Large Pressure‑Induced Emission Enhancement in Rigid Lanthanide Coordination Polymers

Lanthanide coordination polymers (Ln-CPs) hold significant potential for advanced optical applications due to their tunable structures and distinctive emission properties. However, their performance is hindered by the inherently low direct transition efficiency of Ln³⁺ ions, which requires optimizing the ligand-to-metal energy transfer (LMET) process. Herein, we demonstrate that pressure can enhance the energy transfer efficiencies in two isostructural rigid lanthanide compounds, [Tb(HCOO)(C₂O₄)]_n and [Eu(HCOO)(C₂O₄)]_n. Our in-situ high-pressure spectroscopy experiments reveal that hydrostatic compression significantly improves their photoluminescence (PL) intensities, with a 15- and 12-fold increase for the terbium and europium compounds, respectively. Synchrotron high-pressure x-ray diffraction and first principles calculations reveal the enhanced emission results from pressure-induced increase in ligand electron delocalization, which effectively lowers the ligands' excited-state energy levels. This increases the orbital overlap between the excited states of the ligands and Ln³⁺ ions, hence substantially boosting energy transfer efficiency. Additionally, the rigid lattice effectively suppresses thermally activated non-radiative transitions under high pressure, minimizing energy loss during the energy transfer process. Our findings offer a convenient approach to achieve efficient rare-earth phosphorescence through pressure management.