Researchers are investigating how a specific chemical can selectively capture certain molecules within its structure, even though it appears to lack cavities under normal conditions. This groundbreaking material, characterized by its disappearing and reappearing holes, has the potential to significantly improve the efficiency of chemical separation and capture across various industries.
In some instances, the holes, or pores, that form within the molecular structure of a chemical emerge only when subjected to specific conditions or when interacting with certain ‘guest’ molecules. This phenomenon plays an essential role in the field of separation, a critical industrial process, yet researchers are still in the early stages of understanding it.
Researchers have investigated how a specific chemical has the ability to selectively trap certain molecules in its structural cavities, despite the absence of such cavities under typical conditions. This unique material, which features holes that can appear and disappear, could pave the way for more efficient techniques for separating and capturing chemicals in various industries.
The study detailing these researchers’ discoveries was published in Nature Communications on September 27.
While the idea of separating one substance from another might seem straightforward, separation techniques are vital across the economy. Most natural and synthetic materials start off impure. Whether it’s extracting metal ores from undesired rocks in mining, distinguishing recyclable materials, delivering drugs, treating the environment, or storing gases, separation is fundamental to modern industry, and researchers are continually looking for improved methods.
Recently, there has been growing interest in creating synthetic materials with pores—tiny openings that are part of the molecules themselves. These pores have specific sizes, shapes, and chemical properties, allowing only certain compounds that fit these characteristics to pass through. Imagine the classic toddler toy where variously shaped pegs fit only into their matching holes; in this case, the fit is determined by various attributes beyond just shape, which allows specific pores to preferentially allow some substances over others—what chemists refer to as the “selectivity” of “molecular encapsulation,” often shortened to selective encapsulation.
Some synthetic porous materials that interest chemists in this selective encapsulation realm include terms such as metal-organic frameworks, covalent organic frameworks, hydrogen-bonded organic frameworks, and zeolites. However, one material has particularly caught researchers’ attention: macrocyclic molecular crystals. These are solid structures made up of large molecules that consist of many atoms, typically including elements like carbon, nitrogen, or oxygen, arranged in a ring configuration. The interior of this ring generally creates a cavity or pore that certain substances can “fit” into.
Moreover, there are specific macrocyclic molecular crystals where the pores only become evident under certain conditions, such as exposure to heat, pressure, or the presence of other “guest” molecules. At other times, these pores do not exist. This variable presence is termed a “latent pore.”
“By designing materials with latent pores, we could enable systems that respond dynamically to changes in their environments, thus enhancing their functionality and selectivity,” said Takeharu Haino, the lead author of the study and a materials scientist at the Graduate School of Advanced Science and Engineering at Hiroshima University. “The challenge has been that until now, we’ve had limited understanding of why this latency occurs.”
To delve deeper into this subject, the Hiroshima researchers focused on a specific type of macrocyclic molecular crystal with latent pores known as planar tris(phenylisoxazolyl)benzene. This material features a benzene ring at its core and is characterized as planar due to its thin, sheet-like structure. They selected it for study because, unlike other candidates that involve much larger molecules, planar tris(phenylisoxazolyl)benzene is simpler and has been utilized in creating organic semiconductors, light-emitting diodes (LEDs), and other established industrial applications.
The team aimed to explore the latent pores of this substance to separate two distinct forms of decalin, which is also known as decahydronaphthalene. This colorless liquid, used frequently as a solvent and in manufacturing various resins and polymers, exists in two structural forms. Cis-decalin has its hydrogen and carbon atom groups on the same side, while trans-decalin has them positioned on opposite sides. This structural difference alters their physical and chemical properties, making them excellent candidates for studying selective encapsulation.
Using two types of x-ray diffraction techniques, they examined the encapsulation process. By directing x-rays at the material, researchers could determine the arrangement of atoms based on how the rays were diffracted.
The findings revealed that planar tris(phenylisoxazolyl)benzene is highly effective at selecting one form of decalin over the other, achieving this correct encapsulation 96 times out of 100. Additionally, they identified that the intermolecular forces—between molecules that are strong but not atomic—contributed to the pore’s stability and the material’s impressive selectivity. While other materials may exhibit porosity and selectivity, they often lack the stability needed for industrial use. This substance successfully meets all the criteria for selective encapsulation.
This proof of concept could be applicable in diverse areas such as gas trapping, oil separation, and the extraction of trace elements from water. However, the researchers aspire to discover unique encapsulation capabilities that can only be achieved with latent pores.
There is still much to explore in understanding the “supramolecular” chemistry behind latent pores, and the researchers believe they are just beginning to map this exciting territory.
