Ligand-Dependent Luminescence Properties of Lanthanide-Titanium Oxo Clusters

A series of lanthanide-titanium oxo clusters (LnTOCs), Ln₂Ti₈-Ac, Ln₂Ti₈-p-Toluic, and Ln₂Ti₈-Anthra (Ln = Eu and Tb), were prepared based on acetic acid (HAc), p-toluic acid (Hp-Toluic), and anthracene-9-carboxylic acid (HAnthra). Crystal structural analysis showed that these clusters possess the same metal topology framework, in which eight Ti⁴⁺ ions form a cube and two Ln³⁺ ions are located on the opposite faces of the cube. The luminescence investigation discovered that the Eu₂Ti₈-Ac displays the highest quantum yields with 15.6%, and the conjugation effect of ligand substituents can lower the triplet state energy of ligands, thus regulating the luminescence quantum yield of the Ln₂Ti₈ clusters. These results suggest that the triplet excited-state energy of the ligands should match well with the energy levels of Ln³⁺ to enhance the luminescence.