Coupled linker and electric field induced dynamics in metal-organic frameworks
Monique van der Veen a
a Department of Chemical Engineering, Delft University of Technology Van der Maasweg, Van der Maasweg, 9, Delft, Netherlands
Proceedings of Dynamic Materials, Crystals and Phenomena Conference (DynaMIC23)
Fribourg, Switzerland, 2023 March 22nd - 24th
Organizers: Jovana Milic and Simon Krause
Keynote, Monique van der Veen, presentation 020
Publication date: 15th February 2023

The structure of metal-organic frameworks is all but static. Rotational dynamics of linkers and guest-induced deformations are very prevalent. Awareness about their potential impact in gas adsorption, sensors and as stimuli-responsive materials[1] and crystalline molecular machines is growing.[2]

Recent achievements in rotor-MOFs include engineering ultrafast rotation by decreasing the rotation energy barrier via molecular design, and ample free space in the MOF structure to avoid steric hindrance.[3] Yet, MOFs have, in fact, the potential of displaying much more intricate dynamics, similar to other dynamic materials containing closely interacting molecules, such as crowded movement of proteins in lipid bilayers, or concerted molecular motion in liquid crystals. MOFs in fact provide an exceptional playing ground, as they are offer a regular arrangement of rotors with defined intermolecular distances that can be tuned through the choice of building blocks. This means that the free pore space and inter-rotor distance can be tuned to a desired level of “crowdedness” of the rotors for cooperative motion.

Here, we present how correlated motions emerge in linker dynamics as the steric environment of the rotors is gradually modified. We use linker functionalization in the MIL-53 family of materials to tune both the pore dimensions and the rotor–rotor interactions. Via a combined experimental and computational approach, including broadband dielectric spectroscopy, solid-state 2H NMR, DFT calculations, and molecular dynamics simulations up to 6x3x3 supercells, we comprehensively determine the linker dynamics. For steric interactions that inhibit independent but do not prohibit rotor motion, we identify the emergence of correlated rotation modes between different linkers, as well long range A-B-A-B-A-B… conformational ordering.

Metal-organic frameworks could be very promising, yet are hardly explored, for harvesting mechanical energy through their piezoelectric properties. Indeed, the energy harvesting efficiency, which scales with 1/ εr, should indeed be boosted by the  very low dielectric permittivity εr MOFs can have due to the porosity.[4] Moreover, their structural variability should allow for optimization of the piezoelectric constant ‘e’ and ‘d’. While it can be expected that these properties would make them competitive with the materials mostly studied nowadays, namely ferroelectric ceramics, polymers and composites, very few studies have studied the piezoelectric response of MOFs.[5]

To understand the structure-property relationship of piezoelectric constant ‘e’ in MOFs, we perform high throughput DFT calculations for ~1500 non-centrosymmetric MOFs starting from a computational ready, experimental MOF database (QMOF database).[6] Key factors that can influence the piezoelectric constant ‘e’ are Born effective charges, symmetry, coordination environment around the metal node and the presence of polar  linkers and counterions. For the MOFs which show the highest piezoelectric constant ‘e’ we discuss in detail the key factors contributing to their high performance. For some structures we found values of ‘e’ of the same order of magnitude as those of the best performing ceramics. MOFs have higher elasticity due to their flexible frameworks and orders of magnitude lower dielectric constant of MOFs compared to ceramics. This means that piezoelectric metal-organic frameworks should be able to outperform existing piezoelectric materials for mechanical energy harvesting.

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