If you’ve ever built complex structures with Lego bricks, you probably started with laying the foundation and building the framework before introducing anything fancy. Otherwise, the structure might become unstable or require significant rearrangements during construction—easy enough to fix for toys, but not so much for molecular building blocks.
This is a major challenge in assembling covalent organic frameworks (COFs). In their sponge-like crystal structure, COFs can be modified to hold metal ions as tiny reaction centres, offering a home for light-driven reactions used in chemical manufacturing, energy technologies and advanced electronics. But these chemical houses are often built first and only modified later.
“It’s similar to building a house first then trying to install important support beams later. It can work, but could also lead to incomplete or damaged structures,” explained Le Yang, a Principal Scientist at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE). These post-synthetic modifications often reduce the framework’s structural order, making it harder to control where metal ions end up.
Alternative strategies try to assemble everything at once. However, these one-pot reactions require precise coordination and often involve irreversible chemical bond formation. “Once a bond forms incorrectly, it cannot easily break and correct itself, like bricks lodged into the wrong position during construction,” said Jun Zhu, a Scientist at A*STAR IMRE.
Seeking a more orderly manufacturing process, Yang, Zhu and the A*STAR IMRE team, together with researchers from the National University of Singapore, turned to a classic chemical reaction called the Friedländer annulation.
During the reaction, molecular building blocks are locked together into stable ring-shaped structures, while nickel ions are simultaneously captured by the metal binding sites inside the COF. The fused rings make the overall framework more chemically robust, with the added nickel also playing a part in guiding the structure into a more packed, ordered arrangement.
Each part is also crucial for enabling the photocatalytic performance. “The organic framework absorbs light and moves charges, while the nickel ions activate molecules and help form new chemical bonds,” said Zhu. “This integrated design drives efficient reactions without the need for external chemical photosensitisers to help with light absorption.”
Using visible light under mild conditions, the team’s nickel-chelated frameworks powered carbon-chalcogen bond formation, an important process in the production of pharmaceuticals, dyes and organic electronics. The catalysts also remained stable and reusable over multiple cycles.
“Our work provides a platform for designing COFs where light absorption, structure and catalytic activity can be tuned together,” said Yang. By combining high performance, stability and recyclability, the team hopes their design strategy can be expanded to other reaction types and materials, leading to more practical and sustainable chemical manufacturing approaches.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
