Bipolar Molecular Torque Wrench Modulates the Stacking of Two-Dimensional Metal-Organic Framework Nanosheets.

The precise modulation of nanosheet stacking modes introduces unforeseen properties and creates momentous applications but remains a challenge. Herein, we proposed a strategy using bipolar molecules as torque wrenches to control the stacking modes of 2-D Zr-1,3,5-(4-carboxylphenyl)-benzene metal-organic framework (2-D Zr-BTB MOF) nanosheets. The bipolar phenyl-alkanes, phenylmethane (P-C1) and phenyl ethane (P-C2), predominantly instigated the rotational stacking of Zr-BTB-P-C1 and Zr-BTB-P-C2, displaying a wide angular distribution. This included Zr-BTB-P-C1 orientations at 0, 12, 18, and 24° and Zr-BTB-P-C2 orientations at 0, 6, 12, 15, 24, and 30°. With reduced polarity, phenyl propane (P-C3) and phenyl pentane (P-C5) introduced steric hindrance and facilitated alkyl hydrophobic interactions with the nanosheets, primarily resulting in the modulation of eclipsed stacking for Zr-BTB-P-C3 (64.8%) and Zr-BTB-P-C5 (93.3%) nanosheets. The precise angle distributions of four Zr-BTB-P species were in agreement with theoretical calculations. The alkyl induction mechanism was confirmed by the sequential guest replacement and 2-D 13C-1H heteronuclear correlation (HETCOR). In addition, at the single-particle level, we first observed that rotational stacked pores exhibited similar desorption rates for xylene isomers, while eclipsed stacked pores showed significant discrepancy for xylenes. Moreover, the eclipsed nanosheets as stationary phases exhibited high resolution, selectivity, repeatability, and durability for isomer separation. The universality was proven by another series of bipolar acetate-alkanes. This bipolar molecular torque wrench strategy provides an opportunity to precisely control the stacking modes of porous nanosheets.

Anisotropic flexibility and rigidification in a TPE-based Zr-MOFs with scu topology.

Tetraphenylethylene (TPE)-based ligands are appealing for constructing metal-organic frameworks (MOFs) with new functions and responsiveness. Here, we report a non-interpenetrated TPE-based scu Zr-MOF with anisotropic flexibility, that is, Zr-TCPE (H4TCPE = 1,1,2,2-tetra(4-carboxylphenyl)ethylene), remaining two anisotropic pockets. The framework flexibility is further anisotropically rigidified by installing linkers individually at specific pockets. By individually installing dicarboxylic acid L1 or L2 at pocket A or B, the framework flexibility along the b-axis or c-axis is rigidified, and the intermolecular or intramolecular motions of organic ligands are restricted, respectively. Synergistically, with dual linker installation, the flexibility is completely rigidified with the restriction of ligand motion, resulting in MOFs with enhanced stability and improved separation ability. Furthermore, in situ observation of the flipping of the phenyl ring and its rigidification process is made by 2H solid-state NMR. The anisotropic rigidification of flexibility in scu Zr-MOFs guides the directional control of ligand motion for designing stimuli-responsive emitting or efficient separation materials.

Insight into Drug Loading Regulated Micellar Rigidity by Nuclear Magnetic Resonance.

The rigidity of polymeric micelles plays an important role in their biological behaviors. However, how drug loading affects the rigidity of polymeric micelles remains elusive. Herein, the indomethacin (IMC)-loaded Pluronic F127 micelle is used as a model system to illustrate the impact of drug loading on the rigidity and biological behaviors of polymeric micelles. Against expectations, micelles with moderate drug loading show higher cellular uptake and more severe cytotoxicity as compared to both high and low drug loading counterparts. Extensive one- and two-dimensional nuclear magnetic resonance (NMR) measurements are employed to reveal that the higher drug loading induces stronger interaction between IMC and hydrophilic block to boost the micellar rigidity; consequently, the moderate drug loading imparts micelles with appropriate rigidity for satisfactory cellular uptake and cytotoxicity. In summary, NMR spectroscopy is an important tool to gain insight into drug loading regulated micellar rigidity, which is helpful to understand their biological behaviors.

Molecular identification and quantification of defect sites in metal-organic frameworks with NMR probe molecules.

The defects in metal-organic frameworks (MOFs) can dramatically alter their pore structure and chemical properties. However, it has been a great challenge to characterize the molecular structure of defects, especially when the defects are distributed irregularly in the lattice. In this work, we applied a characterization strategy based on solid-state nuclear magnetic resonance (NMR) to assess the chemistry of defects. This strategy takes advantage of the coordination-sensitive phosphorus probe molecules, e.g., trimethylphosphine (TMP) and trimethylphosphine oxide (TMPO), that can distinguish the subtle differences in the acidity of defects. A variety of local chemical environments have been identified in defective and ideal MOF lattices. The geometric dimension of defects can also be evaluated by using the homologs of probe molecules with different sizes. In addition, our method provides a reliable way to quantify the density of defect sites, which comes together with the molecular details of local pore environments. The comprehensive solid-state NMR strategy can be of great value for a better understanding of MOF structures and for guiding the design of MOFs with desired catalytic or adsorption properties.

Defect-Assisted Loading and Docking Conformations of Pharmaceuticals in Metal-Organic Frameworks.

Understanding of drug-carrier interactions is essential for the design and application of metal-organic framework (MOF)-based drug-delivery systems, and such drug-carrier interactions can be fundamentally different for MOFs with or without defects. Herein, we reveal that the defects in MOFs play a key role in the loading of many pharmaceuticals with phosphate or phosphonate groups. The host-guest interaction is dominated by the Coulombic attraction between phosphate/phosphonate groups and defect sites, and it strongly enhances the loading capacity. For similar molecules without a phosphate/phosphonate group or for MOFs without defects, the loading capacity is greatly reduced. We employed solid-state NMR spectroscopy and molecular simulations to elucidate the drug-carrier interaction mechanisms. Through a synergistic combination of experimental and theoretical analyses, the docking conformations of pharmaceuticals at the defects were revealed.

Probing Nonuniform Adsorption in Multicomponent Metal-Organic Frameworks via Segmental Dynamics by Solid-State Nuclear Magnetic Resonance.

The guest adsorption phenomena in multicomponent metal-organic frameworks (MOFs) are intricate due to their structural complexities. In this work, we studied two members of the isostructural series of MUF-77 frameworks that consist of long or short alkyl groups. The adsorption of methanol, N,N-dimethylaniline (DMA) and acridine orange (AO) in two structures of MUF-77 has been investigated. 2H solid-state nuclear magnetic resonance (SSNMR) and two-dimensional 1H-13C NMR spectroscopy were used to probe the dynamics of various compartments of MUF-77. Through the analyses of dynamic behavior by SSNMR and molecular dynamics simulations, we elucidate the spatial distribution of guest molecules are nonuniform around different chemical components, in different pore structures, and across different parts of MOF lattice. In addition, we reveal that the framework flexibility of MUF-77 with short alkyl groups is reduced upon guest adsorption yet the framework flexibility of MUF-77 with long alkyl groups increases upon loading with methanol.

In situ solid-state NMR characterization of pharmaceutical materials: An example of drug-polymer thermal mixing.

Pharmaceutical amorphous solid dispersions, a multicomponent system prepared by dispersing drug substances into polymeric matrix via thermal and mechanical processes, represent a major platform to deliver the poorly water-soluble drug. Microscopic properties of drug-polymer contacts play mechanistic roles in manipulating long-term physical stability as well as dissolution profiles. Although solid-state nuclear magnetic resonance has been utilized as an indispensable tool to probe structural details, previous studies are limited to ex situ characterizations. Our work provides likely the first documented example to investigate comelting of ketoconazole and polyacrylic acid, as a model system, in an in situ manner. Their physical mixture is melted and mixed in the solid-state nuclear magnetic resonance rotor under magic angle spinning at up to approximately 400 K. Critical structural events of molecular miscibility and interaction have been successfully identified. These results design and evaluate the instrumental and experimental protocols for real-time characterizations of the comelting of pharmaceutical materials.

Giant anisotropic thermal expansion actuated by thermodynamically assisted reorientation of imidazoliums in a single crystal.

Materials demonstrating unusual large positive and negative thermal expansion are fascinating for their potential applications as high-precision microscale actuators and thermal expansion compensators for normal solids. However, manipulating molecular motion to execute huge thermal expansion of materials remains a formidable challenge. Here, we report a single-crystal Cu(II) complex exhibiting giant thermal expansion actuated by collective reorientation of imidazoliums. The circular molecular cations, which are rotationally disordered at a high temperature and statically ordered at a low temperature, demonstrate significant reorientation in the molecular planes. Such atypical molecular motion, revealed by variable-temperature single crystal X-ray diffraction and solid-state NMR analyses, drives an exceptionally large positive thermal expansion and a negative thermal expansion in a perpendicular direction of the crystal. The consequent large shape change (~10%) of bulk material, with remarkable durability, suggests that this complex is a strong candidate as a microscale thermal actuating material.