Hofmeister effects on electron-transfer reactions of 1-pyrenesulfonic acid radical cation with nucleophilic anions in nafion membranes

Abstract

The electron-transfer reaction from nucleophilic anions such as SCN-, N₃-, I-, and Br^- to the 1-pyrenesulfonic acid radical cation (Py^•+SA^-) generated via a resonant two-photon ionization process in the Nafion membranes was investigated with transient absorption measurements. The apparent quenching rates observed in the Nafion membrane (k_q^Nf) were almost 2-4 orders smaller than those observed in the bulk solutions (k_q^bulk). The attenuation factor (AF), which is defined as log(k_q^Nf/k_q^bulk), decreased in the order SCN- > N₃^- > I^- > Br^-. This interesting behavior was interpreted in terms of the anionic Hofmeister effects. The effects of hydrophobic organic cations such as tetrabutylammonium ion (Bu₄N⁺) and tetraethylammonium ion (Et₄N⁺) exchanged into the Nafion membranes were also examined.