Triplet excimers of fluoroquinolones in aqueous media

Abstract

Generation of triplet eximers of 6-fluoro-7-piperazinylquinolone-3-carboxylic acids (FQs) have been detected in aqueous media using laser flash photolysis (LFP). These transient species (SS) are generated by self-quenching reactions of FQ triplet excited states such as pefloxacin (PFX), norfloxacin (NFX), the N-acetylated form of NFX (ANFX), and its methyl ester (EANFX) with their ground states. In this context, self-quenching rate constants in the range of (1-7) X 10(8) M-1 s(-1) were determined. The triplet excimers show transient absorption spectra with lambda(max) ca. 710 nm for SSNFX, 740 nm for SSPFX, and 620 nm for SSANFX and E-ANFX, which are red-shifted with respect to their predecessors triplet excited states. These excimers can be also observed in the presence of phosphate buffer (PB). Experiments performed with NFX and ANFX at different PB concentrations showed that deprotonation processes are not involved in the generation of SS. The triplet multiplicity of the FQ excimers was confirmed by energy transfer reactions with naproxen. The correlation between fluorescence, intersystem crossing, excimer and photodegradation quantum yields of (A)NFX indicated that FQ self-quenching reactions are mainly a deactivation pathway. On the other hand, generation of FQ radical anions absorbing at lambda(max) ca. 620 nm has been observed by an efficient electron transfer reaction from Tip to NFX, PFX, and ANFX (rate constants ca. 1 X 109 M-1 s(-1)).