Kinetic study of the riboflavin-sensitised photooxygenation of two hydroxyquinolines of biological interest

4-Hydroxyquinoline (4-OHQ) and 8-hydroxyquinoline (8-OHQ), two compounds of interest because of their bioactivity and their structural relation with bioactive products, are effectively photooxygenated when irradiated with visible light in the presence of riboflavin (Rf) (vitamin B2) in solution in air-saturated water–methanol (9:1). Rf behaves as a dye-sensitiser, since both quinolines are transparent to visible light. 8-OHQ degrades about five times faster than 4-OHQ. Kinetic data obtained through time-resolved and stationary detection of Rf-electronically excited states indicate that a superoxide radical anion-mediated mechanism exclusively operates for 4-OHQ, whereas singlet molecular oxygen – mainly – plus superoxide radical anion is the species that reacts with 8-OHQ. The sensitiser Rf, which is known to photodegrade under visible-light aerobic irradiation, is regenerated in the presence of any of the quinolines through an electron transfer process that produces superoxide radical anion. The overall picture indicates that both quinolines act as sacrificial scavengers of the photogenerated oxygen species, thus preventing the photodegradation of Rf.