Abstract— The 9, lodicyanoanthracene‐sensitized photooxygenation of 2‐methyl‐2‐butene and (+)‐limonene proceeds via the singlet oxygen pathway in carbon tetrachloride as well as in acetonitrile, although the fluorescence of the sensitizer in acetonitrile is quenched by these olefins in an electron transfer quenching mechanism. The 9, 10‐dicyanoanthracene‐sensitized photooxygenation of cis‐ and trans‐ä, ä′‐dimethylstilbenes occurs exclusively via the singlet oxygen pathway in carbon tetrachloride; in acetonitrile, however, singlet oxygen and electron transfer photooxygenation reactions compete with one another. Addition of tetra‐n‐butyl ammonium bromide and increasing oxygen concentrations favor the formation of the singlet oxygen product, whereas addition of anisole, increasing substrate concentrations and decreasing oxygen concentrations favor the electron transfer photooxygenation products. In carbon tetrachloride, exciplexes of the sensitizer and the dimethylstilbenes are formed which give rise to cidrrans‐isomerization of the substrates. In acetonitrile, neither exciplex formation nor cisltrans‐isomerization are observed. A mechanism is proposed which allows us to calculate product distributions of the competing singlet oxygen/electron transfer photooxygenation reactions and thus to determine the efficiencies with which encounters between the singlet excited sensitizer and the substrates finally result in electron transfer photooxygenation products. Using (I) these efficiencies, (2) the β‐value obtained from singlet oxygen photooxygenation sensitized by rose bengal, and (3) the appropriate k‐values determined from fluorescence quenching of 9, 10‐dicyanoanthracene in MeCN by oxygen and the stilbene, allows the calculation of the quantum yield of oxygen consumption by this stilbene. The quantum yield thus calculated is strictly proportional to the rate of oxygen consumption experimentally obtained; this result is considered as convincing evidence for the mechanism proposed.

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