Flash Photolysis of Methylene Blue. I. Reversible Photolysis in the Absence of a Reducing Agent and in the Presence of Certain Kinds of Reducing Agents

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Abstract
Transient spectra which appear in the flash photolysis of methylene blue in aqueous solvents of various pH values have been surveyed both in the absence and in the presence of a reductant. In the absence of a reductant, the triplet state of the dye has been observed to appear in two forms, with different degrees of protonation. The form found in neutral water seems to be formed directly from the excited singlet state of the dye. The pKA value of the acid-base equilibrium has been found to be approximately 6.8. A second transient has been found to be produced in water and in buffer solutions of a moderate pH value at a later stage of the photolysis. Its formation is more marked in buffer solutions, and it may be supposed to be a sort of charge-transfer complex between an excited dye ion and another dye component. The photolysis in the presence of ferrous ions, p-phenylenediamine or benzohydroquinone is nearly reversible, and the absorption of the original dye is restored after the illumination except in an extremely alkaline solution. The transitory formation of leuco methylene blue has been observed when ferrous sulfate or hydroquinone was used as the reductant below a pH value of 8.4, while that was not the case with p-phenylenediamine. The formation of leuco methylene blue from semiquinone has been found to be a bimolecular dismutation process in an acid medium when ferrous ions are the reductant, but when hydroquinone is used as the reductant at moderate pH values, it also reduces the methylene blue semiquinone. When hydroquinone is used as the reductant in a neutral or alkaline solvent, an increase in absorption is often observed at some longer wavelengths; this has tentatively been attributed to a complex between excited dye ions and reductant molecules. A comparison of the transient spectra obtained in various conditions has shown that the semiquinone of methylene blue exists in variously protonated forms in the following ranges: pH>9, pH=3∼8, in 0.1∼1 n sulfuric acid, and in very concentrated sulfuric acid. The pH dependence of the quantum yield of the photoreduction obtained earlier has been tentatively related to the change in the degree of protonation of the triplet state and the semiquinone.