Organic melt, electride, and CVD induced in situ deposition of luminescent lanthanide imidazolate MOFs on nanostructured alumina

Abstract

The highly luminescent MOFs ³_∞[Ce(Im)₃ImH]·ImH, ³_∞[Tb(Im)₃], and the dense framework ³_∞[Sr_0.95Eu_0.05(Im)₂] (Im⁻ = imidazolate-anion, C₃N₂H₃⁻) were grown on nanostructured macroporous aluminium oxide (AAO) membranes. Thereby, luminescent coatings on the membranes were achieved. Three different growth processes unusual for MOFs were investigated and compared: a film growth process in situ to MOF formation from the linker melt, electride induction with solvated electrons, and chemical vapour deposition (CVD) to additionally utilize the gas phase. Deposition from the organic melt has proved to be a fast approach to achieve various film thicknesses of the luminescent frameworks. The electride-based approach offers excellent homogenization at an atomic level for the highest quantum yields of QY > 90% for ³_∞[Sr_0.95Eu_0.05(Im)₂] including the formation of barite rose analogous crystals prior to growth of a complete film on AAO membranes. For ³_∞[Tb(Im)₃] and ³_∞[Ce(Im)₃ImH]·ImH, deposition of bundles of crystals by CVD on AAO is possible while also maintaining the luminescence of the original MOFs but without complete layers. In order to elaborate the divalent character of europium, being the basis of the high efficiency of the luminescence, EPR studies were carried out on ³_∞[Sr_1−xEu_x(Im)₂], x = 0.01, 0.05 and 1.