Visible-light promoted degradation of the commercial antioxidants butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT):: a kinetic study

Visible-light photo-irradiation of the commercial phenolic antioxidants (PhAs) butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), in the presence of vitamin B-2 (riboflavin, Rf), in methanolic solutions and under aerobic conditions, results in the photo-oxidation of the PhAs. The synthetic dye photosensitiser Rose Bengal was also employed for auxiliary experiments. With concentrations of riboflavin and PhAs of ca. 0.02 mM and < 1 mM, respectively, the excited triplet state of the vitamin ((3)Rf*) is quenched by BHT in a competitive fashion with dissolved ground state triplet oxygen. From the quenching of (3)Rf*, the semireduced form of the pigment is generated through an electron transfer process from BHT, with the subsequent production of superoxide anion radical (O-2(center dot-)) by reaction with dissolved molecular oxygen. In parallel, the species singlet molecular oxygen, O-2(1 Delta(g)), is also generated. Both reactive oxygen species produce the photodegradation of BHT. In the case of BHA, the lack of any effect exerted by superoxide dismutase drives out a significant participation of a O-2(center dot-)-mediated mechanism. BHA mainly interacts with O-2(1 Delta(g)) and exhibits a desirable property as an antioxidant - a relatively high capacity for O-2(1 Delta(g)) de-activation and a low photodegradation efficiency by the oxidative species. Electrochemical determinations support the proposed photodegradative mechanism.