Interfacial Electron Transfer Induced by Light in Methylene Blue Incorporated in Nafion Membranes. Laser Kinetic Spectroscopy

Quenching of excited states of methylene blue was studied in Nafion−H +, Nafion−Na + and Nafion−Fe 2+ membranes. Protonated, not protonated, and dimer forms of methylene blue in Nafion showed different fluorescence properties. Fe 2+ is observed to quench the fluorescence of methylene blue embedded in the Nafion membrane. The excitation of monomer of methylene blue molecules induces the triplet excited state of the dye as revealed by laser spectroscopy. Methylene blue radical generation involved excitation of the dye dimeric species or excimers. Nafion−Fe 2+ membrane quenched the excited methylene blue, leading to the formation of Fe 3+ and the semiquinone of the methylene blue radical. The back reaction between Fe 3+ and the semiquinone of the methylene blue radical occurred in the millisecond range and was seen to proceed by second-order kinetics. The latter process involves a dynamic mechanism of reaction including motion of one of the reactants or both. The mechanism of the photoreduction of MB incorporated in the Nafion by the Fe 2+ ions of the double layer ionic aqueous phase is described in detail.