Characterization of Metal-Organic Frameworks Containing Molecular Rotors by Solid-State NMR

The design and fabrication of innovative materials containing fast-rotating elements with low activation energies, such as molecular rotors, that can convert external chemical or physical stimuli into actions is becoming extremely attractive. In this context, Metal-organic frameworks (MOFs) present a perfect playground for the integration of rotors in a controlled structural and chemical environment that allows tuning the interactions with the neighbouring moieties. [1] We successfully constructed a new pillared MOF named FTR-P2, comprising 2D layers of byciclopentane dicarboxylate molecular rotors (BCP) and Zn metal nodes pillared by azobypyridyl ligands to generate a 3D network.[2] The structure consists of two centred interpenetrated frameworks that undergo reciprocal sliding along the pillar direction when guests are in the pores. Solid-state NMR (ssNMR) is the technique of choice for studying the connectivity and dynamics of the molecular moieties comprising the material. Multi-nuclear high-resolution ssNMR techniques demonstrated the purity and symmetry of the crystalline structures and were used to monitor the structural changes induced by temperature variation and adsorption of bulky iodine molecules. In fact, the structure rearranges into a new off-centre motif to fit the guest molecules in the pores (FTR-P2-I2), thus breaking the perfect symmetry of the pyridyl rings and generating a multiplicity of signals (Fig. 1. A). 13C and 1H T1 NMR relaxation times were collected at variable temperatures to study the BCP molecular rotors’ dynamics (Fig. 1. B). The results demonstrated hyper-mobility even at extremely low temperatures, with a maximum relaxation rate (33.8 MHz) for FTR-P2 at 85 K. Interestingly, the structural changes induced by iodine molecules trigger an increase in BCP molecules' cooperativity, enabling even faster mobility with a shift of the maximum to as low as 44 K. [1] J. Perego et al. J. Am. Chem. Soc. 2021, 143, 13082-13090 [2] J. Perego et al. Angew. Chem. Int. ed. 2024, e202317094