Regulation of dipolar rotor dynamics by gas adsorption and photoinduced gas uptake-release in porous materials

Molecular Rotors, especially when bearing dipoles, are an attractive research field entailing a number of useful phenomena, such as switchable ferroelectricity, and the fabrication of dynamic elements of molecular motors in solids. In turn, porous materials are an innovative playground for supporting switchable molecular rotors [1]. We have recently discovered hybrid porous materials with intrinsic dynamics due to the presence of fast molecular rotors in their architectures [2,3]. The highly-organized porous scaffolds supporting organic elements allowed the fabrication of fast molecular rotors (k>108 Hz), such as p-phenylene units and dipolar rotors containing C-F dipoles. Such dipolar rotors face the pores and are entirely exposed to the guest molecules which acted as regulators. A first example of molecular rotors in porous molecular crystals is presented. Disulfonated rotor-containing molecular rods were self-assembled with alkylammonium salts to fabricate porous supramolecular architectures held together by charge-assisted hydrogen bonds[4]. The rotors 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, suggesting the use of porous crystals in sensing and pollutant management. In a further example, the surfaces of inclusion crystals were decorated with regular arrays of dipolar molecular rotors by the insertion of dipolar rotators carrying alkyl chains which are included as guests into channels of a host, tris(o-phenylenedioxy)cyclotriphosphazene (TPP) [5]. The rotors stay at the surface due to a bulky molecular stopper which prevents the rotors from entering the channels while the alkyl chains are included in the crystals. The host-guest relationships were established by 2D solid-state NMR and low rotational barriers were found by dielectric spectroscopy. Furthermore, porosity can be switched on/off in molecular crystals based on star-shaped azobenzene tetramers by photoirradiation [6]. The trans-cis photoisomerization of the azobenzene units takes place in the solid state and converts the porous crystals into a non-porous amorphous phase while crystallinity and porosity are restored upon cis-trans isomerization promoted by visible light irradiation or heating. Photoisomerization enables reversible capture of carbon dioxide from the gas phase as demonstrated by CO2 adsorption isotherms.