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

The quenching of 2,5-diphenyloxazole (PPO) fluorescence by a series of transition metal ions (Mn2+, Fe2+, Co2+ Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+) 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 Fe2+,Hg2+ and Pb2+ are extremely efficient quenchers, Cu2+ and Ni2+ are moderate quenchers and Co2+, Zn2+, Cd2+ and Mn2+ 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.