Porous Crystalline Architectures: Ultrafast Molecular Rotors and Dynamics Control by Gas Stimuli (Keynote)

A challenging issue is the dynamics of nanoporous solids after the insertion of molecular rotors in their building blocks, promising access to the control of rotary motion by chemical and physical stimuli.[1] The combination of porosity with ultra-fast rotor dynamics was discovered in molecular crystals, covalent organic frameworks and MOFs by 2H spin-echo NMR spectroscopy and T1 relaxation times.[2-5] The rotors, as fast as 1011 Hz at 150 K, are exposed to the crystalline channels, which absorb CO2 and I2 from the gas phase, even at low pressures. Interestingly, the rotor dynamics can be switched on and off by vapor absorption/desorption, showing a remarkable change of material dynamics, which, in turn, produces a modulated physical response. Novel mesoporous organosiloxane frameworks allowed us to realize periodic architectures of fast molecular rotors on which C-F dipoles are mounted.[6] These dipolar rotors showed not only rapid dynamics (109 Hz at 325 K) in the solid-state NMR experiments, but also a dielectric response typical of a fast dipole reorientation. Moreover, crystals with permanent porosity were exploited in an unusual way to decorate crystal surfaces with regular arrays of dipolar rotors. The inserted molecules carry alkyl chains which are included as guests into the channel-ends.[7] The rotors stay at the surface due to a bulky molecular stopper which prevents the rotors from entering the channels. The host-guest relationships were established by 2D solid-state NMR and GIAO HF ab initio calculations. In a final example, flexible molecular crystals were fabricated by a series of shape-persistent azobenzene tetramers that form porous molecular crystals in their trans configuration. The efficient trans→cis photoisomerization of the azobenzene units converts the crystals into a non-porous phase but crystallinity and porosity are restored upon Z→E isomerization promoted by visible light irradiation or heating. We demonstrated that the photoisomerization enables reversible on/off switching of optical properties as well as the capture of CO2 from the gas phase.[8] We would like to thank Cariplo Foundation, Lombardy Region/INSTM Consortium and MIUR (PRIN 2016). References: [1] Comotti, A. et al. (2016). Acc. Chem. Res. 49, 1701-1710. [2] Bracco, S. et al. (2017). Chem. Eur. J. 23, 11210. [3] Comotti, A. et al. (2014). J. Am. Chem. Soc. 136, 618. [4] Comotti, A. et al. (2014). Angew. Chem. Int. Ed. 53, 1043-1047. [5] Bracco, S. et al. (2017). Chem. Comm. 53, 7776-7779. [6] Bracco, S. et al. (2015). Angew. Chem. Int Ed. 54, 4773-4777. [7] Kobr, L. et al. (2012). J. Am. Chem. Soc. 134, 10122-10131. [8] Baroncini, M. et al. (2015). Nature Chem. 7, 634-640.