Photodegradation of carbendazim sensitized by aromatic ketones

Carbendazim (1) is a benzimidazole extensively used as post-harvest fungicide on fruits and vegetables. The aim of the present work is to study the photodegradation of 1 sensitized by aromatic ketones, with special attention to mechanistic aspects and to the possible detoxification associated with photochemical treatment. Laser flash photolysis (LPF) λexc = 355 nm of xanthone (XA) and anthraquinone (AQ) was performed in MeCN solutions, in the presence of 1. A new transient absorbing at λmax 500 and 320 nm was obtained and assigned to the semioxidized radical cation 1+. An exergonic thermodynamics for electron transfer quenching was confirmed by means of the Rehm–Weller equation. The same species was observed by direct LFP of 1 at 266 nm in polar solvents. Conversely, when a deoxygenated solution of 1 was submitted to LFP in cyclohexane the transient spectrum presented a band with maximum at 380 nm; it was assigned to the triplet excited state (31*) on the basis of energy transfer to oxygen and β-carotene. The photodegradation of 1 was achieved using XA and AQ as electron acceptors in a solar-simulator, in aerated aqueous medium; the reaction was faster with XA. Formation of a new photoproduct was initially observed; its structure was assigned as carbendazim N-C5 dimer (2). A balance of the total organic carbon (TOC) after prolonged irradiation indicated that mineralization does not occur to a significant extent, pointing to oxidative fragmentation of 1 and 2 to give a variety of low molecular weight products. To check whether the observed photodegradation of 1 results in a decreased toxicity, biological assays were performed using an established model based on the inhibition of mobility of Daphnia magna. The results demonstrate that photodegradation leads to a diminished toxicity, indicating that the photoproducts are less toxic than the parent compound.