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Photoluminescent Lanthanide-Organic Bilayer Networks with 2,3-Pyrazinedicarboxylate and Oxalate

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posted on 05.04.2010, 00:00 by Paula C. R. Soares-Santos, Luís Cunha-Silva, Filipe A. Almeida Paz, Rute A. S. Ferreira, João Rocha, Luís D. Carlos, Helena I. S. Nogueira
The hydrothermal reaction between lanthanide nitrates and 2,3-pyrazinedicarboxylic acid led to a new series of two-dimensional (2D) lanthanide-organic frameworks: [Ln2(2,3-pzdc)2(ox)(H2O)2]n [where 2,3-pzdc2− = 2,3-pyrazinedicarboxylate, ox2− = oxalate, and Ln(III) = Ce, Nd, Sm, Eu, Gd, Tb, or Er]. The structural details of these materials were determined by single-crystal X-ray diffraction (for Ce3+ and Nd3+) that revealed the formation of a layered structure. Cationic monolayers of {2[Ln(2,3-pzdc)(H2O)]+} are interconnected via the ox2− ligand leading to the formation of neutral 2[Ln2(2,3-pzdc)2(ox)(H2O)2] bilayer networks; structural cohesion of the crystalline packing is reinforced by the presence of highly directional O−H···O hydrogen bonds between adjacent bilayers. Under the employed hydrothermal conditions 2,3-pyrazinedicarboxylic acid can be decomposed into ox2− and 2-pyrazinecarboxylate (2-pzc), as unequivocally proved by the isolation of the discrete complex [Tb2(2-pzc)4(ox)(H2O)6]·10H2O. Single-crystal X-ray diffraction of this latter complex revealed its co-crystallization with an unprecedented (H2O)16 water cluster. Photoluminescence measurements were performed for the Nd3+, Sm3+, Eu3+, and Tb3+ compounds which show, under UV excitation at room temperature, the Ln3+ characteristic intra-4fN emission peaks. The energy level of the triplet states of 2,3-pyrazinedicarboxylic acid (18939 cm−1) and oxalic acid (24570 cm−1) was determined from the 12 K emission spectrum of the Gd3+ compound. The 5D0 and 5D4 lifetime values (0.333 ± 0.006 and 0.577 ± 0.017 ms) and the absolute emission quantum yields (0.13 ± 0.01 and 0.05 ± 0.01) were determined for the Eu3+ and Tb3+ compounds, respectively. For the Eu3+ compound the energy transfer efficiency arising from the ligands' excited states was estimated (0.93 ± 0.01).