Rotor dynamics and light-driven motors in nanoporous architectures by solid state nmr

Molecular rotors, motors and photo-switches in the solid state find a favourable playground in nanoporous materials, such as MOFs and POPs, thanks to their large free volume. In this field a particularly prominent application of solid state NMR is the study of dynamic processes in solids and in the gas phase. We have realized a ultra-fast molecular rotor based on bicyclo[1.1.1]pentane–dicarboxylate moiety, assembled in a cubic Zn-MOF structure. The rotors experience fast molecular reorientation even at temperature below 2 K, with a negligible activation barrier as low as 6 cal mol-1, resulting from the symmetry mismatch between the tri-fold geometry of the rotor and the four-fold symmetry imposed by the frameworks [1]. Two distinct ultrafast and interacting molecular rotors can be arranged in pillar-and-layer 3D arrays MOF, wherein the rotors undergo sequential motional behaviour activated at distinct temperatures, as supported by 2H solid-echo, T1(1H) relaxation NMR and DFT modelling [2]. Attractive functional properties, such as dielectric switchable property, can be activated by incorporating fast-reorientable dipoles onto molecular rotors. Fluorinated Al-MOFs, comprising a wheel-shaped ligand with geminal rotating fluorine atoms, produced a benchmark dynamics of correlated dipolar rotors. Gas accessibility, shown by hyperpolarized 129Xe NMR, allowed for chemical stimuli intervention: CO2 triggered dipole reorientation, reducing their collective dynamics and stimulating a dipole configuration change in the crystal [3]. CO2 diffusion in a porous crystalline material, in which the channels are decorated by double helices of electrostatic charges, has been described by 2D 1H-13C HETCOR MAS NMR experiments and by anisotropic line-shape analysis of 13C, providing peculiar details about the role of electrostatic interactions in gas transport phenomena [4]. Moreover, the insertion of photo-responsive molecular motors with active dynamical behavior allowed the generation of POPs with on command responsive properties. Upon light irradiation, the quantitative molecular photoisomerization of an overcrowded alkene photoswitch has been demonstrated by 13C solid state NMR [5]. The sophisticated engineering of chromophores integrated in MOF architectures enabled the fine-tuning of absorption-emission properties. Specifically, we achieved the formation of mixed-ligand photonic nanocrystals, which integrate diphenyl-anthracene moieties, thus realizing fast scintillation and large Stokes shift materials [6]. The accessibility of the pores to the gas phase under a flow of a noble gas such as Xe was demonstrated by hyperpolarized 129Xe NMR and exploited to obtain visible emission stimulated by radioactive gas selective adsorption [7, 8]. References [1] J. Perego, S. Bracco, M. Negroni, C. X. Bezuidenhout, et al. Nat. Chem.,12, 845-851 (2020) [2] J. Perego, C. X. Bezuidenhout, S. Bracco, G. Prando, et. al. J. Am. Chem. Soc., 143, 13082-13090 (2021) [3] J. Perego, C. X. Bezuidenhout, S. Bracco, S. Piva, et al. Angew. Chem. Int. Ed., 62, e202215893, (2023) [4] G. Xing, I. Bassanetti, S. Bracco, M. Negroni, et al. Chem. Sci., 10, 730-736 (2019) [5] F. Castiglioni, W. Danowski, J. Perego, F.K-C. Leung, et al. Nat. Chem., 12, 595-602 (2020) [6] J. Perego, C. X. Bezuidenhout, I. Villa, F. Cova, et al. Nat. Commun., 13, 3504 (2022) [7] S. Mauree, V. Villemot, M. Hamel, B. Sabot, et al. Adv. Funct. Mater. 2302877 (2023) [8] Orfano, M., Perego, J., Cova, F. et al. Nat. Photon. (2023). https://doi.org/10.1038/s41566-023-01211-2