Another approach recently introduced in the design of materials with novel responses to guest adsorption is the concept of "frustrated flexibility" proposed by Pallach et al. in 2021. These authors reported a series of functionalized MOF-5 derivatives exhibiting a new type of guest-and temperatureresponsive structural flexibility: while the materials retained their framework connectivity and topology (i.e., no bonds were broken), they could undergo reversible crystal-to-amorphous structural transitions -depicted on Figure . Pallach and coworkers used a combination of experimental and computational methods to explain this phenomenon as follows: the structural flexibility is due to the frustration of the system. This frustration arises from the conflict of the framework lattice, which is rigid due to intrinsic geometrical constraints of the inorganic building units (as in parent compound MOF-5), with strong intraframework dispersion interactions, which are a driving force towards densification of the structure. Upon changes in temperature, this conflict leads to an entropy-driven transition between an ordered phase (crystalline) and a disordered phase (a semicrystalline, aperiodically distorted structure). This new concept is a very promising direction of research, although it remains to be seen whether this frustrated flexibility can be generalized to other framework types, and identified (or designed) in other material families. On a closely related topic, Chen et al. have proposed a novel method to gain fine control over the flexibility and pore size of an ultramicroporous MOF through the introduction of frustration of the intramolecular interactions. This new CU-4 material (CU = Cambridge University) combines an In 3+ -based inorganic node prone to flexibility (similar to that of the MIL-53(In) family ), with a bulky cubane-derived ligand, and due to the balance of interactions, its pore cavity is conducive to the exclusive recognition of CO 2 . The molecular frustration occurs upon binding of CO 2 , where the multiple H bonds and weak van der Waals interactions minimize framework dynamics, therefore favoring high selectivity. This new idea for MOF design, illustrated on Figure and which the authors called "flexibility-frustrated porosity", is related to the "frustrated flexibility" of Pallach et al. -but not exactly identical.