Engineering molecular rotors and switches in low-density architectures: from single-molecule motion to framework dynamics

The arrangement of dynamic building blocks, such as molecular rotors and switches, in close proximity to one another with precise geometry and topology, is realised in low-density framework materials, including Metal-Organic Frameworks (MOFs) and Porous Aromatic Frameworks (PAFs). We reported on the benchmark dynamics of the isolated di-carboxylate bicyclo[1.1.1]pentane (BCP) molecular rotor within a cubic zinc MOF.1 Indeed, 1H T1 relaxation NMR and muon-spin spectroscopy revealed hyperfast rotary motion in the gigahertz regime even at temperatures as low as 2 K, with a negligible energy barrier (Ea) of 6.2 cal·mol-1. Pillared-layer MOFs comprising bipyridine-based co-ligands generate 3D structures where the BCP rotors can interact with their neighbours.2 Indeed, these rotors navigate the rotational potential energy landscape to produce co-rotating pairs of rotors. These geared molecular rotors have very low energy barriers for rotation (24 cal·mol-1) owing to the synchronicity of their rotation. Fluorinated 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-1), promising innovative applications as electric-field-responsive porous materials.3 Light-responsive Porous Switchable Frameworks (PSFs) based on bistable chiroptical overcrowded alkenes exhibit high surface area (up to 3950 m²·g-1) and reversible bulk photoisomerization in the solid state. Notably, their porosity and gas sorption properties can be reversibly modulated in response to light and heat, mimicking a sponge-like behaviour.4 Spiropyranbased materials, created via in-situ solid-state grafting, maintain high porosity and reversibly switch to zwitterionic merocyanine under chemical or physical stimuli.5 These materials show promise for oncommand pH control, gas uptake/release, ion capture, and water harvesting. Orthogonal PSFs combining different molecular switches enable the selective activation of four states using distinct light wavelengths, paving the way for complex, responsive frameworks with emergent properties.6 REFERENCES 1. Perego, J. et al. "Fast motion of molecular rotors in metal–organic framework struts at very low temperatures." Nat. Chem. 2020, 12, 845-851. 2. Perego, J. et al. "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, 33, 13082-13090. 3. Perego, J. et al. " Benchmark Dynamics of Dipolar Molecular Rotors in Fluorinated Metal-Organic Frameworks." Angew. Chem. Int. Ed. 2023, 62, 5, e202215893. 4. Sheng, J. et al. " Construction of a three-state responsive framework from a bistable photoswitch." Chem 2023, 9, 2701-2716. 5. Sheng, J. et al. " Construction of Multi‐Stimuli Responsive Highly Porous Switchable Frameworks by In Situ Solid‐State Generation of Spiropyran Switches." Adv. Mater. 2024, 36, 2305783. 6. Sheng, J. et al. " Orthogonal photoswitching in a porous organic framework." Angew. Chem. Int. Ed. 2024, 63, e202404878.