Amphipathic and amphidynamic crystalline materials: an XRD and MAS NMR study

Molecular self-assembled crystalline materials are promising in several fields, including gas storage, selective recognition and modulation of functions of active molecules. The tandem X-ray diffraction and solid state NMR approach allowed us to study amphipathic or amphidynamic materials i.e. crystalline structures that exhibit an intrinsic duality within the same periodic architecture [1-4]. In particular, we realized self-assembled crystalline architectures with guest molecules compartimentalized in two amphipathic nanospaces with distinct geometries and polarities [5]. The effect of these distinct environments on the NMR properties of the guest molecules is evident from chemical shift data and 2D heterocorrelated NMR techniques that could discriminate identical guest molecules embedded in distinct structural environments - one highly polar and the other nonpolar. The large magnetic susceptibility effect, due to ring currents of the aromatic host, enabled the determination of the host-guest distances and corroborated the variable-temperature crystal structure resolution. A dual behavior was also highlighted in a block copolymer. The molecular recognition of specific blocks of triblock copolymers by a host molecule led to the formation of hierarchical periodic structures [2]. The end blocks of the triblock copolymer were locked into the inclusion crystals whilst the central block was excluded, creating a new material of assembled nanocrystals regularly superimposed on one another. The formation of the supramolecular architectures was followed in situ synchrotron X-ray diffraction while fast-1H MAS NMR provided direct evidence of selective inclusion of the blocks. Notably, amphidynamic materials could be recognized in hybrid organic-inorganic crystalline materials [1]. The precise engineering of highly-organized porous materials containing organic elements pivoted on inorganic layers enabled the fabrication of fast molecular rotors entirely exposed to the guest molecules exploring the cavities. Powder Xray diffraction highlighted the crystalline order on both the meso- and molecular scales. Spin-echo deuterium NMR gave direct proof of the reorientation rate of the aligned rotors and demonstrated the active role of the guests in modulating the rotor dynamics.