Among the numerous fascinating properties of metal–organic frameworks (MOFs), their rotational dynamics is perhaps one of the most intriguing, with clear consequences for adsorption and separation of molecules, as well as for optical and mechanical properties. A closer look at the rotational mobility in MOF linkers reveals that it is not only a considerably widespread phenomenon, but also a fairly diverse one. Still, the impact of these dynamics is often understated. In this review, we address the various mechanisms of linker rotation reported in the growing collection of literature, followed by a highlight of the methods currently used in their study, and we conclude with the impacts that such dynamics have on existing and future applications.
The inclusion of Ti(IV) in water-stable metal-organic frameworks has been proposed as a strategy for the generation of high surface area heterogeneous photocatalysts, with UiO-66 being a promising candidate. We find that the site of binding of Ti(IV) is at linker defect sites and not incorporated into the inorganic secondary building unit through metal exchange. We also demonstrate the choice of titanium source is critical in the generation of an active hydrogen evolution catalyst, and explain the observed activity using density functional theory calculations, which also enable rational design of future Ti(IV) coordination environments in MOF catalysts.
The organic components in metal−organic frameworks (MOFs) are unique: they are embedded in a crystalline lattice, yet, as they are separated from each other by tunable free space, a large variety of dynamic behavior can emerge. These rotational dynamics of the organic linkers are especially important due to their influence over properties such as gas adsorption and kinetics of guest release. To fully exploit linker rotation, such as in the form of molecular machines, it is necessary to engineer correlated linker dynamics to achieve their cooperative functional motion. Here, we show that for MIL-53, a topology with closely spaced rotors, the phenylene functionalization allows researchers to tune the rotors' steric environment, shifting linker rotation from completely static to rapid motions at frequencies above 100 MHz. For steric interactions that start to inhibit independent rotor motion, we identify for the first time the emergence of coupled rotation modes in linker dynamics. These findings pave the way for function-specific engineering of gear-like cooperative motion in MOFs.
A modulated synthesis approach based on the chelating properties of oxalic acid (H2C2O4) is presented as a robust and versatile method to achieve highly crystalline Al‐based metal‐organic frameworks. A comparative study on this method and the already established modulation by hydrofluoric acid was conducted using MIL‐53 as test system. The superior performance of oxalic acid modulation in terms of crystallinity and absence of undesired impurities is explained by assessing the coordination modes of the two modulators and the structural features of the product. The validity of our approach was confirmed for a diverse set of Al‐MOFs, namely X‐MIL‐53 (X=OH, CH3O, Br, NO2), CAU‐10, MIL‐69, and Al(OH)ndc (ndc=1,4‐naphtalenedicarboxylate), highlighting the potential benefits of extending the use of this modulator to other coordination materials.
The anisotropic alignment of chromophores in a cobalt-based metal–organic framework produces a dichromatic dichroic effect, showing a range of colors from blue to yellow depending on the orientation of polarized light relative to the crystal.
We report the synthesis and dielectric characterization of novel polyvinylidene fluoride−trifluoroethylene P(VDF−TrFE) composite films containing [(CH 3) 2 NH 2 ][Mg(HCOO) 3 ] (DMAMg) and [NH 4 ][Zn(HCOO) 3 ] (AmZn) dense metal−organic frameworks (MOFs). The optical camera and Raman microscopies are used to map the distribution of the MOF fillers in the prepared films. The dielectric spectroscopy experiments of the DMAMg/P(VDF−TrFE) composite performed in a broad temperature range demonstrate rich dielectric behavior originating from the dipolar dynamics of the (CH 3) 2 NH 2 + molecular cations and glassy behavior of the copolymer matrix. An anomalous behavior of the complex dielectric permittivity is also observed because of the structural phase transition of DMAMg fillers. The dielectric properties of the AmZn/P(VDF−TrFE) composite film are mainly determined by the dipolar glass relaxation of the P(VDF−TrFE) polymer. The frequency-dependent dielectric spectra of both composites allow us to characterize the observed dipolar relaxation processes. The (CH 3) 2 NH 2 + cation dynamics follows the Arrhenius law, whereas the glassy behavior of P(VDF−TrFE) is described by the Vogel−Fulcher equation. For both composites, we observe a significant increase of the dielectric permittivity compared with the P(VDF−TrFE) film without MOF fillers.
Am odulateds ynthesis approach based on the chelating properties of oxalic acid (H 2 C 2 O 4 )i sp resented as a robust andv ersatile methodt oa chieveh ighly crystalline Albased metal-organic frameworks. Ac omparatives tudy on this method and the already established modulation by hydrofluoric acid was conducted using MIL-53 as test system. The superior performance of oxalic acid modulation in terms of crystallinity and absence of undesired impurities is ex-plained by assessing the coordination modes of the two modulators and the structural features of the product. The validity of our approach was confirmed for ad iverse set of Al-MOFs,n amely X-MIL-53 (X = OH, CH 3 O, Br,N O 2 ), CAU-10, MIL-69, andA l(OH)ndc (ndc = 1,4-naphtalenedicarboxylate), highlighting the potentialb enefits of extending the use of this modulator to other coordination materials.[a] Dr.[ + + ] These authors contributed equally.[**] Ap revious versiono ft his manuscript has been deposited on ap reprint server (https://doi.org/10.26434/chemrxiv.9999572.v1).Supporting information and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.org/10.Scheme2.The reaction formulas for MIL-53 synthesis modulated by oxalic acid (a) and hydrofluoric acid (b).
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