Photodegradation of the herbicide Norflurazon sensitised by Riboflavin. A kinetic and mechanistic study

The kinetics and mechanism of the Riboflavin (Rf)-promoted photochemical degradation with visible light of the herbicide Norflurazon (NF) has been studied by time-resolved and stationary techniques. Using light of wavelength higher than 400 nm––a region where NF is totally transparent––and with concentrations of Rf and NF of ca. 0.02 and < 1 mM, respectively, only the excited triplet state of Rf (3Rf*) is quenched by NF, in competition with dissolved ground state triplet oxygen, O2(image3Σg-). NF degradation mainly occurs by reaction with superoxide radical anion image formed through two electron transfer steps: from NF to 3Rf*, yielding Rf radical anion, and from this anion to O2(image), regenerating ground state Rf. Although singlet molecular oxygen is also produced, NF only quenches this oxidative species in a physical mode. The global result is the photoprotection of the sensitiser and the photodegradation of NF.