Molecular-Rotor-Driven Advanced Porous Materials

Optimizing Iodine Enrichment through Induced-Fit Transformations in a Flexible Ag(I)-Organic Framework: From Accelerated Adsorption Kinetics to Record-High Storage Density

Efficient capture and storage of radioactive I2 is a prerequisite for developing nuclear power but remains a challenge. Here, two flexible Ag-MOFs (FJI-H39 and 40) with similar active sites but different pore sizes and flexibility are prepared; both of them can capture I2 with excellent removal efficiencies and high adsorption capacities. Due to the more flexible pores, FJI-H39 not only possesses the record-high I2 storage density among all the reported MOFs but also displays a very fast adsorption kinetic (124 times faster than FJI-H40), while their desorption kinetics are comparable. Mechanistic studies show that FJI-H39 can undergo induced-fit transformations continuously (first contraction then expansion), making the adsorbed iodine species enrich near the Ag(I) nodes quickly and orderly, from discrete I- anion to the dense packing of various iodine species, achieving the very fast adsorption kinetic and the record-high storage density simultaneously. However, no significant structural transformations caused by the adsorbed iodine are observed in FJI-H40. In addition, FJI-H39 has excellent stability/recyclability/obtainability, making it a practical adsorbent for radioactive I2. This work provides a useful method for synthesizing practical radioactive I2 adsorbents.

Nanofibrous Porous Organic Polymers and Their Derivatives: From Synthesis to Applications

Engineering porous organic polymers (POPs) into 1D morphology holds significant promise for diverse applications due to their exceptional processability and increased surface contact for enhanced interactions with guest molecules. This article reviews the latest developments in nanofibrous POPs and their derivatives, encompassing porous organic polymer nanofibers, their composites, and POPs-derived carbon nanofibers. The review delves into the design and fabrication strategies, elucidates the formation mechanisms, explores their functional attributes, and highlights promising applications. The first section systematically outlines two primary fabrication approaches of nanofibrous POPs, i.e., direct bulk synthesis and electrospinning technology. Both routes are discussed and compared in terms of template utilization and post-treatments. Next, performance of nanofibrous POPs and their derivatives are reviewed for applications including water treatment, water/oil separation, gas adsorption, energy storage, heterogeneous catalysis, microwave absorption, and biomedical systems. Finally, highlighting existent challenges and offering future prospects of nanofibrous POPs and their derivatives are concluded.

A Universal and Programmable Platform based on Fluorescent Peptide-Conjugated Probes for Detection of Proteins in Organelles of Living Cells

Realizing protein analysis in organelles of living cells is of great significance for developing diagnostic and therapeutic methods of diseases. Fluorescent-labeled antibodies with well imaging performance and high affinity are classical biochemical tools for protein analysis, while due to the inability to effectively enter into cells, not to mention organelles and the uncontrollable reaction sites that might cause antibodies inactivation when chemically modification, they are hard to apply to living cells. Inspired by the structure of fluorescent-labeled antibodies, we designed as a universal detection platform that was based on the peptide-conjugated probes (PCPs) and consisted of three parts: a) a rotor type fluorescent molecular scaffold for conjugation and signal output; b) the cell penetration protein recognition unit; c) the subcellular organelle targeting unit. In living cells, PCPs could firstly localize at organelles and then proceed protein specific recognition, thus jointly leading to the restriction of twisted intramolecular charge transfer and activation of fluorescence signal. As a proof-of-concept, six different proteins in three typical intracellular organelles could be detected by our platform through simply replacing the recognition sequence of proteins and matching organelle targeting units. The position and intensity of fluorescence signals demonstrated specificity of PCPs and universality of the platform.

Solid State Machinery of Multiple Dynamic Elements in a Metal-Organic Framework

Engineering coordinated rotational motion in porous architectures enables the fabrication of molecular machines in solids. A flexible two-fold interpenetrated pillared Metal-Organic Framework precisely organizes fast mobile elements such as bicyclopentane (BCP) (107  Hz regime at 85 K), two distinct pyridyl rotors and E-azo group involved in pedal-like motion. Reciprocal sliding of the two sub-networks, switched by chemical stimuli, modulated the sizes of the channels and finally the overall dynamical machinery. Actually, iodine-vapor adsorption drives a dramatic structural rearrangement, displacing the two distinct subnets in a concerted piston-like motion. Unconventionally, BCP mobility increases, exploring ultra-fast dynamics (107  Hz) at temperatures as low as 44 K, while the pyridyl rotors diverge into a faster and slower dynamical regime by symmetry lowering. Indeed, one pillar ring gained greater rotary freedom as carried by the azo-group in a crank-like motion. A peculiar behavior was stimulated by pressurized CO2, which regulates BCP dynamics upon incremental site occupation. The rotary dynamics is intrinsically coupled to the framework flexibility as demonstrated by complementary experimental evidence (multinuclear solid-state NMR down to very low temperatures, synchrotron radiation XRD, gas sorption) and computational modelling, which helps elucidate the highly sophisticated rotor-structure interplay.

A confinement-regulated (H3C-NH3)+ ion as a smallest dual-wheel rotator showing bisected rotation dynamics

On the basis of variable-temperature single-crystal X-ray diffraction, rotational energy barrier analysis, variable-temperature/frequency dielectric response, and molecular dynamics simulations, here we report a new crystalline supramolecular rotor (CH3NH3)(18-crown-6)[CuCl3], in which the (H3C-NH3)+ ion functions as a smallest dual-wheel rotator showing bisected rotation dynamics, while the host 18-crown-6 macrocycle behaves as a stator that is not strictly stationary. This study also provides a helpful insight into the dynamics of ubiquitous -CH3/-NH3 groups confined in organic or organic-inorganic hybrid solids.

Molecular rotators anchored on a rod-like anionic coordination polymer adhered by charge-assisted hydrogen bonds

On the basis of variable-temperature single-crystal X-ray diffraction, variable-temperature/frequency dielectric analysis, variable-temperature solid-state nuclear magnetic resonance spectroscopy, and molecular dynamics simulations, here we present a new model of crystalline supramolecular rotor (i-PrNHMe2)[CdBr3], where a conformationally flexible near-spherical (i-PrNHMe2)+ cation functions as a rotator and a rod-like anionic coordination polymer {[CdBr3]-}∞ acts as the stator, and the adhesion of them is realized by charge-assisted hydrogen bonds.

Giant Crystalline Molecular Rotors that Operate in the Solid State

Molecular motion in the solid state is typically precluded by the highly dense environment, and only molecules with a limited range of sizes show such dynamics. Here, we demonstrate the solid-state rotational motion of two giant molecules, i.e., triptycene and pentiptycene, by encapsulating a bulky N-heterocyclic carbene (NHC) Au(I) complex in the crystalline media. To date, triptycene is the largest molecule (surface area: 245 Å2 ; volume: 219 Å3 ) for which rotation has been reported in the solid state, with the largest rotational diameter among reported solid-state molecular rotors (9.5 Å). However, the pentiptycene rotator that is the subject of this study (surface area: 392 Å2 ; volume: 361 Å3 ; rotational diameter: 13.0 Å) surpasses this record. Single-crystal X-ray diffraction analyses of both the developed rotors revealed that these possess sufficient free volume around the rotator. The molecular motion in the solid state was confirmed using variable-temperature solid-state 2 H spin-echo NMR studies. The triptycene rotor exhibited three-fold rotation, while temperature-dependent changes of the rotational angle were observed for the pentiptycene rotor.

In-situ encapsulation and construction of Lac@HOFs/hydrogel composite for enhancing laccase stability and azo dyes decolorization efficiency

Enzymes with high catalytic activity and stability have been used for the sustainable development of green chemical applications, such as water remediation. Immobilized laccase can be used to construct a synergistic system for adsorption and degradation, which has great potential for water remediation. Herein, a hydrogen-bonded organic framework was installed onto laccase in-situ to form a net-carboxylate-arranged defective cage, which enhanced its catalytic stability. Thereafter, the CMC/PVA/Lac@HOF-101 hydrogel was fabricated by freeze-thaw cycles using sodium carboxymethylcellulose and polyvinyl alcohol as carriers and copper (II) as a cross-linker. Notably, the MOFs/hydrogel as a protective carrier of laccase maintain long-term recyclability and catalytic stability. After the fifth catalytic cycle, approximately 66.7 % activity of the CP-Lac@HOF-101 was retained. When both free laccase and CP-Lac@HOF-101 were used for decolorization of Acid Orange 7 (AO), the removal rates were 10.9 % and 82.5 % after 5 h, respectively. Furthermore, even in the presence of metal cations, almost 60.0 % of the AO removal efficiency was achieved. The relationship between the structure of the azo dyes and decolorization efficiency of the synergistic system was further investigated. This study offers a method for constructing enzyme@HOF-based composite hydrogels and provides a promising water remediation strategy.

Optimizing Sieving Effect for CO2 Capture from Humid Air Using an Adaptive Ultramicroporous Framework

Excessive CO2 in the air can not only lead to serious climate problems but also cause serious damage to humans in confined spaces. Here, a novel metal-organic framework (FJI-H38) with adaptive ultramicropores and multiple active sites is prepared. It can sieve CO2 from air with the very high adsorption capacity/selectivity but the lowest adsorption enthalpy among the reported physical adsorbents. Such excellent adsorption performances can be retained even at high humidity. Mechanistic studies show that the polar ultramicropore is very suitable for molecular sieving of CO2 from N2 , and the distinguishable adsorption sites for H2 O and CO2 enable them to be co-adsorbed. Notably, the adsorbed-CO2 -driven pore shrinkage can further promote CO2 capture while the adsorbed-H2 O-induced phase transitions in turn inhibit H2 O adsorption. Moreover, FJI-H38 has excellent stability and recyclability and can be synthesized on a large scale, making it a practical trace CO2 adsorbent. This will provide a new strategy for developing practical adsorbents for CO2 capture from the air.

Metallocavitins as Advanced Enzyme Mimics and Promising Chemical Catalysts

The supramolecular approach is becoming increasingly dominant in biomimetics and chemical catalysis due to the expansion of the enzyme active center idea, which now includes binding cavities (hydrophobic pockets), channels and canals for transporting substrates and products. For a long time, the mimetic strategy was mainly focused on the first coordination sphere of the metal ion. Understanding that a highly organized cavity-like enzymatic pocket plays a key role in the sophisticated functionality of enzymes and that the activity and selectivity of natural metalloenzymes are due to the effects of the second coordination sphere, created by the protein framework, opens up new perspectives in biomimetic chemistry and catalysis. There are two main goals of mimicking enzymatic catalysis: (1) scientific curiosity to gain insight into the mysterious nature of enzymes, and (2) practical tasks of mankind: to learn from nature and adopt from its many years of evolutionary experience. Understanding the chemistry within the enzyme nanocavity (confinement effect) requires the use of relatively simple model systems. The performance of the transition metal catalyst increases due to its retention in molecular nanocontainers (cavitins). Given the greater potential of chemical synthesis, it is hoped that these promising bioinspired catalysts will achieve catalytic efficiency and selectivity comparable to and even superior to the creations of nature. Now it is obvious that the cavity structure of molecular nanocontainers and the real possibility of modifying their cavities provide unlimited possibilities for simulating the active centers of metalloenzymes. This review will focus on how chemical reactivity is controlled in a well-defined cavitin nanospace. The author also intends to discuss advanced metal–cavitin catalysts related to the study of the main stages of artificial photosynthesis, including energy transfer and storage, water oxidation and proton reduction, as well as highlight the current challenges of activating small molecules, such as H2O, CO2, N2, O2, H2, and CH4.

Optimizing Acetylene Sorption through Induced-fit Transformations in a Chemically Stable Microporous Framework

Developing practical storage technologies for acetylene (C₂ H₂ ) is important but challenging because C₂ H₂ is useful but explosive. Here, a novel metal-organic framework (MOF) (FJI-H36) with adaptive channels was prepared. It can effectively capture C₂ H₂ (159.9 cm³  cm^-3 ) at 1 atm and 298 K, possessing a record-high storage density (561 g L^-1 ) but a very low adsorption enthalpy (28 kJ mol^-1 ) among all the reported MOFs. Structural analyses show that such excellent adsorption performance comes from the synergism of active sites, flexible framework, and matched pores; where the adsorbed-C₂ H₂ can drive FJI-H36 to undergo induced-fit transformations step by step, including deformation/reconstruction of channels, contraction of pores, and transformation of active sites, finally leading to dense packing of C₂ H₂ . Moreover, FJI-H36 has excellent chemical stability and recyclability, and can be prepared on a large scale, enabling it as a practical adsorbent for C₂ H₂ . This will provide a useful strategy for developing practical and efficient adsorbents for C₂ H₂ storage.

Benchmark Dynamics of Dipolar Molecular Rotors in Fluorinated Metal-Organic Frameworks

Fluorinated Metal-Organic Frameworks (MOFs), comprising a wheel-shaped ligand with geminal rotating fluorine atoms, produced benchmark mobility of correlated dipolar rotors at 2 K, with practically null activation energy (E_a =17 cal mol^-1 ). ¹ H T₁ NMR revealed multiple relaxation phenomena due to the exchange among correlated dipole-rotor configurations. Synchrotron radiation X-ray diffraction at 4 K, Density Functional Theory, Molecular Dynamics and phonon calculations showed the fluid landscape and pointed out a cascade mechanism converting dipole configurations into each other. Gas accessibility, shown by hyperpolarized-Xe NMR, allowed for chemical stimuli intervention: CO₂ triggered dipole reorientation, reducing their collective dynamics and stimulating a dipole configuration change in the crystal. Dynamic materials under limited thermal noise and high responsiveness enable the fabrication of molecular machines with low energy dissipation and controllable dynamics.

Confinement-Driven Photophysics in Hydrazone-Based Hierarchical Materials

Confinement-imposed photophysics was probed for novel stimuli-responsive hydrazone-based compounds demonstrating a conceptual difference in their behavior within 2D versus 3D porous matrices for the first time. The challenges associated with photoswitch isomerization arising from host interactions with photochromic compounds in 2D scaffolds could be overcome in 3D materials. Solution-like photoisomerization rate constants were realized for sterically demanding hydrazone derivatives in the solid state through their coordinative immobilization in 3D scaffolds. According to steady-state and time-resolved photophysical measurements and theoretical modeling, this approach provides access to hydrazone-based materials with fast photoisomerization kinetics in the solid state. Fast isomerization of integrated hydrazone derivatives allows for probing and tailoring resonance energy transfer (ET) processes as a function of excitation wavelength, providing a novel pathway for ET modulation.

Surface Thermal Behavior and RT CO Gas Sensing Application of an Oligoacenaphthylene with p-Hydroxyphenylacetic Acid Composite

The current work describes room-temperature gas sensing performances using an oligoacenaphthylene (OAN)/p-hydroxyphenylacetic acid (p-HPA) composite. Based on inverse gas chromatography (IGC), the London dispersive surface energy γ_s ^d is calculated by using 14 representative models. Even when the γ_s ^d values of both OAN and the OAN/p-HPA composite are decreased as the temperature increases, the surface of OAN shows a higher value than that of the composite. The Gibbs surface free energy values of both are decreased with an increasing temperature. In our results, higher Lewis basic characters are observed in OAN and the OAN/p-HPA composite and the OAN/p-HPA surface exhibits a higher basicity compared to OAN. Because of the presence of phenolic groups in the OAN/p-HPA composite, the more important basic character drives a significant CO gas sensing ability with a sensitivity of 8.96% and good cycling stability as compared to the pristine counterparts. It is expected that the current study sheds light on a new pathway to exploring polymer composite materials for futuristic diverse and multiple applications, including IGC and gas sensor applications.

Transformation of Porous Organic Cages and Covalent Organic Frameworks with Efficient Iodine Vapor Capture Performance

The reaction of 5,5'-([2,2'-bipyridine]-5,5'-diyl)diisophthalaldehyde (BPDDP) with cyclohexanediamine and [benzidine (BZ)/[2,2'-bipyridine]-5,5'-diamine (BPDA)], respectively, affords a nitrogen-rich porous organic cage BPPOC and two two-dimensional (2D) covalent organic frameworks (COFs), USTB-1 and USTB-2 (USTB = University of Science and Technology Beijing), under suitable conditions. Interestingly, BPPOC with a single-crystal X-ray diffraction structure is able to successfully transform into USTB-1 and USTB-2 (newly converted COFs denoted as USTB-1c and USTB-2c, respectively) upon exchange of the imine unit of cyclohexanediamine in the cage by BZ and BPDA. Such a transformation also enables the isolation of analogous COFs (USTB-3c and USTB-4c) on the basis of an isostructural organic cage, BTPOC, which is derived from 5,5'-([2,2'-bithiophene]-4,4'-diyl)diisophthalaldehyde (BTDDP) and cyclohexanediamine. However, the conventional solvothermal reaction between BTDDP and BPDA leads to an impure phase of USTB-4 containing incompletely converted aldehyde groups due to the limited solubility of the building block. The newly prepared COFs have been characterized by nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. In particular, BPPOC is able to absorb the iodine vapor with an uptake of 5.64 g g^-1, breaking the porous organic cage's (POC's) record value of 3.78 g g^-1. Nevertheless, the cage-derived COFs exhibit improved iodine vapor adsorption capability in comparison with the directly synthesized counterparts, with the highest uptake of 5.80 g g^-1 for USTB-1c. The mechanism investigation unveils the superiority of nitrogen atoms to sulfur atoms for POCs in iodine vapor capture with the assistance of definite crystal structures. This, in combination with porosity, synergistically influences the iodine vapor capture capacity of COFs.

Smart Tetraphenylethene-Based Luminescent Metal-Organic Frameworks with Amide-Assisted Thermofluorochromics and Piezofluorochromics

Luminescent metal-organic frameworks (MOFs) are appealing for the design of smart responsive materials, whereas aggregation-induced emission (AIE) fluorophores with twisted molecular rotor structure provide exciting opportunities to construct MOFs with new topology and responsiveness. Herein, it is reported that elongating AIE rotor ligands can render the newly formed AIE MOF (ZnETTB) (ETTB = 4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-3,5-dicarboxylic acid))) with more elasticity, more control for intramolecular motion, and specific amide-sensing capability. ZnETTB shows specific host-guest interaction with amide, where N,N-diethylformamide (DEF), as an example, is anchored through CH···O and CH···π bonds with Zn cluster and ETTB8- ligand, respectively. DEF anchoring reduces both the distortion level and the intramolecular motions of ETTB8- ligand to lead a blueshifted and intensified emission for DEF ∈ ZnETTB. Moreover, amide anchoring also affords the DEF ∈ ZnETTB with the excellent thermofluorochromic behavior, and further increases the piezofluorochromic sensitivity at low-pressure ranges on the basis of its elastic framework. This work is one of the rare examples of amide-responsive smart materials, which shall shed new lights on design of smart MOFs with twisted AIE rotors for further sensing and detection applications.

Highly Efficient Blue Phosphorescence from Pillar-Layer MOFs by Ligand Functionalization

Room temperature phosphorescence (RTP) has been extensively researched in heavy-metal containing complexes and purely organic systems. Despite the rapid blossom of RTP materials, it is still a tremendous challenge to develop highly efficient blue RTP materials with long-lived lifetimes. Taking the metal-organic framework (MOF) as a model, herein, a feasible strategy of ligand functionalization is proposed, including two essential elements, to develop blue phosphorescence materials with high efficiency and long-lived lifetimes simultaneously under ambient conditions. One is isolation of the chromophores with assistance of another predefined co-ligands, the other is restriction of the chromophores' motions through coordination and host-guest interactions. Remarkably, it renders the MOFs with highly efficient blue phosphorescence up to 80.6% and a lifetime of 169.7 ms under ambient conditions. Moreover, a demo of the crown is fabricated with MOFs ink by 3D printing technique. The potential applications for anti-counterfeiting and fingerprint visualization have been also demonstrated. This finding not only outlines a universal principle to design and synthesize highly efficient RTP materials, but also endows traditional MOFs with fresh vitality for potential applications.

Emergence of Coupled Rotor Dynamics in Metal-Organic Frameworks via Tuned Steric Interactions

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.