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Issue 14, 2012
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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

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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:Ce3+. 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 Eu2+ or Tb3+. 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 AE2+ the coordinative demand is also increasing from Mg2+ to Ba2+, 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 η5-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.

Graphical abstract: 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

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Supplementary files

Article information


Submitted
27 Oct 2011
Accepted
30 Nov 2011
First published
20 Jan 2012

Dalton Trans., 2012,41, 4067-4078
Article type
Paper

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

A. Zurawski, J.-Christoph Rybak, L. V. Meyer, P. R. Matthes, V. Stepanenko, N. Dannenbauer, F. Würthner and K. Müller-Buschbaum, Dalton Trans., 2012, 41, 4067
DOI: 10.1039/C2DT12047J

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