Chemical sensitized photoreduction

The second reaction above was verified by thermal production of a-hydroxy-radicals 117>. There are now many reports of chemical sensitization by ketones, wherein a a-hydroxyradical photoproduct reduces a ground state of another compound. a-Diketones 118>, a,/ -unsaturated ketones 119>, and imines 120> all undergo such chemically sensitized photoreductions. 

Recent work on the photochemistry of ketimines has shown that they do not undergo reduction unless ketones are present. Thus chemical sensitization is entirely responsible for the photoreduction of benzophenone methylimine (22)105 while intramolecular chemical sensitization has been suggested as the mechanism for reduction of the acylketimine (23).119 In related work,... 

The use of ketones with (ir, n) triplet states to sensitize photoreductions will be attended by chemical sensitization for example, with benzophenone in isopropyl alcohol and 0.5M acceptor, 1% of the sensitizer triplet will still abstract from the solvent even if the acceptor quenches at the diffusion controlled rate. Failure to determine that the quantum yield of reduction was greater than 0.01 might lead to the conclusion that triplet sensitization was occurring. 

A novel method for generating semidione radicals has recently been reported by Monroe, Weiner, and Hammond 178) who found that the quantum yield for photoreduction of camphorquinone in 2-propanol was markedly enhanced when benzophenone was added and the solution irradiated at wavelength 3660 A where most of the light was absorbed by benzophenone. Instead of benzpinacol formation, the dione underwent photoreduction. Similar enhancement was not observed with jw-methoxy-acetophenone which does not abstract hydrogen from 2-propanol. The conclusion was that the ketyl radical, formed in the efficient H-abstraction reaction of benzophenone, transferred a hydrogen atom to camphorquinone to generate the semidione radical. It was suggested that this phenomenon be called "chemical sensitization . 

Photoreduction of the herbicide paraquat dichloride in aqueous propan-2-ol is more efficient in the presence of a sensitizer such as benzophenone than on direct irradiation.84 Hyde and Ledwith84 propose that the paraquat cation radical is formed by electron transfer from ketyl radicals, in turn produced during the conventional photoreduction of the sensitizer ketone. The suggested mechanism is given in reactions (23)—(25), where PQ2+ is the paraquat dication. The reduction process therefore involves chemical sensitization, rather than electronic energy transfer. 

It seems clear that benzophenone is here functioning as a chemical sensitizer for photoreduction of paraquat, the steps in the mechanism being set out in (189)- (191). 

The mechanism of the photoreduction of azoxybenzene to azobenzene in the presence of benzophenone was rationalized by Monroe and Wamser to involve the transfer of a proton from the ketyl radical, generated from excited benzophenone via hydrogen abstraction from the solvent to azoxybenzene, followed by dehydration (chemical sensitization). ... 

FIG. 11 General mechanism for the heterogeneous photoreduction of a species Q located in the organic phase by the water-soluble sensitizer S. The electron-transfer step is in competition with the decay of the excited state, while a second competition involved the separation of the geminate ion-pair and back electron transfer. The latter process can be further affected by the presence of a redox couple able to regenerate the initial ground of the dye. This process is commonly referred to as supersensitization. (Reprinted with permission from Ref. 166. Copyright 1999 American Chemical Society.)... 

Topics which have formed the subjects of reviews this year include photosubstitution reactions of transition-metal complexes, redox photochemistry of mononuclear and polynuclear" complexes in solution, excited-state electron transfer processes, transition-metal complexes as mediators in photochemical and chemiluminescence reactions, lanthanide ion luminescence in coordination chemistry, inorganic photosensitive materials," and photocatalytic systems using light-sensitive co-ordination compounds. Reviews have also appeared on the photoreduction of water.Finally, various aspects of inorganic photochemistry have been reviewed in a single issue of the Journal of Chemical Education. 

In marked contrast to the direct photoreduction of PQ2+ by aqueous isopropyl alcohol, where a limiting yield of PQ + is formed, the same process sensitized by benzophenone proceeds efficiently with complete conversion of PQ2+ to PQ +. Chemical analysis shows that very little benzophenone is consumed during this part of the reaction, the quantum yield for formation of PQ + is 0-6, and the oxidation product is acetone. Under identical conditions the quantum yield for photoreduction of benzophenone in the absence of PQ2+was 1-4. 

Topics which have formed the subjects of reviews this year include excited state chemistry within zeolites, photoredox reactions in organic synthesis, selectivity control in one-electron reduction, the photochemistry of fullerenes, photochemical P-450 oxygenation of cyclohexene with water sensitized by dihydroxy-coordinated (tetraphenylporphyrinato)antimony(V) hexafluorophosphate, bio-mimetic radical polycyclisations of isoprenoid polyalkenes initiated by photo-induced electron transfer, photoinduced electron transfer involving C o/CjoJ comparisons between the photoinduced electron transfer reactions of 50 and aromatic carbonyl compounds, recent advances in the chemistry of pyrrolidino-fullerenes, ° photoinduced electron transfer in donor-linked fullerenes," supra-molecular model systems,and within dendrimer architecture,photoinduced electron transfer reactions of homoquinones, amines, and azo compounds, photoinduced reactions of five-membered monoheterocyclic compounds of the indigo group, photochemical and polymerisation reactions in solid Qo, photo- and redox-active [2]rotaxanes and [2]catenanes, ° reactions of sulfides and sulfenic acid derivatives with 02( Ag), photoprocesses of sulfoxides and related compounds, semiconductor photocatalysts,chemical fixation and photoreduction of carbon dioxide by metal phthalocyanines, and multiporphyrins as photosynthetic models. 

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