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Polyoxometalate sensitization in mechanistic studies of photochemical reactions: The decatungstate anion as a reference sensitizer for photoinduced free radical oxygenations of organic compounds

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Abstract

The photosensitized oxygenation of organic molecules plays a key role in numerous processes of biological and industrial significance, such as, for example, photodynamic action and photodegradation of polymers. These reactions proceed either via quenching by the substrate of photophysically generated singlet oxygen, O2(1Δg), or via addition of ground state oxygen to photochemically generated radicals derived from the substrate, or via both pathways. The evaluation of the contributions of both mechanisms to the overall process requires reference sensitizers that exclusively induce one of the corresponding reactions. Some compounds are known to produce singlet oxygen with unit efficiency, but no references to sensitizers producing free radicals but no singlet oxygen have been found so far. In this work, we propose to use the decatungstate anion, W10O324-, as a first reference sensitizer for free radical oxygenations of organic molecules. A combination of time-resolved and steady-state studies has been performed to compare the photo-oxygenation of simple reference compounds, including 2-methyl-2-pentene and 2,3-dimethylbutene, by W10O324- and by classical O2(1Δg) sensitizers, such as methylene blue and ruthenium complexes. It is demonstrated that W10O324- sensitized oxygenation of organic compounds occurs exclusively by a free radical pathway, which differs clearly from both Type I and Type II oxygenations. Comparison with Type II reactions shows that: (i) in spite of their weaker reactivity, singlet oxygen mediated reactions are associated with larger photo-oxygenation yields than W10O324- induced processes, due to the longer lifetime of the reactive species; and (ii) reaction of alkenes with both singlet oxygen and decatungstate features charge transfer interactions, whose magnitude is larger in the case of O2(1Δg).

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Author notes

  1. Claude Schweitzer

    Present address: Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada

Authors and Affiliations

  1. Laboratoire de Photochimie Ecole Européenne de Chimie Polymères et Matériaux de Strasbourg, 25, rue Becquerel, 67087 Strasbourg Cedex 2, France

    Charles Tanielian, Claude Schweitzer, Rachid Seghrouchni, Marc Esch & Robert Mechin

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  1. Charles Tanielian
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  2. Claude Schweitzer
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  3. Rachid Seghrouchni
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Dedicated to Professor Jean Kossanyi on the occasion of his 70th birthday.

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Tanielian, C., Schweitzer, C., Seghrouchni, R. et al. Polyoxometalate sensitization in mechanistic studies of photochemical reactions: The decatungstate anion as a reference sensitizer for photoinduced free radical oxygenations of organic compounds. Photochem Photobiol Sci 2, 297–305 (2003). https://doi.org/10.1039/b210786b

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