Urothermal Synthesis of Luminescent Alkaline Earth MOFs

The urothermal approach in 1,3-dimethylimidazolidin-2-one (e-murea) was employed for the successful preparation of a series of nine alkaline earth (AE) metal-organic frameworks (AE-MOFs) with two different dihydroxy terephthalic acid ligands (2,5-dobdcH₄ and 2,3-dobdcH₄; dobdcH₄ = dihydroxyterephthalic acid). The compounds were characterized by single-crystal and powder X-ray diffraction as well as elemental analysis and infra-red spectroscopy to confirm their purity. With the 2,5-dobdcH₂ ^2- ligand, two isostructural 3D MOFs were obtained with Mg(II) and Ca(II), whereas 2D architectures featuring coordination of the hydroxyl groups were isolated with Sr(II) and Ba(II). Binding of the hydroxyl groups in a chelate fashion is consistently observed in AE-MOFs incorporating the 2,3-dobdcH₂ ^2- ligand. These AE-MOFs present strong luminescence in the solid state with quantum yields ranging between 0.45 and 0.84, remarkable for this type of compounds. Interestingly for the AE-MOFs based on the 2,5-dobdcH₂ ^2- ligand, a dependence on the nature of the alkaline earth metal cation was observed, as expressed for the heavier Sr(II) and Ba(II) systems in the presence of two different emission processes in thermal equilibrium as confirmed by investigation of the solid-state luminescence at 77K. For the AE-MOFs based on the 2,3-dobdcH₂ ^2- ligand, such metal- and temperature-dependence is not observed.