Xanthone-photosensitized detoxification of the veterinary anthelmintic fenbendazole

Fenbendazole (1) is a common veterinary anthelmintic, toxic to water living microorganisms. Fluorescence quantum yields of 1 were found to be 0.11 in acetonitrile, 0.068 in methanol, 0.034 in cyclohexane, and 0.013 in water. The singlet excited state energy was ca. 96 kcal mol−1 in all solvents. The phosphorescence spectrum of 1 in ethanol at 77 K displayed a maximum at 450 nm, leading to a triplet energy of 75 kcal mol−1. Experimental excited state energies agree well with the results of DFT calculations at the time-dependent B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level. Laser flash photolysis (LFP) of 1 at 266 nm led to transients absorbing in the 300–700 nm range, ascribed to radical cation 1+, which were also observed upon 355 nm LFP of xanthone (XA) in the presence of 1. Solar-simulated photolysis revealed XA-enhanced photodegradation of 1 and led to decreased toxicity, as shown by Daphnia magna assays.