Photoinduced oxygen uptake of diphenylamines in solution and their ring closure revisited.

The photochemistry of diphenylamine (DPA), N-methyldiphenylamine (MeDPA), and triphenylamine (TPA) was studied in solution at room temperature. The major photoprocess was cyclization, and the quantum yields were Phicyc = 0.02-0.6. The photoinduced oxygen uptake/consumption, studied in air-saturated acetonitrile-water or methanol-water, was efficient. Three subsequent transients, the polyphenylamine triplet state, the 4a,4b-dihydrocarbazole triplet state, and its labile ground state, were accessible by laser flash photolysis prior to carbazole formation. Their yields were determined and compared with the Phicyc values. Oxygen can reduce or enhance Phicyc since one step, quenching of the polyphenylamine triplet state, blocks cyclization and one step, scavenging of dihydrocarbazole, favors cyclization since it competes successfully with the back-reaction to the substrate. The former is dominant for DPA in solvents with low and high polarity and the latter is dominant for MeDPA preferentially in nonpolar solvents as well as for TPA in all solvents.