Nanochannels are becoming an attractive area of research. They are present in nature in biological membranes and minerals. Some can be created synthetically with inorganic structures and organic crystals. Materials containing nanochannels of 0.5 up to 4 nanometer were prepared and characterized by us1. The unusual environment can be filled up by gas atoms, light molecules or polymer chains, which stay isolated one from another. A molecular crystal containing a nanoscale honeycomb formed by 0.5 nm empty channels was described for the first time and defined as a molecular zeolite2,3. Unlike most zeolites the channels are axially symmetric, without cavities or side channels and form an ideal straight tube surrounded by aromatic carbons. Attempts were reported in the literature to build up similar structure by organic components, but no such compounds were obtained by molecular components held together by weak interactions
Gas atoms, like xenon, can diffuse into the porous material as single atoms running along the channels and creating a one-dimensional gas. Single isolated atoms and groups of two or three of them can be there observed by 129-Xe nuclear magnetic resonance (both plain and optically pumped). The atoms fit a single site in a row of adjacent sites, which are occupied for a short time, resembling a nanoscale abacus. The gas atoms show anisotropic deformation due to the confinement. Single-direction collisions to the next atoms and to the channel walls cause xenon atoms to squeeze. The results are related to supermolecular crystal engeneering and to the most recent achievements about electron-rich aromatic carbon nanotubes.
References
1) M. Farina, G. Di Silvestro, P. Sozzani. PHTP a D3 symmetry host in Comprehensive Supermolecular Chemistry editor Mac Nicol et al.,Vol.6 (1996).
2) Comotti, A.; Gallazzi M.C., Simonutti, R; Sozzani, P., "13C and 31P Magic-Angle spinning Nuclear magnettic resonance Spectroscopy of Tris(2,3-naphtalenedioxy)cyclotriphosphazene Inclusion Compounds" Chemistry of Materials 10,3589 (1998).
3) Comotti, A.; Simonutti, R; Sozzani, P., Magic angle spinning nuclear magnetic resonance of TPP and TNP nanotubes. Nanotechnology 1 (1999).