Exciton Diffusion in Solid Solutions of Luminescent Lanthanide β-Diketonates

In this Article, a series of luminescent lanthanide β-diketonate solid solutions, in the formula of TBAEuxM1-x(TTA)4} (TBA = tetrabutylammonium; M = Eu or Gd; TTA = 2-thenoyltrifluoroacetonate), are synthesized by co-precipitation. In the solid solutions, the emission efficiency of Eu3+ is significantly increased with the presence of non-luminescent chelates TBALa(TTA)4 and TBAGd(TTA)4. Low temperature luminescent spectroscopy studies indicate that the TTA- ligands in these non-luminescent chelates do emit phosphorescence with long lifetime. However, the ligand phosphorescence is strongly quenched in solid solutions with the luminescent chelate TBAEu(TTA)4, providing a strong evidence for intermolecular energy transfer through the triplet excited states of the ligands. A quantitative analysis of Eu3+ emission enhancement and TTA- phosphorescence quenching reveals that each Eu3+ center may receive excitation energy from about 30 TTA- ligands, suggesting that the excitation energy has become exciton-like in the solid solutions. Based on the crystallography analysis of TBALn(TTA)4, it is discovered that TTA- ligands in neighboring Ln(TTA)4- units may form π-π stacks with intermolecular distance ≤ 3.5 A, thus enabling efficient triplet exciton diffusion via exchange interaction.