Large Negative Linear Compressibility in InH(BDC)2 from Framework Hinging

Hydrogen-bond-modulated negative linear compressibility in a V-shaped molecular crystal

A material with the "hidden" negative linear compressibility (NLC) will expand along a specific crystal direction upon uniformly compression to a critical pressure; such materials are thought to be promising candidates for non-linear actuators, switches and sensors. Herein, we use density functional theory (DFT) calculations to uncover the hidden NLC in a V-shaped molecular crystal, bis(5-amino-1,2,4-triazol-3-yl)methane (BATZM). The calculations indicate that the crystal is normally compressed over the pressure range of 0-3 GPa while it expands along the b-axis when the external hydrostatic pressure exceeds 3 GPa. The compressive behavior of the BATZM crystal is modulated by inter-molecular hydrogen bonds, which act as highly compressible springs at low pressures but robust struts at high pressures. Hence, the crystal prefers to compress the hydrogen bonds coupled with PLC at first and flatten the molecules, coupled with later NLC to resist the increasing external pressure. The compressive behavior of BATZM provides a strategy to design more hidden NLC materials via the rational use of the hydrogen bonds.

Charge-Assisted Ionic Hydrogen-Bonded Organic Frameworks: Designable and Stabilized Multifunctional Materials

Hydrogen-bonded organic frameworks (HOFs) are a class of crystalline framework materials assembled by hydrogen bonds. HOFs have the advantages of high crystallinity, mild reaction conditions, good solution processability, and reproducibility. Coupled with the reversibility and flexibility of hydrogen bonds, HOFs can be assembled into a wide diversity of crystalline structures. Since the bonding energy of hydrogen bonds is lower than that of ligand and covalent bonds, the framework of HOFs is prone to collapse after desolventisation and the stability is not high, which limits the development and application of HOFs. In recent years, numerous stable and functional HOFs have been developed by π-π stacking, highly interpenetrated networks, charge-assisted, ligand-bond-assisted, molecular weaving, and covalent cross-linking. Charge-assisted ionic HOFs introduce electrostatic attraction into HOFs to improve stability while enriching structural diversity and functionality. In this paper, we review the development, the principles of rational design and assembly of charge-assisted ionic HOFs, and introduces the different building block construction modes of charge-assisted ionic HOFs. Highlight the applications of charge-assisted ionic HOFs in gas adsorption and separation, proton conduction, biological applications, etc., and prospects for the diverse design of charge-assisted ionic HOFs structures and multifunctional applications.

Realizing Persistent Zero Area Compressibility over a Wide Pressure Range in Cu2 GeO4 by Microscopic Orthogonal-Braiding Strategy

Zero area compressibility (ZAC) is an extremely rare mechanical response that exhibits an invariant two-dimensional size under hydrostatic pressure. All known ZAC materials are constructed from units in two dimensions as a whole. Here, we propose another strategy to obtain the ZAC by microscopically orthogonal-braiding one-dimensional zero compressibility strips. Accordingly, ZAC is identified in a copper-based compound with a planar [CuO4 ] unit, Cu2 GeO4 , that possesses an area compressibility as low as 1.58(26) TPa-1 over a wide pressure range from ≈0 GPa to 21.22 GPa. Based on our structural analysis, the subtle counterbalance between the shrinkage of [CuO4 ] and the expansion effect from the increase in the [CuO4 ]-[CuO4 ] dihedral angle attributes to the ZAC response. High-pressure Raman spectroscopy, in combination with first-principles calculations, shows that the electron transfer from in-plane bonding dx 2 -y 2 to out-of-plane nonbonding dz 2 orbitals within copper atoms causes the counterintuitive extension of the [CuO4 ]-[CuO4 ] dihedral angle under pressure. Our study provides an understanding on the pressure-induced structural evolution of copper-based oxides at an electronic level and facilitates a new avenue for the exploration of high-dimensional anomalous mechanical materials.

Negative linear compressibility and strong enhancement of emission in Eu[Ag(CN)2]3·3H2O under pressure

The framework material Eu[Ag(CN)2]3·3H2O exhibits a negative linear compressibility (NLC) of -4.2(1) TPa-1 over the largest pressure range yet observed (0-8.2 GPa). High-pressure single-crystal X-ray diffraction data show that the rapid contraction of the Kagome silver layers under compression causes the wine-rack lattice to expand along the c-axis. The hydrogen bonds between the water molecules and the main frameworks constrain the structural deformation under pressure and eventually a weak NLC effect generated. Furthermore, we found that the pressure-induced emission intensity increases almost 800-fold at 4.0 GPa, followed by a gradual decrease and disappearance at 8.1 GPa. Under compression, high pressure significantly tunes the triplet level positions near the Eu3+ ions, and horizontal displacement between a quenching excited state and the excited levels of Eu3+ facilitates the energy transfer process to the 5D0 excited state and limits the nonradiative corssover at elevated pressures, thus increasing the emission intensity. In addition, we observe a gradual band gap reduction with increasing pressure, and the sample could not be returned to the initial state after the pressure was completely released. By controlling the structural flexibility, we observe a coupled NLC and pressure-induced strong enhancement of the emission properties of Eu[Ag(CN)2]3·3H2O, which provides a new route for the design of new optical devices with intriguing luminescence properties under extreme environments.

Unexpected giant negative area compressibility in palladium diselenide

Negative area compressibility (NAC) is a counterintuitive 'squeeze-expand' behavior in solids that is very rare but attractive due to possible pressure-response applications and coupling with rich physicochemical properties. Herein, NAC behavior is reported in palladium diselenide with a large magnitude and wide pressure range. We discover that, apart from the rigid flattening of layers that has been generally recognized, the unexpected giant NAC effect in PdSe2 largely comes from anomalous elongation of intralayer chemical bonds. Both structural variations are driven by intralayer-to-interlayer charge transfer with enhanced interlayer interactions under pressure. Our work updates the mechanical understanding of this anomaly and establishes a new guideline to explore novel compression-induced properties.

Multiple responses of 1,6-diphenyl-1,3,5-hexatriene to mechanical stimulation: emission enhancement, piezochromism and negative linear compressibility

The properties of mechanoresponsive materials are mainly affected by intermolecular interaction, in which anisotropic grinding and hydrostatic high-pressure compression are the powerful tools used for modulation. Upon applying high pressure to 1,6-diphenyl-1,3,5-hexatriene (DPH), the reduced molecular symmetry results in the originally forbidden S₀ → S₁ transition to become allowed that then leads to a 13-times emission enhancement, and π-π interactions result in piezochromism (red-shifted up to 100 nm). With increasing pressure, high-pressure-stiffened H⋯C/C⋯H and H⋯H interactions enable the DPH molecules to generate a NLC mechanical response (9-15 GPa) with K_b = -5.8764 TPa^-1 along the b-axis. As a contrast, upon destroying the intermolecular interactions by grinding, the DPH luminescence blue-shifts from cyan to blue. Based on this research, we investigate a new pressure-induced emission enhancement (PIEE) mechanism and enabled NLC phenomena by controlling weak intermolecular interactions. In-depth research of the evolution of intermolecular interactions has important reference value for developing new fluorescence materials and structural materials.

Negative Thermal Expansion of Undulating Coordination Layers through Interlayer Interaction

The negative thermal expansion (NTE) of solid-state materials is of significance in various fields, but a very rare phenomenon. In this study, we carried out a meta-analysis for the anisotropic thermal expansion behavior of fifteen two-dimensional coordination polymers [M(salen)]₂[M'(CN)₄(solvent)] (M = Mn, Fe; M' = MnN, ReN, Pt, Pt(I₂)_x; x = 0.18, 0.45, 0.85, 1.0; solvent = H₂O, MeOH, MeCN) with a newly synthesized [Fe(salen)]₂[MnN(CN)₄(MeCN)]. Consequently, we successfully demonstrate the unusual NTE of the undulating coordination layers by an expansion deformation of the layers via strong interlayer interaction within the layer stacking. Notably, the layer volume of [Mn(salen)]₂[ReN(CN)₄] with its powder form decreases with a large NTE coefficient, α_layer-volume = -27 × 10^-6 K^-1 (100-500 K). This is a significantly large value despite the increase in layer thickness along the layer contraction based on the anisotropic transformation of undulating layers. Conversely, the analysis demonstrates that the chemical modification of the layers to enhance intralayer interaction rather than interlayer interaction switches a direction of the layer anisotropy, yielding positive thermal expansion materials with the coefficient of the layer volume reaching +92 × 10^-6 K^-1.

Tunable uniaxial, area, and volume negative thermal expansion in quartz-like and diamond-like metal-organic frameworks

This paper proposes that it will be an effective way to discover and explore organic negative thermal expansion (NTE) materials based on the specific topologies in inorganic NTE materials. Various NTE behaviors from the uniaxial, area, and volume-NTE can be achieved by adjusting the topology, for instance, quartz-like and diamond-like. Zn(ISN)₂ and InH(BDC) metal-organic frameworks (MOFs) with quartz-like topology have been studied by first principles calculations. The calculated area-NTE of Zn(ISN)₂ and uniaxial-NTE of InH(BDC) within quasi-harmonic approximation (QHA) agree well with the experimental data. Through the calculation of Grüneisen parameters, it is shown that low-frequency optical phonons appear dominant resulting in their NTE, but the coupling to high-frequency phonons is of greater ultimate importance. The lattice vibrational modes of great contribution to area-NTE of Zn(ISN)₂ and uniaxial-NTE of InH(BDC) are analyzed in detail. Also, four MOFs with diamond-like topology are predicted to exhibit volume-NTE behavior. Moreover, it is found that there is a bulk modulus anomaly in some studied MOFs with the quartz-like and diamond-like framework, where the temperature dependence of bulk modulus does not follow the inverse dependence on that of volume. These specific topologies provide key geometric frameworks for various NTE behaviors of MOFs, and meanwhile, the local structure and bond environment in MOFs can lead to abnormal interatomic force, i.e., bulk modulus anomaly. This abnormal elastic property also deserves more attention.

Abnormal Compressive Behaviors of Metal-Organic Frameworks under Hydrostatic Pressure

Intuition indicates that materials will contract in all crystal directions under hydrostatic pressure, i.e., all of their axes exhibit positive linear compressibility (PLC) when uniformly compressed. However, some exceptions have been found to exhibit negative linear compressibility (NLC), negative area compressibility (NAC), zero linear compressibility (ZLC), or zero area compressibility (ZAC); these materials are thought to have promising applications under high-pressure conditions, such as would be needed for highly sensitive pressure detectors and deep-sea optical devices. In this Perspective, we summarize the four kinds of abnormal compressive behaviors of MOFs under hydrostatic pressure, including the mechanisms and effects of guest, PTM, and metal atomic radius, which we hope to be helpful to develop practical future application of abnormal compressive materials.

Inflation Negative Compressibility during Intrusion-Extrusion of a Non-Wetting Liquid into a Flexible Nanoporous Framework

Negative compressibility (NC) is a phenomenon when an object expands/shrinks in at least one of its dimensions upon compression/decompression. NC is very rare and is of great interest for a number of applications. In this work a gigantic (more than one order of magnitude higher compared to the reported values) NC effect was recorded during intrusion-extrusion of a non-wetting liquid into a flexible porous structure. For this purpose, in situ high-pressure neutron scattering, intrusion-extrusion experiments, and DFT calculations were applied to a system consisting of water and a highly hydrophobic Cu₂(tebpz) metal-organic framework (MOF), which upon water penetration expands in a and c directions to demonstrate NC coefficients more than order of magnitude higher compared to the highest values ever reported. The proposed approach is not limited to the materials used in this work and can be applied to achieve coefficients of negative linear compressibility of more than 10³ TPa^-1.

Giant Negative Compressibility by Liquid Intrusion into Superhydrophobic Flexible Nanoporous Frameworks

Materials or systems demonstrating negative linear compressibility (NLC), whose size increases (decreases) in at least one of their dimensions upon compression (decompression) are very rare. Materials demonstrating this effect in all their dimensions, negative volumetric compressibility (NVC), are exceptional. Here, by liquid porosimetry and in situ neutron diffraction, we show that one can achieve exceptional NLC and NVC values by nonwetting liquid intrusion in flexible porous media, namely in the ZIF-8 metal-organic framework (MOF). Atomistic simulations show that the volumetric expansion is due to the presence of liquid in the windows connecting the cavities of ZIF-8. This discovery paves the way for designing novel materials with exceptional NLC and NVC at reasonable pressures suitable for a wide range of applications.

Thinking about the Development of High-Pressure Experimental Chemistry

High-pressure chemistry is an interdisciplinary science which uses high-pressure experiments and theories to study the interactions, reactions, and transformations among atoms or molecules. It has been extensively studied thus far and achieved rapid development over the past decades. However, what is next for high-pressure chemistry? In this Perspective, we mainly focus on the development of high-pressure experimental chemistry from our own viewpoint. An overview of the series of topics is as follows: (I) high pressure used as an effective tool to help resolve scientific disputes regarding phenomena observed under ambient conditions; (II) high-pressure reactions of interest to synthetic chemists; (III) utilizing chemical methods to quench the high-pressure phase; (IV) using high pressure to achieve what chemists want to do but could not do; (V) potential applications of in situ properties under high pressure. This Perspective is expected to offer future research opportunities for researchers to develop high-pressure chemistry and to inspire new endeavors in this area to promote the field of compression chemistry science.

Amino Acid Residues Determine the Response of Flexible Metal-Organic Frameworks to Guests

Flexible metal-organic frameworks (MOFs) undergo structural transformations in response to physical and chemical stimuli. This is hard to control because of feedback between guest uptake and host structure change. We report a family of flexible MOFs based on derivatized amino acid linkers. Their porosity consists of a one-dimensional channel connected to three peripheral pockets. This network structure amplifies small local changes in linker conformation, which are strongly coupled to the guest packing in and the shape of the peripheral pockets, to afford large changes in the global pore geometry that can, for example, segment the pore into four isolated components. The synergy among pore volume, guest packing, and linker conformation that characterizes this family of structures can be determined by the amino acid side chain, because it is repositioned by linker torsion. The resulting control optimizes noncovalent interactions to differentiate the uptake and structure response of host-guest pairs with similar chemistries.

Colossal Negative Linear Compressibility in Porous Organic Salts

Negative linear compressibility (NLC) is a common sense violation (that is, crystal phases expand in one or more directions under hydrostatic compression). The excellent NLC performance of crystal materials is intrinsically related to the geometric structure of its skeleton. Here, we discovered a crystalline porous organic salt (CPOS-1); high-pressure X-ray diffraction experiments reveal that the CPOS-1 shows colossal NLC (K_c = -90.7 T Pa^-1) behavior along the c axis. This incredible performance arises from the flexible "supramolecular spring" formed by the charge-enhanced N-H⁺···^-O-S hydrogen bond interaction between the anionic sulfonate and the cationic ammonium ion. Furthermore, we reveal the relationship between this rare NLC behavior and single crystal proton conductivity using high-pressure electrochemical impedance spectroscopy (EIS) method. We believe that NLC behavior research on such inexpensive and readily available porous organic materials is of great significance for accelerating the research and application of NLC materials, especially in organic system.

Stimuli-responsive structural changes in metal–organic frameworks

This feature article mainly summarizes how the structure of MOFs changes under external stimuli.

Pressure-Induced Emission Enhancement and Multicolor Emission for 1,2,3,4-Tetraphenyl-1,3-cyclopentadiene: Controlled Structure Evolution

Mechanoresponsive luminescent (MRL) materials have attracted considerable attention because of their potential applications in mechanical sensors, memory chips, and security inks; MRL materials possessing high efficiency and multicolor emission qualities are especially interesting. In this Letter, we found 1,2,3,4-tetraphenyl-1,3-cyclopentadiene (TPC) crystal exhibited both pressure-induced emission enhancement (PIEE) and multicolor behavior. In addition, infrared spectroscopy analysis indicated that the ring-opening reaction of the phenyl ring occurred when pressure was beyond 24.7 GPa. The reaction was promoted from 24.7 to 35.9 GPa, which resulted in the redder irreversible color change for the sample released from 35.9 GPa than from 24.7 GPa. The results regarding the mechanoresponsive behavior of TPC offered a deep insight into PIEE and multicolor properties from the structural point of view and inspired the idea of capturing different colors by hydrostatic pressure, which will facilitate the design of and search for high-performance MRL materials.

Negative area compressibility of a hydrogen-bonded two-dimensional material

Very few materials expand two-dimensionally under pressure, and this extremely rare phenomenon, namely negative area compressibility (NAC), is highly desirable for technological applications in pressure sensors and actuators. Hitherto, the few known NAC materials have dominantly been limited to 2D crystals bonded via coordination interactions while other 2D systems have not been explored yet. Here, we report the large NAC of a hydrogen-bonded 2D supramolecular coordination complex, Zn(CH3COO)2·2H2O, with a synergistic microscopic mechanism. Our findings reveal that such an unusual phenomenon, over a wide pressure range of 0.15-4.44 GPa without the occurrence of any phase transitions, arises from the complex cooperation of intra-layer coordination and hydrogen-bonding interactions, and inter-layer van der Waals forces. In addition, we propose that these NAC crystals could have important applications as pressure-converting materials in ultrasensitive pressure sensing devices.

Anomalous Lattice Dynamics in AgC4N3: Insights From Inelastic Neutron Scattering and Density Functional Calculations

We have performed temperature dependent inelastic neutron scattering measurements to study the anharmonicity of phonon spectra of AgC4N3. The analysis and interpretation of the experimental spectra is done using ab-initio lattice dynamics calculations. The calculated phonon spectrum over the entire Brillouin zone is used to derive linear thermal expansion coefficients. The effect of van der Waals interaction on structure stability has been investigated using advanced density functional methods. The calculated isothermal equation of states implies a negative linear compressibility along the c-axis of the crystal, which also leads to a negative thermal expansion along this direction. The role of elastic properties inducing the observed anomalous lattice behavior is discussed.

Positive and Negative Two-Dimensional Thermal Expansion via Relaxation of Node Distortions

The ability to tune physical properties is attractive for the development of new materials for myriad applications. Understanding and controlling the structural dynamics in complicated network structures like coordination polymers (CPs) is particularly challenging. We report a series of two-dimensional CPs [Mn(salen)]₂[M(CN)₄]· xH₂O (M = Pt (1), PtI₂ (2), and MnN (3)) incorporating zigzag cyano-network layers that display composition-dependent anisotropic thermal expansion properties. Variable-temperature single-crystal X-ray structural analyses demonstrated that the thermal expansion behavior is caused by double structural distortions involving [Mn(salen)]⁺ units incorporated into the zigzag layers. Thermal relaxations produce structural transformations resulting in positive thermal expansion for 2·H₂O and negative thermal expansion for 3. In the case of 1·H₂O, the relaxation does not occur and zero thermal expansion results in the plane between 200 to 380 K. The present study proposes a new strategy based on structural distortions in coordination networks to control thermal responsivities of frameworks.

Near Zero Area Compressibility in a Perovskite-Like Metal-Organic Frameworks [C(NH2)3][Cd(HCOO)3]

Materials with zero area compressibility (ZAC) can keep their crystal uncompressed in two specific directions upon uniform compression. High-pressure angle-dispersive X-ray powder diffraction (ADXRD) experiments reveal a ZAC phenomenon in the ab-plane in crystal of a formate-based perovskite, [C(NH₂)₃][Cd(HCOO)₃]. The ZAC behavior is ascribed to the unique rhombohedral [Cd(HCOO)₃]^- frameworks and confirmed by density functional theory (DFT) calculations. For the first time, a near ZAC single material is explicitly reported. This study opens up an exciting research field on pressure-resistant materials. We anticipate more ZAC materials to be discovered in the following explorations under the inspiration of this work.

Unusual and Tunable Negative Linear Compressibility in the Metal-Organic Framework MFM-133(M) (M = Zr, Hf)

High-pressure single-crystal X-ray structural analyses of isostructural MFM-133(M) (M = Zr, Hf) of flu topology and incorporating the tetracarboxylate ligand TCHB^4- [H₄TCHB = 3,3',5,5'-tetrakis(4-carboxyphenyl)-2,2',4,4',6,6'-hexamethyl-1,1'-biphenyl] and {M₆(μ₃-OH)₈(OH)₈(COO)₈} clusters confirm negative linear compressibility (NLC) behavior along the c axis. This occurs via a three-dimensional wine-rack NLC mechanism leading to distortion of the octahedral cage toward a more elongated polyhedron under static compression. Despite the isomorphous nature of these two structures, MFM-133(Hf) shows a higher degree of NLC than the Zr(IV) analogue. Thus, for the first time, we demonstrate here that the NLC property can be effectively tuned in a framework material by simply varying the inorganic component of the frameworks without changing the network topology and structure.

Pressure-induced phase transition of 4-aminobenzonitrile: the formation and enhancement of N–H⋯N weak hydrogen bonds

A reversible pressure-induced structural phase transition of 4-aminobenzonitrile was found at about 0.3 GPa by conducting in situ high-pressure synchrotron angle-dispersive X-ray diffraction (ADXRD) experiments. The discontinuous changes of Raman modes at 0.2 GPa confirmed the occurrence of phase transition. In situ high-pressure Raman spectra indicated that the molecular arrangement and intermolecular interactions changed abruptly. The process of this phase transition continued up to about 1.0 GPa. When the pressure reached 1.1 GPa, the initial N–H⋯N interaction transformed into a new weak hydrogen bond, which was enhanced by further compression. The ab initio calculations and Hirshfeld surfaces were used to illustrate the above views. This study gives an example that demonstrates that the pressure can induce the formation of hydrogen bonds, which contributes to the development of supramolecular chemistry.

The initial N–H⋯N interactions in 4-aminobenzonitrile crystals are enhanced and changed into weak hydrogen bonds by high pressure.

Disorder–order transitions in the perovskite metal–organic frameworks [(CH3)2NH2][M(HCOO)3] at high pressure

Compression of dimethylammonium metal formates results in distorted metal formate frameworks and loss of dynamic disorder of dimethylammonium simultaneously.