Inter- and intra-molecular energy transfer during sensitization of Eu(DBM)3Phen luminescence by Tb(DBM)3Phen in PMMA

A series of Eu(DBM)3Phen and Tb(DBM)3Phen co-doped poly(methyl methacrylate) (EuxTby-PMMA, subscript x and y denote molar ratio of the ion to repeating unit of PMMA in each sample) were prepared with different Eu/Tb contents. During luminescence measurement of these samples, it was found that for Eu0.010Tb0-PMMA the maximum excitation was found to be at 363 nm, and for Eu0Tb0.010-PMMA the maximum excitation at 274 nm. This difference reveals that there is different intra-molecular energy transfer mechanism for these two complexes, which contain the same ligands but different central ions. For Eu0.0050Tby-PMMA (y = 0, 5.0 × 10−6, 3.0 × 10−5, 5.0 × 10−5, 3.0 × 10−4, 4.0 × 10−4, 1.0 × 10−3, 4.0 × 10−3, 1.0 × 10−2, 2.0 × 10−2, 4.0 × 10−2, 6.0 × 10−2 and 8.0 × 10−2, respectively), enhanced luminescence of Eu3+ at 612 nm for 5D0 → 7F2 was observed at the concentration of Tb3+ higher than 0.0048 mol/L. At this Tb3+ content, the minimum intensity of the luminescence of Eu(DBM)3Phen was observed, which comes from the balance between absorption consumption of ligands in Tb(DBM)3Phen and sensitizing in luminescence of Eu3+ by Tb3+. EuxTb0.010-PMMA (x = 0, 5.0 × 10−3, 8.0 × 10−3, 9.0 × 10−3, 0.013, 0.020, 0.030, 0.040 and 0.060) showed a gradually increasing emission of Eu3+ at 535 nm along with decreasing of Tb3+ emission at 546 nm, which resulted from a fast intra-molecular energy transfer from 5D1 to 5D0 of Eu3+ and an inter-molecular energy transfer from the triplet state of DBM in Tb(DBM)3Phen to 5D1 of Eu3+, and the latter energy transfer process is directly dependent on the distance between two kinds of ions. Based on observation above, it was realized that there will be a critical distance between two kinds of complexes in each EuxTby-PMMA sample, which determines the minimum concentration of Tb(DBM)3Phen for the sensitization in luminescence of the sample. To give a vivid picture for this complex system, a model for the sensitization of EuxTby-PMMA was established in terms of Perrin formulation. The radius of the quenching sphere, which corresponds to the critical distance between Eu3+ and Tb3+, was found to be 1.633 nm for the sensitization in EuxTby-PMMA.