Ligand-to-Metal Energy Transfer in terbium and europium oxalate heptahydrate crystals: Understanding the influence of oxalate ligand on the photoluminescent properties

The intense green and red emission of terbium and europium oxalates are attributed to the cross-relaxation process between lanthanide ions. However, the role of the organic ligand as a sensitizing agent for the emission has not been fully elucidated, leaving the photoluminescent (PL) properties relatively unexplored. This work presents a comprehensive study of the Ligand-to-Metal Energy Transfer (LMET) in terbium and europium oxalates heptahydrate. Single crystals were grown using the hydro-silica gel technique, and a novel improvement of the synthesis procedure which allowed growing substantially larger europium oxalate crystals than in previous studies in the field is also reported. X-ray diffraction (XRD), Fourier-Transform Infrared spectroscopy (FTIR), and thermogravimetric analysis confirmed the chemical composition RE₂(C₂O₄)₃⋅7H₂O. PL studies provided reliable evidence of a sensitized emission via the antenna effect, indicating that the LMET contributes to the population of the ⁵D_j emissive levels of Ln³⁺ ions, assisting the cross-relaxation process. These findings enhance our understanding of the PL properties of terbium and europium oxalates and demonstrate that the oxalate ligand is a more effective luminescent sensitizer for Tb³⁺ ions than for Eu³⁺ ions. Additionally, PL excitation studies on terbium and europium oxalate decahydrate crystals were conducted to contrast the emission properties between both forms of hydrate oxalates. Notably, terbium oxalate heptahydrate crystals exhibit a significant improvement in the Charge Transfer (CT) due to higher intramolecular charge transfer.