Dynamics of CO2 and Xe and Ultra-fast Molecular Rotors in Porous Crystals

The enormous interest manifested in recent years for porous materials has generated efficient systems for adsorbing gases of great interest for energy and the environment, such as CO2, CH4 and H2. Our project is based on the design of porosity in combination with switchable dynamics and flexibility for gaining control over gas capture and selectivity. This approach is made possible by fabricating tetrahedral building blocks and rotor-on-axel molecular struts. The interaction of tetrahedral-shaped polyanions with linear difunctional organic cations (CPOS-5) produced tailored sub-nanometer channels with double helices of electrostatic charges governed the association and transport of CO2 molecules.1 The unique screwing dynamics of CO2 travelling along the ultramicropores with a single-file 106 step/s transport rate was revealed by in-situ 13C-NMR combined with CO2 DFT modelling. Highly symmetrical tetrahedral elements were designed to construct swellable porous adamantoid frameworks through co-operation of hydrogen bonds mounted on conformationally flexible groups.2 The flexibility of the porous crystals manifests itself in response to stimuli of selected gases: the contact with CO2, Xe and hexane triggers the enlargement of channel cross-section and capacity. The accomodation of CO2 and Xe in the channel chambers was revealed by synchrotron-light XRD, combined with Molecular Dynamics and DFT calculations. 129Xe NMR highlights gas dynamics while receiving the encoding of the shape and orientation of each visited cage. Ultra-fast molecular rotors were realized in porous crystals by engineering crystalline frameworks containing rod-like linkers as amphidynamic elements.3 The porous frameworks promise access to the control of rotary motion by chemical and physical stimuli. Rotor dynamics as fast as 1011 Hz (in the regime of conventional liquids) in properly designed porous crystals was hampered by a gas or vapor diffused to the cavities, such as CO2, iodine and hydrocarbon vapors. In turn, when C-F dipoles were mounted on the rotors, they induced fast oscillating dipoles. The dipole reorientation interacts with an applied electric field with the final aim to produce switchable ferroelectric properties. References 1 Xing, G.; Bassanetti, I.; Bracco, S.; Negroni, M.; Bezuidenhout, C.; Ben, T.; Sozzani, P.; Comotti, A. Chem. Sci. 2019, 10, 730-736 (Highlighted in Nature Nanotechnology). 2 Bassanetti, I.; Bracco, S.; Comotti, S.; Negroni, M.; Bezuidenhout, C.; Canossa, S.; Mazzeo, P. P.; Marchio’, L.; Sozzani, P. J. Mater. Chem. A 2018, 6, 14231-14239. 3 Comotti, A.; Bracco, S.; Sozzani, P. Acc. Chem. Res. 2016, 49, 1701-1710.