In principle, polymerization tends to produce amorphous or poorly crystalline materials. Efficiently producing high-quality single crystals by polymerization in solvent remains as an unsolved issue in chemistry, especially for covalent organic frameworks (COFs) with highly complex structures. To produce μm-sized single crystals, the growth time is prolonged to >15 days, far away from the requirements in practical applications. Here, we find supercritical CO₂ (sc-CO₂) accelerates single-crystal polymerization by 10,000,000 folds, and produces two-dimensional (2D) COF single crystals with size up to 0.2 mm within 2~5 min. Although it is the fastest single-crystal polymerization, the growth in sc-CO₂ leads to not only the largest crystal size of 2D COFs, but also higher quality with improved photoconductivity performance. This work overcomes traditional concept on low efficiency of single-crystal polymerization, and holds great promise for future applications owing to its efficiency, industrial compatibility, environmental friendliness and universality for different crystalline structures and linkage bonds.

Fig. Schematic illustration of polymerization in organic solvent or sc-CO₂. In sc-CO₂, the ultra-fast diffusion of monomers, oligomers, and byproducts in medium and micro-pores accelerates polymerization as well as error-checking and proof-reading, while the suitable solubility accelerates vertically epitaxial growth, resulting in minutes-growth of single crystals.

This work solves the problem and finds that the high quality and single-crystalline nature of sc-COFs give rise to highly polarized PL, showing that the 2D COF single crystals produced in sc-CO₂ have great potential in special application fields which previous polycrystalline samples haven’t. It is worth noting that there is a general belief that slow crystallization is necessary to grow high-quality single crystals1. In contrary, this result indicates ultra-fast polymerization in sc-CO₂ can produce single crystals with both larger size and higher quality, compared with slow crystallization, overcoming the contradiction between growth time and product quality. This work provides a new understanding of ultra-fast single-crystal polymerization in supercritical liquid. Besides boronate ester and imine 2D COFs, it also holds great promise in the efficient and precise construction of various COFs and other covalent crystalline materials, opening up other opportunities for not only fundamental research but also practical application of these materials and their single crystals.

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links：https://www.nature.com/articles/s41467-021-24842-x