Pulsed-laser flash and continuous photolysis of ion-pair complexes of methyl viologen and ethylenediamine tetra-acetic acid in aqueous solution

Abstract

Methyl viologen (MV²⁺) and EDTA form ion-pair complexes in aqueous solution that exhibit enhanced tail absorption in the u.v.-visible spectral region. Continuous photolysis of the system yields MV^˙+ with values of ϕ(MV^˙+) that are dependent on pH, the concentrations of the various species in solution, temperature and the wavelength of excitation. Pulsed-laser excitation at 355 mm generates initially a spectrum that is not that of free MV^˙+; its enhanced absorption in the 440–540 nm region is reminiscent of that displayed by reduced viologen dimers and aggregates. The conversion of the initial absorption to that of free MV^˙+ occurs via[MV²⁺]-independent first-order kinetics in the µs time frame; k_obs increases as [EDTA] is decreased and the pH of the solution is increased. It is suggested that the ion-pair complexes of MV²⁺ exist as ‘pseudo-micellar’ aggregates, so that the absorption of light generates MV^˙+ and an oxidized EDTA radical, with the electron localized on an amine moiety, within the aggregate. The reducing EDTA radical, formed via the deprotonation of the extremely strong carbon-acid alpha to the amine radical site and a carboxylate group, causes the generation of another equivalent of MV^˙+ within the aggregate in competition with the decay modes of the radical; the MV^˙+ species therein are ultimately released into bulk solution as the aggregate structure equilibrates; the rate of equilibration depends on [EDTA] in bulk solution and the degree of repulsion among the anionic species in the outer hydrophilic sheath of the ‘pseudo-micelle’. The observed values of Φ(MV^˙+) reflect the competition within the aggregates between geminate-pair back electron transfer (radiationless deactivation in photophysical terms) and radical degradation processes.