Recognition of weak interactions in porous supramolecular architectures

Nanoporous molecular crystals are attracting large attention in the recent literature for their applications in the field of gas storage, selective recognition and molecular confinement. We could obtain empty-pore hexagonal crystalline structures held together by weakly directional interactions and fabricate supramolecular architectures that cooperatively stabilizes gases or polymers confined in one-dimensional spaces [1]. Multinuclear NMR of the solid and the gas phase is a method of choice for characterizing nanostructured materials, the size and the shape of the cavities, and their interfaces. The recognition of CH··· π specific interactions that contribute to the overall stabilization is demonstrated by the diamagnetic susceptibility effect of the host aromatic ring currents exerted on the guest molecules and by 2D advanced NMR techniques, which provide highly resolved spectra in hydrogen, carbon and phosphorous dimensions [1]. This multinuclear approach allows a detailed description of the role of weak interactions cooperating to fabricate nanostructured materials that exhibit exceptional thermal stability. The large upfield shift provided a tool for understanding the topology of gases or polymer chains interacting with the inner surface of the porous host [2]. For example, we have demonstrated that flexible polymer chains and suitable aromatic host self-assemble in varied ratios to fabricate crystalline and semi-crystalline materials with robust architectures. The high melting macromolecular adducts were successfully prepared by solvent-free mechanochemical or thermal treatment of the crystalline host and few polymers [3]. This unconventional strategy has been applied for the selective confinement of copolymer sequences in the aromatic host, creating novel architectures composed by alternated crystalline lamellae and amorphous layers. In this case the driving force for the fabrication of the nanostructured materials was based on the establishment of a network of cooperative noncovalent intermolecular interactions between the host and some sequences of the copolymer, while steric effects prevent the formation of the inclusion-crystal with the remaining blocks [4].