Journal
CHEMISTRY-A EUROPEAN JOURNAL
Volume 19, Issue 51, Pages 17369-17378Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/chem.201302504
Keywords
energy transfer; metal-organic frameworks; lanthanides; photoluminescence; rare earths
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Funding
- Deutsche Forschungsgemeinschaft
- Bavarian State Ministry of Science, Research
- Arts for the Collaborative Research Network Solar Technologies go Hybrid
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A series of 12 dinuclear complexes [Ln(2)Cl(6)(-4,4-bipy)(py)(6)], Ln=Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, (1-12, respectively) was synthesized by an anhydrous solvothermal reaction in pyridine. The complexes contain a 4,4-bipyridine bridge and exhibit a coordination sphere closely related to luminescent lanthanide MOFs based on LnCl(3) and 4,4-bipyridine. The dinuclear complexes therefore function as a molecular model system to provide a better understanding of the luminescence mechanisms in the Ln-N-MOFs [Ln(2)Cl(6)(4,4-bipy)(3)]2(4,4-bipy). Accordingly, the luminescence properties of the complexes with Ln=Y, Sm, Eu, Gd, Tb, Dy, (1, 4-8) were determined, showing an antenna effect through a ligand-metal energy transfer. The highest efficiency of luminescence is observed for the terbium-based compound 7 displaying a high quantum yield (QY of 86%). Excitation with UV light reveals typical emission colors of lanthanide-dependent intra 4f-4f-transition emissions in the visible range (Tb-III: green, Eu-III: red, Sm-III: salmon red, Dy-III: yellow). For the Gd-III- and Y-III-containing compounds 6 and 1, blue emission based on triplet phosphorescence is observed. Furthermore, ligand-to-metal charge-transfer (LMCT) states, based on the interaction of Cl- with Eu-III, were observed for the Eu-III compound 5 including energy-transfer processes to the Eu-III ion. Altogether, the model complexes give further insights into the luminescence of the related MOFs, for example, rationalization of Ln-independent quantum yields in the related MOFs.
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