Bismuth (shown here) is the heaviest stable element, and it's in the pnictogen group of elements. An international team of researchers has synthesized stable halogen-bonded cocrystals using heavy pnictogens, excluding lighter elements and making the crystals water-resistant. These cocrystals may be useful in formulating drug delivery systems or waterproof devices.

‘It Is a Very Exciting Time to Be a Chemist’

Cocrystals that can connect heavy elements are pushing chemistry into new territory, which may lead to innovations like waterproof devices.


Waterproof devices might be one step closer to reality thanks to new cocrystals that use halogen bonding to form strong and stable attraction between heavier elements like phosphorus, arsenic, and antimony — these are the pnictogens: increasingly heavy elements that fall into the 15th column in the periodic table.

Until now, similar interactions have always required at least one atom of a lighter element like hydrogen, nitrogen, oxygen, or fluorine. By excluding these lighter elements, the halogen-bonded cocrystals have difficulty engaging in hydrogen bonding, and are therefore naturally resistant to water.

Led by researchers at the University of Zagreb, McGill University, and the University of Birmingham, the study was published in Nature Communications.

Halogen-bonded cocrystals stretch the limits of chemistry

Cocrystals are crystalline materials that combine two or more components. For instance, these could be a drug and a coformer that helps give it better properties, like improved solubility, stability, or mechanical strength.

The pnictogen cocrystals rely on halogen bonding, which is emerging as an important force in designing functional materials that can be used in applications like drug delivery and electronics. The components recognize each other and self-organize into complex structures.

Cocrystals made exclusively from heavier elements are important because they stretch the limits of chemistry into uncharted territory, enabling new properties and patterns of molecular assembly.

“Quite apart from the potentially practical applications of this discovery, it is a big advance in fundamental chemistry,” said co-author Tomislav Friščić in a statement.

“For the first time, researchers have demonstrated molecular recognition events including only heavier elements located in the 4th and 5th periods (rows). This is significantly deeper in the periodic table than has been seen until now. It is a very exciting time to be a chemist — it’s as though we were explorers moving closer to the South Pole of the periodic table — and who knows what we will find there.”

The team already has plans to dig even deeper into the periodic table, attempting to incorporate the bismuth — another pnictogen and the heaviest stable element — into a cocrystal. The previously unexplored architectures that can be built using heavier elements expand the toolkit for designing materials for all kinds of applications.

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Karyn Ho is a science animator and engineer who thrives at the interface between science, engineering, medicine, and art. She earned her MScBMC (biomedical communications) and PhD (chemical engineering and biomedical engineering) at the University of Toronto. Karyn is passionate about using cutting edge discoveries to create dynamic stories as a way of supporting innovation, collaboration, education, and informed decision making. By translating knowledge into narratives, her vision is to captivate people, spark their curiosity, and motivate them to share what they learned.