Ultra-fast Molecular Rotor Dynamics and their Regulation in Nanoporous Architectures

Nanoporous materials are excellent candidates for the fabrication of molecular rotors in the solid state and promise access to the control of rotary motion by chemical and physical stimuli.[1] The combination of remarkable porosity with fast rotor dynamics was discovered in molecular crystals, covalent organic frameworks and metal-organic frameworks (MOFs) by 2H spin-echo NMR spectroscopy and spin lattice T1 relaxation times.[2-4] A microporous MOF engineered to contain in its scaffold rod-like ligands [1,4-bis(1H-pyrazol-4-ylethynyl)benzene] (BPEB) showed extremely rapid 180° flip reorientation of the central p-phenylene unit with rotational rates of 1011 Hz at 150 K. Molecular rotors are exposed to the crystalline channels, which absorb CO2 and I2 even at low pressure. Interestingly, dynamics could be tuned by gas absorption/desorption, showing a remarkable change of material dynamics, which, in turn, produces a modulated NMR response. Crystal-pore accessibility of the MOF allowed the CO2 molecules to enter the cavities and control the molecular rotor spinning speed down to 105 Hz at 150 K (Figure). This strategy enabled the regulation of rotary motion by gas diffusion in the channels and the determination of the energetics of rotary dynamics in the presence of CO2, enlarging perspectives in the field of sensors and gas detection. Moreover, the insertion of dipoles onto molecular rotors in mesoporous organosilica architectures permits to obtain fast molecular rotors containing dynamic C-F dipoles. The dipolar rotors show not only rapid dynamics of the aromatic rings (109 Hz at 325 K) in the solid state NMR experiments, but also dielectric response typical of a fast dipole-reorientation under the stimuli of an applied electric field.[5] Regular arrays of dipolar molecular rotors can be also mounted on crystal surfaces in an unusual way, exploiting the formation of surface inclusion compounds. Guest molecule is comprised of a rotator, a stopper and a shaft: 2D 1H-13C HETCOR NMR spectroscopy identified the moieties (shafts) which are inserted into the bulk crystal, although they represent a minor part of the nanostructured material.[6] Acknowledgements: Cariplo Foundation, INSTM Consortium, Lombardy Region and PRIN 2016 are acknowledged for financial support. References: [1] A. Comotti, et al., Acc. Chem. Res. 2016, 49, 1701. [2] A. Comotti, et al., J. Am. Chem. Soc. 2014, 136, 618. [3] A. Comotti, et al., Angew. Chem. Int. Ed. 2014, 53, 1043. [4] S. Bracco, et al., Chem. Eur. J. 2017, 23, 11210. [5] S. Bracco, et al., Angew. Chem. Int Ed. 2015, 54, 4773. [6] L. Kobr, et al., J. Am. Chem. Soc. 2012, 134, 10122.