Photophysical properties of rufloxacin in neutral aqueous solution

The photophysical properties of rufloxacin, 9-fluoro-2,3-dihydro-10-(4-methyl-1-pyrazinyl)-7-oxo- do[1,2,3-de]-1,4-benzothiazin-6-carboxyl acid, a fluoroquinolone antibacterial drug exhibiting photosensitizing action toward biological substrates, were studied in aqueous solutions at neutral pH. The lowest excited electronic states of the zwitterion were characterized by both experimental techniques and theoretical methods, Steady-state and time-resolved emission, triplet-state absorption and singlet oxygen production were investigated. The results indicate that the lowest excited singlet is a fluorescent, relatively long-lived state (Phi(f) = 0,075, tau(f) congruent to 4.5 ns) with an efficient intersystem crossing to the triplet manifold (Phi(ISC) congruent to 0.7), The lowest triplet is a long-lived state (tau(T) congruent to 10 mu s at 295 K in 0.01 M phosphate buffer), with properties that make it a good candidate for being the precursor of the photodecarboxylation of the drug. It is quenched by oxygen at a rate of 1.7 x 10(9) M-1 s(-1) and singlet oxygen is formed with a quantum yield of 0.32 in air-saturated solutions.