From IR to x-rays: gaining molecular level insights on metal-organic frameworks through spectroscopy

Computational Modeling of Reticular Materials: The Past, the Present, and the Future

Reticular materials rely on a unique building concept where inorganic and organic building units are stitched together giving access to an almost limitless number of structured ordered porous materials. Given the versatility of chemical elements, underlying nets, and topologies, reticular materials provide a unique platform to design materials for timely technological applications. Reticular materials have now found their way in important societal applications, like carbon capture to address climate change, water harvesting to extract atmospheric moisture in arid environments, and clean energy applications. Combining predictions from computational materials chemistry with advanced experimental characterization and synthesis procedures unlocks a design strategy to synthesize new materials with the desired properties and functions. Within this review, the current status of modeling reticular materials is addressed and supplemented with topical examples highlighting the necessity of advanced molecular modeling to design materials for technological applications. This review is structured as a templated molecular modeling study starting from the molecular structure of a realistic material towards the prediction of properties and functions of the materials. At the end, the authors provide their perspective on the past, present of future in modeling reticular materials and formulate open challenges to inspire future model and method developments.

A Critical Assessment on Calculating Vibrational Spectra in Nanostructured Materials

Vibrational spectroscopy is an omnipresent spectroscopic technique to characterize functional nanostructured materials such as zeolites, metal-organic frameworks (MOFs), and metal-halide perovskites (MHPs). The resulting experimental spectra are usually complex, with both low-frequency framework modes and high-frequency functional group vibrations. Therefore, theoretically calculated spectra are often an essential element to elucidate the vibrational fingerprint. In principle, there are two possible approaches to calculate vibrational spectra: (i) a static approach that approximates the potential energy surface (PES) as a set of independent harmonic oscillators and (ii) a dynamic approach that explicitly samples the PES around equilibrium by integrating Newton's equations of motions. The dynamic approach considers anharmonic and temperature effects and provides a more genuine representation of materials at true operating conditions; however, such simulations come at a substantially increased computational cost. This is certainly true when forces and energy evaluations are performed at the quantum mechanical level. Molecular dynamics (MD) techniques have become more established within the field of computational chemistry. Yet, for the prediction of infrared (IR) and Raman spectra of nanostructured materials, their usage has been less explored and remain restricted to some isolated successes. Therefore, it is currently not a priori clear which methodology should be used to accurately predict vibrational spectra for a given system. A comprehensive comparative study between various theoretical methods and experimental spectra for a broad set of nanostructured materials is so far lacking. To fill this gap, we herein present a concise overview on which methodology is suited to accurately predict vibrational spectra for a broad range of nanostructured materials and formulate a series of theoretical guidelines to this purpose. To this end, four different case studies are considered, each treating a particular material aspect, namely breathing in flexible MOFs, characterization of defects in the rigid MOF UiO-66, anharmonic vibrations in the metal-halide perovskite CsPbBr3, and guest adsorption on the pores of the zeolite H-SSZ-13. For all four materials, in their guest- and defect-free state and at sufficiently low temperatures, both the static and dynamic approach yield qualitatively similar spectra in agreement with experimental results. When the temperature is increased, the harmonic approximation starts to fail for CsPbBr3 due to the presence of anharmonic phonon modes. Also, the spectroscopic fingerprints of defects and guest species are insufficiently well predicted by a simple harmonic model. Both phenomena flatten the potential energy surface (PES), which facilitates the transitions between metastable states, necessitating dynamic sampling. On the basis of the four case studies treated in this Review, we can propose the following theoretical guidelines to simulate accurate vibrational spectra of functional solid-state materials: (i) For nanostructured crystalline framework materials at low temperature, insights into the lattice dynamics can be obtained using a static approach relying on a few points on the PES and an independent set of harmonic oscillators. (ii) When the material is evaluated at higher temperatures or when additional complexity enters the system, e.g., strong anharmonicity, defects, or guest species, the harmonic regime breaks down and dynamic sampling is required for a correct prediction of the phonon spectrum. These guidelines and their illustrations for prototype material classes can help experimental and theoretical researchers to enhance the knowledge obtained from a lattice dynamics study.

MOFs in the time domain

Many of the proposed applications of metal-organic framework (MOF) materials may fail to materialize if the community does not fully address the difficult fundamental work needed to map out the 'time gap' in the literature - that is, the lack of investigation into the time-dependent behaviours of MOFs as opposed to equilibrium or steady-state properties. Although there are a range of excellent investigations into MOF dynamics and time-dependent phenomena, these works represent only a tiny fraction of the vast number of MOF studies. This Review provides an overview of current research into the temporal evolution of MOF structures and properties by analysing the time-resolved experimental techniques that can be used to monitor such behaviours. We focus on innovative techniques, while also discussing older methods often used in other chemical systems. Four areas are examined: MOF formation, guest motion, electron motion and framework motion. In each area, we highlight the disparity between the relatively small amount of (published) research on key time-dependent phenomena and the enormous scope for acquiring the wider and deeper understanding that is essential for the future of the field.

ResponZIF Structures: Zeolitic Imidazolate Frameworks as Stimuli-Responsive Materials

Zeolitic imidazolate frameworks (ZIFs) have long been recognized as a prominent subset of the metal-organic framework (MOF) family, in part because of their ease of synthesis and good thermal and chemical stability, alongside attractive properties for diverse potential applications. Prototypical ZIFs like ZIF-8 have become embodiments of the significant promise held by porous coordination polymers as next-generation designer materials. At the same time, their intriguing property of experiencing significant structural changes upon the application of external stimuli such as temperature, mechanical pressure, guest adsorption, or electromagnetic fields, among others, has placed this family of MOFs squarely under the umbrella of stimuli-responsive materials. In this review, we provide an overview of the current understanding of the triggered structural and electronic responses observed in ZIFs (linker and bond dynamics, crystalline and amorphous phase changes, luminescence, etc.). We then describe the state-of-the-art experimental and computational methodology capable of shedding light on these complex phenomena, followed by a comprehensive summary of the stimuli-responsive nature of four prototypical ZIFs: ZIF-8, ZIF-7, ZIF-4, and ZIF-zni. We further expose the relevant challenges for the characterization and fundamental understanding of responsive ZIFs, including how to take advantage of their flexible properties for new application avenues.

Enhanced Catalysis under 2D Silica: A CO Oxidation Study

Interfacially confined microenvironments have recently gained attention in catalysis, as they can be used to modulate reaction chemistry. The emergence of a 2D nanospace at the interface between a 2D material and its support can promote varying kinetic and energetic schemes based on molecular level confinement effects imposed in this reduced volume. We report on the use of a 2D oxide cover, bilayer silica, on catalytically active Pd(111) undergoing the CO oxidation reaction. We "uncover" mechanistic insights about the structure-activity relationship with and without a 2D silica overlayer using in situ IR and X-ray spectroscopy and mass spectrometry methods. We find that the CO oxidation reaction on Pd(111) benefits from confinement effects imposed on surface adsorbates under 2D silica. This interaction results in a lower and more dispersed coverage of CO adsorbates with restricted CO adsorption geometries, which promote oxygen adsorption and lay the foundation for the formation of a reactive surface oxide that produces higher CO₂ formation rates than Pd alone.

Rational design of a high-efficiency, multivariate metal-organic framework phosphor for white LED bulbs

Developing rare-earth element (REE) free yellow phosphors that can be excited by 455 nm blue light will help to decrease the environmental impact of manufacturing energy efficient white light-emitting diodes (WLEDs), decrease their cost of production, and accelerate their adoption across the globe. Luminescent metal-organic frameworks (LMOFs) demonstrate strong potential for use as phosphor materials and have been investigated intensively in recent years. However, the majority are not suitable for the current WLED technology due to their lack of blue excitability. Therefore, designing highly efficient blue-excitable, yellow-emitting, REE free LMOFs is much needed. With an internal quantum yield of 76% at 455 nm excitation, LMOF-231 is the most efficient blue-excitable yellow-emitting LMOF phosphor reported to date. Spectroscopic studies suggest that this quantum yield could be further improved by narrowing the material's bandgap. Based on this information and guided by DFT calculations, we apply a ligand substitution strategy to produce a semi-fluorinated analogue of LMOF-231, LMOF-305. With an internal quantum yield of 88% (λ _em = 550 nm) under 455 nm excitation, this LMOF sets a new record for luminescent efficiency in yellow-emitting, blue-excitable, REE free LMOF phosphors. Temperature-dependent and polarized photoluminescence (PL) studies have provided insight on the mechanism of emission and origin of the significant PL enhancement.