Charge-transfer complexation between methyl viologen and sacrificial electron donors EDTA, triethanolamine, and cysteine

Abstract

Methylviologen dication (MV/sup 2 +/) forms charge-transfer (CT) complexes with EDTA, triethanolamine (TEOA), and cysteine (Cys) in aqueous solution. The complexes of EDTA and TEOA show enhanced tail absorption from 320 nm well into the visible region; the Cys complex shows a discrete absorption band in the 350-380-nm region. By use of the Benesi-Hildebrand and other treatments for the variations of the absorbances of the complexes with the concentrations of the components, values of K/sub eq/ and epsilon/sub lambda/ for the complexes are obtained as functions of pH. Values of K/sub eq/ range from 0.29 M/sup -1/ for the Cys complex at pH 3.5 to 18 M/sup -1/ for the EDTA complex at pH 11.2; K/sub eq/ values are significantly higher in alkaline solution where the deprotonated donors are better reducing agents. The emission at 525 nm observed in these and other MV/sup 2 +/ systems is believed to arise from a highly luminescent impurity, not a CT complex. The ramifications of CT complex formation on the photochemistry of systems containing MV/sup 2 +/ and sacrificial electron donors are discussed. 57 references, 4 figures, 1 table.