Structures and Luminescent Sensing Properties of Terbium Metal–Organic Frameworks with Methyl-Decorated Phenanthroline Ligand

Two new Tb(III) metal–organic frameworks based on 4,7-dimethylphenanthroline (dmphen) and flexible ligand trans-1,4-cyclohexanedicarboxylate (chdc2−) were synthesized and characterized. Their crystallographic formulae are [Tb2(dmphen)2(H2O)2(chdc)3]·2DMF (1; DMF = N,N-dimethylformamide) and [Tb2(dmphen)2(NO3)2(chdc)2]·2DMF (2). Among some differences in their synthetic conditions, the most important one is apparently the using of terbium(III) nitrate instead of terbium(III) chloride as a metal precursor in the synthesis of 2, providing a nitrate coordination to Tb3+, and its subsequent notable structural differences to 1. Compound 1 was found to have a layered hcb structure with intralayer windows ca. 10 × 8 Å2 in size. Its layer-to-layer packing leaves narrow channels running across these windows, with 18% as a total solvent-accessible volume in the coordination structure. Compound 2 was found to have a layered sql structure with smaller intralayer windows of ca. 8 × 6 Å2 in size. Methyl substituents on the phen ligands do not affect the topology of the framework but seem to have a substantial effect on the packing density, as well as the pore volume of the resulting MOF. A high 18.4% luminescence quantum yield was found for 2. Its emission lifetime of 0.695(12) ms belongs to a typical range for phosphorescent Tb(III)-carboxylate complexes. A quenching of its emission by different nitroaromatic molecules was found. A linear concentration dependence on 3-nitrotoluene and 4-nitro-m-xylene at micromolar concentrations was found during luminescent titration experiments (LOD values ca. 350 nM), suggesting this MOF to be a viable and highly sensitive luminescent sensor for such substrates.