By tuning the steric environment and free pore space in metal-organic frameworks, a large variety of rotor dynamics of the organic linkers can appear. Nitrofunctionalized MIL-53 is a terephthalate-linker-based MOF that shows coupled rotor dynamics between the neighboring linkers along the pore direction. Here, we use classical molecular dynamics up to 6 × 2 × 2 supercells to investigate the range of the correlated linker dynamics. Interestingly, we observe an PNPNPNPN... conformational arrangement (P = nearly planar and N = nonplanar) for the conformations of the linkers in a row along the pore direction in the MOF. We identified correlated linker dynamics emerging among the direct and next nearest neighboring linkers along the pore. Due to 180° rotational flips of the planar linkers along the pore, (1) a change in the width of librations in their direct neighbors (PN) is observed; (2) intriguingly, their next nearest planar neighbors (PP) rotate between 0° and ±180° to reattain aligned (0°, 0°) or (±180°, ±180°) conformations. The presence of correlated dynamics in such linkers over long-length scales occurring at nanoseconds time scales is desirable for applications like ferroelectric switching or diffusion control via geared linker rotation, and this work provides insight into the design for such applications.

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Metal–organic frameworks (MOFs) are a class of hybrid materials with metal-based inorganic nodes connected by organic linkers via strong coordination bonds. These building blocks can be arranged in 3-D crystalline lattices to synthesize structures with varying pore sizes and a variety of structures (tuneability). These hybrid materials possess exceptional porosity and large surface areas, making them suitable for applications in gas separation and storage, catalysis, and biomedical fields. MOFs also exhibit remarkable flexibility, which is determined by the topology of the framework and the degrees of freedom between bond angles in the organic linkers or coordination bonds between the organic linkers and inorganic nodes. One among the major categories of flexibility in MOFs is the rotational dynamics of organic linkers. The structural dynamics can have a pronounced influence on gas adsorption, diffusion and optical properties. Chapter 4 and Chapter 5 study the rotational dynamics of terephthalate linkers in functionalized MIL-53 MOFs by varying the steric interactions between the linkers using computational methods like ab initio molecular dynamics and classical molecular dynamics. Using the remarkable porosity, structural flexibility, and tuneability features of MOFs as central handles, in this thesis, we aim to study the (a) Piezoelectric properties in MOFs for their application as energy harvesters and (b) Rotational dynamics of linkers in MOFs. It is well-known that MOFs possess a high degree of flexibility and permanent porosity. High porosity of MOFs leads to low dielectric constants. This, together with higher flexibility of MOFs, makes them promising candidates for piezoelectric energy harvesting. Although all non-centrosymmetric MOFs are piezoelectric, their piezoresponse has hardly been studied thoroughly. Chapter 2 and Chapter 3 of this thesis will investigate the structure-property relationships of piezoelectric properties in MOFs through computational methods, and provide design guidelines that can contribute to the development of high-performing piezoelectrics. @en

Metal-organic frameworks (MOFs) are a class of nanoporous crystalline materials with very high structural tunability. They possess a very low dielectric permittivity ϵr due to their porosity and hence are favorable for piezoelectric energy harvesting. Even though they have huge potential as piezoelectric materials, a detailed analysis and structure-property relationship of the piezoelectric properties in MOFs are lacking so far. This work focuses on a class of cubic non-centrosymmetric MOFs, namely, zeolitic imidazolate frameworks (ZIFs) to rationalize how the variation of different building blocks of the structure, that is, metal node and linker substituents affect the piezoelectric constants. The piezoelectric tensor for the ZIFs is computed from ab initio theoretical methods. From the calculations, we analyze the different contributions to the final piezoelectric constant d14, namely, the clamped ion (e140) and the internal strain (e14int) contributions and the mechanical properties. For the studied ZIFs, even though e14 (e140 + e14int) is similar for all ZIFs, the resultant piezoelectric coefficient d14 calculated from piezoelectric constant e14 and elastic compliance constant s44 varies significantly among the different structures. It is the largest for CdIF-1 (Cd2+ and -CH3 linker substituent). This is mainly due to the higher elasticity or flexibility of the framework. Interestingly, the magnitude of d14 for CdIF-1 is higher than II-VI inorganic piezoelectrics and of a similar magnitude as the quintessential piezoelectric polymer polyvinylidene fluoride.

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The organic components in metal-organic frameworks (MOFs) are unique: they are embedded in a crystalline lattice, yet, as they are separated from each other by tunable free space, a large variety of dynamic behavior can emerge. These rotational dynamics of the organic linkers are especially important due to their influence over properties such as gas adsorption and kinetics of guest release. To fully exploit linker rotation, such as in the form of molecular machines, it is necessary to engineer correlated linker dynamics to achieve their cooperative functional motion. Here, we show that for MIL-53, a topology with closely spaced rotors, the phenylene functionalization allows researchers to tune the rotors' steric environment, shifting linker rotation from completely static to rapid motions at frequencies above 100 MHz. For steric interactions that start to inhibit independent rotor motion, we identify for the first time the emergence of coupled rotation modes in linker dynamics. These findings pave the way for function-specific engineering of gear-like cooperative motion in MOFs.

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