Engineering dynamics in porous materials: ultrafast rotational motion at 2K and modulation of porosity by light-triggered molecular switches

Extensive development in microporous materials led to interesting physical properties and promising applications. Their structure enclose high free volume that enables fast dynamics even in condensed matter and allows the effective inclusion of molecular switches and machines with full retention of their molecular dynamical behavior in the solid state. Metal organic frameworks (MOFs) are characterized by a versatile chemistry and crystalline structures. A fast-rotating molecular rotor based on bicyclo[1.1.1]pentane–dicarboxylate moiety was installed in the 3D cubic structure of a highly porous zinc MOF1, thus isolating the individual rotor from each another. Moreover, the mismatch between the symmetry of the rotor and the symmetry of the carboxylate imposed by the cubic crystal structure generated a very shallow 12 fold potential well for rotational motion. Solid state NMR relaxation measurements and muon-spin spectroscopy performed at temperatures as low as 2 K allowed the determination of an activation energy as low as 6.2 cal mol-1, consistent with fast molecular reorientation in the GHz regime even at the lowest temperatures. Molecular dynamics simulations provides an understanding of the rotational motion at the nanoscale level consisting of continuous rotations above very shallow energy minima. This exciting results paved the way for the generation of materials endowed with fast dynamical properties even at very low temperatures. Installation of molecular machines in the solid state can upgrade stimuli-controlled nanometric changes into useful material properties due to the cooperative response of collection of molecular motors. A bistable overcrowded alkene was introduced in the main framework architecture of porous aromatic frameworks through the co-polymerization reaction of a photoswitch moiety and tetraphenylmethane molecule3. The frameworks displayed high porosity with surface areas as high as 4800 m2/g and provided enough free volume for the photoisomerization of molecular switches in the solid material. UV light irradiation induced the quantitative photoisomerization of molecular switches and modulated the adsorptive properties of the porous material. Specifically, the adsorption capacity towards CO2 and N2 can be reversibly increased or lowered of about 20% (Figure 1). The process can be reversed by irradiation or heating, allowing the control of gas adsorption properties using external stimuli and opening possible application as “on demand” sorbents for gaseous species and controlled drug release. 1Perego, J.; Bracco, S.; Negroni, M.; Bezuidenhout, C. X.; Prando, G.; Carretta, P.; Comotti, A. and Sozzani, P. Nature Chem, 2020, 12, 845-851. 2Prando, G.; Perego, J.; Negroni, M.; Riccò, M.; Bracco, S.; Comotti, A.; Sozzani, P. and Carretta, P. Nano Lett. 2020, 20 (10), 7613-7618. 3Castiglioni, F.; Danowski, W.; Perego, J.; Leung, F. K.-C.; Sozzani, P.; Bracco, S.; Wezenberg, S. J.; Comotti, A. and Feringa, B. L. Nature Chem., 2020, 12, 595-602.