Solvatomorphic Phase Transitions and Tunable Luminescence Emission in Lanthanide Metal-Organic Frameworks

Four new metal-organic frameworks with the formulae [Sm₂(phen)₂(NO₃)₂(chdc)₂]·2Solv, where Solv = N,N-dimethylformamide (DMF; 1), N,N-dimethylacetamide (DMA; 2), N,N-diethylformamide (DEF; 3), N-formylpiperidine (NFP; 4), phen = 1,10-phenanthroline and chdc^2– = trans-1,4-cyclohexanedicarboxylate were synthesized and structurally characterized. These compounds are based on similar binuclear samarium(III)-carboxylate blocks, bound by flexible chdc linker into layered sql-type coordination networks. Different amide solvents drive different intralayer block orientations between 1 and 2–4 and different layer-to-layer packings in all the described compounds. A strong dependence of the emission color upon varying excitation wavelength was determined for 1–4, while relative impacts of Sm³⁺ and phen emission into overall luminescence were found to depend strongly on these packings, and their reasonable correlation to the distances between closest π-π-stacked phen moieties in the structures was revealed. Phase transitions between compounds 1–4 were studied by means of powder X-ray diffraction. Additionally, two-metal near-white luminophores were obtained for phases 3 and 4 by doping their synthetic systems by minor (~5%) additive of Tb³⁺. In general, this study shows a possibility of tuning the luminescence properties of porous metal-organic frameworks by minor structural differences induced by solvent-driven dynamics with no apparent quenching or other direct impact of the optical properties of the solvent included.