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The 355 nm laser flash photolysis of nalidixic acid at pH 9.2 leads to the formation of the nalidixate anion triplet state (absorption lambda max = 620 nm; 5700 less than or equal to epsilon T less than or equal to 9000 M-1cm-1; 0.6 less than or equal to phi T less than or equal to 1). The first order triplet state decay (kT = 7.7 x 10(3) s-1) is accompanied by a diffusion controlled triplet-triplet annihilation. Oxygen efficiently quenches the triplet state (k = 3.2 x 10(9) M-1s-1). The nalidixate radical dianion (absorption lambda max = 650 nm; epsilon = 3000 M-1cm-1) is produced by the diffusion controlled reductive quenching of the triplet state by tryptophan and tyrosine. The superoxide anion (O2-.) is produced by diffusion controlled reaction of the radical dianion with oxygen. The O2-. is characterized by its reactions with ferricytochrome c and superoxide dismutase. The physiological form of nalidixic acid is thus a good Type I and Type II photosensitizer.

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Triplet absorption spectra. extinction coefficients (e., ), decay rates ( k , ) . oxygen quenching rates (kcl) and intersystem crossing yields (Q ,) for 3-carbethoxypsoralen (3-CPs). X-methoxypsoralcn (%MOP) and 5-methoxypsoralen (5-MOP) in methanol are reportcd. For 8-MOP and 3-CPs corresponding values are also reported with water as the solvent. Some photophysical data are also reported for 5-MOP in water, but E , and Q I were not obtained.The phosphorescence spectra for these furocoumarin derivatives in ethanol at 77 K are reported together with the corresponding lowest triplet energy and lifetime. The values 0 1 the various photophysical properties obtained are compared with values reported by prcviouh workers.