The ease with which molecular building blocks can be ordered in metal–organic frameworks is an invaluable asset for many potential applications. In this work, we exploit this inherent order to produce chromatic polarizers based on visible-light linear dichroism via cobalt paddlewheel chromophores.@en

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.

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Metal–organic frameworks (MOFs) are ordered arrays of polytopic organic ligands, commonly called linkers, which interconnect metal-based inorganic building units via coordination bonds. The highly precise assembly of well defined building blocks into extended 3-D networks, known as reticular chemistry, has allowed researchers in this field to produce tens of thousands of different frameworks. As an obvious consequence of their unprecedented porosity and tunability, these versatile materials are continuously studied with various potential applications in mind. An important portion of MOF research has been invested in the interaction between molecules and the framework, aiming for applications such as gas storage and separation, as well as catalysis, drug delivery, heat exchange, and water harvesting. MOFs are considered soft or flexible materials, a characteristic that includes structural dynamics or large amplitude deformations. This flexibility can usually be attributed to the framework’s topology and the degrees of freedom of some of its bonds. However, the linkers themselves may also have degrees of freedom allowing independent molecular dynamics, in particular in the form of rotation. This type of dynamics is particularly common in MOFs because their porous architectures often provide enough space for the rotation of a molecular fragment to occur. It is this type of dynamics that this thesis is centered on, starting from the fact that, although it is an intriguing phenomenon that occurs in MOFs, it has remained relatively unexplored. Nevertheless, the past four years have seen an increase in researchers’ interest in rotational dynamics in MOFs. This may be due to two main reasons: First, linker rotation influences MOF properties, not only when guest molecule interactions are involved, but also in optical and mechanical properties. Development of our knowledge on linker rotation is therefore essential for a more complete understanding of these materials’ properties and how they may be modified to enhance a specific trait. Second, the exploitation of linkers’ rotational freedom could potentially lead to important technological advances. The latter category includes various innovative ideas, such as the design of ferroelectric MOFs by means of controllable dipolar rotors, or the realization of crystalline molecular machines able to produce useful work.@en

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.

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We report the synthesis and dielectric characterization of novel polyvinylidene fluoride-trifluoroethylene P(VDF-TrFE) composite films containing [(CH₃)₂NH₂][Mg(HCOO)₃] (DMAMg) and [NH₄][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 (CH₃)₂NH₂⁺ 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 (CH₃)₂NH₂⁺ 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.

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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.

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