This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Crystals are usually imagined as rigid objects with flat faces and straight edges. A research team led by the University of Osaka has recently created a porous crystal that grows into a spring-like helix.
The team also found that the crystal can change shape as solvent molecules move in and out of its internal spaces. Their findings are published in Angewandte Chemie International Edition. Helical or twisted crystals are known in some organic materials, but in porous materials, they have mostly been reported as very small crystals, bundles or aggregates.
This has made it difficult to understand how a crystal's internal arrangement is connected to its overall shape. The team used an organic molecule based on pyrene and changed the crystallization conditions. At 60°C, the material formed straight, needle-like crystals.
At 120°C, when the solvent evaporated more quickly, it formed helical crystals. X-ray measurements indicated that both forms have essentially the same porous internal structure. The researchers propose that, during rapid crystal growth, small irregularities in the molecular arrangement may become trapped and cause uneven growth, producing the helical shape.
This mechanism remains a hypothesis: Direct observation during crystal growth is still needed, and other effects, such as solvent effects and convection in the solution, have not been ruled out. The helical crystals have a measured surface area of 1,185 m² per gram, reflecting their high porosity, and retain their porous structure even when heated above 300°C. When solvent molecules were removed from the pores, the helices loosened and extended; when solvent was reintroduced, they contracted slightly.
This combination of porosity and shape change could inspire microscale materials that move in response to chemicals or materials that arrange molecules and ions along helical pathways. "This finding grew out of careful observation by Yuzuki Murata, a graduate student in my laboratory," says corresponding author Ichiro Hisaki. "It overturned our assumption that crystals must be rigid and straight.
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