Enhancing the quantum yield of lanthanide clusters via single-site ligand modulation

The optical properties of trivalent lanthanide ions (Ln³⁺) are restricted by their lattice sites, and there are still challenges in regulating their lattice sites and coordination environment at the atomic scale. This work proposes and validates a strategy of ligand engineering unit site regulation that achieves effective regulation of local coordination symmetry by precisely replacing a single coordination molecule on an atomically precise Eu₄Ti₉ cluster. We synthesized an Eu₄Ti₉ cluster protected by 4-trifluoromethyl benzoic acid (4-TFMBA), in which Eu³⁺ exists in two coordination configurations (Eu1 adopts a D_4d arrangement, while Eu2 changes from D_4d in Eu4Ti9-H to D_2d in Eu4Ti9-P). Without changing the coordination environment of the Eu1 site, replacing the terminal water molecule ligand at the Eu2 site with triphenylphosphine oxide (TPPO) can significantly reduce the local symmetry and increases the photoluminescence quantum yield of the cluster from 11.32% to 28.94%. By combining crystallographic analysis, the Judd-Ofelt theory, and temperature-dependent spectroscopy, we clarify the quantitative structure-activity relationship between the local symmetry change, crystal field parameter evolution, and luminescence enhancement. This work not only reports creation of a class of luminescent clusters but also, more importantly, provides a new way to control the luminescence dynamics of lanthanide materials at atomic precision through rational ligand design, which is of great significance to the development of lanthanide-based materials.