| Abstract
| - Photoreaction of 1-ethyl-6-fluoro-7-(1-piperazinyl)-1,4-dihydro-4-oxoquinoline-3-carboxylic acid (norfloxacin, NFX) and other 6-fluoroquinolones in aqueous solution gives rise to the corresponding6-hydroxy derivatives. Although two mechanisms have been proposed for this photonucleophilicaromatic substitution, direct evidence for any of them is still missing. Obtaining such evidencerequires work in basic media, where intramolecular electron transfer from the piperazine ring tothe quinolone system is the almost exclusive singlet deactivation pathway. To overcome this problem,the 4‘-N-acetyl derivative of norfloxacin (ANFX) has been employed in the present paper due tothe lower availability of the N lone pair. The photochemical and photophysical properties of ANFXhave been studied in aqueous solutions at pH between 7.4 and 13. As expected, fluorescence ofANFX is not significantly quenched in basic media. Furthermore, the excited triplet state (λmax =620 nm) reacts with hydroxide anions with a rate constant of (0.3 ± 0.1) × 106 M-1 s-1. This supportsa direct attack by hydroxide anions to the excited triplet state with subsequent release of fluorideas the operating mechanism. The fact that the reaction is inhibited by the presence of naproxen (awater-soluble naphthalene derivative) as triplet quencher clearly confirms the mechanisticassignment.
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