Six alkyl peroxides and two acyl peroxides have been used to quench the excited triplet state of a series of photosensitizers. The quenching rate constants (kq) determined by laser flash photolysis were found to decrease at lower sensitizer triplet energies; however, the falloff slope was only 16% as steep as the -0.73 mol kcal'1 characteristic of normal energy transfer. The low slope is rationalized in terms of a repulsive peroxide excited state. Peroxides which are more thermally labile are generally better quenchers of a given sensitizer. Electron transfer is considered but it is concluded that the peroxides studied here undergo ordinary exchange energy transfer.
The thermal decomposition of azomethane has been reinvestigated i n the temperature range 390450" by the " toluene-carrier " flow technique. The rate constant for the decomposition in 15 mm. of toluene is given by K = 1014e-*6WJ/RT sec.-l. The experiments provide strong evidence that the primary step in the decomposition is the splittingoff of a methyl radical. The reason for the discrepancy between these results and those obtained in static systems and the bond strengths in azomethane are discussed.
The direct and triplet-sensitized photolysis of two acyclic azoalkanes 1 and 2 has been examined in some detail. Whereas triplet sensitization usually leads to no reaction for acyclic azoalkanes, the triplet state of compounds 1 and 2 decomposes moderately efficiently without the intermediacy of the cis isomers. It was found that the cage effect for direct irradiation is much greater than that for sensitized decomposition. Although this result would normally be interpreted as a spin correlation effect (SCE), the difference is better explained as arising from stepwise decomposition of the azoalkane triplet state. The product distribution from 1 and 2 and the mechanism of azoalkane photolysis are discussed.
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