Molecular Rotors in Nanoporous Periodic Architectures

New mesoporous hybrid covalent frameworks were prepared to realize a periodic architecture of fast molecular rotors containing dynamic C-F dipoles in their structure.[1] The mobile elements, designed on the basis of fluorinated p-divinylbenzene moieties, were integrated into the robust covalent structure through siloxane bonds, and showed not only the rapid dynamics of the aromatic rings (ca. 108 Hz at 325 K), as detected by solid-state NMR spectroscopy, but also a dielectric response typical of a fast dipole reorientation under the stimuli of an applied electric field. The nanochannels are open and accessible to diffusing-in gas molecules, and rotor mobility could be individually regulated by I2 vapors. The iodine enters the channels of the periodic architecture and selectively reacts with the pivotal double bonds of the divinyl-fluoro-phenylene rotors without disrupting the structure, affecting their motion and the dielectric properties.The combination of porosity with rotor dynamics was also discovered in molecular crystals. Disulfonated rotor-containing molecular rods were self-assembled with alkylammonium salts to fabricate porous supramolecular architectures held together by charge-assisted hydrogen bonds (Figure 1) [2]. The rotors, as fast as 108 Hz at 240 K, are exposed to the crystalline channels, which absorb CO2 and I2 vapors at low pressure. The rotor dynamics could be switched off and on by I2 absorption/desorption, showing remarkable change of material dynamics by the interaction with gaseous species and suggesting the use of molecular crystals in sensing and pollutant management. Moreover, porosity can be switched on/off in molecular crystals based on star-shaped azobenzene tetramers by photoirradiation[3]. Photoinduced trans-cis isomerization of molecules changes intermolecular interactions triggerring the formation of a non porous material which can be reverted to porous crystals by visible light irradiation or thermal treatment