Terahertz insights into MOF dynamics

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 3^rd 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.

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.

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)

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.

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