Multifaceted Photoreactivity of 6-Fluoro-7-aminoquinolones from the Lowest Excited States in Aqueous Media: A Study by Nanosecond and Picosecond Spectroscopic Techniques

Abstract

Nanosecond and picosecond absorption and emission spectroscopic techniques were applied to the investigation of the reactivity from the lowest excited states of some 6-fluoro-7-piperazino-4-quinolone-3-carboxylic acids (FQs) in aqueous media at neutral pH, in the absence and presence of different sodium salts. Following the detection of various transients, we proposed a mechanism for the cleavage of the carbonÀfluorine bond that proceeded through different reaction pathways, dependent on the molecular structure and the characteristics of the medium. The drug lomefloxacin (LOM), a 6,8-difluoroquinolone derivative, underwent heterolytic cleavage of the C8ÀF bond from the excited singlet state. With the 6-monofluoroquinolone norfloxacin (NOR) and the corresponding 1,8-naphthyridinone enoxacin (ENX), the lowest singlet state was not significantly reactive and an important deactivation channel was intersystem crossing (ISC) to the triplet manifold. The lowest triplet state underwent cleavage of the C6ÀF bond through a solvent mediated process possibly via a cyclohexadienyl anionic adduct. In the presence of sulfite or phosphate buffer a novel defluorination mechanism, induced by electron transfer from the inorganic anions to the FQ triplet state, was observed. The correlation between the transients observed and the final photoproducts in the different media was elucidated.

Enhanced Photostability of Fluoroquinolone Antibacterials Capped on Silver Nanoparticles

“…Detailed literature data show that the photodegradation of NF and RF mainly involve their lowest excited triplet state ( 3 FQ Ã ), formed after effective intersystem crossing (isc), as the key reactive intermediate in the drugs photodegradation. [32,33] The triplet states of NF and RF undergo two different deactivation pathways in phosphate buffer solution, which, for sake of clarity, are recalled in Scheme 3. In the case of NF, a photoinduced electron transfer involving the phosphate ions of the buffer leads to the NF radical anion, which initiates the radical-mediated drug photodecomposition.…”

“…The choice of these two compounds is not casual but it is was suggested by the different mechanism involved in their photosensitization reactions in buffered water solution. In fact, while a type I process (radical mediated) occurs for NF, [32] a type II pathway (singlet oxygen mediated) mainly occurs for RF. [33] Results and Discussion…”

“…In the case of NF, a photoinduced electron transfer involving the phosphate ions of the buffer leads to the NF radical anion, which initiates the radical-mediated drug photodecomposition. [32] In contrast, the lowest triplet state of RF is unreactive toward phosphate ions and, under aerobic conditions is effectively quenched by molecular oxygen through energy transfer mechanism leading to the effective generation of the highly reactive singlet oxygen ( 1 O 2 ). [33] In this view, the remarkable photostability of the Ag@FQs can be the result of significant quenching by the metal on the reactive triplets of the FQs.…”

“…Figure 6(A) shows the transient absorption spectra of the free NF recorded in aqueous buffered solution at different delay time with respect to the excitation laser pulse. According to literature, [32] we observed the excited triplet state of NF with maximum at 620, which with elapsing time evolved into the NF radical anion (Scheme 3 path a), mainly absorbing around 700 nm. Given the largely superimposed spectra of the two species, the time profiles in the whole spectral range explored are well-described by a biexponential function with time constants of ca.…”

Femtosecond Transient Absorption Spectroscopy Study of the Early Events of Norfloxacin in Aqueous Solutions with Varying pH Values

“…A strong reduction of the photodegradation quantum yield and sensitization by benzophenone have been reported as evidence for a long-lived excited state as the main precursor species. 11 Nanosecond flash photolysis and picosecond spectroscopy of EX, reveal that ISC accounts for an efficient deactivation process from the excited singlet state. Effective oxygen quenching [rate constants in the range of (2 ± 3) × 10 9 M −1 s −1 ] is consistent with the effect of air equilibration on the photodegradation quantum yields.…”