Fluorescence quenching of liquid scintillation counters by metal ions

Abstract

The fluorescence quenching of four secondary liquid scintillation counters—POPOP, PBBO, α-NPO and BBO—by transition metal ions has been carried out in 40% methanol–water mixture at room temperature. The quenching is found to be appreciable and purely dynamic. The quenching rate constants were determined using the Stern–Volmer equation. The mechanism of quenching by a series of transition metal ions on these scintillation counters was studied. A reasonable linear fit of the logarithm of the bimolecular quenching constant (log k q ) with halfwave reduction potential ( E 1/2) of the various metal ions was obtained. A similar correlation between the bimolecular quenching rate constant (log k q ) and the oxidation potential of the scintillation counters was also observed. These results indicate the mechanism of quenching to be electron transfer in nature, mediated by the formation of a nonemissive exciplex and the direction of electron transfer is ascertained to be from the excited fluorophore to the metal d-orbitals, The values of Δ G ↕ 0 (the activation energy for the electron transfer process at Δ G=0) and the free energy at which the quenching is purely diffusion limited have been approximately estimated.