Photoionization and Ion-Radical Decay of Anthracene in a Water Swollen Nafion Network. Effect of Different Counterions on the SO3-−Water Clusters

Abstract

The formation of anthracene ion radicals in the H + -, Na + -, Fe 2 + -, and Fe 3 + -Nafion membrane was established by laser kinetic spectroscopy. The formation of ion radicals was observed to be due to (1) the two photon ionization of the anthracene molecule with the formation of the radical cation An + . radical and a fast electron scavenging by H + , Na + , Fe 2 + , and Fe 3 + acting as efficient electron traps in solution and (2) the quenching of the excited anthracene by Fe 3 + that leads by a redox process to the formation of An + . and Fe 2 + . It is shown that the ensuing kinetics of the ion-radical decay depends on the chemical nature of the traps. The lifetime of T An becomes shorter after Fe 2 + or Fe 3 + is introduced in the Nafion. The steady-state anthracene fluorescence is quenched by Fe 3 + or Fe 2 + and followed the logarithmic decay law ln(I 0 /I) where the decay in solution was seen to be proportional to the [Fe 3 + ] or [Fe 2 + ]. The counterions of SO 3 - -water clusters as well as of oxygen in the reaction media strongly affect the kinetics of ion-radical reactions occurring in the Nafion membrane. The counterions and oxygen are suggested to be the traps for the generated electrons in solution. The excited state of An was shown to react with Fe 3 + through electron transfer. Triplet excited anthracene molecules are quenched by Fe 3 + and Fe 2 + with rate constants k q (Fe 3 + ) = (1.9 ′ 0.19) × 10 8 M - 1 s - 1 and k q (Fe 2 + ) = (1.4 ′ 0.11) × 10 9 M - 1 s - 1 . Anthracene ion radicals are formed in the reaction with Fe 3 + but not with Fe 2 + on thermodynamic grounds. Fe 2 + or Fe 3 + being different chemical species quench with similar rates the An probe inside the Nafion membrane. Treatment of Fe 3 + -Nafion with NaOH leads to precipitation of iron particles in the membrane having as consequences (1) the decrease of the observed rate for the triplet excited anthracene quenching with a concomitant decrease in the observed An + . yields and (2) a significant decrease of the An + . decay time because of the lowering of the mobility of the iron ions in solution.