The Kinetics and Mechanism of the Photo-oxidation of N,N′-Diphenyl-p-phenylenediamine. Electron-transfer from the Upper Excited Singlet State to Oxygen

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The photo-oxidation of N,N′-diphenyl-p-phenylenediarnine (DPPD) has been studied spectrophotometrically. The quantum yield for the diimine formation, Φ, in aerated cyclohexane at 2537 Å and 20°C did not change with variations of the irradiation time, the concentration of DPPD, or the light intensity. However, the Φ values increased, as did the decrease in the fluorescence quantum yields, as the excitation energies increased. The dissolved oxygen had the optimum concentration (1.2×10−4M), giving the maximum value of Φ in aerated cyclohexane at 2537 Å, and none of the product, the diimine, was obtained in the degassed system. When about 0.13M of ethanol or methanol was added to aerated cyclohexane, the maximum value of Φ was measured, and the addition of a highlypolar solvent, acetonitrile or H2O, decreased regularly in Φ. On the basis of these results, it can be said that the photo-oxidation of DPPD originates from the Franck-Condon state of the upper excited singlet state, 1(RH2)n. It seems that, in the primary processes of the reaction, the electron transfers take place from the Franck-Condon state of 1(RH2)n directly to the oxygen molecule (3Σg−) and to the solvent, resulting in a trapped electron which immediately jumps into the oxygen molecule. Then, the (RH2)+O2− ion pair may be formed, and subsequently the intermediate, (RH2)+O2−, may be converted to the product or the starting materials. The dissolved oxygen and the polar solvents are responsible for the electron-transfer processes, but they deactivate the ion pair, resulting in the decrease in Φ.
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The Kinetics and Mechanism of the Photo-oxidation of <I>N</I>,<I>N</I>′-Diphenyl-<I>p</I>-phenylenediamine. Electron-transfer from the Upper Excited Singlet State to Oxygen

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