Structures, luminescence, and slow magnetic relaxation of eight 3D lanthanide–organic frameworks

Abstract

Eight three-dimensional lanthanide–organic frameworks: [Ln(BPDC)_1.5(DMF)(H₂O)₂]·2H₂O (Ln = Eu (1), Gd (2), Tb (3), Dy (4); BPDC = 4,4′-dicarboxylate-2,2′-dipyridine anion), [Ln(BPDC)(DMF)₂(NO₃)] (Ln = Eu (5), Gd (6), Tb (7), Dy (8)) were fabricated and structurally characterized. Compounds 1–4 are isostructural, belonging to the triclinic system with space group P, while compounds 6–8 belong to the monoclinic system with space group C2/c. Structural differences between two types of compounds may be caused by different reaction conditions. Magnetic properties of 2–4 and 6–8 have been investigated and only compounds 4 and 8 display significant frequency-dependence, albeit without reaching the characteristic maxima above 2 K, implying slow magnetic relaxation behaviors in 4 and 8. After the application of a DC field, good peak shapes of AC signal were obtained and got the energy barrier for 4, ΔE/k_B = 79.80 K and the pre-exponential factor τ₀ = 1.28 × 10⁻¹⁰ s, for 8, ΔE/k_B = 38.15 K and τ₀ = 2.47 × 10⁻⁹ s. Geometrical differences in the crystal fields of Dy³⁺ in 4 and 8 seem to be responsible for the large divergence of their magnetic behaviors. Luminescence analyses were performed on coordination polymers containing Eu³⁺, Tb³⁺, and Dy³⁺, which exhibit the characteristic transitions of corresponding lanthanide ions, and give the lifetime (τ₀) of 1, 3, 4, 5, 7 and 8 are 0.56 ms, 0.89 ms, 8.48 μs, 0.60 ms, 0.75 ms and 36.35 μs, respectively.