Mechanistic studies on the photodegradation of azoarenes

The photochemical stability of a series of coloured azobenzenes and azothiophenes containing both electron donors and attractors has been explored in methanol under anaerobic conditions using a xenon light source for irradiation. 4-Dialkylaminoazobenzenes containing a nitro group in the 4′-position of the chromophore are the most stable, while those containing the same group in the 2′-position are the least stable and found to have similar lifetimes to azothiophenes. The photodegradation reaction or loss of colour in each case is attributable to the irradiation of the second absorption band alone, which leads to the formation of a reactive triplet state. This is believed to behave as a biradical and abstract hydrogen from the solvent as the relative rate of photodegradation increases sharply in moving from methanol to propan-2-ol. The reaction is inhibited by radical scavengers, such as the stable 2,2,6,6-tetramethyl-1-piperidinyloxy free radical and enhanced by photochemical radical generators such as acetone or benzophenone. A combination of steric and electronic effects are believed to be responsible for the low photochemical stability of 2′-nitroazobenzenes versus the corresponding 4′-isomers.