Homoleptic imidazolate frameworks 3∞[Sr_1−xEu_x(Im)₂]—hybrid materials with efficient and tuneable luminescence

Abstract

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...

Synthesis, Crystal Structure, Polymorphism, and Magnetism of Eu(CN3H4)2 and First Evidence of EuC(NH)3

“…Compared to both SrC(NH)3 [30] and YbC(NH)3 [31], EuC(NH)3 shows a shorter a-and a longer c-axis, while the volume falls in-between the two. The C-N bonds are found to be somewhat short at [30] and YbC(NH) 3 [31], EuC(NH) 3 shows a shorter a-and a longer c-axis, while the volume falls in-between the two. The C-N bonds are found to be somewhat short at 1.328(1) Å, close to a double bond although the bond order should be 1 1 / 3 .…”

“…This compound is isostructural to SrC(NH) 3 [30] and YbC(NH) 3 [31] and crystallizes in the hexagonal space group P6 3 /m with a = 5.1634(7) Å, c = 7.1993(9) Å, V = 166.23(4) Å 3 , and Z = 2 ( Figure 8; Table 2). As for YbC(NH) 3 , DFT calculations were used to locate the hydrogen atoms, a method validated in reference [46] (Table 4). So far, EuC(NH) 3 was only obtained together with an unidentified side phase.…”

“…Finally, we want to give a preliminary account of EuC(NH) 3 . This compound is isostructural to SrC(NH) 3 [30] and YbC(NH) 3 [31] and crystallizes in the hexagonal space group P6 3 /m with a = 5.1634(7) Å, c = 7.1993(9) Å, V = 166.23(4) Å 3 , and Z = 2 ( Figure 8; Table 2).…”

“…For Eu 2+ , there are a number of simple amides, thiocyanates, and carbodiimides such as Eu(NH 2 ) 2 [14,15], Eu(NCS) 2 [16], and EuNCN [17], but also a growing number of more exotic and intriguing examples including Eu 2 Si 5 N 8 [18,19], Eu 3 [NBN] 2 [20], Eu 2 Cl 2 NCN [21], and EuSi 2 O 2 N 2 [22]. Low-dimensional magnetic properties have been reported in Eu 2+ coordination polymers with 2,2'-bipyridime showing 1D ferromagnetic interactions [23] and in LiEu 2 (NCN)I 3 and LiEu 4 (NCN) 3 I 3 [24], also with low-dimensional ferromagnetic ordering and possibly conflicting antiferromagnetic interactions at very low temperatures.…”

“…Within the recent decades, several technological innovations disrupted the rare-earth market [2], in turn stimulating the scientific quest for future materials. One vibrant field is the study of Eu 2+ compounds whose complex crystal structures are coupled with application-relevant properties including, to name only some recent examples, luminescence [3][4][5][6], field-induced reversal of the magnetoresistive effect [7], and complex magnetism [8,9]. The most renowned magnetic compounds are the europium chalcogenides that are considered ideal 3D Heisenberg systems [10].…”