Rotors, motors and luminescent properties in metal organic frameworks

Rotors, motors and switches in the solid state find a favorable playground 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 in the presence of a concerted dance of dipoles with practically null activation energy of 17 cal mol-1[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]. We demonstrated that fluorescent metal–organic framework (MOF) nanocrystals can work as fast scintillators [6]. The MOF comprised 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 enabled fast sensitization of the ligand fluorescence, showing an ultrafast scintillation rise time of ~50 ps. Additionally, 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 resulted in high efficiency luminescence with significant Stokes shift and benchmark performances [7]. Moreover, scintillating MOFs were simultaneously capable of concentrating radioactive gases and efficiently producing visible light revealed with high sensitivity. We demonstrated the capability of a hafnium-based MOF incorporatingdicarboxy-9,10-diphenylanthracene as a scintillating conjugated ligand to detect gas radionuclides [8]. Metal–organic frameworks showed fast scintillation, a fluorescence yield of ∼40%, and accessible porosity suitable for hosting noble gas atoms and ions. Adsorption and detection of 85Kr, 222Rn and 3H radionuclides were explored through a newly developed device, suggesting the use of scintillating porous MOFs to fabricate sensitive detectors of natural and anthropogenic radionuclides. [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 Photonics 2021, 15, 393-400; [7] Nature Communications 2022, 13, 3504. [8] Nature Photonics 2023, https://doi.org/10.1038/s41566-023-01211-2.