Europium and terbium tris(dipicolinates) as secondary standards for quantum yield determination

Aqueous. solutions of europium(III) and terbium(III) tris(dipicolinates), around physiological pH are shown to be convenient secondary standards in the determination of quantum yields of lanthanide complexes containing these ions. Conditions for which a strict linearity is observed between the concentration of the solutions and the emission intensity are established. The speciation in these solutions, which contain a non-negligible amount of bis species is presented and discussed on the basis of both stability con stants and lifetime determinations of the Eu(D-5(0)) level. The Q(L)(Eu) quantum yield Q(L)(Eu) displays a strong pH-dependence: for a solution with an absorbance of 0.30, it increases sharply from about 2% at pH 2.5 to reach 11.5-12.5% in the pH range of 6-9. The proposed standard for Eu-III is a solution Of Cs-3[Eu(dpa)(3)] 7.5 x 10(-5) M in tris buffer 0.1 M (absorbance = 0.20) with a quantum yield of 13.5% +/- 1.5% under excitation at 279 nm. For Tb-III, we propose a standard solution of Cs-3[Tb(dpa)(3)] 6.5 x 10(-5) M in tris buffer 0.1 M (absorbance = 0. 18) with a quantum yield of 26.5% +/- 2% under excitation at 279nm. Despite the speciation between his and tris complexes, these two standard buffered solutions present a constant quantum yield within a reasonably large range of concentration and they are easy to handle, which makes them adequate for laboratory use.