One fundamental aspect of the embedding of molecular rotors into framework materials of low density is the following: the constraints induced by the crystalline framework allow for the spacing of the rotors, the reduction of their interactions (although we will see exceptions below), and the promotion of fast dynamics. The design and optimization of structure for fast rotor motion is therefore a key area of research in this field. This is well exemplified by the work of Perego et al. , where the authors reported very fast rotation of bicyclopentane rotors in a highly porous zinc-based MOF. They ascribed this unusual dynamics in Zn 4 O(bicyclo[1.1.1]pentane-1,3-dicarboxylate) 3 to a frustration between the three-fold bipyramidal symmetry of the rotor and the four-fold symmetry of the inorganic struts (see Figure ), limiting the formation of conformations with preferential orientation and therefore lowering the energy barrier for rotation. In a subsequent study, Prando et al. confirmed the highly dynamical properties of that system at cryogenic temperatures, through the use of muon-spin relaxation (µSR) spectroscopy. They calculated an exceptionally low activation energy of 30 cal.mol -1 .