid in controlling the photosensitized electron transfer process and -evolution.

Different aspects involved in the design of artificial photosynthetic systems have been discussed. Charged colloids and water-oil microemulsions provide effective organized media for controlling photosensitized electron transfer processes. Development of catalysts capable of utilizing the photoproducts in chemical routes, particularly in multi-electron fixation processes is of major... 

Adam W, Arnold MA, Nau WM, Pischel U, Saha-Moller CR (2001b) Structure-dependent reactivity of oxyfunctionalized acetophenones in the photooxidation of DNA base oxidation and strand breaks through photolytical radical formation (spin trapping, EPR spectroscopy, transient kinetics) versus photosensitization (electron transfer, hydrogen-atom-abstraction). Nucleic Acids... 

A recently reported regioselective photoreduction of benzoates by photosensitized electron transfer reaction was applied to nucleosides [85]. In presence of jV-methyl-carbazole as the electron donor sensitizer and in an isopropanol, water solution, w-trifluoromethylbenzoates of adenosine 83 or benzoates of uridine 84 give deoxygenated products in good yields (73 %). 

As a first example, the photochemical synthesis of substituted 1,2-dihydro-[60]fullerenes will be discussed. These compounds can be synthesized by various photochemical reaction pathways. In the first one the radical cation Qo is involved in the reaction. In 1995, Schuster et al. reported the formation of C6o radical cations by photosensitized electron transfer that were trapped by alcohols and hydrocarbons to yield alkoxy or alkyl substituted fullerene monoadducts as major products [211], Whereas Foote et al. used N-methylacridinium hexafluorophos-phate NMA+ as a sensitizer and biphenyl as a cosensitizer [167], Schuster et al. used 1,4-dicyanoanthracene (DCA) as a sensitizer for the generation of C 6o- The... 

Photosensitized electron transfer reactions conducted in the presence of molecular oxygen occasionally yield oxygenated products. The mechanism proposed to account for many of these reactions [145-147] is initiated by electron transfer to the photo-excited acceptor. Subsequently, a secondary electron transfer from the acceptor anion to oxygen forms a superoxide anion, which couples with the donor radical cation. The key step, Eq. (18), is supported by spectroscopic evidence. The absorption [148] and ESR spectra [146] of trans-stilbene radical cation and 9-cyanophenanthrene radical anion have been observed upon optical irradiation and the anion spectrum was found to decay rapidly in the presence of oxygen. 

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