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Kinetic Control of Metal–Organic Framework Crystallization Investigated by Time‐Resolved In Situ X‐Ray Scattering

Metal–organic frameworks (MOFs) are among the most sophisticated nanostructured solids: they often possess high surface areas and pore volumes, with the possibility of finetuning their chemical environment by either selecting the appropriate building blocks or by postsynthetic functionalization. For many frameworks, flexibility of the lattice allows them to undergo a significant transformation in solid state.[1] All these features make MOFs a special class of solids with the potential of transcending many common limitations in different technological disciplines, such as ferromagnetism,[2] semiconductivity, gas separation,[3] storage,[4] sensing,[5] catalysis,[ 6] drug delivery,[7] or proton conductivity.[8] However, the crystallization mechanism of these complex structures is far from understood. Notwithstanding the plethora of publications that present new MOFs,[9] and the effectiveness of the high-throughput approach,[10] serendipity still governs the synthesis of new structures.

NH2-MIL-53(Al): A High-Contrast Reversible Solid-State Nonlinear Optical Switch

The metal-organic framework NH(2)-MIL-53(Al) is the first solid-state material displaying nonlinear optical switching due to a conformational change upon breathing. A switching contrast of at least 38 was observed. This transition originates in the restrained linker mobility in the very narrow pore configuration.

Highly Selective Chemical Sensing in a Luminescent Nanoporous Magnet

Among the wide variety of properties of interest that a given material can exhibit, luminescence is attracting an increasing attention due to its potential application in optical devices for lighting equipment and optical storage, [ 1a − c] optical switching, [ 1d ,e] and sensing. [ 1f − i ] At this respect, many scientists, working in the multidisciplinary fi eld of the materials science, have directed their efforts to the obtention of luminescent materials with potential sensing applications. For instance, sensitive and selective detection of gas and vapor phase analytes can result specially interesting because of the variety of applications that can be found in many different fi elds. A key principle concerning the luminescent chemosensors [ 2 ] is that they must be able to detect differences between small molecules, [ 2 , 3 ] and sequentially implement a recognition– transduction protocol. [ 2b ] In this sense, the remarkable shape selectivity of a class of highly porous materials, the so-called metal-organic frameworks (MOFs) [ 4 ] which have already shown applications in different fi elds (gas storage and separation, molecular recognition and catalysis, molecular electronics and spintronics, molecular photonics, etc) [ 4–6 ] has converted them in excellent candidates for the fabrication of chemical sensors. [ 2 , 3 ] The key point responsible for the high potential success of MOFs as chemo-sensors is the exceptional tunability of their structures and properties.

Adsorption and Separation of Light Gases on an Amino-Functionalized Metal–Organic Framework: An Adsorption and In Situ XRD Study

The NH(2)-MIL-53(Al) metal-organic framework was studied for its use in the separation of CO(2) from CH(4), H(2), N(2)C(2)H(6) and C(3)H(8) mixtures. Isotherms of methane, ethane, propane, hydrogen, nitrogen, and CO(2) were measured. The atypical shape of these isotherms is attributed to the breathing properties of the material, in which a transition from a very narrow pore form to a narrow pore form and from a narrow pore form to a large pore form occurs, depending on the total pressure and the nature of the adsorbate, as demonstrated by in situ XRD patterns measured during adsorption. Apart from CO(2), all tested gases interacted weakly with the adsorbent. As a result, they are excluded from adsorption in the narrow pore form of the material at low pressure. CO(2) interacted much more strongly and was adsorbed in significant amounts at low pressure. This gives the material excellent properties to separate CO(2) from other gases. The separation of CO(2) from methane, nitrogen, hydrogen, or a combination of these gases has been demonstrated by breakthrough experiments using pellets of NH(2)-MIL-53(Al). The effect of total pressure (1-30 bar), gas composition, temperature (303-403 K) and contact time has been examined. In all cases, CO(2) was selectively adsorbed, whereas methane, nitrogen, and hydrogen nearly did not adsorb at all. Regeneration of the adsorbent by thermal treatment, inert purge gas stripping, and pressure swing has been demonstrated. The NH(2)-MIL-53(Al) pellets retained their selectivity and capacity for more than two years.

Interplay of metal node and amine functionality in NH2-MIL-53: modulating breathing behavior through intra-framework interactions.

A series of amino-functionalized MIL-53 with different metals as nodes has been synthesized. By determining adsorption properties and spectroscopic characterization, we unequivocally show that the interaction between the amines of the organic linker and bridging μ(2)-OH of the inorganic scaffold modulates metal organic framework (MOF) flexibility. The strength of the interaction has been found to correlate with the electropositivity of the metal.

Experimental evidence of negative linear compressibility in the MIL-53 metal–organic framework family

We report a series of powder X-ray diffraction experiments performed on the soft porous crystals MIL-53(Al) and NH2-MIL-53(Al) in a diamond anvil cell under different pressurization media. Systematic refinements of the obtained powder patterns demonstrate that these materials expand along a specific direction while undergoing total volume reduction under an increase in hydrostatic pressure. The results confirm for the first time the Negative Linear Compressibility behaviour of this family of materials recently predicted from quantum chemical calculations.

High compressibility of a flexible metal–organic framework

The metal–organic framework NH2-MIL-53(In) shows a very high amorphization resistance (>20 GPa) together with a large compressibility (K0 = 10.9 GPa).

Post-synthetic cation exchange in the robust metal–organic framework MIL-101(Cr)

In this work an unambiguous proof of post-synthetic solvent-assisted cation exchange in the robust metal–organic framework MIL-101(Cr) is reported. Such substitution can alter directly the secondary building unit, which often defines the properties of the material.

Evidence for a chemical clock in oscillatory formation of UiO-66

Chemical clocks are often used as exciting classroom experiments, where an induction time is followed by rapidly changing colours that expose oscillating concentration patterns. This type of reaction belongs to a class of nonlinear chemical kinetics also linked to chaos, wave propagation and Turing patterns. Despite its vastness in occurrence and applicability, the clock reaction is only well understood for liquid-state processes. Here we report a chemical clock reaction, in which a solidifying entity, metal–organic framework UiO-66, displays oscillations in crystal dimension and number, as shown by X-ray scattering. In rationalizing this result, we introduce a computational approach, the metal–organic molecular orbital methodology, to pinpoint interaction between the tectonic building blocks that construct the metal–organic framework material. In this way, we show that hydrochloric acid plays the role of autocatalyst, bridging separate processes of condensation and crystallization.

Nanocarrier-Mediated Photochemotherapy and Photoradiotherapy

Photothermal therapy (PTT) and photodynamic therapy (PDT) both utilize light to induce a therapeutic effect. These therapies are rapidly gaining importance due to the noninvasiveness of light and the limited adverse effect associated with these treatments. However, most preclinical studies show that complete elimination of tumors is rarely observed. Combining PDT and PTT with chemotherapy or radiotherapy can improve the therapeutic outcome and simultaneously decrease side effects of these conventional treatments. Nanocarriers can help to facilitate such a combined treatment. Here, the most recent advancements in the field of photochemotherapy and photoradiotherapy, in which nanocarriers are employed, are reviewed.