S. Kaskel

The role of phonons in switchable MOFs: a model material perspective

A.E.J. Hoffman, I. Senkovska, L. Abylgazina, V. Bon, V. Grzimek, A.M. Dominic, M. Russina, M.A. Kraft, I. Weidinger, W.G. Zeier, V. Van Speybroeck, S. Kaskel
Journal of Materials Chemistry A
11, 28, 15286-15300


The large cell volume changes of switchable metal–organic frameworks (MOFs) render them as promising functional materials. Low-frequency phonon modes are known to influence the dynamic response of these materials. The pillared layer DUT-8(M) materials are prototypical examples of switchable MOFs, enabling switching between the closed and open pore phases, largely depending on the metal ions constituting the paddle wheel unit. However, the role of specific phonon modes in the softness of these materials is still rather unexplored. This study combines complementary spectroscopic techniques such as Raman spectroscopy, inelastic neutron scattering, and phonon acoustic spectroscopy (PAS) with density functional theory calculations (DFT) to unravel the vibrational properties of DUT-8(M) with different metal nodes (M = Ni, Co, Zn, Cu) to address these open questions. After analysis of the various experimental and theoretical spectroscopic data, the closed pore phase of DUT-8(Ni) appeared to be stiffer than that of the materials with Co and Zn. Experiments also show that the open pore phase of the Ni based compound is softer than those containing Zn and Co, although these findings could not be supported by theory. Nevertheless, DFT calculations could explain that changing the metal atom has mainly an impact on the phonon modes inducing changes in the paddle wheel unit. These results yield valuable insights into the role of the metal node on the observed flexibility in DUT-8(M) materials and can help to understand the mechanisms behind the phase transition in switchable MOFs.

Unfolding the terahertz spectrum of soft porous crystals: rigid unit modes and their impact on phase transitions

A.E.J. Hoffman, I. Senkovska, J. Wieme, A. Krylov, S. Kaskel, V. Van Speybroeck
Journal of Materials Chemistry A
10 (33), 17254-17266


Phase transitions in exible metal-organic frameworks or soft porous crystals are mediated by low-frequency phonons or rigid-unit modes. The alteration of specic building blocks may change the lattice dynamics of these frameworks, which can inuence the phase transition mechanism. In this work, the impact of building block substitution on the rigid-unit modes in exible MIL-53 analogs with a winerack topology will be investigated via ab initio lattice dynamics calculations. First, the accuracy of the theoretical simulations is veried via experimental Raman measurements, which provide unique ngerprint vibrations in the terahertz range to characterize the phase transition. Following analysis of the low-frequency vibrations shows that there exists a set of universal rigid-unit modes inducing translations and/or rotations of the building blocks. The theoretical results demonstrate that linker substitutions have a large eect on the rigid-unit mode frequencies, whereas this is less so for inorganic chain substitutions. These ndings may help to rationally tune the phonon frequencies in soft porous crystals.

Gold Open Access

Charting the Complete Thermodynamic Landscape of Gas Adsorption for a Responsive Metal-Organic Framework

R. Goeminne, S. Krause, S. Kaskel, T. Verstraelen, J.D. Evans
JACS (Journal of the American Chemical Society)
143, 11, 4143–4147


New nanoporous materials have the ability to revolutionize adsorption and separation processes. In particular, materials with adaptive cavities have high selectivity and may display previously undiscovered phenomena, such as negative gas adsorption (NGA), in which gas is released from the framework upon an increase in pressure. Although the thermodynamic driving force behind this and many other counterintuitive adsorption phenomena have been thoroughly investigated in recent years, several experimental observations remain difficult to explain. This necessitates a comprehensive analysis of gas adsorption akin to the conformational free energy landscapes used to understand the function of proteins. We have constructed the complete thermodynamic landscape of methane adsorption on DUT-49. Traversing this complex landscape reproduces the experimentally observed structural transitions, temperature dependence, and the hysteresis between adsorption and desorption. The complete thermodynamic description presented here provides unparalleled insight into adsorption and provides a framework to understand other adsorbents that challenge our preconceptions.

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