Quenching of 2,5-diphenyloxazole (PPO) fluorescence by metal ions

Abstract

The quenching of 2, 5-diphenyloxazole (PPO) fluorescence by a series of transition metal ions (Mn 2+, Fe 2+, Co 2+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, Hg 2+, Pb 2+ has been studied. From a reasonably linear fit of the logarithm of the bimolecular quenching constant ( log k q) with the half-wave reduction potential ( E 1 2 in V versus SCE) of the various metal ions, the quenching was proposed to proceed via the formation of a non-emissive exciplex with charge transfer taking place from the excited fluorophore (PPO) to the metal ions. Ions like Fe 2+, Hg 2+ and Pb 2+ are extremely efficient quenchers, Cu 2+ and Ni 2+ are moderate quenchers and Co 2+, Zn 2+, Cd 2+ and Mn 2+ are very poor quenchers of PPO fluorescence. Various other probable mechanisms like ground-state complex formation, heavy atom quenching, paramagnetic effect, resonance energy transfer, etc., do not appear to be important in the fluorescence quenching of (PPO) though their presence cannot be completely excluded.