The photo-oxidation of an aromatic amine studied by flash photolysis

Photo-oxidation of the amine N-/β-hydroxyethyl N-methyl aniline (HEMA) by daylight and oxygen leads to interesting discoloration effects: when the amine is exposed as a thin film a blue product is formed. The formation of this blue product in solution has been studied by the flash photolysis technique. Maximum formation of the blue product occurs when HEMA is flash-photolysed in air-saturated aqueous solution and the pH is about 3.5. Examination of the flash photolysis of HEMA by kinetic spectroscopy shows that two transient species are associated with the formation of this product. These transients are the radical cations + ϕN+R1R2 and dimer* +. The second transient dimer•+ is formed by the reaction ϕN+R1R2 + ϕNR1R2 → dimer•+ and in nitrogen-saturated solution it further reacts by 2 dimer *+ → dimer2+ + dimer. Values are obtained for th e rate constants of these reactions. A reaction scheme involving the reaction of dimer •+ and dimer2+ with oxygen is given for the formation of the blue product in aqueous solution and an explanation is put forward to account for the daylight discoloration behaviour of HEMA.