Photosynthesis-Inhibiting Activity of Fluorinated 2‑Hydroxynaphthalene-1-carboxanilides

2-Hydroxy-N-phenylnaphthalene-1-carboxamide, three fluoro monosubstituted and five fluoro disubstituted 2-hydroxynaphthalene-1-carboxanilides were prepared by microwave-assisted synthesis and characterized. All the compounds were evaluated for their ability to inhibit photosynthetic electron transport (PET) in spinach (Spinacia oleracea L.) chloroplasts. The PET inhibitory activity of the discussed compounds proved to be in a wide range, from inactive N-(2,6-difluorophenyl)-2-hydroxynaphthalene-1-carboxamide with an IC₅₀ = 904 μM to N-(2,5-difluorophenyl)-2-hydroxynaphthalene-1-carboxamide with an IC₅₀ of 44.2 μM, which was the most potent isomer of the series of evaluated compounds. Based on previous studies, it can be assumed that the mechanism of PET inhibition of these compounds is the inhibition of photosystem II in the thylakoid membrane.