Background and problem

Next-generation nanosensing and -actuating materials should be able to efficiently and reliably transform well-defined atomic-level stimuli into a macroscopic response or change in its structure. Soft porous crystals (SPCs) or flexible metal-organic frameworks (MOFs) are framework materials that are ideally suited for this task, as they undergo large-amplitude phase transformations between multiple metastable phases under the influence of external stimuli such as temperature, pressure, or adsorption [1, 2, 3]. For instance, DUT-49(Cu) has been proposed for gas-releasing rescue systems given its unique potential for ‘anomalous’ negative gas adsorption (NGA). However, in order to design these promising materials for specific applications, it is crucial to fundamentally understand the origin of this cooperative behavior and rationalize how this macroscopic spatiotemporal response can be tuned by introducing specific building blocks in the material. The NGA phenomenon in DUT-49(Cu), for instance, was experimentally observed to critically depend on the size of the MOF crystal [4], an effect that remains elusive to date and precludes its full exploitation in applications.

To obtain insight in these elusive but highly attractive size-dependent phase transitions in SPCs, we recently investigated a series of MOFs (see Figure 1a) at length scales going substantially beyond the nanocells that are typically simulated (see Figure 1c) [5]. In this way, we demonstrated that phase transformations in MOFs do not occur collectively, but rather proceed in a stepwise fashion in which part of the MOF crystal undergoes a phase transition first. During the transformation, multiple phases temporarily coexist in the same MOF crystal, leading to interfacial defects (see Figure 1b) and associated strain fields that affect the materials’ spatiotemporal response. We furthermore rationalized that this observed phase coexistence, a new type of spatial disorder in MOFs, could be stabilized in the material through carefully controlling the external stimuli, resulting in a MOF crystal that exhibits the attractive properties of both single phases. Given that phase coexistence was moreover demonstrated to depend on the crystallite size, it forms a possible explanation for the size-dependent structural flexibility in SPCs.

Goal

In order to fully understand and design phase coexistence in MOFs as a new pathway to design the spatiotemporal response of defect-engineered MOFs [6], three key questions need to be answered (see Figure 2):

1. Can the introduction of small alterations in the MOF building blocks, such as linker functionalization or multivariate inorganic nodes, expand the window of thermodynamic conditions under which phase coexistence can be stabilized?
2. Can phase coexistence exist in MOFs exhibiting topologies other than the winerack topology (e.g., ZIF-4, ZIF-7, DUT-49), thus providing evidence for a more fundamental mechanism through which phase coexistence and the associated strain fields nucleate and propagate through the crystal?
3. Do there exist experimentally accessible spectroscopic fingerprints linked to phase coexistence that can be predicted via operando theoretical approaches and that reveal the spatiotemporal response of the material under realistic external conditions?

In this thesis, it is the goal to explore these fundamental questions by extending our earlier thermodynamic protocol using flexible and ab initio-derived force fields for mesosized MOF crystals [5]. To reach these challenging length scales (> 10 nm), both in-house developed software as well as the versatile OpenMM simulation toolkit will be used, as the latter may provide an extra order-of-magnitude increase in length scale [7]. First, the impact of functionalizing the organic ligands with, e.g., –OH and –Br groups on the emergence of phase coexistence in MIL-53 will be rationalized. These groups provide both steric hindrance–favoring the open-pore configuration–as well as attractive interactions that favor the closed-pore configuration; judiciously positioning these groups in the framework may hence culminate in a substantially expanded stability window for phase coexistence. Second, ZIF-4 and ZIF-7 will be investigated as MOF architectures that both exhibit a rich polymorphism that is fundamentally different from the earlier investigated winerack-type MOFs, allowing for more complicated forms of phase coexistence. The obtained thermodynamic insight will form a crucial step forward to consciously design spatial disorder in MOFs and exploit the phenomenon for practical applications.

The student will be actively coached to make him/her acquainted with the several advanced simulation and coarse-graining techniques early in the thesis year, and to transfer necessary programming skills needed to perform the research.