Weizhao Cai

Giant negative linear compression positively coupled to massive thermal expansion in a metal–organic framework

Materials with negative linear compressibility are sought for various technological applications. Such effects were reported mainly in framework materials. When heated, they typically contract in the same direction of negative linear compression. Here we show that this common inverse relationship rule does not apply to a three-dimensional metal-organic framework crystal, [Ag(ethylenediamine)]NO3. In this material, the direction of the largest intrinsic negative linear compression yet observed in metal-organic frameworks coincides with the strongest positive thermal expansion. In the perpendicular direction, the large linear negative thermal expansion and the strongest crystal compressibility are collinear. This seemingly irrational positive relationship of temperature and pressure effects is explained and the mechanism of coupling of compressibility with expansivity is presented. The positive coupling between compression and thermal expansion in this material enhances its piezo-mechanical response in adiabatic process, which may be used for designing new artificial composites and ultrasensitive measuring devices.

Giant Negative Area Compressibility Tunable in a Soft Porous Framework Material

A soft porous material [Zn(L)2(OH)2]n·Guest (where L is 4-(1H-naphtho[2,3-d]imidazol-1-yl)benzoate, and Guest is water or methanol) exhibits the strongest ever observed negative area compressibility (NAC), an extremely rare property, as at hydrostatic pressure most materials shrink in all directions and few expand in one direction. This is the first NAC reported in metal-organic frameworks (MOFs), and its magnitude, clearly visible and by far the highest of all known materials, can be reversibly tuned by exchanging guests adsorbed from hydrostatic fluids. This counterintuitive strong NAC of [Zn(L)2(OH)2]n·Guest arises from the interplay of flexible [-Zn-O(H)-]n helices with layers of [-Zn-L-]4 quadrangular puckered rings comprising large channel voids. The compression of helices and flattening of puckered rings combine to give a giant piezo-mechanical response, applicable in ultrasensitive sensors and actuators. The extrinsic NAC response to different hydrostatic fluids is due to varied host-guest interactions affecting the mechanical strain within the range permitted by exceptionally high flexibility of the framework.

Pressure effects on H-ordering in hydrogen bonds and interactions in benzoic acid

High-pressure structures of benzoic acid, C6H5COOH, reveals the interplay of proton disorder in O–H⋯O hydrogen bonds, molecular orientation and O⋯O distance. The benzoic acid structures have been determined by single crystal X-ray diffraction up to 2.21 GPa at 296 K. The initial ordering of H atoms in the hydrogen bonds of benzoic acid dimers is reversed above 0.30 GPa. These opposite effects are due to the H-stabilization by enhanced interactions of the molecule in the asymmetric crystal environment, and to the compressed O⋯O distance, lowering the potential energy barrier for the H-hopping, respectively. Pressure facilitates the esterification of benzoic acid in methanol and ethanol solutions, but no symmetry, structure or crystal volume changes indicative of phase transition postulated for this compound have been observed.

Enantiomeric crystallization of (±)-trans-1,2-diaminocyclohexane under pressure

The isothermal and isochoric crystallizations of (±)-trans-1,2-diaminocyclohexane, C6H10(NH2)2, (±)-DACH, yield enantiomeric conglomerates of orthorhombic symmetry, space groupP21212, the same as obtained by isobaric crystallization. Despite the compression to 73% of ambient pressure volume, no racemic compound of DACH was formed. This contradicts for this compound the possibility of reverse causality between density and preferred enantiomorphic/racemic crystallization in Wallachs rule. The crystal structures have been determined at 0.36, 0.52, 0.65, 1.19 and 2.04 GPa at 296 K by single-crystal X-ray diffraction. It has been established that the enantiomeric-conglomerate crystallization of DACH at ambient pressure is mainly due to the molecular arrangement, governed by the close-packing rule in the crystal. High pressure enhances the close packing, and it does not change the course of crystallization of (±)-DACH. The shortest intermolecular contacts, NH⋯N and CH⋯N, become most compressed between 0.36 and 2.04 GPa, indicating that such weak hydrogen bonds are enhanced by pressure. The phase diagram of (±)-DACH has been outlined to 2.04 GPa.

CCDC 979869: Experimental Crystal Structure Determination

Related Article: Weizhao Cai, Jiangang He, Wei Li, Andrzej Katrusiak|2014|J.Mater.Chem.C|2|6471|doi:10.1039/C4TC00654B

Anomalous compression of a weakly CH center dot center dot center dot O bonded nonlinear optical molecular crystal

The organic nonlinear optical crystal, 3-methyl-4-nitropyridine N-oxide (POM), exhibits a negative-linear-compressibility (NLC) region as well as exceptionally large positive thermal expansion. High-pressure single crystal X-ray diffraction measurements have revealed an anomalous reversal of NLC at 0.12 GPa, induced by the collapse of the CH...O bonded supramolecular framework and subtle rotations of the nitro group. The initial compression of the weak supramolecular network in the molecular POM crystal is analogous to the hydrostatic responses of the framework crystals with much stronger cohesion forces. Density functional theory (DFT) calculations show that both the subtle conformational distortions and the crystal compression modify the second-harmonic generation (SHG) efficiency of POM.