A series of lanthanide–quinoxaline-2,3(1H,4H)-dione complexes containing 1D chiral Ln2O3 (Ln = Eu, Tb, Sm, Dy) chains: luminescent properties and response to small molecules

Abstract

Five new isostructural lanthanide–organic complexes, [Ln₂O₂(OH)(HQXD)(H₂QXD)₂]·H₂O (Ln = Eu 1, Tb 2, Sm 3 Dy 4 and Gd 5; H₂QXD = quinoxaline-2,3(1H,4H)-dione), have been synthesized under hydrothermal conditions. These complexes are characterized by powder X-ray diffraction (PXRD), infrared spectroscopy (IR), elemental analysis (EA), thermogravimetric-differential thermal analysis (TG-DTA) and photo-luminescent spectroscopy. Single crystal X-ray diffraction analysis of complex 1 revealed that the structure featured in 1D chiral “Eu₂O₃” chains surrounded by coordinating organic ligands. These chains are interconnected via hydrogen bonding and offset π⋯π stacking interactions of the ligands to form the 3D supramolecular frameworks. The photo-luminescence studies for complexes 1–5 disclosed that the ligand (H₂QXD) showed an antenna effect to transfer energy toward the lanthanide cations. The energy transfer mechanism investigations show that the energy transition from the triplet energy level (³ππ*) of ligand H₂QXD to the Tb³⁺ cation is more effective than to the Eu³⁺, Sm³⁺ and Dy³⁺ ions; therefore it has been selected as a representative to examine the potential for sensing small molecules. Complex 2′, which was obtained by the heating treatment of 2 at 150 °C, displayed a high luminescence sensitivity towards small solvent molecules. Tertiary butanol (t-butanol) was found to be an excellent sensitizer, while tetrahydrofuran (THF) was a highly quenching species. Complex 2′ could regain a higher photo-luminescence intensity after treating for 5 cycles with t-butanol, revealing a prospect for reusability.