The variety of functionalities and porous structures inherent to metal-organic frameworks (MOFs) together with the facile tunability of their properties makes these materials suitable for a wide range of existing and emerging applications. Many of these applications are based on processes involving interaction of MOFs with guest molecules. To optimize a certain process or successfully design a new one, a thorough knowledge is required about the physicochemical characteristics of materials and the mechanisms of their interaction with guest molecules. To obtain such important information, various complementary analytical techniques are applied, among which vibrational spectroscopy (IR and Raman) plays an important role and is indispensable in many cases. In this review, we critically examine the reported applications of IR and Raman spectroscopies as powerful tools for initial characterization of MOF materials and for studying processes of their interaction with various gases. Both the advantages and the limitations of the technique are considered, and the cases where IR or Raman spectroscopy is preferable are highlighted. Peculiarities of MOFs interaction with specific gases and some inconsistent band assignments are also emphasized. Summarizing the broad analytical possibilities of the IR and Raman spectroscopies, we conclude that it can be applied in combinations with other techniques to explicitly establish the structure, properties, and reactivity of MOFs.

Power of Infrared and Raman Spectroscopies to Characterize Metal-Organic Frameworks and Investigate Their Interaction with Guest Molecules

Defect engineering is a versatile approach to modulate band and electronic structures as well as materials performance. Herein, metal-organic frameworks (MOFs) featuring controlled structural defects, namely UiO-66-NH2 -X (X represents the molar equivalents of the modulator, acetic acid, with respect to the linker in synthesis), were synthesized to systematically investigate the effect of structural defects on photocatalytic properties. Remarkably, structural defects in MOFs are able to switch on the photocatalysis. The photocatalytic H2 production rate presents a volcano-type trend with increasing structural defects, where Pt@UiO-66-NH2 -100 exhibits the highest activity. Ultrafast transient absorption spectroscopy unveils that UiO-66-NH2 -100 with moderate structural defects possesses the fastest relaxation kinetics and the highest charge separation efficiency, while excessive defects retard the relaxation and reduce charge separation efficiency.

Switching on the Photocatalysis of Metal–Organic Frameworks by Engineering Structural Defects

Owing to their molecular building blocks, yet highly crystalline nature, metal-organic frameworks (MOFs) sit at the interface between molecule and material. Their diverse structures and compositions enable them to be useful materials as catalysts in heterogeneous reactions, electrical conductors in energy storage and transfer applications, chromophores in photoenabled chemical transformations, and beyond. In all cases, density functional theory (DFT) and higher-level methods for electronic structure determination provide valuable quantitative information about the electronic properties that underpin the functions of these frameworks. However, there are only two general modeling approaches in conventional electronic structure software packages: those that treat materials as extended, periodic solids, and those that treat materials as discrete molecules. Each approach has features and benefits; both have been widely employed to understand the emergent chemistry that arises from the formation of the metal-organic interface. This Review canvases these approaches to date, with emphasis placed on the application of electronic structure theory to explore reactivity and electron transfer using periodic, molecular, and embedded models. This includes (i) computational chemistry considerations such as how functional, k-grid, and other model variables are selected to enable insights into MOF properties, (ii) extended solid models that treat MOFs as materials rather than molecules, (iii) the mechanics of cluster extraction and subsequent chemistry enabled by these molecular models, (iv) catalytic studies using both solids and clusters thereof, and (v) embedded, mixed-method approaches, which simulate a fraction of the material using one level of theory and the remainder of the material using another dissimilar theoretical implementation.

Electronic Structure Modeling of Metal-Organic Frameworks.

Metal clusters on supports: synthesis, structure, reactivity, and catalytic properties.

This article reviews recent fabrication methods for surface-enhanced Raman spectroscopy (SERS) substrates with a focus on advanced nanoarchitecture based on noble metals with special nanospaces (round tips, gaps, and porous spaces), nanolayered 2D materials, including hybridization with metallic nanostructures (NSs), and the contemporary repertoire of nanoarchitecturing with organic molecules. The use of SERS for multidisciplinary applications has been extensively investigated because the considerably enhanced signal intensity enables the detection of a very small number of molecules with molecular fingerprints. Nanoarchitecture strategies for the design of new NSs play a vital role in developing SERS substrates. In this review, recent achievements with respect to the special morphology of metallic NSs are discussed, and future directions are outlined for the development of available NSs with reproducible preparation and well-controlled nanoarchitecture. Nanolayered 2D materials are proposed for SERS applications as an alternative to the noble metals. The modern solutions to existing limitations for their applications are described together with the state-of-the-art in bio/environmental SERS sensing using 2D materials-based composites. To complement the existing toolbox of plasmonic inorganic NSs, hybridization with organic molecules is proposed to improve the stability of NSs and selectivity of SERS sensing by hybridizing with small or large organic molecules.

New Trends in Nanoarchitectured SERS Substrates: Nanospaces, 2D Materials, and Organic Heterostructures.

The metal organic framework (MOF) UiO-66, which incorporates Zr6O8 nodes, exhibits high stability under a wide range of conditions that commends it for potential applications. The MOF properties can be tuned by the choice of the groups bonded to the node defect sites and to the linkers. We report data for UiO-66 incorporating formate, acetate, benzoate, and trifluoroacetate on the nodes and −NH2, −OH, −NO2, and phenyl substituents on the benzene rings of the benzene-1,4-dicarboxylic acid-derived linkers. The MOFs were characterized by IR and 1H NMR spectroscopies, thermal gravimetric analysis, N2 adsorption, X-ray diffraction crystallography, scanning and transmission electron microscopy, and electronic structure calculations. The ligands on the nodes were identified and quantified by 1H NMR spectra of the MOFs digested in NaOH/D2O solutions. The effects of the node and linker groups on the electronic properties of the nodes have been quantified with IR spectra of the node μ3-OH groups and by density func...

Tuning the Properties of Zr 6 O 8 Nodes in the Metal Organic Framework UiO-66 by Selection of Node-Bound Ligands and Linkers

A family of NaY zeolite-supported iridium clusters was prepared by reductive carbonylation of Ir(CO) 2 (acac) sorbed in the zeolite pores to form Ir 6 (CO) 16 , which was decarbonylated by treatment in He. This method allowed preparation of samples with Ir contents as high as 33.0wt.%. Extended X-ray absorption fine structure spectra of the decarbonylated samples indicate that the clusters are well approximated as octahedral Ir 6 in the zeolite cages. The high loadings of clusters in the zeolite allowed characterization of the hydride ligands on the clusters by 'H NMR spectroscopy and inelastic neutron scattering spectroscopy. These ligands exist on the clusters even in the absence of H 2 in the gas phase, presumably equilibrated with OH groups on the zeolite. The data indicate both terminal and bridging hydride ligands on the clusters. The H/Ir atomic ratio is estimated to be approximately 0.01 on the basis of the 'H NMR data and thermogravimetric analysis of the sample.

Zeolite-encaged iridium clusters with hydride ligands : Characterization by extended X-ray absorption fine structure, NMR, and inelastic neutron scattering vibrational spectroscopies

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Tuning the Properties of Zr 6 O 8 Nodes in the Metal Organic Framework UiO-66 by Selection of Node-Bound Ligands and Linkers

Zeolite-encaged iridium clusters with hydride ligands : Characterization by extended X-ray absorption fine structure, NMR, and inelastic neutron scattering vibrational spectroscopies