Unveiling self-sensitized photodegradation pathways by DFT calculations: A case of sunscreen p-aminobenzoic acid

Self-sensitized photodegradation has been observed for diverse aquatic organic pollutants. However, photodegradation pathways have not been clarified in previous experimental studies. Here, we attempted to probe self-sensitized photodegradation pathways of organic pollutants employing both photolytic experiments and density functional theory calculations. By performing photolytic experiments, we found that singlet state oxygen (1O2) play an essential role in photodegradation of a sunscreen p-aminobenzoic acid (PABA). PABA can photogenerate 1O2 and react fast with 1O2. We hypothesized that PABA underwent 1O2 induced self-sensitized photodegradation. By calculating transition states, intermediates and reaction barriers, we found that 1O2 can oxidize PABA through electrophilic attacks on the benzene ring to abstract one H atom of the amino group following a 1,3-addition mechanism or to induce decarboxylation. Either pathway produces a hydroperoxide. O-O bond cleavage of the hydroperoxides occurring at ground states or the lowest triplet excited states can produce phenoxyl radical precursors of 4-amino-3-hydroxybenzoic acid and 4-aminophenol, which are photodegradation products detected in experiments. Thus, a viable 1O2 self-sensitized photodegradation mechanism was unveiled for PABA.