Covalent organic frameworks (COFs) are new materials that were first reported in 2005, and research on these has advanced in recent years. COFs are crystalline materials composed of organic molecules connected by covalent bonds in two-dimensional or three-dimensional structures, and they have numerous fine pores. Their applications are studied in various fields, such as for catalysts, battery materials, and gas adsorption. If nanotube structures could be formed with COFs, unique properties arising from one-dimensionality would be expected to emerge, but examples of achieving such structures have been extremely limited.
A research team led by Professor Shu Seki of the Graduate School of Engineering at Kyoto University has now discovered that two-dimensional sheet-like COFs based on diazapyrene—an organic molecule created by substituting two carbon atoms in pyrene (C16H10) with nitrogen atoms—spontaneously roll up at the interface between two liquids to form one-dimensional nanotube structures. Furthermore, they confirmed that when ultrasonic waves are applied to the liquid containing these dispersed nanotubes, the rolled-up nanotube structures return to their original sheet structures. The team also evaluated the proton (hydrogen ion) conduction properties of these nanotubes and demonstrated that they exhibit world-class conductivity.
The research team had previously worked on forming COFs with near-planar structures using diazapyrene. In this research, they discovered that when synthesizing COFs, changing the ratio of solvents used and the temperature resulted in structures that are decisively different from sheets. What might have been overlooked as a "failed COF synthesis" was carefully tracked through detailed analysis of the synthesized material's ultrafine structure, revealing that it had become one-dimensional nanotubes. These nanotubes possess high stability due to covalent bonding while simultaneously exhibiting extremely high electron and proton conductivity, making them promising for various applications, including for membranes and clean energy materials.
(TEXT: Masanori Nakajo)
