Dipolar molecular rotors in fluorinated MOFs and CO2 capture

Within the ever-expanding field of porous materials, Metal-Organic Frameworks (MOFs) are well-known for their synthetic versatility and ability to capture greenhouse gases such as CO2. MOFs have also been shown to be able to support extensive dynamics without disrupting their primary architecture. Lately, MOFs were successfully employed to insert bicyclopentane-based rotors in the frameworks as ligands bridging the metal ions or cluster nodes, demonstrating extremely fast dynamics down to 2 K. [1]. The next step of the study would be to impose or modulate the properties of the material by controlling the rotation of said ligands inside the framework. [2] MOFs containing dipolar molecular rotors as linkers are emerging as an attractive class of compounds for their ability to interact with static or oscillating electric fields. Therefore, we built a bicyclopentane dicarboxylate-based (FTR) aluminum MOF (Al-FTR) and its dipolar, fluorinated analogue (Al-FTR-F2) (Figure 1, A), explored the rotor dynamics and CO2 capture ability. [3] To solve the crystal structures, we considered the arrangements of the CF2 dipoles as determined by the combination of molecular mechanics and PW-DFT in tandem with Rietveld refinement. Computationally-informed structural analysis of the two MOFs reveals very fast dynamics down to temperatures as low as 4 K, reminiscent of liquid state mobility, with strong agreements between 1H T1 spin-lattice NMR relaxation times and computational analysis. Notably, the mobility in Al-FTR-F2, especially at those extreme temperatures, is due to a concerted rotation of dipoles prompted by the interactions among nearest-neighbor rotors, interconverting into each other thanks to a cascade mechanism (Figure 1, B). The subsequent analysis of the CO2-filled Al-FTR-F2 reveals that gas filling can induce a coherence switch of the dipolar rotors, drastically impacting the overall configurational landscape (Figure 1, C). Similar compounds endowed with such controllable motional phenomena may find application in sensing or switching, the translation of light irradiation into movement and the control of solid-state dynamics with electrical fields minimizing energy dissipation. [1] J. Perego et al. Nature Chem. 2020, 12, 845. [2] J. Perego et a. J. Am. Chem. Soc. 2021, 143, 13082. [3] J. Perego et al. Angew. Chem. Int. Ed. 2023, 62, e202215893.