The Influence of Chemical Modification on Linker Rotational Dynamics in Metal–Organic Frameworks

Exploring the effect of molecular size and framework functionalisation on transport in metal-organic frameworks using pulsed-field gradient nuclear magnetic resonance

Molecular transport is an important aspect in metal-organic frameworks (MOFs) as it affects many of their applications, such as adsorption/separation, drug delivery and catalysis. Yet probing the fundamental diffusion mechanisms in MOFs is challenging, and the interplay between the MOF's features (such as the pore structure and linker dynamics) and molecular transport remains mostly unexplored. Here, the pulsed-field gradient nuclear magnetic resonance (PFG NMR) technique is used to probe the diffusion of several probe molecules, i.e., water, xylenes and 1,3,5-triisopropylbenzene (TIPB), within the UiO-66 MOF and its derivatives (UiO-66NH2 and UiO-66Br). Exploiting differences in the size of probe molecules we were able to probe the diffusion rate selectively in the different pore environments of the MOFs. In particular, when relatively small molecules, such as water and small hydrocarbons, were used as probes, the PFG NMR log attenuation plots were non-linear with two distinctive diffusion regions, suggesting faster diffusion in the inter-crystalline space and slower diffusion within crystal aggregates, the latter occurring mostly inside the framework of the MOFs. Conversely, experiments with a larger probe molecule, i.e., TIPB, with a kinetic diameter of 0.95 nm, which makes it unable to access the framework windows of the MOF crystals, showed linear PFG NMR log attenuation plots, which indicates diffusion occurring in a single environment, most likely in the inter-crystalline space. Analysis of the apparent tortuosity values of the systems under investigation highlights the role of linker functionalisation in influencing the molecular diffusion of the probe molecules, which affects both intra-molecular interactions and pore accessibility within the MOF crystals. The findings of this work demonstrate that the diffusion behaviour of probe molecules within MOFs is influenced by the pore size, structure, functionalisation of the MOF linker and molecular interactions. Our study contributes to further advance the understanding of mass transport in MOFs by PFG NMR and provides insights that can inform the design and optimisation of MOF-based materials for various applications.

Exploring Spin-Phonon Coupling in Magnetic 2D Metal-Organic Frameworks

Layered magnetic metal-organic frameworks (MOFs) are an emerging class of materials that can combine the advantages of both MOFs and 2D magnetic crystals. The recent discovery of large coercivity and long-range magnetic ordering up to 515 K in a layered MOF of general formula MCl₂(pyz)₂ (M = transition metal, pyz = pyrazine) offers an exciting versatile platform to achieve high-T_C magnetism at the 2D limit. In this work, we investigate the exfoliation feasibility down to the monolayer of VCl₂(pyz)₂ and CrCl₂(pyz)₂ by means of first-principles calculations. We explore their structural, electronic, magnetic and vibrational properties, as well as the effect of halide substitution. Then, we provide a full analysis of the spin-phonon coupling (SPC) in both 2D derivatives. Our calculations reveal a low SPC and thermal evolution of the magnetic exchange interactions and single-ion anisotropy mainly governed by low-frequency phonon modes. Finally, we provide chemical insights to improve the performance of these magnetic 2D MOFs based on the effective manipulation of the phonon modes that can present a major impact on their magnetic properties.

Output Enhancement of Triboelectric Nanogenerators Based on Hierarchically Regular Cadmium Coordination Polymers for Photocycloaddition

Exploiting the well-arranged and tunable frameworks of crystalline materials, we herein report coordination polymers (CPs) with modulated hierarchical structures as triboelectric materials to construct and extend the application scope of triboelectric nanogenerators (TENGs). Different lengths and shapes of bridging ligands [4,4'-bpa = 1,2-bis(4-pyridyl)ethane, 4,4'-bpe = 1,2-bis(4-pyridyl)ethene, and 4,4'-bpp = 1,3-di(2-pyridyl)propane for 1, 2, and 3, respectively] were used to construct Cd-CP-based hierarchical frameworks. These compounds were used as triboelectric materials, and their electronic structure contributions were determined by the output of the corresponding TENGs. The results indicated that 2-TENG with the 4,4'-bpe ligand had the highest output, attributed to the improvement in the electron activity due to the π-conjugation group in the bridging ligand, which was further verified by density functional theory calculations. Furthermore, 2@PVDF (PVDF = polyvinylidene fluoride) composite films with different concentrations of Cd-CP were prepared. Detailed electrical characterizations revealed that the arrangement of 12% active constituents of Cd-CP-2 effectively enhanced the output performance of 2@PVDF-TENG, which could light up an ultraviolet lamp plate to successfully execute the [2 + 2] photocycloaddition.

Breathing Effect via Solvent Inclusions on the Linker Rotational Dynamics of Functionalized MIL-53

The breathing effects of functionalized MIL-53-X (X=H, CH₃ , NH₂ , OH, and NO₂ ) induced by the inclusions of water, methanol, acetone, and N,N-dimethylformamide solvents were comprehensively investigated by solid-state NMR spectroscopy. 2D homo-nuclear correlation NMR provided direct experimental evidence for the host-guest interaction between the guest solvents and the MOF frameworks. The variations of the ¹ H and ¹³ C NMR chemical shifts in functionalized MIL-53 from the narrow pore phase transitions to large pore forms due to solvent inclusions were clearly identified. The influence of functionalized linkers and their host-guest interactions with the confined solvents on the rotational dynamics of the linkers was examined by separated-local-field MAS NMR experiments in conjunction with DFT theoretical calculations. It is found that the linker rotational dynamics of functionalized MIL-53 in narrow pore form is closely related to the computational rotational energy barrier. The BDC-NO₂ linker of activated MIL-53-NO₂ undergoes relatively faster rotation, whereas the BDC-NH₂ and BDC-OH linkers of activated MIL-53-NH₂ and MIL-53-OH exhibit relatively slower rotation. The host-guest interactions between confined solvents and MIL-53-NO₂ , MIL-53-CH₃ would significantly induce an increase of the order parameters of unsubstituted carbon and reduce the rotational frequency of linkers. This study provides a spectroscopic approach for the investigation of linker rotation in functionalized MOFs at natural abundance with solvents inclusions.

Monitoring Dynamics, Structure, and Magnetism of Switchable Metal-Organic Frameworks via 1 H-Detected MAS NMR

We present a toolbox for the rapid characterisation of powdered samples of paramagnetic metal-organic frameworks at natural abundance by 1 H-detected solid-state NMR. Very fast MAS rates at room and cryogenic temperatures and a set of tailored radiofrequency irradiation schemes help overcome the sensitivity and resolution limits often associated with the characterisation of MOF materials. We demonstrate the approach on DUT-8(Ni), a framework containing Ni2+ paddle-wheel units which can exist in two markedly different architectures. Resolved 1 H and 13 C resonances of organic linkers are detected and assigned in few hours with only 1-2 mg of sample at natural isotopic abundance, and used to rapidly extract information on structure and local internal dynamics of the assemblies, as well as to elucidate the metal electronic properties over an extended temperature range. The experiments disclose new possibilities for describing local and global structural changes and correlating them to electronic and magnetic properties of the assemblies.

Molecular-Rotor-Driven Advanced Porous Materials

Advanced porous materials (APMs)-such as metal-organic frameworks (MOFs) and porous organic polymers (POPs)-have emerged as an exciting research frontier of chemistry and materials science. Given their tunable pore size and extensive diversity, APMs have found widespread applications. In addition, adding dynamic functional groups to porous solids furthers the development of stimuli-responsive materials. By incorporating moving elements-molecular rotors-into the porous frameworks, molecular-rotor-driven advanced porous materials (MR-APMs) can respond reversibly to chemical and physical stimuli, thus imparting dynamic functionalities that have not been found in conventional porous materials. This Minireview discusses exemplary MR-APMs in terms of their design, synthesis, rotor dynamics, and potential applications.

Atomistic Insight Into the Host-Guest Interaction of a Photoresponsive Metal-Organic Framework

Photoresponsive functional materials have gained increasing attention due to their externally tunable properties. Molecular switches embedded in these materials enable the control of phenomena at the atomic level by light. Metal-organic frameworks (MOFs) provide a versatile platform to immobilize these photoresponsive units within defined molecular environments to optimize the intended functionality. For the application of these photoresponsive MOFs (pho-MOFs), it is crucial to understand the influence of the switching state on the host-guest interaction. Therefore, we present a detailed insight into the impact of molecular switching on the intermolecular interactions. By performing atomistic simulations, we revealed that due to different interactions of the guest molecules with the two isomeric states of an azobenzene-functionalized MOF, both the adsorption sites and the orientation of the molecules within the pores are modulated. By shedding light on the host-guest interaction, our study highlights the unique potential of pho-MOFs to tailor molecular interaction by light.

Resolution enhancement and proton proximity probed by 3D TQ/DQ/SQ proton NMR spectroscopy under ultrafast magic-angle-spinning beyond 70 kHz

Proton nuclear magnetic resonance (NMR) in solid state has gained significant attention in recent years due to the remarkable resolution and sensitivity enhancement afforded by ultrafast magic-angle-spinning (MAS). In spite of the substantial suppression of ¹H-¹H dipolar couplings, the proton spectral resolution is still poor compared to that of ¹³C or ¹⁵N NMR, rendering it challenging for the structural and conformational analysis of complex chemicals or biological solids. Herein, by utilizing the benefits of double-quantum (DQ) and triple-quantum (TQ) coherences, we propose a 3D single-channel pulse sequence that correlates proton triple-quantum/double-quantum/single-quantum (TQ/DQ/SQ) chemical shifts. In addition to the two-spin proximity information, this 3D TQ/DQ/SQ pulse sequence enables more reliable extraction of three-spin proximity information compared to the regular 2D TQ/SQ correlation experiment, which could aid in revealing the proton network in solids. Furthermore, the TQ/DQ slice taken at a specific SQ chemical shift only reveals the local correlations to the corresponding SQ chemical shift, and thus it enables accurate assignments of the proton peaks along the TQ and DQ dimensions and simplifies the interpretation of proton spectra especially for dense proton networks. The high performance of this 3D pulse sequence is well demonstrated on small compounds, L-alanine and a tripeptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (MLF). We expect that this new methodology can inspire the development of multidimensional solid-state NMR pulse sequences using the merits of TQ and DQ coherences and enable high-throughput investigations of complex solids using abundant protons.