Emergent rotational dynamics and optical properties of metal–organic frameworks

Metal–organic frameworks (MOFs) are ordered arrays of polytopic organic ligands, commonly called linkers, which interconnect metal-based inorganic building units via coordination bonds. The highly precise assembly of well defined building blocks into extended 3-D networks, known as reticular chemistry, has allowed researchers in this field to produce tens of thousands of different frameworks. As an obvious consequence of their unprecedented porosity and tunability, these versatile materials are continuously studied with various potential applications in mind. An important portion of MOF research has been invested in the interaction between molecules and the framework, aiming for applications such as gas storage and separation, as well as catalysis, drug delivery, heat exchange, and water harvesting.

MOFs are considered soft or flexible materials, a characteristic that includes structural dynamics or large amplitude deformations. This flexibility can usually be attributed to the framework’s topology and the degrees of freedom of some of its bonds. However, the linkers themselves may also have degrees of freedom allowing independent molecular dynamics, in particular in the form of rotation. This type of dynamics is particularly common in MOFs because their porous architectures often provide enough space for the rotation of a molecular fragment to occur. It is this type of dynamics that this thesis is centered on, starting from the fact that, although it is an intriguing phenomenon that occurs in MOFs, it has remained relatively unexplored.

Nevertheless, the past four years have seen an increase in researchers’ interest in rotational dynamics in MOFs. This may be due to two main reasons: First, linker rotation influences MOF properties, not only when guest molecule interactions are involved, but also in optical and mechanical properties. Development of our knowledge on linker rotation is therefore essential for a more complete understanding of these materials’ properties and how they may be modified to enhance a specific trait. Second, the exploitation of linkers’ rotational freedom could potentially lead to important technological advances. The latter category includes various innovative ideas, such as the design of ferroelectric MOFs by means of controllable dipolar rotors, or the realization of crystalline molecular machines able to produce useful work.