Photophysical properties of rufloxacin in neutral aqueous solution

The photophysical properties of rufloxacin, 9-fluoro-2,3-dihydro-10-(4-meth yl-1-pyrazinyl)-7-oxo- do[1,2,3-de]-1,4-benzothiazin-6-carboxyl acid, a flu oroquinolone antibacterial drug exhibiting photosensitizing action toward b iological substrates, were studied in aqueous solutions at neutral pH. The lowest excited electronic states of the zwitterion were characterized by bo th experimental techniques and theoretical methods, Steady-state and time-r esolved emission, triplet-state absorption and singlet oxygen production we re 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 propertie s that make it a good candidate for being the precursor of the photodecarbo xylation 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-sat urated solutions.