Molecular containers go supersized

For chemists hungry to load up on larger items from the molecular buffet, help is at hand—in the form of an enormous bowl.

The commodious structure is a cavitand, a type of hollow molecular container that has an opening on only one side. Cavitands are used to capture or carry guest molecules in applications such as sensing, environmental clean-up, or catalysis.

A team led by Konrad Tiefenbacher at the University of Basel has now produced the largest cavitand to date. With an internal volume of 814 ų, it’s about twice as large as the previous record holders (Angew. Chem., Int. Ed. 2022, DOI: 10.1002/anie.202209885).

A cavitand’s volume depends on the diameter of its base and the height of its walls. Many cavitands use a macrocycle called resorcin[4]arene as their base, which is about 7 Å across. Instead, Tiefenbacher’s team created acridane[4]arenes that provide a 14 Å-wide base for their supersized cavitands. “It’s really a much larger copy of resorcin[4]arene, which has been used in thousands of publications,” Tiefenbacher says.

The acridane[4]arenes, made in a simple six-step synthesis, feature eight hydroxy groups that serve as anchor points for various wall molecules. “They’ve built this modular platform that you can actually put different units on top of, to start exploring the encapsulation of bigger molecules,” explains Jovica Badjic, a cavitand researcher at the Ohio State University, who was not involved in the research. “It’s absolutely brilliant.”

The largest of Tiefenbacher’s cavitands features high walls formed from bulky triptycene-quinoxaline groups. It is big enough to bind C₆₀ and C₇₀ fullerene molecules, and it can preferentially trap C₇₀ from a mixture of the two fullerenes, probably because the larger molecule fits better into the host’s cavity. “The selectivity is extremely high, which is rather unusual,” says Enrico Dalcanale of the University of Parma, who was involved in creating what was previously one of the largest cavitands. “Distinguishing between C₆₀ and C₇₀ in this way is very nice.”

This separation is a proof of principle, however; Tiefenbacher doesn’t expect his cavitands to replace chromatography as the routine way to purify fullerenes. Instead, he is interested in using the new acridane[4]arenes to construct water-soluble cavitands that can bind and deliver insoluble drug molecules. “And if you construct even larger molecular containers, we could think about binding biomolecules” such as proteins or messenger RNA, he adds.

The researchers are now installing different walls onto their acridane[4]arene bases and also hope to create wider macrocycles that will form the foundations of even larger cavitands.