Abstract
Hydrothermal reactions of oxalic acid (H2ox) and 2-sulfobenzoic acid (H2L) with lanthanide oxide or hydroxide yielded five lanthanide oxalatosulfocarboxylates. They have a general formula [Ln2L2(ox)(H2O)6]n [Ln = La (1), Nd (2), Eu (3), Gd (4), Tb (5)]. Their structures were characterized by single crystal X-ray diffraction, powder X-ray diffraction (PXRD), infrared (IR) spectroscopy, elemental analysis (EA), and thermogravimetric analysis (TGA). 1–5 are isostructural coordination polymers and feature one-dimensional (1D) chains, which are extended into 3D supramolecular frameworks through inter-chain hydrogen bonds between coordinated H2O donors and oxygen acceptors from carboxylate, sulfonate and oxalate groups. In addition, the solid-state photoluminescence properties of the Nd (2), Eu (3) and Tb (5) derivatives have been measured at room temperature. The Eu and Tb-containing coordination polymers (CPs) exhibit intense red and green luminescence emissions, respectively, with lifetimes in the order of millisecond.
Highlights
Hydrothermal reactions of oxalic acid (H2ox) and 2-sulfobenzoic acid (H2L) with lanthanide oxide or hydroxide yielded five lanthanide oxalatosulfocarboxylates
Solid-state photoluminescence investigations revealed the π-conjugated 2-sulfobenzoic acid ligand can be a good chromophore as lanthanide luminescence sensitizer
Single-crystal X-ray diffractions, powder X-ray diffractions (Figure 1), elemental analyses and IR spectroscopic studies performed on 1–5 reveal that they are isostructural, with variations in the crystal unit cell volumes and Ln-O bond lengths decreasing from La (1) to Tb (5) (Tables 1 and 2), because of the lanthanide contraction effect
Summary
Hydrothermal reactions of oxalic acid (H2ox) and 2-sulfobenzoic acid (H2L) with lanthanide oxide or hydroxide yielded five lanthanide oxalatosulfocarboxylates. F-f transitions are parity forbidden according to the Laporte selection rules, which results in low molar absorptivities, making direct photo-excitation of Ln (III) ions inefficient, unless high power laser excitation sources are utilized [4] This can be solved via indirect excitation using a π-conjugated ligand chromophore as luminescence sensitizer. In a typical sensitization process, the ligand is excited through high absorptivity π-π* transition, which transfers energy to the Ln (III) ion, populating the Lncentered excited state, giving the lanthanide emission [5] This so called “antenna effect” has been well documented for lanthanide complexes based on β-diketonates, polyazine ligands, microcycle polyamines and aromatic carboxylate ligands [6–18]. Solid-state photoluminescence investigations revealed the π-conjugated 2-sulfobenzoic acid ligand can be a good chromophore as lanthanide luminescence sensitizer
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