Molecular Rotors in Porous Covalent Frameworks and Supramolecular Architectures

Highly porous materials are attracting large attention in the recent literature for their applications in the field of gas storage, selective recognition and molecular confinement. The discovery of ultra-fast molecular rotors in porous covalent frameworks allowed us to look at them from a new perspective [1,2]. The unusual combination of remarkable porosity with fast dynamics enabled the reversible speed-regulation of matrix rotors by the interaction with I2 vapors entering the galleries of the porous compounds. Molecular rotors, especially when bearing dipoles, are a challenging research field entailing a number of useful phenomena, such as switchable ferroelectricity, and the fabrication of dynamic elements of molecular motors in solids. Recently, fast rotating organic elements bearing carbon-fluorine dipoles have been fabricated in porous organic-inorganic hybrid periodic architectures [3]. The reactivity of the pivot bonds allowed halogen addition and motion regulation. 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. 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 [5]. 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 low rotational barriers were found by dielectric spectroscopy. [1] A. Comotti, S. Bracco, P. Valsesia, M. Beretta, P. Sozzani Angew. Chem. Int Ed., 2010, 49, 1760. [2] A. Comotti, S. Bracco, T. Ben, S. Qiu, P. Sozzani Angew. Chem., Int. Ed., 2014, 53, 1043. [3] S. Bracco, M. Beretta, A. Cattaneo, A. Comotti, A. Falqui, K. Zhao, C. Rogers, P. Sozzani, Angew. Chem. Int Ed., 2015, 54, 4773. [4] A. Comotti, S. Bracco, A. Yamamoto, M. Beretta, T. Hirukawa, N. Tohnai, M. Miyata, P. Sozzani J. Am. Chem. Soc., 2014, 136, 618. [5] L. Kobr, K. Zhao, Y. Shen, A. Comotti, S. Bracco, R. K. Shoemaker, P. Sozzani, N. A. Clark, J. Price, C. Rogers, J. Michl J. Am. Chem. Soc. 2012, 134, 10122.