Exciton diffusion in solid solutions of luminescent lanthanide β-diketonates

Abstract

In this article, a series of luminescent lanthanide β-diketonate solid solutions, with the formula of TBAEu_xM_1−x(TTA)₄ (TBA = tetrabutylammonium; M = La 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 luminescence 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 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 Å, thus enabling efficient triplet exciton diffusion via exchange interaction.