Terahertz insights into MOF dynamics

Exciting discovery of MOF mechanics through THz vibrations

Researchers from the University of Oxford, University of Turin, and Diamond have been able to observe for the first time the surprising structural dynamics of a synthetic nanoporous material: trampoline-like flexibility giving rise to ‘negative thermal expansion‘ (shrinking under heat instead of expanding), and framework cluster rotations resulting in ‘auxeticity’ (getting wider in the middle when stretched from one direction).
 
These nanoporous materials, called metal-organic frameworks (MOFs), are micro-engineered compounds made up of metal ions (or clusters) bound together by organic ligands. Known for their ability to capture and store specific molecules (useful in drug delivery, catalysis, and carbon capture), understanding their structural dynamics is important when it comes to designing MOFs for practical applications.
 
In a novel experimental setup, the team recorded terahertz (THz) absorption and Raman spectra to study the material’s behaviour under very low-energy vibrational motions. Ab initio quantum mechanical simulations were then conducted to unravel the basic mechanisms responsible for the mechanical anomalies.
 
The results of the study, published in CrystEngComm’s New Talent special issue, demonstrate the suitability of Terahertz Spectroscopy as a method for not only probing structural dynamics, but also understanding how the makeup of a MOF can give rise to its peculiar mechanical properties.
Metal-organic frameworks
Because of their highly-ordered porosity and well-defined architecture, MOFs have an exceptionally large internal surface area, typically between 1,000 – 10,000 m2 g-1. In addition to the rich structural diversity of MOFs, their highly adaptable physical functionalities and chemical affinities enable a wide range of technological applications, e.g. sensors, optoelectronics, gas separations and capture, drug delivery, catalysis, energy storage and conversion, amongst others.1,2
 
The MOF material studied in this paper is termed HKUST-1 (or Cu3BTC2 ; BTC = benzene-1,3,5-tricarboxylate);3 it has a topical copper paddle-wheel framework structure (Fig.1a) with a large surface area surpassing 2,000 m2 g-1. Engineering this self-assembling copper-based hybrid system is chemically straightforward using a one-pot synthesis. While a lot is known about HKUST-1’s chemical structure and porosity, little is known about the fundamental vibrational motions (modes) controlling many of its functionalities. Diamond’s Multimode InfraRed Imaging and Microspectroscopy (MIRIAM) beamline was used for its ability to obtain broadband measurements from mid-Infrared (IR) and THz (also known as far IR) regions simultaneously.
 

Figure 1: (a) Crystal structure of HKUST-1 material. (b) Experimental SR-FIR and Raman spectra up to 18 THz region (0–600 cm−1), consistent with the DFT spectra calculated using quantum mechanical approach. Reproduced from Ryder et al (2016) with permission from the Royal Society of Chemistry.

 

A novel experimental setup
Synchrotron Radiation Infrared (SR-IR) and Raman spectroscopic techniques were used on MIRIAM to probe the little-understood low-energy vibrational motions present in HKUST-1. An HKUST-1 powder sample was placed on top of a diamond crystal for Attenuated Total Reflection (ATR) and held in position by pressure under a high-density polyethylene disk. The SR-IR beam entered the ATR crystal and was reflected at the interface of the crystal and the sample, then redirected to the detector. The IR absorption spectrum was thus measured as a modulation of this reflected light due to the evanescent wave penetrating the specimen at the diamond surface, with no other sample preparation and under vacuum.

Matthew Ryder, lead author of the report and 3rd year DPhil candidate at the University of Oxford, carried out the experiments at Diamond. He said: “SR-IR spectroscopy allowed for high-resolution absorption spectra at 1 cm-1, which could be measured rapidly in approximately two minutes per scan. Fourier Transform Infrared (FTIR) spectroscopy enabled us to collect THz data with high signal-to-noise ratio, which is challenging using conventional laboratory equipment. Data in this frequency region gave us crucial information about the collective lattice dynamics of the MOF structure.”

The detailed experimental results were revealed using first-principle density functional theory (DFT) calculations. Matthew Ryder continued: “DFT calculations not only provided excellent match to the spectroscopic measurements (see Fig.1b), but also predicted the complete elastic properties of the MOF single crystals. The latter let us pinpoint any unusual mechanical behaviour, whose source can be linked to its basic THz collective dynamics.”

Matthew Ryder, Jin-Chong Tan, and Gianfelice Cinque at the MIRIAM beamline.

MIRIAM Beamline
In this long-term collaboration, the beamline team was involved in the full scientific experiment. Dr Gianfelice Cinque, Principal Beamline Scientist for MIRIAM said, “The optimised IR-to-THz spectroscopy method applied here will be appealing to those interested in studying the properties of MOF structural properties, and the correlation with low-energy dynamics. This approach is used to establish the vibrational spectra of complex frameworks and elasticity of porous networks and soft matter.”
 
MIRIAM can distinctively bridge the so-called ‘THz-gap’ with the complete IR spectral range. This allows seamless and full molecular characterisation at the same instrument over three decades of wavelength, from circa 1 µm to 1 mm.

Figure 2: (a) Molecular rotor dynamics of the Cu-based paddle-wheel moiety occurring at 1.7 THz (58 cm−1), viewed down the (a) <110> and (b) <011> crystallographic axes. (b) Trampoline-like vibrational motion identified at 2.4 THz (81 cm−1), viewed down the (a) <111> and (b) <110> crystal axes. This mode is linked to the anomalous negative thermal expansion (NTE) phenomenon. Reproduced from Ryder et al (2016) with permission from the Royal Society of Chemistry.

 

Surprising structural dynamics
The experiment revealed previously unreported collective vibrations – behaviours displayed by the HKUST-1 MOF:
 
  1. Rotational deformations to the Cu paddle-wheel section (linked to ‘rotor dynamics’) at 1.7 THz (Fig.2a)
  2. An intriguing trampoline-like deformation mode (at 2.4 THz) that is the source of recently reported negative thermal expansion (NTE)4 phenomenon(Fig.2b).
  3. At just 0.5 THz, a synchronous cluster mechanism (Fig. 3a) was observed that could be linked to an anomalous elastic deformation called auxeticity (when a framework counterintuitively becomes wider when stretched uniaxially) (Fig. 3b)
 
Professor Jin-Chong Tan, from the University of Oxford’s Department of Engineering Science, who led the team, said: “Broadband THz spectroscopy using synchrotron radiation has opened up numerous new possibilities for conducting challenging experiments. It can help us to disentangle the elastic complexities and structural dynamics underpinning the performance of next generation materials, such as MOFs,5 hybrid nanostructures, and composite systems.”
 
To find out more about the MIRIAM beamline, or to discuss potential applications, please contact Principal Beamline Scientists Dr Gianfelice Cinque: gianfelice.cinque@diamond.ac.uk
 

a)

b)

Figure 3: a) Cluster rotation mode at 0.5 THz; b) Demonstration of the plausible mechanism associated with cluster rotational dynamics responsible for the auxetic response. Reproduced from Ryder et al (2016) with permission from the Royal Society of Chemistry.

References:

  1. Ryder MR & Tan JC, Mater Sci Tech. 30, 1598-1612 (2014)
  2. V. Stavila V, Talin AA, & Allendorf MD, Chem Soc Rev. 43, 5994-6010 (2014)
  3. Chui SS et al, Science 283, 1148-1150 (1999).
  4. Peterson VK et al, Angew. Chem. Int. Ed. 49, 585-588 (2010).
  5. Ryder MR, Cinque G, Tan JC et al, Phys Rev Lett. 113, 215502 (2014)

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