Oxidative quenching

The correlation of oxidative quenching rates with redox potentials for a series of nitroaromatic compounds in acetonitrile solvent is shown in Table 5.4. The photoreaction involves the formation of the complex Ru(bpy and the nitroaromatic anion ArNOI  

For these bipyridinium ions (byrn ) there is now no electrostatic attraction between the like-charged quenching products Ru(bpy) and bpym  

MeNC5H4CH=CHC5H4NMe that has a triplet energy of 210 kJ mol , a significant contribution to the quenching reaction comes from an energy transfer pathway. 

In addition to organic compounds, the triplet excited state of Ru(bpy)3 undergoes oxidative quenching with inorganic compounds and ions. Since the reduction potential for the Ru(bpy)3 couple is estimated as -0.86 V versus NHE in aqueous solution, the excited state has sufficient energy to reduce any ion that has a more positive value than this. Simple aqueous ions such as Fe, Cu, and 

Eu act as oxidative quenchers of Ru(bpy)3, as do tris chelate complexes such as Ru(bpy)3 and Cr(bpy)3. A compilation of the rate constants for many such electron transfer reactions should be consulted for individual examples. One of the earliest examples of the oxidative quenching of Ru(bpy)3 by an inorganic compound is the photoreduction of the kinetically inert cobalt(III) complex, CoC1(NH3)5 (Ref. 83)  

Based on the behavior of 17 Co(III) complexes including several ammines in the luminescence quenching of Ru(bpy)32+ and Os(phen)32+, Sandrini et a/.[35] proposed electron transfer quenching. Based on the quantum yields for the production of Co(Il), Haim et a/.[36] concluded that the dominant quenching for 00(00)3 + is energy transfer and parallel electron and energy transfer in the case of Co(NH3)6 +.  

Oxidative quenching of Ru(bpy)32+ by nitroaromatic compounds such as dinitrobenzene illustrates typical behaviour expected for cases of pure electron transfer quenching [37]  

Oxidative quenching by electron acceptors such as bipyridinium salts (also known as viologens), polypyridine complexes of Rh(III) and Co(IIl) and cage complexes have been widely studied [38-40]  

These quenchers are of special interest in connection with sensitized photoreduction of water to molecular hydrogen. The reduced form of these quenchers (A ) have enough reducing power to reduce water to molecular hydrogen and in fact they do so efficiendy in the presence of suitable redox catalysts  

Caged Amine complexes of Co(III) The reduced form of Co(III) complexes such as Co(III)(NH3)5X2+ are substitutionally labile. A number of caged cobalt(II) compounds have been found to be fairly inert to substitution. The sar, sep or the sulfur-containing capten cage complexes have [Co(III)/Co(II)] potentials -0.2V vs NHE are and efficient quenchers of Ru(bpy)3 [41-43]. Some of them have been found use as relays for water photoreduction. 

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Allyl aryl ethers undergo accelerated Claisen and [1,3] rearrangements in the presence of a mixture of trialkylalanes and water or aluminoxanes. The addition of stoichiometric quantities of water accelerates both the trimethylaluminum-mediated aromatic Claisen reaction and the chiral zirconocene-catalyzed asymmetric carboalumination of terminal alkenes. These two reactions occur in tandem and, after oxidative quenching of the intermediate trialkylalane, result in the selective formation of two new C-C bonds and one C-0 bond (Eq. 12.70).153 Antibodies have also been developed to catalyze Claisen154 and oxy-Cope155 rearrangements. 

A regenerative photogalvanic cell with oxidative quenching (Fig. 5.58b) is based, for example, on the Fe3+-Ru(bpy)2+ system. In contrast to the iron-thionine cell, the homogeneous photoredox process takes place near the (optically transparent) cathode. The photoexcited Ru(bpy)2+ ion reduces Fe3+ and the formed Ru(bpy)3+ and Fe2+ are converted at the opposite electrodes to the initial state. 

In a second type of experiment, oxidative quenching is achieved by use of [Co(NH3)5C1]2+ as the quencher. In the one example reported the ethyl-phenyl derivative of the substrate was used, and the Rum so generated oxidized the heme with k = 6x 103 s l. Prom spectroscopic studies it is believed that the heme is oxidized to a porphyrin n-cation radical and has an axial water ligand. One might anticipate the generation of other oxidized states with the use of other substrate derivatives. 

Ru(bpy)3, while the powerful oxidant Rufbpy) is produced by oxidative quenching. 

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+ ... 

These features are consistent with the oxidative quenching mechanism of Ru(bpy)3+ ... 

A number of furoquinoline alkaloids are available by taking advantage of in-between metalation of 2,4-dimethoxy quinoline derivatives, as established in model studies (Scheme 103). To illustrate, the trimethoxyquino-line 571, upon metalation and ethylene oxide quench, afforded the carbinol 572 which, upon mild hydrolysis, furnished the alkaloid dihydro-y-fagarine (573) together with the quinolone 574 (Scheme 172) (71T1351). 

In such reactions, both the reductive and oxidative quenching pathways are possible for the photocatalytic reactions between Dred and Aox. In general, one reaction pathway, either reductive or oxidative quenching, becomes dominant when the reactant pair Dox and Aox is fixed. However, acid catalysis can alter the operating quenching pathway by accelerating one pathway and/or retarding the other pathway (vide infra). 

Photochemical hydrogen production via oxidative quenching of [Ru(bipy)3]2+ by... 

Hydrogen production via oxidative quenching of ruthenium(H) complexes containing chemically... 

Despite the irreproducibility of Whitten s results, his and Sutin s observations have spawned enormous interest in [Ru(bipy)3]2+ as a sensitizer since it has Amax at 452 nm — close to the emission maximum of the sun. Furthermore, its photochemically excited state has a long lifetime and is both strongly oxidizing and reducing. Finally, [Ru(bipy)3]3+, formed by oxidative quenching, is sufficiently oxidizing ( °= 1.26 V) to oxidize water.116... 

The mechanism of this photochemical reaction is as shown in equations (91)-(94) and relies upon oxidative quenching of [Ru(bipy)3]2+ by the cobalt complex to give [Ru(bipy)3]3+ and highly labile cobalt(II) amine complexes which decompose at a rate which competes with back... 

A mechanism that accounts for the oxidative addition of halocarbons has been proposed for the two d8-d8 dimers (Figure 4) (23). The mechanism involves the oxidative quenching of the triplet excited state of the metal dimer as the primary photoprocess. This gives a radical anion species that dissociates a halide, thereby producing an organic radical. The dissociated halide adds to the partially oxidized metal dimer to form a mixed valence Ir -Ir -X or Pt 1-Pt -X intermediate. This intermediate reacts further with the remaining organic radical (presumably in a second, thermal electron transfer step) to form the final d2-d2 dihalide dimer. 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

Deactivation of an excited species can proceed through radiation or radiationless decays, energy transfer quenching, or electron transfer routes. The operation of artificial photosynthetic devices relies mainly on electron-transfer (ET) processes induced by an excited species [16, 17]. Two general mechanisms can be involved in the ET process of an excited species Reductive ET quenching of an excited species, S, by an electron donor D, results in the redox products S- and D+ (Fig. 4 a). Alternatively, oxidative quenching of the excited species by an electron acceptor, A, can occur (Fig. 4b), resulting in the electron transfer products S+ and A-. 

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