UiO-66 framework with an encapsulated spin probe: synthesis and exceptional sensitivity to mechanical pressure.

Probes sensitive to mechanical stress are in demand for the analysis of pressure distribution in materials, and the design of pressure sensors based on metal-organic frameworks (MOFs) is highly promising due to their structural tunability. We report a new pressure-sensing material, which is based on the UiO-66 framework with trace amounts of a spin probe (0.03 wt%) encapsulated in cavities. To obtain this material, we developed an approach for encapsulation of stable nitroxide radical TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) into the micropores of UiO-66 during its solvothermal synthesis. Pressure read-out using electron paramagnetic resonance (EPR) spectroscopy allows monitoring the degradation of the defected MOF structure upon pressurization, where full collapse of pores occurs at as low a pressure as 0.13 GPa. The developed methodology can be used in and ex situ and provides sensitive tools for non-destructive mapping of pressure effects in various materials.

Stability of ZIF-8 Nanoparticles in Most Common Cell Culture Media.

Zeolite imidazolate framework-8 (ZIF-8) is a promising platform for drug delivery, and information regarding the stability of ZIF-8 nanoparticles in cell culture media is essential for proper interpretation of in vitro experimental results. In this work, we report a quantitative investigation of the ZIF-8 nanoparticle's stability in most common cell culture media. To this purpose, ZIF-8 nanoparticles containing sterically shielded nitroxide probes with high resistance to reduction were synthesized and studied using electron paramagnetic resonance (EPR). The degradation of ZIF-8 in cell media was monitored by tracking the cargo leakage. It was shown that nanoparticles degrade at least partially in all studied media, although the degree of cargo leakage varies widely. We found a strong correlation between the amount of escaped cargo and total concentration of amino acids in the environment. We also established the role of individual amino acids in ZIF-8 degradation. Finally, 2-methylimidazole preliminary dissolved in cell culture media partially inhibits the degradation of ZIF-8 nanoparticles. The guidelines for choosing the proper cell culture medium for the in vitro study of ZIF-8 nanoparticles have been formulated.

Blatter-Radical-Grafted Mesoporous Silica as Prospective Nanoplatform for Spin Manipulation at Ambient Conditions.

Quantum computing and quantum information processing (QC/QIP) crucially depend on the availability of suitable quantum bits (qubits) and methods of their manipulation. Most qubit candidates known to date are not applicable at ambient conditions. Herein, we propose radical-grafted mesoporous silica as a versatile and prospective nanoplatform for spin-based QC/QIP. Extremely stable Blatter-type organic radicals are used, whose electron spin decoherence time is profoundly long even at room temperature (up to Tm ≈2.3 µs), thus allowing efficient spin manipulation by microwave pulses. The mesoporous structure of such composites is nuclear-spin free and provides additional opportunities of embedding guest molecules into the channels. Robustness and tunability of these materials promotes them as highly promising nanoplatforms for future QC/QIP developments.

Structural Dynamics and Adsorption Properties of the Breathing Microporous Aliphatic Metal-Organic Framework.

A mixed-ligand metal-organic framework [Zn2(chdc)2(dabco)]·2NMP (chdc2- = trans-1,4-cyclohexanedicarboxylate; dabco = 1,4-diazabicyclo[2.2.2]octane; NMP = N-methylpyrrolidone) was synthesized under solvothermal conditions. This coordination compound demonstrates a guest-driven framework breathing due to a conformational change between e,e-chdc and a,a-chdc forms of the linkers with a reversible restoration of crystallinity. Both the local and longer-range coordination environment of the metal centers were extensively studied by electron paramagnetic resonance on a Cu(II)-doped compound. This approach allowed the detailed investigation of the ligand structural conformations and the framework structural dynamics, supported by an X-ray diffraction method. Carbon dioxide and methane adsorption measurements as well as vapor sorption of benzene and cyclohexane at 298 K of the activated compound were studied. While adsorption of small gas molecules, such as CO2, CH4, and N2, is moderate and does not induce the phase transition, the multistepped character of C6H6 and C6H12 adsorption isotherms characterize the breathing nature of [Zn2(chdc)2(dabco)]. The uptake of benzene from the vapor phase reaches 125 mL·g-1 at 298 K, which surpasses most of benzene uptake values reported for microporous metal-organic frameworks.

Precise Measurement and Controlled Tuning of Effective Window Sizes in ZIF-8 Framework for Efficient Separation of Xylenes.

Metal-organic frameworks (MOFs) are the promising nanomaterials for separation of molecules with close dimensions and structures, such as various types of isomers. The efficiency of separation can be greatly enhanced if the apertures of the nanosized windows, controlling the diffusion of a particular molecule inside the cavities, are fine-tuned by external stimuli. We report the new approach for precise measurement of window sizes in ZIF-8 MOF and employ it in efficient separation of xylenes, which is of high practical importance. For this sake, we synthesized ZIF-8 with embedded stable nitroxides in the pores and applied electron paramagnetic resonance spectroscopy for in situ kinetic measurement of the diffusion of various guest molecules through the material. Slight variation of temperature within 298-333 K allowed tuning of the windows and reaching optimum conditions for separation of p-, m-, and o-xylenes with the efficiency up to 92-95%. The developed methodology provides deeper understanding of steric and kinetic aspects of molecular diffusion in ZIF-8 and paves the way to rational optimization of other MOF-based separation applications.

A Titanium Metal-Organic Framework with Visible-Light-Responsive Photocatalytic Activity.

The one-step synthesis and characterization of a new and robust titanium-based metal-organic framework, ACM-1, is reported. In this structure, which is based on infinite Ti-O chains and 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl) tetrabenzoic acid as a photosensitizer ligand, the combination of highly mobile photogenerated electrons and a strong hole localization at the organic linker results in large charge-separation lifetimes. The suitable energies for band gap and conduction band minimum (CBM) offer great potential for a wide range of photocatalytic reactions, from hydrogen evolution to the selective oxidation of organic substrates.

Radical-Doped Metal-Organic Framework: Route to Nanoscale Defects and Magnetostructural Functionalities.

Nanosized structural defects in metal-organic frameworks (MOFs) attract growing attention and often remarkably enhance functional properties of these materials for various applications. In this work, a series of MOFs [Cu2(TPTA)1- x(BDPBTR) x] (H4TPTA, [1,1':3',1″-terphenyl]-3,3'',5,5''-tetracarboxylic acid; H4BDPBTR, 1,3-bis(3,5-dicarboxyphenyl)-1,2,4-benzotriazin-4-yl radical)) with a new stable radical linker doped into the structure has been synthesized and investigated using Electron Paramagnetic Resonance (EPR). Mixed linkers H4TPTA and H4BDPBTR were used to bridge copper(II) paddle-wheel units into a porous framework, where H4BDPBTR is the close structural analogue of H4TPTA. MOFs with various x = 0-0.4 were investigated. EPR studies indicated that the radical linker binds to the copper(II) units differently compared to diamagnetic linker, resulting in the formation of nanosized structural defects. Moreover, remarkable kinetic phenomena were observed upon cooling of this MOF, where slow structural rearrangements and concomitant changes of magnetic interactions were induced. Thus, our findings demonstrate that doping of structurally mimicking radical linkers into MOFs represents an efficient approach for designing target nanosized defects and introducing new magnetostructural functionalities for a variety of applications.

Understanding Hysteresis in Carbon Dioxide Sorption in Porous Metal-Organic Frameworks.

Two new isostructural microporous coordination frameworks [Mn3(Hpdc)2(pdc)2] (1) and [Mg3(Hpdc)2(pdc)2] (2) (pdc2- = pyridine-2,4-dicarboxylate) showing primitive cubic (pcu) topology have been prepared and characterized. The pore aperture of the channels is too narrow for the efficient adsorption of N2; however, both compounds demonstrate substantially higher uptake of CO2 (119.9 mL·g-1 for 1 and 102.5 mL·g-1 for 2 at 195 K, 1 bar). Despite of their structural similarities, 2 shows a typical reversible type I isotherm for adsorption/desorption of CO2, while 1 features a two-step adsorption process with a very broad hysteresis between the adsorption and desorption curves. This behavior can be explained by a combination of density functional theory calculations, sorption, and X-ray diffraction analysis and gives insights into the further development of new sorbents showing adsorption/desorption hysteresis.

Confined Carbon Mediating Dehydroaromatization of Methane over Mo/ZSM-5.

Non-oxidative dehydroaromatization of methane (MDA) is a promising catalytic process for direct valorization of natural gas to liquid hydrocarbons. The application of this reaction in practical technology is hindered by a lack of understanding about the mechanism and nature of the active sites in benchmark zeolite-based Mo/ZSM-5 catalysts, which precludes the solution of problems such as rapid catalyst deactivation. By applying spectroscopy and microscopy, it is shown that the active centers in Mo/ZSM-5 are partially reduced single-atom Mo sites stabilized by the zeolite framework. By combining a pulse reaction technique with isotope labeling of methane, MDA is shown to be governed by a hydrocarbon pool mechanism in which benzene is derived from secondary reactions of confined polyaromatic carbon species with the initial products of methane activation.

Nanoscale solvent organization in metal-organic framework ZIF-8 probed by EPR of flexible ß-phosphorylated nitroxides.

Metal-organic frameworks (MOFs) draw increasing attention as nanoenvironments for chemical reactions, especially in the field of catalysis. Knowing the specifics of MOF cavities is decisive in many of these cases; yet, obtaining them in situ remains very challenging. We report the first direct assessment of the apparent polarity and solvent organization inside MOF cavities using a dedicated structurally flexible spin probe. A stable ß-phosphorylated nitroxide radical was incorporated into the cavities of a prospective MOF ZIF-8 in trace amounts. The spectroscopic properties of this probe depend on local polarity, structuredness, stiffness and cohesive pressure and can be precisely monitored by Electron Paramagnetic Resonance (EPR) spectroscopy. Using this approach, we have demonstrated experimentally that the cavities of bare ZIF-8 are sensed by guest molecules as highly non-polar inside. When various alcohols fill the cavities, remarkable self-organization of solvent molecules is observed leading to a higher apparent polarity in MOFs compared to the corresponding bulk alcohols. Accounting for such nanoorganization phenomena can be crucial for optimization of chemical reactions in MOFs, and the proposed methodology provides unique routes to study MOF cavities inside in situ, thus aiding in their various applications.

Blatter Radical-Decorated Silica as a Prospective Adsorbent for Selective NO Capture from Air.

Nitrogen oxides are adverse poisonous gases present in the atmosphere and having detrimental effects on the human health and environment. In this work, we propose a new type of mesoporous materials capable of capturing nitrogen monoxide (NO) from air. The designed material combines the robust Santa Barbara Amorphous-15 silica scaffold and ultrastable Blatter-type radicals acting as NO traps. Using in situ electron paramagnetic resonance spectroscopy, we demonstrate that NO capture from air is selective and reversible at practical conditions, thus making Blatter radical-decorated silica highly promising for environmental applications.

Fine-Tuning Window Apertures in ZIF-8/67 Frameworks by Metal Ions and Temperature for High-Efficiency Molecular Sieving of Xylenes.

Separation of structurally similar components from their mixtures is one of the most promising applications of metal-organic frameworks (MOFs). A high efficiency of such molecular sieving requires fine tuning of the MOF structure. In this work, we investigate subtle metal- and temperature-induced changes in window dimensions of zeolitic imidazolate frameworks (ZIF-8(Zn) and ZIF-67(Co)) and apply such structural tuning for efficient separation of xylene isomers (p-, m-, and o-xylenes). The use of Co instead of Zn favorably modifies window geometry: it accelerates the diffusion of all components by a factor of 2-3 while maintaining closely the same separation efficiency as that of ZIF-8(Zn). Outstanding selectivity above 18:1 and faster isolation of demanded p-xylene from the ternary mixture using ZIF-67(Co) have been demonstrated at room temperature, opening new horizons for its energy-efficient xylene separation. More generally, our findings suggest the prospective ways to tune various MOFs for target liquid-state separations.

Mitigation of Pressure-Induced Amorphization in Metal-Organic Framework ZIF-8 upon EPR Control.

Pressure-induced amorphization is one of the processes inhibiting functional properties of metal-organic frameworks (MOFs). Such amorphization often occurs when MOFs are being shaped for practical applications, as well as during certain exploitations. Typically, the porosity of MOFs, which is crucial for sorption, separation, and catalysis, suffers under external pressure. We report a new experimental approach for efficient monitoring of pressure-induced processes in MOFs that employs trace amounts of spin probes (stable nitroxide radicals) embedded in the pores of MOF and detection by electron paramagnetic resonance (EPR). EPR spectra of spin probes in MOF ZIF-8 demonstrate significant changes upon pressure-induced amorphization, whose extent can be quantitatively determined from the spectral shapes. Moreover, stabilization of ZIF-8 against amorphization via reversible adsorption of various guests was studied using this approach. Mitigation effect depends on diffusion parameters and localization of guest molecules in the cavity, and maintaining of the structure and permeability up to 80% was achieved even at 1.15 GPa applied. Therefore, the proposed methodology allows significant mitigation of MOF amorphization under external pressure and conveys further perspectives of the controlled adjustment of stabilizing agents for various MOFs and their applications.

Nanoconfinement effects on structural anomalies in imidazolium ionic liquids.

Imidazolium Ionic Liquids (ILs) have been found to exhibit unusual nanostructuring behavior below their glass transition temperatures (Tg), which is ascribed to rearrangements in nonpolar domains formed by segregated alkyl chains. However, the dimensions required for such highly cooperative bulk phenomena are still unknown. In this work, we for the first time, investigate the effect of nanoconfinement on structural anomalies in imidazolium ILs. For this purpose, a series of ILs were embedded into the cavities of metal-organic framework (MOF) ZIF-8 and investigated using spin probes and Electron Paramagnetic Resonance (EPR) spectroscopy. The unusual nanostructuring near Tg, previously known for bulk ILs, was also observed for such nanoconfined ILs, and the amplitude of the anomaly was found to be dependent on the structure of the IL, thus showing the effects of molecular packing inside the MOF cavity. The first observation of structural anomalies in nanoconfined ILs opens perspectives for designing smart materials exhibiting these phenomena, and engaging MOFs as platforms creates the basis for potential applications of such functionalities.

Structural Transitions of the Metal-Organic Framework DUT-49(Cu) upon Physi- and Chemisorption Studied by in Situ Electron Paramagnetic Resonance Spectroscopy.

Flexible metal-organic frameworks (MOFs) exhibit a variety of phenomena attractive for basic and applied science. DUT-49(Cu) is one of the remarkable representatives of such MOFs, where phase transitions are coupled to pressure amplification and "negative gas adsorption". In this work we report important insights into structural transitions of DUT-49(Cu) upon physi- and chemisorption of gases and volatile liquids obtained by in situ electron paramagnetic resonance (EPR) spectroscopy. In this method, phase transitions are detected via the zero-field splitting in dimeric copper(II) units. First, a new approach was validated upon physisorption of n-butane. Then, using diethyl ether, we for the first time demonstrated that chemisorption can also trigger phase transition in DUT-49(Cu). On the basis of the EPR results, the transition appears completely reversible. The developed EPR-based approach can also be extended to other flexible MOFs containing paramagnetic metal paddlewheels, where high sensitivity and spectral resolution allow in situ studies of stimuli-induced structural variability.