Benchmark Rotor Dynamics and Light-driven Motors Engineered in 3D Porous Architectures

Rotors, motors and switches in the solid state find a favorable playground in porous materials, especially in Metal Organic Frameworks (MOFs), thanks to their large free volume, which allows for fast dynamics. We focused on dynamic materials endowed with high responsiveness and under limited thermal noise conditions which enable the fabrication of molecular machines with low energy dissipation and controllable dynamics. We have realized a fast molecular rotor in the solid state whose rotation speed approaches that of unhindered rotations in organic moieties even at very low temperatures (2 K) [1,2]. The three-fold bipyramidal symmetry of the rotator conflicts with the four-fold symmetry of the struts within the cubic crystal cell of the zinc metal–organic framework, frustrating the formation of stable conformations. This allows for the hyperfast rotation of the bicyclic units persistent for several continuous turns, with an energy barrier of 6.2 cal mol−1 and a high frequency even at very low temperatures (1010 Hz below 2 K). Geared molecular rotors with negligible energy-requirements in MOFs enabled fast yet controllable and correlated rotary motion[3]. A MOF architecture was capable of supporting fast motional regimes (107 Hz), even at extremely cold temperatures, of two distinct and hypermobile rotors arranged in pillar-and-layer 3D arrays. The rotors explored multiple configurations of conrotary and disrotary relationships, switched on and off by thermal energy, an unprecedented cascade mechanism modulated by distinct energy barriers. Notably, fluorinated Metal-Organic Frameworks (MOFs), comprising a wheel-shaped ligand with geminal rotating fluorine atoms, produced a benchmark mobility of correlated dipolar rotors at 2 K, with practically null activation energy (Ea = 17 cal/mol), as supported by 1H T1 relaxation NMR, synchrotron radiation X-ray diffraction at 4 K and DFT modeling (Angew. Chem. Int. Ed. 2023 in press). Gas accessibility, shown by hyperpolarized-Xe NMR, allowed for chemical stimuli intervention: CO2 triggered dipole reorientation, reducing their collective dynamics and stimulating a dipole configuration change in the crystal. Furthermore, motors were inserted into porous frameworks and metal-organic frameworks wherein two distinct linkers with complementary light absorption-emission properties were integrated into the same material. Unidirectional motion was achieved by exposure to sun-light of the solid particles, which thus behave as autonomous nanodevices.[4] The visible-light-driven rotation of an overcrowded alkene-based molecular motor strut in a dual-function metal−organic framework (MOF) was realized by the use of two types of functional linkers. Visible light-driven rotation of the molecular motor was proved to be in the solid state at rates similar to those observed in solution.[5] [1] [1] Perego, J.; Bracco, S.; Negroni, M.; Bezuidenhout, C.; Prando, G.; Carretta, P.; Comotti, A.; Sozzani, P. Fast Motion of Molecular Rotors in Metal-Organic Framework Struts at Very Low Temperatures. Nature Chem. 2020, 12, 845-851. [2] Prando, G.; Perego, J.; Negroni, M.; Ricco’, M.; Bracco, S.; Comotti, A.; Sozzani, P.; Carretta, P. Molecular Rotors in a Metal-Organic Framework: Muons on a Hyper-Fast Carousel. Nanoletters 2020, 20, 7613-7618. [3] Perego, J.; Bezuidenhout, C.; Bracco, S.; Prando, G.; Marchio’, L.; Negroni, M.; Carretta, P.; Sozzani, P.; Comotti, A. Cascade Dynamics of Multiple Molecular Rotors in a MOF: Benchmark Mobility at a Few Kelvins and Dynamics Control by CO2. J. Am. Chem. Soc. 2021, 143, 13082-13090. [4] Castiglioni, F.; Danowski, W.; Perego, J.; Leung, F. K-C.; Sozzani, P.; Bracco, S.; Wezenberg, S. J.; Comotti, A.; Feringa, B. L. Modulation of Porosity in a Solid Material Enabled by Photoisomerization of an Overcrowded Alkene. Nature Chem. 2020, 12, 595-604. [5] Danowski, W.; Castiglioni, F.; Sardjan, A. S.; Kause, S.; Pfeifer, L.; Roke, D.; Comotti, A.; Browne, W. R.; Feringa, B. L. Visible-Light-Driven Rotation of Molecular Motors in a Dual-Function Metal-Organic Framework Enabled by Energy Transfer. J. Am. Chem. Soc. 2020, 142, 9048-9056.