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 = <i>N</i>,<i>N</i>-dimethylformamide (DMF; 1), <i>N</i>,<i>N</i>-dimethylacetamide (DMA; 2), <i>N</i>,<i>N</i>-diethylformamide (DEF; 3), <i>N</i>-formylpiperidine (NFP; 4), phen = 1,10-phenanthroline and chdc^2- = <i>trans</i>-1,4-cyclohexanedicarboxylate were synthesized and structurally characterized. These compounds are based on similar binuclear samarium(III)-carboxylate blocks, bound by flexible chdc linkers into layered <i>sql</i>-type coordination networks. The amide solvents drive different intralayer block orientations between 1 and 2-4 and different layer-to-layer packings in all the described compounds. A pronounced dependence of the emission color upon the excitation wavelength variation was determined for 1-4, while the relative impacts of Sm³⁺ and phen emission on overall luminescence were found to depend strongly on these packings, and their reasonable correlation to the distances between the 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, bimetallic near-white luminophores were obtained for phases 3 and 4 by doping their synthetic systems with a minor (∼5%) Tb³⁺ additive. 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 on the optical properties of the included solvent.