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Abstract The intermediate species which are produced in the course of the photoreduction of methylene blue have been investigated by flash photolysis in plain aqueous, borate and phosphate buffer solutions with and without reducing agents. In an acidic solution, an absorption band with the lifetime of 30μsec. appears at 375 mμ, while in neutral and basic solutions, two bands appear, at 415 and 520 mμ, with lifetimes of 70∼90 μsec. The bands at 375 and 415 mμ are considered to be due, respectively, to the acidic and basic forms of the triplet state, since the transient spectra change with pH, making an isosbestic point. The band at 415 mμ is overlapped by another one with a lifetime longer by several hundred microseconds; this is presumed to be semiquinone. The intensity of the band at 520 mμ decreases upon the addition of reducing agents and increases upon the introduction of oxygen. Hence, this band may be due to the halfoxidized form of the dye. These results support the reaction scheme that the excited state of the dye reacts with the ground state, which has been proposed in the investigation of steady light photobleaching.
Kato et al. (Wed,) studied this question.