Precise quantum yields for generation of singlet molecular oxygen, O2(1Δg), have been measured for tetrakis(4-sulfonatophenyl)porphyrin (TSPP), closely-related water-soluble porphyrin derivatives, and tris(2,2‘-bipyridyl)ruthenium(II) (bpy3Ru2+) in O2-saturated water. Under the experimental conditions, TSPP is present in an aggregated state, but the measured quantum yield for formation of O2(1Δg), ΦΔ = 0.51, remains on the same order as that of the corresponding monomer in methanol, ΦΔ = 0.70. Comparison with the quantum yield for formation of the triplet state indicates that quenching of the triplet by O2 gives O2(1Δg) with an efficiency of ca. 80%. There is a small but significant increase in ΦΔ when D2O is used in place of H2O. For the other porphyrin derivatives, the ability of the aggregate to sensitize formation of O2(1Δg) appears to depend on the total electronic charge resident on the molecule, decreasing with decreasing negative charge. A strong solvent dependence noted for O2(1Δg) production with (bpy3Ru2+) is explained in terms of competition between triplet energy transfer and photoinduced electron (or partial charge) transfer. Similar behavior could be responsible for the relatively low ΦΔ value observed for TSPP.

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(1deltag), 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, W10O32(4-), 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 W10O32(4-) and by classical O2(1deltag) sensitizers, such as methylene blue and ruthenium complexes. It is demonstrated that W10O32(4-) 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 W10O32(4-) 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(1deltag).

No abstract