Flash Photolysis of Methylene Blue. II. Photolysis in an Aqueous Solvent in the Presence of N-Phenylglycines and Photolysis in Organic Solvents

Abstract

Abstract The flash photolysis of methylene blue in an aqueous medium with added N-phenylglycines or photolysis in organic solvents results in a more or less apparent bleaching of the dye. In the former case, the general features of the transient spectra are similar to those in the reversible photolysis treated in Part I. An absorption peak at 400 mμ in a solvent with a pH of 8.0 shifts to 430 mμ in a solvent with a pH of 8.8 ; it is to be attributed to the semiquinone of methylene blue. The absorption around 255 mμ increases at a later stage of the reaction, indicating the formation of leuco methylene blue. The photolysis seems to proceed in a way similar to that of the reversible cases. The results of flash photolysis with several derivatives of N-phenylglycine support the general idea of their reactivity obtained in a study with continuous illumination. There is a difference between the ways in which N-p-bromophenylglycine and N-p-methoxyphenylglycine retard the photoreduction, although this may be superficial. In the former case, some early intermediate (probably the triplet state of the dye) is deactivated almost catalyticaly, while in the latter case the deactivation takes place at the stage of a long-lived semiquinone-like intermediate. Peculiar features of photolysis with α-methyl-N-phenylalanine and N-phenyl-β-alanine have also been noted. The degree of irreversible bleaching in an organic solvent has no regular relation to the ease with which the solvent molecule is dehydrogenated by an ordinary oxidant. The main part of the transient absorption in organic solvents is attributed to the semiquinone of the dye. The peak has somewhat red-shifted compared with that in water. Triplet absorption seems also to appear at an early stage of photolysis. The photolysis in 1- or 2-propanol reveals the reversible feature observed when ferrous ions were used as reductants.