Highly porous three-dimensional Al- and Ga-MOFs with radical catecholate linker molecules were synthesized and characterized by electron diffraction, molecular simulation, Rietveld refinement and N2 sorption measurements.
A new scandium metal−organic framework (Sc-MOF) with the composition of [Sc(OH)(OBA)], denoted as Sc-CAU-21, was prepared under solvothermal reaction conditions using 4,4′-oxidibenzoic acid (H 2 OBA) as the ligand. Single-crystal structure determination revealed the presence of the new inorganic building unit (IBU) {Sc 8 (μ-OH) 8 (O 2 C) 16 }. It is composed of cisconnected ScO 6 polyhedra forming an eight-membered ring through bridging μ-OH groups. The connection of the IBUs leads to a 3D framework, containing 1D pores with a diameter between 4.2 and 5.6 Å. Pore access is limited by the size of the IBU, and in contrast to the isoreticular aluminum compound Al-CAU-21 [Al(OH)(OBA)], which is nonporous toward nitrogen at 77 K, Sc-CAU-21 exhibits a specific surface area of 610 m 2 g −1 . The title compound is thermally stable in air up to 350 °C and can be employed as a host for photoluminescent ions. Sc-CAU-21 exhibits a ligand-based blue emission, and (co)substituting Sc 3+ ions with Ln 3+ ions (Eu 3+ , Tb 3+ , and Dy 3+ ) allows the tuning of the emitting color of the phosphor from red to green. Single-phase white-light emission with CIE color coordinates close to the ideal for white-light emission was also achieved. The luminescence property was utilized in combination with powder X-ray diffraction to study in situ the crystallization process of Sc-CAU-21:Tb and Sc-CAU-21:Eu. Both studies indicate a two-step crystallization process, with a crystalline intermediate, prior to the formation of Sc-CAU-21:Ln.
In situ monitoring of the formation of emissive complexes is essential to enable the development of rational synthesis protocols, to provide accurate control over the generation of structure-related properties (such as luminescence) and to facilitate the development of new compounds. In situ luminescence analysis of coordination sensors (ILACS) utilizes the sensitivity of the spectroscopic properties of lanthanide ions to their coordination environment to detect structural changes during crystallization processes. Here, ILACS was utilized to monitor the formation of [Eu(bipy)(NO)] (bipy = 2,2'-bipyridine) during co-precipitation synthesis. Validity of the ILACS results was ensured by concomitant utilization of in situ monitoring of other reaction parameters, including in situ measurements of pH value, ionic conductivity, and infrared spectra, as well as ex situ and synchrotron-based in situ X-ray diffraction analyses. Gradual desolvation of the Eu ions and attachment of ligands were detected by an exponential increase of the intensity of the D → F (J = 0-4) transitions in the emission spectrum. Additionally, the in situ emission spectra show a decrease in the crystallization rate and an increase in the induction time in response to a reduction in the concentration of the starting solutions from 12 mM until crystallization ceased at starting reactant concentrations <6 mM. An increase to a three-fold higher concentration leads to the formation of a reaction intermediate, and its stability was determined to be highly concentration-dependent. The in situ luminescence measurements also demonstrated the existence of a ligand exchange process within the [Eu(bipy)(NO)] complex upon addition of a phen (phen = 1,10'-phenanthroline) solution and the generation of a new phen-containing emissive complex. In attempting to solve the structure of this new phen-containing complex, a different, but nevertheless previously unsynthesized complex, [Eu(phen)(NO)]bipy, was obtained, which shows characteristic Eu luminescence in the red spectral range.
Three structurally related metal–organic frameworks (MOFs) of composition [Sc2(OH)2(L)] denoted Sc-CAU-9-PTCDA, Sc-CAU-9-TCPB, and Sc-CAU-9-H2TCPP containing the tetratopic linker molecules L1 4– = PTCDA4– (perylene-3,4,9,10-tetracarboxylate), L2 4– = TCPB4– (1,2,4,5-tetrakis(4-carboxylatophenyl)-benzene), or L3 4– = H2TCPP4– (meso-tetra(4-carboxylatophenyl)-porphyrin), have been obtained from solvothermal syntheses. The crystal structures were determined from powder X-ray diffraction data. The title compounds contain chains of trans corner-sharing [ScO6] octahedra, connected by the linker molecules to form three-dimensional networks. Depending on the shape of the respective linker the compounds crystallize in a frz or fry topology and with increasing size of the linker, an expansion of the framework and the formation of accessible channels are observed. The compounds were thoroughly characterized and exhibit high thermal stabilities up to 500 °C. Sc-CAU-9-TCPB and Sc-CAU-9-H2TCPP possess permanent porosity toward nitrogen with specific surface areas of asBET = 1350 m2/g and 650 m2/g, respectively. Luminescence measurements revealed a ligand-based emission in the blue spectral region for Sc-CAU-9-TCPB.
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