Understanding the Anisotropic Elastic Properties of Metal–Organic Frameworks at the Nanoscale: The Instructive Example of MOF-74

Metal–organic
frameworks (MOFs) are a particularly intriguing
class of self-assembled materials, whose elastic properties crucially
impact many of their envisioned applications. Thus, we here present
an in-depth, “nanoscale” discussion of these properties
for the prototypical class of MOF-74 derivatives. These provide a
particular wealth of insights due to their pronounced anisotropy with
fundamentally different building blocks connecting the structures
parallel and perpendicular to the pore direction. To go beyond solely
reporting macroscopic parameters, we trace their values back to atomistic
displacements under stress employing state-of-the-art dispersion-corrected
density functional theory. Interestingly, all of the studied MOFs
exhibit qualitatively different responses to either unidirectional
or isotropic stress, which can be ascribed to distinctly different
atomic rearrangements for stress parallel or perpendicular to the
channel direction. In the former case, one primarily observes a lateral
expansion and rotation of the nodes, which can be impeded, e.g., by an exoskeleton formed by an adsorbed water layer.
Conversely, for stress perpendicular to the channel, the MOFs comply
with a deformation of the hexagonal pores, which causes a significant
expansion perpendicular to the stress direction. We also show that
the details of these atomistic motions impact the structure-to-property
relationships for a variety of MOF-74 variants beyond the expectations
based on bonding strengths and the degree of porosity.