Photoredox system

Photoredox systems involving carbonyl compounds and amines are used in many applications. Carbonyl compounds employed include benzophenone and derivatives, a-diketones [e.g. benzil, cainphoroquinone (85),2W 291 9,10-phenanthrene quinone], and xanthone and coumarin derivatives. The amines are tertiary and must have a-hydrogens [e.g. N,A7-dimethylani 1 ine, Michler s ketone (86)]. The radicals formed are an a-aminoalkyl radical and a ketyl radical. 

Metal complex-organic halide redox initiation is the basis of ATRP. Further discussion of systems in this context will be found in Section 9.4, The kinetics and mechanism of redox and photoredox systems involving transition metal complexes in conventional radical polymerization have been reviewed by Bam ford. 

The metal complexes most commonly used in these photoredox systems are manganese and rhenium carbonyls. The proposed mechanism of the photoredox... 

In heterogeneous photoredox systems also a surface complex may act as the chromophore. This means that in this case not a bimolecular but a unimolecular photoredox reaction takes place, since electron transfer occurs within the lightabsorbing species, i.e. through a ligand-to-metal charge-transfer transition within the surface complex. This has been suggested for instance for the photochemical reductive dissolution of iron(III)(hydr)oxides (Waite and Morel, 1984 Siffert and Sulzberger, 1991). For continuous irradiation the quantum yield is then ... 

Figure 7.15 Photoredox system for the reduction of water to hydrogen based on...

Figure 7.15 shows a photoredox system capable of producing hydrogen from water. Photon absorption by Ru(bpy)2+ results in formation of Ru(bpy)3+, which undergoes oxidative quenching by methylviologen (MV2+). 

Figure 7.16 Photoredox system for the oxidation of water to oxygen based on oxidative quenching of excited Ru(bpy)2+ by the sacrificial Co(NH3)5C12+ Reprinted from C. Kutal, Photochemical Conversion and Storage of Solar Energy , Journal of Chemical Education, Volume 60 (10), 1983. American Chemical Society...

A photoredox system for the production of oxygen from water using Ru(bpy)3+ as a sensitiser is shown in Figure 7.16. This system involves oxidative quenching of excited Rufbpy) by the Co(III) complex Co(NH3)5C12+ ... 

When a photochemical process in solution gives a photoresponse at the electrode, the system can form a photogalvanic cell. The photoinduced redox reaction is typical for photogalvanic cells. The most well known photoredox system is thionine (TH+ 23) and ferrous ion88). The excited TH+ is reduced by Fe2+ to give TH and Fe3+ (Eq. (25)). 

A photoredox system of Fe2+ and polycation polymer pendant TH+ (26) induced a much higher photopotential than the monomeric system 92). A thin layer cell... 

Classes Of Sensitization, a. Photoreducible Dye Sensitization. In 1954 Oster (7) reported the first documentation of a dye-sensitized photoredox system. During the course of his work, Oster identified several classes of effective dyes, termed by him "photoreducible." These included examples of the classes of acridine, xanthine, and thiazine dyes. Figure 3 illustrates an example of each class, chosen in such a manner that the entire visible spectrum is covered by their absorption spectra. In Oster s work, identification of suitable activators (reduc-tants) to use in conjunction with the dyes was empirically determined. 

In this context, it maybe of interest that hydroxymethylation of l,3Me2Ura (and its derivatives) by the Eu(III)/Eu(II) photoredox system in MeOH affords 5,6-dihydro-l,3-dimethyl-6-hydroxymethyluracil in close to 100% yield (Ishida et al. 1985). Apparently, the one-electron-reduced l,3Me2Ura, the 0(4)-protonated l,3Me2Ura-4-yl radical, does not undergo sufficiently rapid isomerization into the thermodynamically more stable l,3-dimethyl-5-hydro-uracil-5-yl radical under these conditions. Otherwise, 5,6-dihydro-l,3-dimethyl-5-hydroxymethy-luracil would have been observed. 

A comparison has been made between the degree of asymmetry of the quenching reaction and of the photoreaction using the well-known photoredox system in which A-(-)-[Ru(bipy)3]Cl2 and A-(-l-)-[Ru(bipy)3]Cl2 serve as donors and l-methyl-l -[(3S)-(-)-[methylpinanyl)]-4,4 -bipyridinium dichloride serves as acceptor. Much of the asymmetry found in the quenching step is lost in the competing back electron transfer and ion pair dissociation steps.Solution medium control of the photoredox yield in the system [Ru(bipy)3f -methyl viologen-EDTA has been examined and (MV ) was found to be where tJct is the efficiency of release of redox products from... 

One particular photoredox system, that involving Thionine (245 X = S) and ferrous ions, originally subjected to investigation by the flash-photolysis technique by Hatchard and Parker, is of current interest for photo-galvanic systems. ... 

M. Sykora and J.R. Kincaid, Photochemical Energy Storage in a Spatially Organized Zeolite-based Photoredox System. Nature London), 1997, 387, 162-164. 

Addition Reactions.- The photoelectron transfer process of the iminium salt (38) with the 3-butenoate anion results in the formation of the allylated product (39). The reaction involves decarboxylation of the 3-butenoate followed by a radical coupling reaction. The photoaddition of halogenated alkenes to the tetraraza phenanthrene (40) yields products (41) of (2+2)-addition. The Eu(III)/Eu(II) photoredox system has been studied with regards to its reactivity toweu ds a-methylstyrene. Irradiation of the system at > 280 nm in methanol yielded the products (42) and(43). ... 

The author will first review some general concepts of photoredox reactions related to homogeneous and to heterogeneous photoredox systems and will then discuss the possible roles of surface-bound ligands and of surface compounds... 

A homogeneous, bimolecular photoredox system, which has been extensively investigated, involves ruthenium(II) trisbipyridyl [Ru(bpy)2 + ] as photocatalyst and methylviologen (MV2+) as electron acceptor, where triethanolamine (TEOA) or ethylenediamintetraacetate (EDTA) are used as sacrificial electron donors for the reduction of Ru(bpy)2 + (Tazuke et al, 1987) ... 

An application of photoredox systems for fuel production from light encounters three major problems summarized in Fig. 3.2. First, sensitizer/relay couples have to be found which are suitable from the viewpoint of light absorption and redox potentials and undergo no chemical side reactions in the oxidation state of interest. The sensitizer... 

The majority of the recently investigated photoredox systems involve principally three components a photosensitizer, an electron acceptor (or relay) and an electron... 

The photoredox system Ru(bipy)2+/MV2+ (with EDTA or triethanolamine (TEOA) as the electron donor and Pt-catalysts) has been thoroughly investigated307-317 as a system leading to light-induced H2 evolution from water. Upon visible light excitation Ru(bipy)2+ reduces MV2+. 

More efficient systems can be constructed by having the two components of the photoredox system in the one molecule." ... 

Measurement of a steady state UV-vis absorption spectrum when the back electron transfer is slow, so that the equilibrium under irradiation shifts to the product side. Such a reaction is called a photoredox system. There are not many examples of this type. When the back electron transfer is rapid, the presence of some sacrificial donor or acceptor is effective in shifting the reaction to the product side and therefore to monitor the reaction. 

Another example of a photoredox system in an organized molecular assembly was reported on hydrogen evolution in LB membrane where porphyrin-violo-gen-linked molecules were assembled on a glass plate with platinum particles [391] (Fig. 20). Photoinduced hydrogen evolution upon visible light irradiation of Zn porphyrin mediated by natural cytochrome Cj and H2ase have also been reported [392]. 

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