Boroxine Nanotubes: Moisture-Sensitive Morphological Transformation and Guest Release

Boroxines, (R-BO)(3), which can be easily synthesized via a dehydration reaction of boronic acids, R-B(OH)(2), selectively self-assemble in toluene into nanofibers, nanorods, nanotapes, and nanotubes, depending on the aromatic substituent (R). Spectroscopic measurements show that the nanotube consists of a J-aggregate of the boroxine. Humidification converts the morphology from the nanotube to a sheet as a result of the hydrolysis of the boroxine components and subsequent molecular-packing rearrangement from the J-aggregate to an H-aggregate. Such a transformation leads to the compulsive release of guest molecules encapsulated in the hollow cylinder of the nanotube. The hydrolysis and the molecular-packing rearrangement described above are suppressed by coordination of pyridine to the boron atom, with the resulting moiety acting as a Lewis acid of the boroxine component. The pyridine-coordinated nanotube is transformed into a helical coil by humidification. Guest release during the nanotube-to-helical-coil transformation is much slower than during the nanotube-to-sheet transformation, but faster than from a nanotube that did not undergo morphological transformation. The storage and release of guest molecules from the boroxine nanotubes can be precisely controlled by adjusting the moisture level and the concentration of Lewis bases, such as amines.