Revealing the secrets of jumping crystals’ motion

When a popcorn kernel heats up, water trapped inside its hard shell turns into steam, building pressure. The kernel expands until—pop!—it explodes, leaping erratically.

Place some types of organic crystals on a heated surface, and they hop and jump around in a similar fashion. Steam, however, doesn’t explain this staccato motion. Researchers are only just uncovering the mechanistic details that underlie the unusual behavior, which not all crystals display. Jumping appears to be limited to molecular crystals, which are collections of well-ordered and weakly interacting molecules, and the motion results from subtle rearrangements in the way the molecules are packed.

Panče Naumov learned of jumping crystals about nine years ago, when a postdoc working with him at Osaka University was having a hard time examining a sample of oxitropium bromide, an organic compound used for treating asthma. The crystals were literally jumping off the heated microscope stage as the postdoc gazed at them, recalls Naumov, now at New York University Abu Dhabi.

Naumov mentioned the strange observation to Joel Bernstein, a crystallographer at Ben-Gurion University of the Negev, who was visiting Osaka as a guest lecturer. As luck would have it, Bernstein was familiar with the phenomenon and pointed Naumov to the work of Margaret C. Etter.

In the early 1980s, Etter, an organic chemist at 3M, was investigating heat-induced phase transformations in a palladium compound with a long name—(phenylazophenyl)palladium hexafluoroacetylacetonate, or PHA. She noticed that depending on the way the crystals were heated in a microscope, sometimes they flew off the hot stage. She published the observation in 1983 (J Am. Chem. Soc., DOI: 10.1021/ja00341a065).

So Naumov wasn’t the first to notice jumping crystals. But after some research, he realized the behavior—called the thermosalient effect—was little studied and somewhat forgotten. Etter moved to the University of Minnesota shortly after her JACS paper was published, and she continued studying solid-state organic compounds but not the thermosalient effect. That left Naumov intrigued as to how some crystals could jump up to several centimeters repeatedly and whether that mechanical motion could be put to use in actuators, microscopic machines, or artificial muscles. So he mounted an investigation.

Naumov encouraged other scientists, such as Elena V. Boldyreva, a solid-state organic chemist at Novosibirsk State University, to return to analyzing these phenomena. Naumov had known about the work of Boldyreva, who first witnessed jumping crystals in 1980 as a master’s student at NSU, since his graduate school days. Together with Bernstein, who died as this C&EN story was being written, Naumov also carried out a detailed study of the thermosalient behavior of oxitropium bromide in 2010 (J. Am. Chem. Soc., DOI: 10.1021/ja105508b). All these efforts helped revive interest in jumping crystals.

Since then, researchers working in this area have cataloged a small but growing number of organic, organometallic, and metal coordination compounds—about 30 so far—that exhibit the thermosalient effect. They have also uncovered additional compounds that jump, spin, and flip in response to light and, in some cases, mechanical agitation, meaning they leap when you poke them.

In all these cases, the salient crystals accumulate strain in their framework, or lattice, when stimulated. When the strain builds to a certain limit, the crystals undergo a rapid structural change, converting from one form to another. The transformation likely begins at a defect—a point in the crystal lattice where one molecule’s environment differs from that of its neighbors. That molecule is the first to succumb to the strain and change its configuration. The motion triggers a molecular domino effect that can propagate at a rate of over 1 billion molecules per second.

The sudden solid-state phase transition releases the mechanical strain in a way that imparts momentum to the crystal, causing self-propulsion. “It’s a very fast transformation of energy,” Naumov says. “That’s what caught our attention.”

These crystals often shatter or disintegrate as a result of the energetic phase change, causing fragments to fly through the air. “That’s not the same as a chemical explosion,” Naumov stresses. When a crystal like potassium permanganate explodes, the crystal-shattering reaction generates oxygen, nitrogen, or other gases and is irreversible. Thermosalient crystals, on the other hand, can undergo configuration changes reversibly, upon both heating and cooling, and the fragments often continue jumping. For instance, as a crystal jumps away from a hot spot, it cools down and then may heat up again depending on where it lands, causing it to flip back and forth between polymorphic forms.

“It’s apparent that this behavior is more common than people thought,” Naumov says of the ever-growing list of heat-, light-, and touch-activated crystals that scientists are discovering. He has some ideas why jumping crystals have been somewhat overlooked.

One reason is that chemists studying solid-state phase changes rely on optical microscopes less often than they used to. Commonly used methods, such as differential scanning calorimetry and X-ray powder diffraction, don’t look at individual crystals, and the jumping needs to be seen to be studied. In addition, those methods call for grinding samples into fine powders, which masks the salient effects because very fine particles barely jump, and their motions are hard to observe and track. Naumov also suspects researchers may have occasionally witnessed crystal jumping but didn’t report it and certainly did not delve into it, perhaps because they felt the behavior was quirky and inexplicable or irrelevant.

To the field’s aficionados, the puzzling behavior isn’t a research deterrent; it’s an invitation to dig for answers about these crystals. Digging into oxitropium bromide to kick off their collaboration, Bernstein and Naumov found that gently heating the pharmaceutical compound causes its unit cell to expand along one axis and shrink along another. As the strain builds, the molecule, made of two rigid cyclic fragments connected by a flexible ester link, responds like a spring. It releases suddenly and forcefully, shifting its molecular packing to a high-temperature configuration that alleviates the strain.

Working with Boldyreva, Naumov uncovered another salient compound’s mechanism. The simple structure of [Co(NH₃)₅(NO₂)]Cl(NO₃) gives no hint that crystals of the material have the potential to jump. But as the pair learned, ultraviolet light makes the crystals hop in a way that’s extremely reminiscent of popcorn popping. Using a high-speed camera coupled to an optical microscope, the team tracked the detailed photosalient behavior of hundreds of prismatic, or box-shaped, crystals. They analyzed a large variety of motions, including spins, flips, jumps, and rolls, as well as various modes of crystal splitting and disintegration.

As a result of the analysis, the team developed a model based on a photoisomerization reaction in which the cobalt-bonded NO₂ group rotates, causing the coordination to switch from Co-NO₂ to Co-ONO. The shift induces lattice strain that reversibly compresses springlike H bonds between NH₃ groups and the Cl^– and NO₃^– anions (Angew. Chem., Int. Ed. 2013, DOI: 10.1002/anie.201303757).

More recently, Naumov joined forces with Rajesh G. Gonnade’s group at the National Chemical Laboratory in Pune, India, to search for thermosalient behavior in naphthalenes. They found it in crystals of a Y-shaped naphthalene double ester. X-ray analysis indicates that in this compound, the strain release is due to reversible opening and closing of the molecule’s arms in a tweezer-like motion (Chem.—Eur. J. 2018, DOI: 10.1002/chem.201705586).

Although scientists haven’t yet discovered many salient compounds, tetrabromobenzene (TBB) is one member of the small class that has been studied repeatedly. Yet the simple molecule still keeps secrets, especially about lattice events that trigger crystal jumping. Researchers had a hunch that low-frequency vibrations—for example, the kind in which a group of molecules in a molecular crystal oscillate in unison relative to neighboring molecules—could play a role. But few chemists have the expertise required to probe them with low-frequency vibrational spectroscopy. So Naumov teamed up with Syracuse University’s Timothy M. Korter, a specialist in that technique.

The group probed TBB’s low-frequency Raman spectra at various temperatures and analyzed the results with solid-state quantum calculations. The results pinpointed a specific subtle motion within the TBB lattice, a vibrational mode with a frequency of about 15 cm^–1, that serves as the molecular event that triggers conversion of one polymorph to the other at a specific temperature and causes the crystals to jump (Chem. Sci. 2018, DOI: 10.1039/c8sc03897j).

Results of these recent studies are helping scientists better understand these materials’ fundamental properties. But with such a small number of examples, it’s not yet possible to predict which crystals will jump and which ones won’t. Researchers are keen to be able to intuit that property from a crystal’s structure. But for now, that’s difficult. It’s also hard to say if and when these materials will be put to use.

In a simple demonstration, Naumov’s group showed that silver-coated TBB crystals can serve as tiny fuses that break at a threshold current, interrupting the flow of electricity in a test circuit. And Boldyreva coauthored a patent describing a photometer based on light-driven crystal bending. Beyond those examples, however, the research community hasn’t said very much about products based on jumping crystals. They won’t be on store shelves anytime soon. The field is small but likely to continue growing as word spreads about these fascinating hyperactive materials.