Electron transfer in the quenching of triplet methylene blue by complexes of iron(II)

Abstract

A Q-switched pulsed ruby laser emitting 1.0-J flashes at 693.4 nm was used in an investigation by means of flash photolysis-kinetic spectrophotometry of the mechanism of quenching of 2 to 10 ..mu..M triplet methylene blue by substitution-inert complexes of Fe(II). Complexes included Fe(H/sub 2/O)/sub 6//sup 2 +/, H/sub 2/Fe/sup II/(CN)/sub 6//sup 2 -/, HFe/sup II/(CN)/sub 6//sup 3 -/, Fe/sup II/(CN)/sub 6//sup 4 -/, Fe/sup II/(CN)/sub 4/bpy/sup 2 -/, Fe/sup II/(CN)/sub 2/(bpy)/sub 2//sup 0/, ferrocene/sup 0/, and Fe/sup II/(bpy)/sub 3//sup 2 +/. System variables included solvent (H/sub 2/O, aqueous EtOH, aqueous CH/sub 3/CN, and aqueous DMF), pH (2.1 to 8.2) and ionic strength (0.01 to 1.6 M). Energy transfer from triplet methylene blue to quencher is not possible in any of the cases investigated, and quenching proceeds via partial or complete electron transfer. Rate constants for quenching, k/sub q/, were diffusion-controlled with all quenchers except Fe/sup II/(H/sub 2/O)/sub 6//sup 2 +/ and, possibly, Fe/sup II/(bpy)/sub 3//sup 2 +/. Efficiency of net electron transfer in the quenching process was measured directly and found to vary with quencher, solvent, and state of protonation of triplet methylene blue and quencher. The kinetics of the reverse electron-transfer reaction between complexes of Fe(III) and semimethylene blue,more » MBH/sup +/, was studied independently by generating MBH/sup +/ by quenching triplet methylene blue with diphenylamine in the presence of a complex of Fe(III). Values of the second-order specific rate of reserve electron transfer were measured directly. Mechanistic features for the diffusion-controlled quenching of /sup 3/MBH/sup 2 +/ by complexes of Fe(II), Fe/sup II/L/sub x//sup m/ consistent with the data are proposed.« less