Single-crystal-to-single-crystal post-synthetic modifications of three-dimensional LOFs (Ln = Gd, Eu): A way to modulate their luminescence and thermometric properties

Single-crystal-to-single-crystal post-synthetic modifications (PSMs) of Lanthanide Organic Frameworks (LOFs) {[Ln2(H2L)3(DMF)4]·2DMF}n(Ln = Gd,1and Eu,2; H4L = 2,5-dihydroxyterephthalic acid; DMF = dimethylformamide), carried out by treatment with (a) chloroform or (b) an imidazole solution in chloroform, yielded respectively isostructural {[Ln2(H2L)3(DMF)4]·CHCl3}n(Ln = Gd,3; Eu,4) or {[Ln2(H2L)3(Im)4][Ln2(H2L)3(Im)2(H2O)2]·6Im·2CHCl3}n(Ln = Gd,5; Eu,6). Single crystal X-ray diffraction studies of5showed two different regularly alternating dimeric units in the LOF and two regularly alternating cavities with different guest molecules. All compounds revert to the parent LOF,1or2, when treated with DMF at 90 °C for 8 h. Europium-LOFs (24and6) showed a faint red emission that progressively gained intensity upon cooling from RT to liquid nitrogen temperature. This behaviour was exploited to develop a family of luminescent thermometers whose characteristics were affected by the structural and coordinative modifications induced by PSMs.