Researchers have prepared flat, porous sheets of just carbon and hydrogen arranged in a hexagonal network. The 2D material is the first crystalline covalent organic framework (COF) with an all-carbon backbone (J. Am. Chem. Soc. 2026, DOI: 10.1021/jacs.5c22787). The COF exhibits photoluminescent properties that are far superior to those of its amorphous counterpart.
Growing crystalline COFs is more art than science. Much of the process depends on bonds between building blocks forming correctly—or breaking and reforming if they didn’t initially join up the right way. “This reversibility is essential in all crystallization,” says Seungkyu Lee, a nanomaterials scientist at the University of Hong Kong (HKU) and the senior author of the new study.
But carbon-carbon bonds are too strong and irreversible. “If misconnected, there is no way you can disassociate and correct the error,” Lee explains. Hydrocarbon COFs have been prepared previously in amorphous forms—configurations that lack long-range order. To make crystalline COFs, scientists had to include noncarbon atoms in the materials’ backbones to allow for reversible bonds, such as imine bonds.
Lee’s group pulled off the hydrocarbon synthesis using the classic olefin metathesis reaction but with an additional ingredient that helps reverse C–C bonds: a free-floating olefin in the solution. The extra olefin helps the metathesis catalyst break up malformed bonds in the growing COF, giving them a second chance to form correctly.
To test their idea, the researchers mixed a triangular hydrocarbon building block, a Grubbs metathesis catalyst, and the common olefin trans-stilbene at 60 °C. Their method not only produced crystals of the COF—dubbed HKU-50—but even converted amorphous HKU-50 to its crystalline form.
The crystalline version of HKU-50 is four times as luminescent as the amorphous form. Lee’s collaborators, David Phillips at HKU and Nak Cheon Jeong at Daegu Gyeongbuk Institute of Science and Technology, expect the 2D material to be useful for flexible electronics or chemical sensing. While they pursue new applications for HKU-50, Lee’s group plans to use the new synthetic strategy to prepare more hydrocarbon COFs, including 3D ones.
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