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

• Published in: Journal of luminescence 2024-08, Vol.272, p.120648, Article 120648
• Main Authors: Vasconcelos, Elaine S., Galdino, Renato H.M., Silva, Iran F., Pesci, Rafaela B.P., Deflon, Victor M., Teotonio, Ercules E.S., Malta, Oscar L., Sinha, Shyama P., Faustino, Wagner M.
• Format: Article
• Language: English
• ISSN: 0022-2313; 1872-7883
• DOI: 10.1016/j.jlumin.2024.120648

This study presents a comprehensive investigation into the structural and luminescent properties of lanthanide 2-hydroxybenzoate compounds, focusing on Eu3+ and Tb3+ ions in both single and mixed systems. The X-ray structures of (Hphen)2[Eu2(2-OHBz)8(H2O)2]·2H2O (1a·2H2O) and [Eu(2-OHBz)3(phen)2]·PhMe (2a·PhMe) were determined, where 2-OHBz: 2-hydroxybenzoate and phen: 1,10-phenantroline, revealing triclinic crystal structures with distinct coordination geometries. Compound 1a·2H2O displayed a centrosymmetric dinuclear anion with a coordination number of eight for each Eu3+ center, while compound 2a·PhMe exhibited a neutral mononuclear complex with a decacoordinated Eu3+ center. A comparative analysis of luminescent properties between these compounds unveiled significant differences in the nonradiative decay rates of the 5D0 level of Eu3+ ions, suggesting potential luminescence-quenching channels. The investigation is extended to mixed Eu3+-Tb3+ systems, demonstrating efficient Tb3+ to Eu3+ 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 5D0 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. The image depicts X-ray structures of (Hphen)2[Eu2(2-OHBz)8(H2O)2]·2H2O (1a) and [Eu(2-OHBz)3(phen)2]·PhMe (2a), sample photos with UV illumination, energy transfer processes, and a luminescence decay curve modeled with three different approaches. These models are used to rationalize the luminescent properties of mononuclear and binuclear Eu/Tb-2-hydroxybenzoate complexes with 1,10-phenanthroline. [Display omitted] •Structural and luminescent properties of the mono and binuclear Eu/Tb-2-hydroxybenzoate complexes with 1,10-phenanthroline.•Luminescence activation and quenching of Eu3+ ions in mixed systems.•Novel models for understanding energy transfer and excited-state lifetimes in mixed lanthanide systems.