Photophysical and chemical processes affecting the stability of the thiazine dye-iron system

• Published in: International journal of hydrogen energy Vol. 4; no. 5; pp. 403 - 410
• Main Authors: Solar, S., Getoff, N.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 1979
• ISSN: 0360-3199; 1879-3487
• DOI: 10.1016/0360-3199(79)90102-2

Photo-electrochemical devices can be used under suitable conditions for production of hydrogen. The thiazine dye-iron redox system has been proposed as a photogalvanic cell, but has not been shown to function very satisfactorily. Some of the photophysical and chemical processes affecting its instability are the subject of the present investigations. The observed fluorescence quenching of electronically excited methylene blue ( λ exc = 300 nm, λ em = 685 nm) by iron(II) ions with a Stern-Volmer constant of, K SV = 2.1 M −1 results in electron and energy transfer reactions. The first process causes production of semiquinone (MBH +), which is reversible by iron(III) ions. As a sequence of the energy transfer from the excited dye in the second band (292 to above 400 nm) to the ferrous ions, solvated electrons ( e aq −) are generated with a quantum yield, Q = 1.4 × 10 −5. This gives rise to the appearance of H atoms, which can attack the dye at various positions with k(MB + + H) = (1.5 ± 0.1) × 10 10 dm 3 mol −1 s −1 and decompose it. Pulse radiolysis experiments showed the formation of semiquinone and various types of H-adducts as well as scission of the chromophore group. For direct excitation of iron(II)-ions from 292 to 313.4 nm a Q( e aq −) = 3.8 × 10 −3 was established and the Q( e aq) = 7 × 10 −2 for 253.7 nm was reproduced.