Electronic absorption and luminescent spectra of nonfluorinated nalidixic (nlqH) and pipemidic acid (pifqH), monofluoroquinolones - norfloxacin (nfqH) and pefloxacin (pfqH) as well as of their difluorinated analogs 1-ethyl-6,8-difluoro-1,4-dihydro-7-(1-(4-methylpiperazinyl) - 4-oxo-3-quinolinecarboxylic (mdfqH) acid and 1-ethyl-6,8-difluoro-1,4-dihydro-7-(1-piperazinyl) - 4-oxo-3-quinolinecarboxylic acid (dfqH) - were investigated. Quantum yields, lifetimes of excited states and rate constants of radiative and nonradiative transitions of the compounds were measured. The Mulliken charges of atoms from these compounds were calculated by quantum-chemical complex GAMESS. Differences in the electronic structures of these compounds and their spectral-luminescent characteristics were compared with the data of the phototoxicity degree of fluoroquinolones. Analysis of the Mulliken charges of the difluoroquinolones points to the changes of the redistribution of the electron density along π-conjugated system, and on the oxygen atoms of the carbonyl and carboxyl groups. The analysis of the molecular orbitals involved in the electronic transitions of the compounds revealed that both defluorination and piperazine photolysis are photodecomposition mechanisms which may take place in the excited states of these compounds. The relationship between the location order of the π-π* excited levels of the FQs and the degree of their phototoxicity has been determined.

The nalidixic acid transient spectra were analyzed and the scheme of photo-conversion of the acid was suggested. At a large delay of laser pulses the transient absorption spectra contain the bands due to the dissociation of the products of the cation-and anion-radical photodecomposition in the micro-and millisecond range. By quantum-chemical calculations we revealed that at the transition to the ion-radical state a proton migration occurs subsequent to the movement of the center of gravity of the negative charge. The phototransformation of nalidixic was found to proceed with a TICT-effect.The antibiotics of quinolone class (fqH) have found wide application in medicine. However, the tendency of some antibiotics to phototoxicity [1] requires the study of the influence of light on the composition and phototransformations of the shortliving intermediates. This paper presents the results of laser flash photolysis and quantum chemical calculations of the anionic, neutral and cationic protolytic forms of nalidixic acid (nlqH). The numbering of atoms in the structure of nlqH is shown in Fig. 1.The primary photochemical processes at the interaction of photons with the compounds of fluoroquinolones is intra-or intermolecular electron transfer between the fragments with the transition of nalidixic acid into the singlet S 1 -excited state.The stepwise photophysical and photochemical processes within nanosecond and microsecond ranges are accompanied by competing processes that include energy migration, nonradiative energy loss, electron and proton transport, and luminescence. The primary photochemical process in the case of nalidixic acid is commonly regarded as the intramolecular electron transfer.We believe that the processes of protonation or deprotonation of quinolones occurring at the intra-or intermolecular electron transfer in the course of the photoexcitation play a key role in the acid-base catalysis of antibiotics. It was of interest to follow the spectral-kinetic and photochemical properties of nalidixic acid in acidic and basic environments for the cationic, neutral and anionic forms. In the study of photolysis of nalidixic acid we found the qualitative Fig. 1. Structure of nalidixic acid with numbering of its atoms (Arabic numerals) and fragments (Roman numerals).