Intense Near‐IR Emission from Nanoscale Lanthanoid Fluoride Clusters

Abstract

Lanthanoid-doped fluoride glasses are intense near-IR (NIR) emission sources because of the low-energy phonon characteristics of fluoride lattices. [1][2][3][4][5] These materials are particularly useful in optical applications, [6][7][8][9][10][11][12] because fluorides are absolutely air-stable. Unfortunately, the extreme insolubility of lanthanoid ions in the presence of fluoride sources [13][14][15] has always presented a barrier to developing alternative synthetic approaches to LnF x materials, particularly in media that would preclude the incorporation of NIR-emission-quenching OH groups. Herein we demonstrate that by using unconventional chalcogen-based ligands we can dramatically alter the solubility characteristics of lanthanoid cations in the presence of fluoride anions. We describe the synthesis, structural characterization, and exceptional NIR emission properties of the largest known lanthanoid cluster.Exposure of in situ prepared Ln(SePh) 3 to fluoride sources does not result in the immediate precipitation of solid LnF 3 . Metathesis reactions of Ln(SePh) 3 with HgF 2 , CsF, or Me 4 NF have yet to deliver crystalline products, but reactions of Ln(SePh) 3 with NH 4 F in pyridine with subsequent filtration and saturation of the solution (either by layering with hexane or slow cooling) result in the crystallization, in 5-20 % yields, of nanoscale lanthanoid fluoride clusters that were shown, by low-temperature single crystal Xray diffraction, [16] molecular structure of 1 and 2 is shown in Figure 1, with significant bond distances for 1 and 2 given in the caption; complete structural information is given as Supporting Information. Clusters 1 and 2 contain a central set of four 12-coordinate Ln ions encapsulated by six triply bridging and six tetrahedral fluoride ions, which coordinate to the next layer of 24 lanthanoid ions that are then further connected through additional m 2 , m 3 , and m 4 fluorides, with the surface of the cluster capped by pyridine and selenolates. While the internal Ln coordination numbers are 12, the surface ions are either 8-or 9-coordinate. The surprising solubility of these LnF 3 -containing materials can be attributed to the relative instability of the selenolate starting materials. Chalcogenolate instability has been used to rationalize the apparent stability of SePhencapsulated lanthanoid oxo cluster compounds, [17] but in that system Ln 2 O 3 starting materials did not appear to react with Ln(SePh) 3 , and so thermodynamic stability of the clusters remained unproven. In the present system, suspended LnF 3 reacts with a solution of Ln(SePh) 3 , but a crystalline product of this reaction has not yet been obtained. Still, the observation of a reaction is additional evidence that selenolate-encapsulated fluoride clusters are thermodynamically stable with respect to precipitation of LnF 3 . Presumably the total energy of the system is lowered when the selenolate ligands are distributed over a number of Ln III ions. The potential utility of these clusters as signal amplificat...

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