​MOFs are crystalline compounds that consist of metal ions coordinated to organic molecules, which have ordered porosity at a very small scale. Due to their capability of adsorbing gases and solvents, they have accumulated substantial interest for their potential use in carbon capture, microelectronics, bioscience, catalysis and more.  

To fulfil the potential use of these compounds, their dynamics must first be understood. A recently published paper in Physical Review Letters describes experiments investigating the low-frequency dynamics of a particular zirconium-based MOF designated as MIL-140A: ZrO(O₂C-C₆H₄-CO₂). The research team led by Professor Jin-Chong Tan from the Engineering Science Department of the University of Oxford has performed high-resolution spectroscopy experiments alongside density functional theory (DFT) calculations to investigate the low-frequency vibrational dynamics within the terahertz region of their MOF.

Matthew Ryder, the lead author of the study, has demonstrated that terahertz vibrations are useful for characterising structural flexibility and the collective lattice dynamics of framework materials. The project recently advanced the use of terahertz vibrations to study the low-symmetry MIL-140A structure. This work results in a deeper and more detailed understanding of the phenomena occurring in MOFs, and the demonstrated approach can be applied to many related framework materials.

DFT is a quantum mechanical computational tool used for calculating the physical properties of systems with many electrons, DFT simulations can match unique terahertz vibrations to physical phenomena. Matthew used DFT to calculate theoretical spectra of MIL-140A, comparing with experimental spectra. The spectra and terahertz vibrational modes of the MOF were experimentally determined using high-resolution inelastic neutron scattering (INS) on the TOSCA and OSIRIS ​instruments, and infra-red spectroscopy on the MIRIAM beamline B22 at the Diamond Light Source.

By utilising each of these techniques, they obtained a definitive quantitative comparison between experimental and calculated spectra for the MOF. This enabled them to confirm phenomena including coordinated shear dynamics, hindered rotational dynamics and 'trampoline-like' motions of the organic linker moieties. These are significant findings, as each of these terahertz modes pinpointed at the nanoscale could potentially explain unusual physical phenomena observed at the macroscale.

The paper gives the first reported experimental evidence of shear dynamics in a MOF, which was confirmed by a particular mode in the INS spectra, whose intensity was found to be matching the DFT calculations predicted. This is a particularly useful finding, as it is a mechanism speculated to be the cause of framework destabilisation of MOFs.

Rotational motions in three-dimensional MOFs have previously been studied using other techniques, such as nuclear magnetic resonance (NMR), and scanning tunnelling microscopy (STM). However, this is the first work to use terahertz vibrations in detecting molecular rotors within a MOF material, as well as the accompanying lattice dynamics. Terahertz vibrations associated with hindered rotor motions were detected, including those corresponding to both symmetric and asymmetric rotations of the organic linkers. These observations have enabled the researchers to gain a detailed understanding of all the rotational motions occurring within the complex structure.​

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​​Fig.1 (left) Symmetric rotor dynamics at 1.48 THz; (right) asymmetric rotor dynamics at 1.34 THz. Reprinted figure with permission from Matthew R. Ryd​er et al. Phys. Rev. Lett. 118, 255502, 2017. Copyright 2017 by the American Physical Society.​​ DOI: 10.1103/PhysRevLett.118.255502.​

Evidence for the trampoline-like motion of the organic linker moieties involved detecting two unique modes corresponding to these motions. These motions have been suggested to cause negative thermal expansion, a phenomenon detected in less complex MOFs.

“We have shown that vibrational modes located in the low-energy terahertz spectral region can possess much structural information regarding the flexibility of MOF materials. It is especially exciting to be able to relate some of these motions to mechanisms responsible for their sometimes counterintuitive mechanical response. I look forward to progressing my work to discover what other exciting discoveries may be hidden within the lattice dynamics of MOFs and other promising materials." Matthew explains.

The characterisation of MOF terahertz vibrational dynamics is now possible, which paves the way to a more accurate recognition of the physical phenomena that occur within framework materials. The experimental data of terahertz vibrations shown in the paper both validates theory, and leads to the drastic increase in understanding of the complex mechanisms that govern the behaviour of MOFs and related framework materials.

“The use of terahertz vibrations that this work demonstrates will encourage their further use to study and search for unknown relationships between structural dynamics and unconventional mechanical properties hidden in contemporary framework materials," Professor Tan comments.

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Further information

The research paper can be found here: DOI: 10.1103/PhysRevLett.118.255502.

Multifunctional Materials and Composites (MMC) Laboratory, Oxford University​.​​​