Molecular-scale Porous Materials

Arrangement of Molecules

The pillar-shaped structures of pillar[n]arenes are suitable for surface modification and formation of one-dimensional (1D) channels. The aromatic ring-rich and regular polygonal structure of pillar[n]arenes makes them ideal building elements of 2D assemblies. In particular, the hexagonal symmetry of pillar[6]arenes can provide long-range ordered hexagonal 2D sheet assemblies owing to the dense packing between adjacent pillar structures. Our group constructed 3D vesicle assemblies by co-assembly of pentagonal pillar[5]arenes and hexagonal pillar[6]arenes. Pillar[6]arenes assemble to form 2D sheet structures because of their highly symmetrical hexagonal structures. However, as the ratio of low-symmetry pentagonal pillar[5]arenes in the system was increased, the 2D sheets converted to 3D vesicle assemblies. A ratio of pillar[5]arenes to pillar[6]arenes of 12∶20 provided uniform vesicles with smooth surfaces. A co-assembled structure with a 12∶20 ratio of five- to six-membered rings is also observed in fullerene. The five-membered rings in fullerene provide the curvature required to form its spherical structure. Thus, pentagonal pillar[5]arenes provided curvature to form a vesicle structure.

Separation of Molecules

The separation of alkane isomers is still a difficult task even though various separation and purification methods and molecular recognition technologies have been developed. However, there is a great demand for separating alkanes. For example, the branched alkanes in the 5–7 carbon number range are major components in high-octane gasoline because they have high research octane numbers (RONs). Quality of gasoline is evaluated in the RON, which is the volume ratio of isooctane in a mixture of isooctane and n-heptane in gasoline. However, the boiling point of isooctane (b.p.: 99 oC) is almost the same as that of n-heptane (b.p.: 98 oC), so the separation of these two alkanes via petroleum distillation is very difficult. We discovered that pillar[6]arene crystals selectively took up branched alkanes into the cavity even in the crystal state. Utilizing this feature, when pillar[6]arene crystals were exposed to a mixture of linear and branched alkanes, the crystals selectively took up branched alkanes, resulting in a significant improvement in gasoline quality. It is easy to improve the quality of gasoline and is expected to contribute to solving energy and environmental problems.

Detection of Molecules

Based on the excellent alkane shape recognition ability and functionality of pillar[n]arenes, the shape of alkane gas could be detected by color change of pillar[5]arene crystals. By using the molecular detection system, we can detect the shapes of alkane gas in the alkane gas mixture and thus use for sensor to evaluate gasoline purity.。