Photoreduction of Methylene Blue by Some Derivatives of N-Phenylglycine. Relation between the Structure and the Reactivity of the Electron Donors

The photobleaching of methylene blue in the presence of various derivatives of N-phenylglycine has been examined to investigate the relation between the structure and the reactivity of the electron donor and to get information about the nature of the photoreaction. The time course of the photobleaching is well expressed by the superposition of two terms decreasing exponentially with time. The hydrogen atom on the nitrogen or α-carbon atom of the N-phenylglycine molecule seems to take no part in the photoreduction process, and the first stage is probably the electron transfer between the interacting species. As judged from the initial quantum yield,Φ, of the photoreductions, the influence of substituents in the ortho- and para-positions of the benzene ring is much larger than that in the meta-position. Both electropositive and electronegative substituents reduce Φ. The small Φ in the former cases is attributed to the ability of such φ·G’s to deactivate the excited dye ions. From spectral data, no sign of complex formation between φ·G’s and unexcited dye molecules can be inferred. The fluorescence of methylene blue is not quenched by φ·G’s in the range of concentrations used in the photochemical reaction. In view of these results, the kinetic data are treated by use of a current kinetic scheme, when a relation akin to Hammett’s rule is found between a certain rate constant and the σ value of the substituent in the benzene ring of the N-phenylglycine. Kinetic data are also used to estimate the life-time of the metastable excited state of the dye, which is calculated to be about 4.2×10−5sec. or longer. This value is in a range reasonable to be interpreted as that of the lowest triplet state of the dye.