Crystals springing into action: metal–organic framework CUK-1 as a pressure-driven molecular spring
Paul Iacomi1, Ji Sun Lee2, Lous Vanduyfhuys3, K.H Cho2, Pierre Fertey4, Jelle Wieme3, Dominique Granier1, Guillaume Maurin1, Veronique an Speybroeck3, Jong-San Chang2, Pascal Yot1
1ICGM, University Montpellier, CNRS, ENSCM, F-34095 Montpellier, France,
2Research Center for Nanocatalysts, KRICT, Yusung, Daejeon 305-600, Korea.
3Centre for Molecular Modeling, Ghent University, Technologiepark 903, B-9052 Zwijnaarde, Belgium
4Synchrotron Soleil, L’orme des Merisiers, Saint-Aubin – BP 48, F-91192 Gif-sur-Yvette, France
We have intensively explored the mechanical properties of flexible MOFs over the last few years [1,2], with the aim of using their high compressibility at medium pressures as a mean of storing mechanical energy. The considerable stored energy associated with open/closed pore transitions, in the range of 30–200 J/g (up to 4 kJ g/1 for shock absorbers) is highly attractive. However, the hysteretic compression-decompression curve characterising known nano-damper MOFs leads to a partial loss of work energy, lowering the potential storage efficiency, as well as creating issues through heat dissipation. Insofar, the search for an ideal spring-like crystalline material, capable of reversible pressure-induced structural switching without any hysteresis has been fruitless, precluding their applicability for efficient, high density energy storage applications.
Herein, we use mercury porosimetry and in situ high pressure single crystal X-ray diffraction to reveal the wine-rack CUK-1 MOF as a unique crystalline material capable of a fully reversible mechanical pressure-triggered structural contraction . The near-absence of hysteresis upon cycling exhibited by this robust MOF, akin to an ideal molecular spring, is associated with a constant work energy storage capacity of 40 J/g. Molecular simulations were further deployed to uncover the free-energy landscape behind this unprecedented pressure-responsive phenomenon in the area of compliant hybrid porous materials.
Fig. 1 Reversible energy storage in CUK-1.
 P G Yot et al. ‘Chemical Science 7, no. 1 (2016): 446–50.
 P G Yot, et al. European Journal of Inorganic Chemistry 2016, no. 27 (September 2016): 4424–29.
 P Iacomi, et al. Chemical Science, 2021, 10.1039.D1SC00205H.