Alkaline earth imidazolate coordination polymers by solvent free melt synthesis as potential host lattices for rare earth photoluminescence: X∞[AE(Im) 2(ImH) 2-3], Mg, Ca, Sr, Ba, x = 1-2
ABSTRACT The series of alkaline earth elements magnesium, calcium, strontium and barium yields single crystalline imidazolate coordination polymers by reactions of the metals with a melt of 1H-imidazole: (1)(∞)[Mg(Im)(2)(ImH)(3)] (1), (2)(∞)[AE(Im)(2)(ImH)(2)], AE = Ca (2), Sr (3), and (1)(∞)[Ba(Im)(2)(ImH)(2)] (4). No additional solvents were used for the reactions. Co-doping experiments by addition of the rare earth elements cerium, europium and terbium were carried out. They indicate (2)(∞)[Sr(Im)(2)(ImH)(2)] as a possible host lattice for cerium(III) photoluminescence showing a blue emission and thus a novel blue emitting hybrid material phosphor 3:Ce(3+). Co-doping with europium and terbium is also possible but resulted in formation of (3)(∞)[Sr(Im)(2)]:Ln, Ln = Eu and Tb (5), with both exhibiting green emission of either Eu(2+) or Tb(3+). The other alkaline earth elements do not show acceptance of the rare earth ions investigated and a different structural chemistry. For magnesium and barium one-dimensional strand structures are observed whereas calcium and strontium give two-dimensional network structures. Combined with an increase of the ionic radii of AE(2+) the coordinative demand is also increasing from Mg(2+) to Ba(2+), reflected by four different crystal structures for the four elements Mg, Ca, Sr, Ba in 1-4. Different linkages of the imidazolate ligands result in a change from complete σ-N coordination in 1 to additional η(5)-π coordination in 4. The success of co-doping with different lanthanide ions is based on a match in the chemical behaviour and cationic radii. The use of strontium for host lattices with imidazole is a rare example in coordination chemistry of co-doping with small amounts of luminescence centers and successfully reduces the amount of high price rare earth elements in hybrid materials while maintaining the properties. All compounds are examples of pure N-coordinated coordination polymers of the alkaline earth metals and were identified by single crystal X-ray analysis and powder diffraction. The degree of co-doping was determined by SEM/EDX. Mid IR, Far IR and Raman spectroscopy and micro analyses as well as simultaneous DTA/TG were also carried out to characterize the products in addition to the photoluminescence studies of the co-doped samples.
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ABSTRACT: The series of anhydrous lanthanide chlorides LnCl3, Ln = Pr-Tb, and 4,4[prime or minute]-bipyridine (bipy) constitute isotypic MOFs of the formula 2[infinity][Ln2Cl6(bipy)3][middle dot]2bipy. The europium and terbium containing compounds both exhibit luminescence of the referring trivalent lanthanide ions, giving a red luminescence for Eu3+ and a green luminescence for Tb3+ triggered by an efficient antenna effect of the 4,4[prime or minute]-bipyridine linkers. Mixing of different lanthanides in one MOF structure was undertaken to investigate the potential of this MOF system for colour tuning of the luminescence. Based on the gadolinium containing compound, co-doping with different amounts of europium and terbium proves successful and yields solid solutions of the formula 2[infinity][Gd2-x-yEuxTbyCl6(bipy)3][middle dot]2bipy (1-8), 0 [less-than-or-equal] x, y [less-than-or-equal] 0.5. The series of MOFs exhibits the opportunity of tuning the emission colour in-between green and red. Depending on the atomic ratio Gd:Eu:Tb, the yellow region was covered for the first time for an oxygen/carboxylate-free MOF system. In addition to a ligand to metal energy transfer (LMET) from the lowest ligand-centered triplet state of 4,4[prime or minute]-bipyridine, a metal to metal energy transfer (MMET) between 4f-levels from Tb3+ to Eu3+ is as well vital for the emission colour. However, no involvement of Gd3+ in energy transfers is observed rendering it a suitable host lattice ion and connectivity centre for diluting the other two rare earth ions in the solid state. The materials retain their luminescence during activation of the MOFs for microporosity.Journal of Materials Chemistry 05/2012; 22(20):10179-10187. DOI:10.1039/C2JM15571K · 7.44 Impact Factor
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ABSTRACT: By reacting iron(III) chloride with a melt of piperazine, the complex [FeCl3(Hpipz)(pipz)] is formed containing equatorial chloride anions and axial piperazine ligands constituting a trigonal bipyramidal coordination polyhedron. The obtained product is an example for the capture of a precursor complex under melt conditions for the formation of coordination polymers and is thus of limited crystallinity. Characterization was achieved by solution and refinement of the crystal structure by a combination of powder X-ray diffraction, geometrical optimization methods, 57Fe-Mößbauer, and IR spectroscopy, magnetic investigations, and elemental analysis. In addition, corroboration by X-ray single crystal data was used to prove the results from powder diffraction as suitable single crystals were finally achieved by modification of the synthesis with the auxiliary solvent pyridine.Zeitschrift für anorganische Chemie 10/2012; 638(12‐13). DOI:10.1002/zaac.201200148 · 1.25 Impact Factor
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ABSTRACT: Four alkali-induced 3D coordination polymers, namely [Sr(HIDC)(H3IDC)(H2O)]n·nH2O (1), [Sr(HIDC)]n (2), [Sr3(HIDC)2(H2IDC)4]n (3), and [Sr2(HIDC)2]n (4), were constructed from strontium(II) and imidazole-4,5-dicarboxylic acid (H3IDC) and were characterized by elemental analysis, IR, TG, PL, powder and single-crystal X-ray diffraction. Complex 1 is an eca net with the Schläfli symbol of (39.412.57) by considering the SrII cation as an 8-connected node. Complex 2 presents a rare 5-connected network with Schäfli symbol of (45.65) when the SrII cation and HIDC2– dianion are viewed as 5-connected nodes. Complex 3 is a complicated 3D network, which can be regarded as a H2IDC– monoanion sustained porous framework filled by another HIDC2– dianion and H2IDC– monoanion. Complex 4 exhibits a 3D porous framework with 1D trigonal hydrophobic channels. The ligand H3IDC in the four complexes exists as the H3IDC molecule, the H2IDC– monoanion, and the HIDC2– dianion. In addition, the μ2 (κ1N:κ1O:κ1O) mode of the H3IDC molecule in complex 1, the μ3 (κ1O:κ1O:κ1O:κ1O) and μ3 (κ1N:κ1O:κ1O:κ1O) modes of the H2IDC– monoanion in complex 3, as well as the μ5 (κ1N:κ2O:κ1O:κ2O:κ1O) and μ6 (κ1N:κ2O:κ2O:κ2O:κ2O) modes of the HIDC2– dianion in complexes 2 and 4 are reported for the first time. Diverse topological architectures of the four complexes depend mainly on the coordination spheres of the SrII cation, the coordination modes of the anions, the different alkali, and the ligand to alkali ratio. TG analysis indicates that complex 2 presents high thermal stability, while the TG curve and temperature-dependent PXRD spectra of complex 4 indicate that its framework is stable up to 350 °C. Moreover, the solid-state luminescent properties demonstrate that complexes 1 to 4 exhibit strong blue emission at room temperature.Berichte der deutschen chemischen Gesellschaft 11/2012; 2012(33). DOI:10.1002/ejic.201200796