A research team synthesized and determined the structure of a borate-linked 3D crystalline covalent organic framework, TCTP-COF, via electron diffraction for the first time. These findings will help scientists determine the structure-property relationships for other 3D COFs and facilitate their tuning for advanced applications.
Overcoming many of society's biggest challenges requires innovations in materials chemistry to achieve specific goals: carbon sequestration to mitigate climate change, absorbing toxic chemicals for environmental remediation, and delivering life-saving compounds to cure disease, for example.
One type of compound, called three-dimensional covalent organic frameworks (3D COFs), shows promise in all of these potential applications. As a class of synthetic, highly ordered, porous crystalline polymers, specific 3D COFs can also be used as battery electrodes and as catalysts to speed chemical reactions, making them the subject of intense research and development.
Synthesizing highly ordered 3D COFs in a reliable manner remains challenging. To date, methods used to produce COFs often result in amorphous or poorly crystalline solids. This occurs because strong, directional covalent bonds form too rapidly, forcing the material into a disordered network instead of an ordered, thermodynamically stable crystal lattice. Because of this, the atomic-level structures have been achieved for only a limited number of 3D COFs, hindering researchers' understanding of each 3D COF structure and its properties.
To address this issue, a group of researchers from the National Institute of Natural Sciences (NINS), Osaka University, Nagoya University, SOKENDAI and the Comprehensive Research Organization for Science and Society in Japan recently synthesized and determined the structure of a spiroborate-linked 3D crystalline COF using microcrystal electron diffraction (microED) methods for the first time, providing a new design strategy for expanding the synthesis and implementation of highly ordered 3D COF architectures.
The team published their article on July 10 in the journal Science Advances.
"In this study, we focused on borate anions as a new linkage motif for constructing 3D crystalline COFs. Borates are known to form tetracoordinate spiro-type structures that are rigid and stable. A previous report of crystalline one-dimensional polymers featuring borate-based linkages further suggests that this motif could be extended to the construction of highly crystalline 3D COFs. Although several 3D COFs using borate linkages have been reported, it has not yet been possible to conduct a single-crystal structural analysis on these COFs," said Yasutomo Segawa, associate professor in the Institute for Molecular Science and The Graduate University for Advanced Studies, SOKENDAI in Okazaki, Japan and senior author of the research paper.
Currently, most crystalline COFs rely on imine linkages, or carbon-nitrogen double bonds, which limit their structural diversity. Exploring new modes of bond formation for crystalline COF synthesis, such as the borate ion linkage motif used by the research team, allows scientists to expand the variety of COFs synthesized and further uncover novel structure-function relationships.
Borate anions contain the elements borate and oxygen, are negatively charged, and are commonly used as building blocks in materials science. In the current study, the tetracoordinate spiro-type structures created by the researchers are formed with a central borate anion to which four tetracyclopentatetraphenylene (TCTP) molecules are bound, forming a tetrahedron with a unique 3D shape that can be tuned for different applications.
The research team successfully synthesized a 3D COF with nbo topology, which refers to a highly symmetrical 3D network structure that mimics the crystal lattice of niobium monoxide (NbO), an inorganic compound. These structures are known for their permanent porosity, high thermal stability, and large, accessible pores. In this study, the 3D COF with NbO topology the team synthesized, TCTP-COF, is a highly porous material with a crystalline structure and an open lattice. "In this study, we selected TCTP to achieve both a square-planar structure and a monomer unit with sufficient solubility," said Segawa.
Importantly, the research team was able to synthesize TCTP-TOF and also determine its structure. "This study not only reports the first structural elucidation of a borate-linked 3D crystalline COF via electron diffraction but also demonstrates the potential of hetero[8]circulene analogues (flat, ring-shaped synthetic molecules) as robust building blocks for complex 3D frameworks. These findings pave the way for both the precise construction of functional ionic COFs and the exploration of their structure-property relationships to enable their use in advanced applications," said Segawa.
Soshi Hirota from the Institute for Molecular Science and The Graduate University for Advanced Studies, SOKENDAI in Okazaki, Japan; Haruki Sugiyama from the Institute for Molecular Science, The Graduate University for Advanced Studies, SOKENDAI, and the Neutron Industrial Application Promotion Center at the Comprehensive Research Organization for Science and Society in Ibaraki, Japan; Nao Hirata and Sachiko Nakano from the Institute for Molecular Science; Junichi Usuba and Yuh Hijikata from the Research Center for Net Zero Carbon Society in the Institute of Innovation for Future Society at Nagoya University in Nagoya, Japan; Ryotaro Matsuda from the Research Center for Net Zero Carbon Society in the Institute of Innovation for Future Society at Nagoya University and the Department of Chemistry and Biotechnology, School of Engineering, and Department of Materials Chemistry in the Graduate School of Engineering at Nagoya University; Takanori Nakane and Akihiro Kawamoto from the Institute for Protein Research and the JEOL YOKOGUSHI Research Alliance Laboratories in the Graduate School of Frontier Biosciences at Osaka University in Osaka, Japan; and Genji Kurisu from the Institute for Protein Research, the JEOL YOKOGUSHI Research Alliance Laboratories in the Graduate School of Frontier Biosciences, the Department of Macromolecular Science in the Graduate School of Science, and the Institute for Open and Transdisciplinary Research at Osaka University also contributed to this research.
This research was supported by the FOREST program (JPMJFR211R) from JST, JSPS KAKENHI (JP25K01758, JP22K14660 and JP25H01265), the Murata Science and Education Foundation, the Iketani Science and Technology Foundation, the Foundation of public interest of Tatematsu, and the Yazaki Foundation.
