2D and 3D lanthanide metal–organic frameworks constructed from three benzenedicarboxylate ligands: synthesis, structure and luminescent properties

Abstract

Nineteen lanthanide–benzenedicarboxylate complexes were obtained under similar hydrothermal conditions: [Ln(IP-Py)(HIP-Py)(H₂O)₂]_n (Ln = Pr (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6)), [Ln₄(IP)₆(H₂O)₄(DMF)·DMF·mH₂O]_n (Ln = Sm, m = 1 (7), Eu, m = 1.5 (8), Gd, m = 1.5 (9), Tb, m = 1.5 (10), Dy, m = 1 (11), Ho, m = 1.5 (12)), [Ln₂(IP-Py)₂(IP)(H₂O)₄·H₂O]_n (Ln = Sm (13), Eu (14), Tb (15), Dy (16)) and [Ln₂(IP-Py)₂(TP)(H₂O)₄·H₂O]_n (Ln = Sm (17), Eu (18), Gd (19)), where: H₂IP-Py = 5-(4-pyridyl)-isophthalic acid), H₂IP = isophthalic acid, H₂TP = terephthalic acid. Complexes 1–6 exhibit a two-dimensional layered structure based on H₂IP-Py. For 7–12, the Ln³⁺ ions are linked into one-dimensional chains with two triple-stranded helices of opposite handedness and further constructed into a two-dimensional layer by coordination of the IP²⁻ ligands. For complexes 13–16 and 17–19, a similar two-dimensional layer was formed by coordination of Ln³⁺ and IP-Py²⁻, and three-dimensional pillar-layered frameworks were formed due to the layers bridged by the auxiliary IP²⁻ or TP²⁻ ligand, respectively. Most complexes show the characteristic narrow-banded lanthanide 4f^N–4f^N emission in combination with luminescence from the ligands, with the relative contribution depending on the energy transfer from the ligands to the lanthanides. The ligands play an important role in the sensitization of the lanthanide emission. The onset of the main excitation band for the Eu³⁺ complexes is found at the lowest energy for 14, enabling excitation by near-UV LEDs. The complexes containing the technologically relevant ions for (back) lighting applications, Tb³⁺ (green) and Eu³⁺ (red), are evaluated in detail in terms of luminescence lifetime and thermal stability. The emission intensity of selected Eu³⁺ and Tb³⁺ compounds is stable up to room temperature.