Homoleptic frameworks of the formula 3 N [Sr 1Àx Eu x (Im) 2 ] (1) (x = 0.01-1.0; Im À = imidazolate anion, C 3 H 3 N 2 À ) are hybrid materials that exhibit an intensive green luminescence. Tuning of both emission wavelength and quantum yield is achieved by europium/strontium substitution so that a QE of 80% is reached at a Eu content of 5%. Even 100% pure europium imidazolate still shows 60% absolute quantum efficiency. Substitution of Sr/Eu shows that doping with metal cations can also be utilized for coordination compounds to optimize materials properties. The emission is finely tuneable in the region 495-508 nm via variation of the europium content. The series of frameworks 3 N [Sr 1Àx Eu x (Im) 2 ] presents dense MOFs with the highest quantum yields reported for MOFs so far.Framework and MOF chemistry 1 have attracted attention, as interesting properties were reported like conductivity, 2 catalytic effects, 3 luminescence 4 and porosity. 5 They are mainly known for oxygen coordinating ligands, mostly metal carboxylates 6 which include the alkaline earth and 4f elements. 7 Because of the oxophilicity of lanthanides oxygen-free multi-dimensional coordination networks are rarely found except for a few rare earth imidazolates and triazolates. 8 Among transition metals the imidazole ring system is of exceptional interest together with several 3d metals as they adopt zeolite structures (ZIFs) 9 that can be used for sorption and gas separation. Different from many solid state phosphors, coordination compounds can exhibit luminescence by metal ions although they contain 100% luminescence centres. 10 An expected quenching by concentration is suppressed by ligand shielding. They are furthermore interesting luminescent hybrid materials, as emission can be achieved either via a fluorescence of the ligand system 11 or the metal centres, mainly by the use of lanthanides. 4 The excitation can benefit from antenna effects, viz. the ligand system is excited primarily followed by a transfer of the energy to the luminescence centres. 12 However there are only little coordination compounds for which effective emission characterized by high quantum efficiencies has been reported. 4,11,13 Mostly, no quantum yields were determined, although luminescence becomes important for MOFs concerning sensoring and lighting from UV to near IR. 14,15 We now report a series of homoleptic imidazolate frameworks containing divalent strontium and europium that shows an exceptional combination of properties: an effective luminescence with the highest quantum yield reported for coordination polymers today, together with multiple excitation options including excitation maxima at the applicationally important wavelengths 370 and 460 nm (for Hg and blue LED excitation). The emission can be finely tuned in the region 495-508 nm (blue-green to green) via variation of the Eu content (Fig. 1). Furthermore a low quenching by concentration is observed, combined to a high thermal stability of the frameworks up to 530 1C. (1) are obtained by reactions of t...

The series of anhydrous lanthanide chlorides LnCl 3 , Ln ¼ Pr-Tb, and 4,4 0 -bipyridine (bipy) constitute (1-8), 0 # x, y # 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 0 -bipyridine, a metal to metal energy transfer (MMET) between 4f-levels from Tb 3+ to Eu 3+ is as well vital for the emission colour. However, no involvement of Gd 3+ 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.

By solvent free syntheses of the rare earth trichlorides LnCl(3), Ln = Pr, Nd, Sm, Eu, Tb with melts of 4,4'-bipyridine two-dimensional frameworks of the formula (2)(infinity)[Ln(2)Cl(6)(4,4'-bipy)(3)] x 2(4,4'-bipy) are obtained, with 4,4'-bipy = C(10)H(8)N(2), 4,4'-bipyridine. 4,4'-Bipyridine acts both as a linker ligand as well as a template and populates all cavities in the structures. The template can be evaporated at temperatures >200 degrees C giving a new high temperature compound with a yet unknown structure. Further heating results in the release of the remaining linking equivalents of the ligand and reformation of LnCl(3). Thus the reaction can be run in cycles if the evaporated ligand is collected. Luminescence in the visible range without quenching by concentration is observed for the lanthanides Eu and Tb, which is identified for Eu(III) with the transitions (5)D(0)-->(7)F(J) and (5)D(4)-->(7)F(J) for Tb(III). The hybrid character of the material is reflected by the additional strong antenna effect of the ligand: main excitation is observed via the 4,4'-bipyridine linkers followed by an energy transfer to the metal centres. It is remarkable that both the template containing MOFs as well as the new high temperature compounds exhibit luminescence properties.

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The blue emitting luminescent MOF ∞³[Ce(Im)3ImH]·ImH forms a 3D-framework with Kagomé net topology. The framework exhibits an intense blue luminescence which can be retained upon activation of the MOF with the formula ∞³[Ce(Im)3ImH]. The luminescence is metal-based due to parity-allowed 5d-4f-transitions. Time-dependent investigations of the interaction with liquid and gas analytes show that the MOF – by utilising 5d-4f-transitions of Ce(3+) – can be used as a high-speed "turn-off" detector for water and oxygen in dry air. Other protic or polar solvents, like methanol, acetone or pyridine, which also show a "turn-off"-effect can be distinguished from water-detection either on a time scale (ranging up to 250,000 : 1) or a shift of the chromaticity, the latter being pronounced for MeOH. The fast time-dependent decrease of the luminescence intensity for water arises from an extremely fast hydrolysis and is irreversible. Polar aprotic molecules like dichloromethane and acetonitrile can also result in a "turn-on"-effect of the luminescence intensity due to their behaviour as additional sensitizers for Ce(3+)-emission. We conclude that the cerium-MOF can be utilised in gas and liquid sensing applications as a detector material for water and oxygen in dry air. The luminescence is intense with good quantum yield between 55% (as-synthesised) and 36% (activated). This implies that only milligram amounts of the material are needed to detect the analyte species and is especially useful, as the MOF can be directly used as-synthesised for water detection in applications for which an irreversible signal change is desired, e.g. preventing a signal change upon unwanted re-drying.