Why are the luminescent properties of the mononuclear and binuclear Eu/ Tb-2-hydroxybenzoate complexes with 1,10-phenanthroline so different? An experimental-theoretical study

This study presents a comprehensive investigation into the structural and luminescent properties of lanthanide 2-hydroxybenzoate compounds, focusing on Eu 3 + and Tb 3 + ions in both single and mixed systems. The X-ray structures of (Hphen) 2 [Eu 2 (2-OHBz) 8 (H 2 O) 2 ] & sdot; 2H 2 O ( 1a & sdot; 2H 2 O) and [Eu(2-OHBz) 3 (phen) 2 ] & sdot; PhMe ( 2a & sdot; PhMe) were determined, where 2-OHBz: 2-hydroxybenzoate and phen: 1,10-phenantroline, revealing triclinic crystal structures with distinct coordination geometries. Compound 1a & sdot; 2H 2 O displayed a centrosymmetric dinuclear anion with a coordination number of eight for each Eu 3 + center, while compound 2a & sdot; PhMe exhibited a neutral mononuclear complex with a decacoordinated Eu 3 + center. A comparative analysis of luminescent properties between these compounds unveiled significant differences in the nonradiative decay rates of the 5 D 0 level of Eu 3 + ions, suggesting potential luminescence-quenching channels. The investigation is extended to mixed Eu 3 + -Tb 3 + systems, demonstrating efficient Tb 3 + to Eu 3 + energy transfer, especially at room temperature. Three models, including two novel ones proposed in this study, were considered to rationalize energy transfer in the mixed systems. Our theoretical-experimental investigation reveals that variations in lifetimes of the emitting level 5 D 0 of Eu-2-OHBz mononuclear and binuclear complexes, or mixed Eu/Tb-2-OHBz complexes, with 1,10-phenanthrolines, primarily result from multiphonon decay processes; however, low-energy ligand-to-metal charge transfer (LMCT) states play a key role in suppressing luminescence in the binuclear complex compare to the mononuclear ones.