Abstract

Nearly thirty years after the first use of the name metal–organic framework (MOF), more than 50,000 different non-disordered MOF structures have so far been reported, according to the Cambridge Structural Database (CSD). Yet, the field still holds immense opportunities for investigation into the nearly infinite variety of linker and metal combinations. These organic linkers and metal nodes combine to form framework structures, which are often three-dimensional, crystalline, and display extensive porosity. Moreover, by carefully designing MOFs by selecting linkers, metals, and targeting specific frameworks, different applications can be envisioned, including gas storage, drug delivery, chemical separations, light-harvesting and energy conversion, catalysis, sensing, and adsorption. Many of these applications take advantage of open metal sites, where the Lewis acidic metals are involved in host-guest interactions and processes that benefit from an ordered site that can accept an electron pair.
Due to the special characteristics of rare-earth (RE) elements, which include scandium, yttrium, and the whole series of lanthanoids, promising new RE-based MOFs have been obtained in the past years. Among these characteristics, it is worth mentioning the high coordination numbers and distinct optical properties of RE(III) ions, which can lead to the generation of materials with interesting photophysical and photochemical properties and unique crystalline structures, potentially featuring stable and accessible open metal sites.
In this work, a series of RE-MOFs based on the archetypical zirconium MOF-808 was obtained through a de novo synthetic approach. These MOFs were named as RE-CU-45 (CU = Concordia University) and feature six-connected hexanuclear RE(III)-clusters bridged by 1,3,5-benzenetricarboxylic acid linkers. Studies were carried out to optimize the synthetic conditions used to obtain the new RE-MOFs, mainly exploring the metal precursors that are used, focusing on a balance between high purity, yield, suitable crystallite sizes, and reproducibility of the synthesis. Additionally, the RE-MOF has been tested in regard to the possibility of accessing its pores and open metal sites through a series of post-synthetic modification by solvent assisted ligand incorporation. The final materials are fully characterized by powder X-ray diffraction (PXRD), N2 sorption, thermogravimetric analysis (TGA), diffuse reflectance infrared Fourier Transform spectroscopy (DRIFTS), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) spectroscopy, and studied regarding their photophysical properties through diffuse reflectance UV-vis spectroscopy (DR-UV-vis) and photoluminescence spectroscopy.