9, 10‐DICYANOANTHRACENE‐SENSITIZED PHOTOOXYGENATION OF Α,α′‐DIMETHYLSTILBENES. MECHANISM AND KINETICS OF THE COMPETING SINGLET OXYGEN AND ELECTRON TRANSFER PHOTOOXYGENATION REACTIONS

Abstract

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.

The photochemistry of EDA complexes in micellar solutions. II. The stilbene–methylviologen system in the presence of oxygen

“…Leest (24) has proposed that the reaction of MV'+ with 0' regenerates MV+' and takes place in two steps, the first equivalent to step [4] in our reaction scheme and a second slow step, MV'+ with 02'-to yield MV". On the other hand, the dependence of @-,, on methylviologen concentration suggests that MV+' can react with 02'-as in step [6], yielding oxidation products from which can arise, at least in part, the green fluorescence observed. Finally, step [7] of the scheme, where HCl is competing with MV+' for 02'-, can account for the dependence of the green fluorescence upon solvent acidity, as observed by Mau et al (19).…”

“…[14] is in accord with the fact that higher concentrations of MV+' than of HCl must be used to obtain a similar effect on quantum yield. It is noteworthy that steps [4] and [6] in the proposed reaction scheme constitute a simplified mechanism for the photoreaction of MV+' with 0 2 , yielding fluorescent products via methylviologen radical cation MV'+ (18,19,24,25). Leest (24) has proposed that the reaction of MV'+ with 0' regenerates MV+' and takes place in two steps, the first equivalent to step [4] in our reaction scheme and a second slow step, MV'+ with 02'-to yield MV".…”

“…From the slopes of both lines, the ratio of rate constants for steps [7] and [6] of the reaction scheme can be obtained as These steps correspond to the reactions of 0;-with H+ and Mv+', respectively, by which the two latter species are supposed to influence as inhibitors. The value obtained in eq.…”

“…Both species, M V +~ and H + , competing with cS'+ by 02'-(steps [6] and [7]), favor the isomerization sequence. This is shown in Table 3 for the case of protons: a decrease in pH supresses the inhibition by oxygen and the quantum yield is raised near to the value obtained in the absence of oxygen.…”

“…The major part of the experimental work in this field has been camed out in homogeneous solution by direct excitation (2,3) and sensitization (4)(5)(6). Some attention has been given to reactions produced by photoexcitation of charge-transfer complexes formed by stilbenes with neutral organic molecules (7,8).…”