Identifying the Role of Terahertz Vibrations in Metal-Organic Frameworks: From Gate-Opening Phenomenon to Shear-Driven Structural Destabilization

Abstract

We present an unambiguous identification of low-frequency terahertz vibrations in the archetypal imidazole-based metal-organic framework (MOF) materials: ZIF-4, ZIF-7, and ZIF-8, all of which adopt a zeolite-like nanoporous structure. Using inelastic neutron scattering and synchrotron radiation far-infrared absorption spectroscopy, in conjunction with density functional theory (DFT), we have pinpointed all major sources of vibrational modes. Ab initio DFT calculations revealed the complex nature of the collective THz modes, which enable us to establish detailed correlations with experiments. We discover that low-energy conformational dynamics offers multiple pathways to elucidate novel physical phenomena observed in MOFs. New evidence demonstrates that THz modes are intrinsically linked, not only to anomalous elasticity underpinning gate-opening and pore-breathing mechanisms, but also to shear-induced phase transitions and the onset of structural instability.

Structure–property relationships in metal-organic frameworks for hydrogen storage

“…ZIF-7 has been reported to have a sorbate-induced gateopening phenomena, involving a narrow-to-large pore phase transition [38][39][40]. This peculiar sorption pattern has been confirmed in CO2 and C2-C4 alkane/alkaline adsorption [40,41].…”

Tracking thermal-induced amorphization of a zeolitic imidazolate framework via synchrotron in situ far-infrared spectroscopy

“…For example, phase transitions in ZIF-7 upon exchange of guest molecules and ZIF-8 upon gas adsorption have been ascribed to "breathing" and "gate-opening" mechanisms respectively. 7,[12][13][14][15] Another topical …”

“…7 Specifically, we related quasi-local vibrations to the deformation of the organic linkers and Zn-based coordination polyhedra (ZnN4) in zeolitic imidazolate frameworks (ZIFs), 8 and linked them to various physical phenomena, including "gate-opening", "breathing" and shear destabilization modes. 7 We also reported molecular rotational dynamics, trampoline-like vibrational motions, and demonstrated how collective vibrations can be connected to the elasticity and mechanical stability of the framework structure, revealing the origin of auxeticity (negative Poisson's ratio), in the copper paddle-wheel MOF, HKUST-1. 9 Unlike in situ MIR spectroscopy, 10,11 which can be performed via benchtop equipment, detailed characterization of the FIR region in situ requires a more sophisticated experimental setup, explaining the relative absence of reports to date.…”

Zeolite membranes – a review and comparison with MOFs

“…In a recent theoretical paper, 338 Density Functional Theory (DFT) has been used to unravel the vibration patterns of selected ZIFs (ZIFs 4, 7 and 8) on a molecular level. Vibrations in the THz region can explain such cooperative phenomena of ''gate opening'' and ''breathing'' (see Fig.…”