Matthew R. Ryder

Nanoporous metal organic framework materials for smart applications

Abstract This review is concerned with the recent advances in metal organic framework (MOF) materials. We highlight the unique combination of physicochemical and thermomechanical characteristics associated with MOF-type materials and illustrate emergent applications in three challenging technological sectors: energy, environmental remediation and biomedicine. MOFs represent an exciting new class of nanoporous crystalline solids constituting metal ions/clusters and multifunctional organic linkages, which self-assemble at molecular level to generate a plethora of ordered 3D framework materials. The most intriguing feature of a MOF lies in its exceptionally large surface area, far surpassing those of the best activated carbons and zeolites. Next generation multifunctional materials encompassing MOF based thin films, coatings, membranes and nanocomposites have potential for exploitation in an immense array of unconventional applications and smart devices. We pinpoint the key technological challenges and basic scientific questions to be addressed, so as to fulfil the translational potential for bringing MOFs from the laboratory into commercial applications.

Discovering connections between terahertz vibrations and elasticity underpinning the collective dynamics of the HKUST-1 metal–organic framework

We employed a combination of theoretical and experimental techniques to study the metal–organic framework (MOF)-mechanics central to the paddle-wheel Cu3(BTC)2 porous structure, commonly designated as HKUST-1. Lattice dynamics of the hybrid framework at below 18 THz were measured by means of Raman and synchrotron far-infrared spectroscopy, and systematically correlated to collective vibrational modes computed from ab initio density functional theory (DFT). We have identified a number of intriguing low-energy framework vibration mechanisms, reminiscent of the ‘trampoline-like’ deformations and new oscillatory motions associated with Cu paddle-wheel ‘molecular rotors’. The three independent single-crystal elastic constants of the HKUST-1 (i.e. C11, C12 and C44) were calculated using the DFT approach, taking into account the effects of dispersion corrections. We established the full elasticity solutions accompanying detailed deformation mechanisms that control its anisotropic mechanical properties, ranging from the Youngs and shear moduli to linear compressibility and Poissons ratio. Our results support the notion that the co-existance of soft modes and intrinsic shear distortions connected to the THz lattice dynamics dictate a range of anomalous elastic phenomena, for example: negative Poissons ratio (auxeticity), negative thermal expansion (NTE), and exceedingly low shear moduli properties.

Communication: Remarkable electrophilicity of the oxalic acid monomer: An anion photoelectron spectroscopy and theoretical study

Our experimental and computational results demonstrate an unusual electrophilicity of oxalic acid, the simplest dicarboxylic acid. The monomer is characterized by an adiabatic electron affinity and electron vertical detachment energy of 0.72 and 1.08 eV (±0.05 eV), respectively. The electrophilicity results primarily from the bonding carbon-carbon interaction in the singly occupied molecular orbital of the anion, but it is further enhanced by intramolecular hydrogen bonds. The well-resolved structure in the photoelectron spectrum is reproduced theoretically, based on Franck-Condon factors for the vibronic anion → neutral transitions.

Intermolecular interactions between molecules in various conformational states: the dimer of oxalic acid.

We considered stability of the dimer of oxalic acid. The global minimum energy structure identified by us is stabilized by two inter- and four intramolecular hydrogen bonds, whereas the most stable structure identified in previous studies is supported by two inter- and three intramolecular hydrogen bonds. The latter structure proves to be less stable by 25 meV than the former. The global minimum stability results from a balancing act between a moderately attractive two-body interaction energy and small repulsive one-body terms. We have analyzed zero-point vibrational corrections to the stability of various conformers of oxalic acid and their dimers. We have found that minimum energy structures with the most stabilizing sets of hydrogen bonds have the largest zero-point vibrational energy, contrary to a naive anticipation based on red shifts of OH stretching modes involved in hydrogen bonds.

Probing Dielectric Properties of Metal–Organic Frameworks: MIL-53(Al) as a Model System for Theoretical Predictions and Experimental Measurements via Synchrotron Far- and Mid-Infrared Spectroscopy

Emerging nanoporous materials, such as metal-organic frameworks (MOFs), are promising low-k dielectrics central to next-generation electronics and high-speed communication. Hitherto, the dielectric characterization of MOFs is scarce, with very limited experimental data for guiding new materials design and synthesis. Herein we demonstrate the efficacy of high-resolution synchrotron infrared (IR) specular reflectance experiments to study the dynamic dielectric properties of a flexible MOF structure: bistable MIL-53(Al) that exhibits switching between a large pore (LP) and a narrow pore (NP) architecture. We show that the ratio of LP:NP content of a polycrystalline sample can be changed via increased mechanical stress applied for pelletizing the MIL-53(Al) powder. We quantify the frequency-dependent dielectric constants over ∼1 to 120 THz, identifying all dielectric transitions as a function of stress and phase mixtures, showing how porosity modifies MOFs dielectric properties.

Dielectric Properties of Zeolitic Imidazolate Frameworks in the Broadband Infrared Regime

The field of metal-organic framework (MOF) materials is rapidly advancing toward practical applications; consequently, it is urgent to achieve a better understanding and precise control of their physical properties. Yet, research on the dielectric properties of MOFs is at its infancy, where studies are confined to the static dielectric behavior or lower-frequency response (kHz-MHz) only. Herein, we present the pioneering use of synchrotron-based infrared reflectivity experiments combined with density functional theory (DFT) calculations to accurately determine the dynamic dielectric properties of zeolitic imidazolate frameworks (ZIFs, a topical family of MOFs). We show, for the first time, the frequency-dependent dielectric response of representative ZIF compounds, bridging the near-, mid-, and far-infrared (terahertz, THz) broad-band frequencies. We establish the structure-property relations as a function of framework porosity and structural change. Our comprehensive results will pave the way for novel ZIF-based terahertz applications, such as infrared optical sensors and high-speed wireless communications.

Large elastic recovery of zinc dicyanoaurate

We report a single-crystal nanoindentation study of the negative compressibility material zinc(II) dicyanoaurate. The material exhibits a particularly strong elastic recovery, which we attribute to the existence of supramolecular helices that function as atomic-scale springs—storing mechanical energy during compressive stress and inhibiting plastic deformation. Our results are consistent with the relationship noted by Cheng and Cheng [Appl. Phys. Lett. 73, 614 (1998)] between elastic recovery and the ratio of material hardness to Young’s modulus. Drawing on comparisons with other framework materials containing helical motifs, we suggest helices as an attractive design element for imparting resistance to plastic deformation in functional materials.