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

The electron-transfer reaction from nucleophilic anions such as SCN(-), N(3)(-), 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(-) > N(3)(-) > I(-) > Br(-). This interesting behavior was interpreted in terms of the anionic Hofmeister effects. The effects of hydrophobic organic cations such as tetrabutylammonium ion (Bu(4)N(+)) and tetraethylammonium ion (Et(4)N(+)) exchanged into the Nafion membranes were also examined.