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.

金属有机框架（metal–organic frameworks, MOFs）是一类格外引人关注的自组装材料，其弹性性能对诸多预期应用具有至关重要的影响。因此，本文针对典型的MOF-74衍生物类别，对其弹性性能展开了深入的“纳米尺度”探讨。该类衍生物因显著的各向异性而蕴含丰富的研究视角：其沿孔道方向与垂直于孔道方向的结构连接基元存在本质差异。为突破仅报道宏观参数的局限，本文采用当前顶尖的色散校正密度泛函理论（density functional theory, DFT），将弹性性能的数值溯源至应力作用下的原子尺度位移。值得注意的是，所有被研究的MOFs对单向应力与各向同性应力均表现出定性迥异的响应，这可归因于沿孔道方向与垂直于孔道方向的应力对应的原子重排过程存在显著差异。就单向应力而言，主要观察到结构节点发生侧向膨胀与旋转，该过程可被诸如吸附水层形成的外骨骼所阻碍。反之，当应力垂直于孔道方向时，MOFs会发生六边形孔道的形变，进而导致垂直于应力方向的显著膨胀。本文还证实，这些原子尺度运动的细节会影响多种MOF-74变体的结构-性能关联，其关联程度超出了基于键合强度与孔隙率所预期的结果。