The Dynamical World of Metal-Organic Frameworks

Rotors, motors and switches in the solid state find a favorable playground in in Metal Organic Frameworks (MOFs), thanks to their large free volume, which allows for fast dynamics. We have realized a fast molecular rotor in a Zn-MOF whose rotation speed approaches that of unhindered rotations in organic moieties even at very low temperatures (2 K) [1,2]. Geared molecular rotors with negligible energy-requirements in pillared MOFs enabled fast yet controllable and correlated rotary motion[3], showing an unprecedented cascade mechanism. Chemical stimuli such as the use of CO2 diffused through the open pores changed dramatically the global rotation mechanism and rotor speed. Attractive functional properties, such as dielectric, optical and ferroelectric switchable properties, can be activated by incorporating fast-reorientable dipoles onto molecular rotors to produce materials responsive to static or oscillating electric fields. In fluorinated MOFs comprising a wheel-shaped ligand with geminal rotating fluorine atoms, we tailored benchmark dipole rotational dynamics even in the presence of cooperative motion with practically null activation energy of 17 cal mol-1. The mobility in the fluorinated MOF, especially in the 2–10 K temperature range, is due to the presence of a concerted dance of dipoles triggered by the interactions among nearest-neighbour rotors[4]. Furthermore, motors were inserted into MOFs wherein two distinct linkers with complementary light absorption-emission properties were integrated into the same material. Unidirectional motion was achieved by exposure to sunlight of the solid material, which thus behaves as an autonomous nanodevice.[5] Responsive porous switchable framework materials based on a molecular switch incorporated in the backbone responds to a light stimulus. The efficient and quantitative bulk photoisomerization of the incorporated molecular switch takes advantage of the porosity of the framework and the consequent gas adsorption capacity can be reversibly modulated in response to light and heat.[6] We demonstrated that fluorescent metal–organic framework (MOF) nanocrystals can work as fast scintillators.[7] The MOF comprises high-Z linking nodes that interact with the ionizing radiation and are arranged in an orderly fashion at a nanometric distance from ligand emitters. Their incorporation in the framework enables fast sensitization of the ligand fluorescence, showing an ultrafast scintillation rise time of ~50 ps. Moreover, two ligands of equal molecular length and connectivity, yet complementary electronic properties, were co-assembled in a Zr-MOF, generating crystalline hetero-ligand MOF nanocrystals which result in high efficiency luminescence with significant Stokes shift and benchmark performances.[8] References [1] Nature Chem. 2020, 12, 845. [2] Nanoletters 2020, 20, 7613. [3] J. Am. Chem. Soc. 2021, 143, 13082. [4] Angew. Chem. Int. Ed. 2023, e202215893. [5] J. Am. Chem. Soc. 2020, 142, 9048. [6] Nature Chem. 2020, 12, 595. [7] Nature Photonics 2021, 15, 393-400; [8] Nature Communications 2022, 13, 3504.