When is a hole not a hole? Researchers investigate the mystery of 'latent pores'

Researchers have explored how a particular chemical can selectively trap certain molecules in the cavities of its structure -- even though in normal conditions it has no such cavities. This innovative material with now-you-see-them-now-you-don't holes could lead to more efficient methods for separating and capturing chemicals right across industry.

A study describing the researchers' findings was published in Nature Communications on September 27.

Separation of one kind of substance from another may sound straightforward as a topic of scientific investigation, but separation techniques are essential right across the economy as most things found in nature or manmade start off impure. From metal ores that are attached to unwanted rock within the field of mining, to distinguishing one material to be recycled from another, to drug delivery, environmental remediation, and gas storage, separation is at the heart of industrial modernity, and researchers are always on the hunt for better ways to do it.

In recent years, there has been increased interest in the fabrication of synthetic materials with pores -- tiny holes -- within the molecules themselves. These pores have specific sizes, shapes and other chemical attributes wherein only certain compounds whose characteristics match the 'hole' can fit. Think of the toddler's classic hammer-and-bench toy, with square, circular, triangular and star-shaped wooden pegs that can each only fit in the correspondingly shaped hole in the bench. But in this case, fitting into a given pore depends on many more characteristics than just the shape of the toddler's peg, allowing certain pores to select for some substances over others -- what chemists call the "selectivity" of "molecular encapsulation," or just selective encapsulation.

These synthetic porous materials of interest to chemists specializing in selective encapsulation include such buzzwords as metal-organic frameworks, covalent organic frameworks, hydrogen-bonded organic frameworks, and zeolites. But recently, one material in particular has piqued the interest of these researchers: macrocyclic molecular crystals. These are solids formed from large molecules with a significant number of atoms, often including elements like carbon, nitrogen or oxygen, arranged in a ring. The interior of this ring -- in general -- forms the cavity or pore where only certain substances "fit."

On top of this, there are types of macrocyclic molecular crystals where the pore only appears in the presence of certain conditions such as heat or pressure or that of other, "guest" molecules. The rest of the time, there is no pore. This now-you-see-it-now-you-don't type of cavity is called a "latent pore."

"By designing materials with latent pores, we potentially can create systems that respond dynamically to environmental changes, enhancing their functionality and selectivity," said Takeharu Haino, the lead author of the study and a materials scientist with the Graduate School of Advanced Science and Engineering at Hiroshima University. "The trouble is: until now, we didn't always know why this latency was happening."

To investigate what was going on, the Hiroshima researchers opted to have a deeper look at a particular type of latent-pore-bearing macrocyclic molecular crystal: planar tris(phenylisoxazolyl)benzene. The circular shape at its heart in this case comes from a ring of benzene, and it is termed planar because it comes in thin, tabular lamination shapes. They chose this one to investigate because other options involve very large molecules, but planar tris(phenylisoxazolyl)benzene is a simple flat molecule. It also has already been used in development of organic semiconductors, light-emitting diodes (LEDs) and a number of other, proven, industrial applications.

They wanted to investigate the ability of the substance's latent pores to separate two different forms of decalin. Also known as decahydronaphthalene, it is a colourless liquid at room temperature that is often used as a solvent, as well as in the production of various resins and polymers.

It also comes in two different structures -- the same number of atoms, but arranged differently. There is cis-decalin, where a grouping of hydrogen and carbon atoms lies on the same side of the molecule, and also trans-decalin, where the hydrogen and the carbon atoms lie on opposite sides. This changes the decalin's physical and chemical properties and so makes the substance a good candidate for exploring selective encapsulation.

They used two types of x-ray diffraction analysis to explore the encapsulation process as it happened. In this form of investigation, x-rays are directed at the object of interest and the angles at which the rays are diffracted tell the researchers the object's arrangement of atoms.

What they found was that planar tris(phenylisoxazolyl)benzene is a superb selector, correctly encapsulating the one form of decalin over the other 96 times out of a hundred. They also discovered that it was the intermolecular forces affecting the substance -- the various interactions between the molecules that are strong but still weaker than atomic within the molecules that contributed to the pore's stability and determines its remarkable selectivity. Other materials may be porous and selective but remain insufficiently stable for industrial applications. This substance ticks all the selective-encapsulation boxes.

This particular proof of concept could be used in a wide range of applications, such as gas entrapment, oil separation, and removal of trace elements from water, but the researchers want to seek out unique encapsulation functions that can only be achieved with latent pores.

And there remains a great deal of territory to explore explaining this "supramolecular" chemistry of latent pores. The researchers feel that they are just beginning their mapping of that territory.