New insights into the photoreactivity of the organophosphorus pesticide fenthion: a sigma aryl cation as a key intermediate in the photodecomposition

The primary photodegradation processes of fenthion (FN), one of the most photosensitive pesticides used in agriculture, have been investigated by combining laser flash photolysis and steady-state measurements. The triplet state of FN is produced quite efficiently (Phi(isc) approximately 0.3). However, this excited state does not seem to trigger the primary photodecomposition pathways of the pesticide. It is demonstrated that FN undergoes photoheterolysis via the excited singlet state and gives the corresponding singlet "sigma aryl cation". Chemical evidence for the generation of this transient species is given by trapping with typical "sigma nucleophiles" such as chloride and methanol. This photodegradation mechanism is, in part, quite different with respect to what is known to date and may account for the formation of the O,O-dimethyl S-3-methyl-4-(methylthio)phenyl] phosphorothioate discovered as a novel photodegradation product of FN.