Taking lanthanides out of isolation: tuning the optical properties of metal-organic frameworks

Metal organic frameworks (MOFs) are increasingly used in applications that rely on the optical and electronic properties of these materials. These applications require a fundamental understanding on how the structure of these materials, and in particular the electronic interactions of the metal node and organic linker, determines these properties. Herein, we report a combined experimental and computational study on two families of lanthanide-based MOFs: Ln-SION-1 and Ln-SION-2. Both comprise the same metal and ligand but with differing structural topologies. In the Ln-SION-2 series the optical absorption is dominated by the ligand and using different lanthanides has no impact on the absorption spectrum. The Ln-SION-1 series shows a completely different behavior in which the ligand and the metal node do interact electronically. By changing the lanthanide in Ln-SION-1, we were able to tune the optical absorption from the UV region to absorption that includes a large part of the visible region. For the early lanthanides we observe intraligand (electronic) transitions in the UV region, while for the late lanthanides a new band appears in the visible. DFT calculations showed that the new band in the visible originates in the spatial orbital overlap between the ligand and metal node. Our quantum calculations indicated that Ln-SION-1 with late lanthanides might be (photo)conductive. Experimentally, we confirm that these materials are weakly conductive and that with an appropriate co-catalysts they can generate hydrogen from a water solution using visible light. Our experimental and theoretical analysis provides fundamental insights for the rational design of Ln-MOFs with the desired optical and electronic properties.

Pyrene-based metal organic frameworks: from synthesis to applications

Christopher Patrick Ireland, Fatma Pelin Kinik, Daniele Ongari, Andres Adolfo Ortega Guerrero, Berend Smit

Pyrene is one of the most widely investigated aromatic hydrocarbons given to its unique optical and electronic properties. Hence, pyrene-based ligands have been attractive for the synthesis of metal–organic frameworks (MOFs) in the last few years. In this review, we will focus on the most important characteristics of pyrene, in addition to the development and synthesis of pyrene-based molecules as bridging ligands to be used in MOF structures. We will summarize the synthesis attempts, as well as the post-synthetic modifications of pyrene-based MOFs by the incorporation of metals or ligands in the structure. The discussion of promising results of such MOFs in several applications; including luminescence, photocatalysis, adsorption and separation, heterogeneous catalysis, electrochemical applications and bio-medical applications will be highlighted. Finally, some insights and future prospects will be given based on the studies discussed in the review. This review will pave the way for the researchers in the field for the design and development of novel pyrene-based structures and their utilization for different applications.

2021

Synthesis, Characterization, and Applications of Visible Light Active Metal-Organic Frameworks

Samantha Lynn Anderson

Understanding how crystalline materials are assembled is important for the rational design of metal-organic frameworks (MOFs). Controlling their formation can allow researchers to streamline the synthesis of new materials as well as control their properties for targeted applications. In the first chapter of this thesis we report the construction of two 3-dimensional TbIII based MOFs (SION-1 and SION-2). Here, SION-2 acts as a metastable MOF and intermediate phase that partially dissolves and transforms into the thermodynamically stable MOF, SION-1. This chemical transformation occurs in a DMF/water solvent mixture, and is triggered when additional energy is provided to the reaction. In situ studies reveal the partial dissolution of the metastable phase after which the MOF components are reassembled into the thermodynamically stable phase. The marked difference in thermal and chemical stability between the kinetically and thermodynamically controlled phases is contrasted by their identical chemical building unit composition. Following the understanding of this transformation, an isostructural family of SION-1 and SION-2 had been constructed using lanthanide metal salts, where LnIII-SION-1 is comprised of Ce, Nd, Eu, Gd, Tb, Dy, Ho, Er, Yb and Lu and LnIII-SION-2 is formed using Ce, Nd, Eu, Gd, Tb and Yb. The electronic properties of lanthanide based MOFs have not been extensively studied due to the general cognizance that LnIII ions behave as isolated free ions as their 4f orbitals do not contribute to bonding. Here, we systematically studied the electronic properties (UV-vis) of LnIII-SION-1 and LnIII-SION-2 and show that the higher the intensity of this absorption band in LnIII-SION-1 is due to the better overlap between Ln(5d) and ligand(pi*) orbitals and reveals that the lowest unoccupied crystalline orbitals are dispersed over an energy range and can be considered as a conduction band. We validated this observation by the non-linear I-V curves collected on CeIII- and YbIII-SION-1, both of which display dielectric properties. In the second chapter of this thesis, we rationally design a novel biologically derived MOF featuring unobstructed Watson-Crick faces of Ade pointing towards the MOF cavities. We show, through a combined experimental and computational approach, that Thy molecules diffuse through the pores of the MOF and become base-paired with Ade. The Ade-Thy pair binding at 40-45% loading reveals that Thy molecules are packed within the channels in a way that fulfill both the Woodward-Hoffmann and Schmidt rules, and upon UV irradiation, Thy molecules dimerize into ThyThy. This study highlights the utility of MOFs as nanoreactors for the synthesis of molecules that are otherwise difficult. Concluding this thesis, a biologically derived multimodal sensor MOF is synthesized and a formed into a device. The MOF, SION-9, is comprised of Ade and a porphyrin based ligand, and has tuneable conductive properties ranged from 2.69-4.20 x 10-8 - 7.43-7.34 x 10-6 S/cm. SION-9 is sensitive to both pressure and temperature simultaneously, and demonstrated both fast response times (< 60 ms) and a long shelf life (> 6 months).

EPFL2019

Electrical and optical properties of Ta-Si-N thin films deposited by reactive magnetron sputtering

David Oezer, Rosendo Sanjines

The electrical and optical properties of TaxSiyNz thin films deposited by reactive magnetron sputtering from individual Ta and Si targets were studied in order to investigate the effects of nitrogen and silicon contents on both properties and their correlation to the film microstructure. Three sets of fcc-TaxSiyNz thin films were prepared: sub-stoichiometric TaxSiyN0.44, nearly stoichiometric TaxSiyN0.5, and over-stoichiometric TaxSiyN0.56. The optical properties were investigated by near-normal-incidence reflectivity and ellipsometric measurements in the optical energy range from 0.375 eV to 6.8 eV, while the d.c. electrical resistivity was measured in the van der Pauw configuration from 20 K to 300 K. The optical and electrical measurements were interpreted using the standard Drude-Lorentz model and the so-called grain boundary scattering model, respectively. The electronic properties were closely correlated with the compositional and structural modifications of the TaxSiyNz films due to variations in the stoichiometry of the fcc-TaNz system and the addition of Si atoms. According to the nitrogen and silicon contents, fcc-TaxSiyNz films can exhibit room temperature resistivity values ranging from 10(2) mu Omega cm to about 6 x 10(4) mu Omega cm. The interpretation of the experimental temperature-dependent resistivity data within the Grain Boundary Scattering model, combined with the results from optical investigations, showed that the mean electron transmission probability G and the free carriers concentration, N, are the main parameters that control the transport properties of these films. The results indicated that the correlation between electrical and optical measurements with the chemical composition and the nanostructure of the TaxSiyNz thin films provides a pertinent and consistent description of the evolution of the Ta-Si-N system from a solid solution to a nanocomposite material due to the addition of Si atoms. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4766904]