Wavelength-Dependent UV-LED Photolysis of Fluorinated Pesticides and Pharmaceuticals

The wavelength dependence of photoproduct formation and quantum yields was evaluated for fluorinated pesticides and pharmaceuticals using UV-light emitting diodes (LEDs) with 255, 275, 308, 365, and 405 nm peak wavelengths. The fluorinated compounds chosen were saflufenacil, penoxsulam, sulfoxaflor, fluoxetine, 4-nitro-3-trifluoromethylphenol (TFM), florasulam, voriconazole, and favipiravir, covering key fluorine motifs (benzylic-CF₃, heteroaromatic-CF₃, aryl-F, and heteroaromatic-F). Quantum yields for the compounds were consistently higher for UV-C as compared to UV-A wavelengths and did not show the same trend as molar absorptivity. For all compounds except favipiravir and TFM, the fastest degradation was observed using 255 or 275 nm light, despite the low power of the LEDs. Using quantitative ¹⁹F NMR, fluoride, trifluoroacetate, and additional fluorinated byproducts were tracked and quantified. Trifluoroacetate was observed for both Ar-CF₃ and Het-CF₃ motifs and increased at longer wavelengths for Het-CF₃. Fluoride formation from Het-CF₃ was significantly lower as compared to other motifs. Ar-F and Het-F motifs readily formed fluoride at all wavelengths. For Het-CF₃ and some Ar-CF₃ motifs, 365 nm light produced either a greater number of or different major products. Aliphatic-CF₂/CF₃ products were stable under all wavelengths. These results assist in selecting the most efficient wavelengths for UV-LED degradation and informing future design of fluorinated compounds.