Shining ultraviolet light on a crystal of a cobalt coordination compound (roughly 0.5 mm long) causes it to jump in various ways, as seen in these high-speed photographs. The crystal can flip orientation and zip off the microscope stage (top row) or spring toward the viewer, spin in midair, and land on the stage (bottom row).
MATERIALS
Revealing the secrets of jumping crystals Some organic crystals hop about under light and heat. There’s renewed interest in finding out why MITCH JACOBY, C&EN CHICAGO
W
hen 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
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C&EN | CEN.ACS.ORG | JANUARY 21, 2019
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.
C R E D I T: A NG EW. C H EM ., I NT. E D.
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C R E D I T: C HE M. —E UR . J . ( NA PH TH A LE NE M OD ELS ) ; C HE M. SC I . ( T ET RABROM O B E N Z E N E M O DE LS)
has some ideas why jumping Naumov encouraged other crystals have been somewhat scientists, such as Elena V. overlooked. Boldyreva, a solid-state organic One reason is that chemists chemist at Novosibirsk State studying solid-state phase University, to return to analyzchanges rely on optical miing these phenomena. Naumov croscopes less often than had known about the work of they used to. Commonly used Boldyreva, who first witnessed methods, such as differential jumping crystals in 1980 as scanning calorimetry and a master’s student at NSU, X-ray powder diffraction, since his graduate school days. don’t look at individual crysTogether with Bernstein, who tals, and the jumping needs died as this C&EN story was to be seen to be studied. In being written, Naumov also addition, those methods call carried out a detailed study for grinding samples into fine of the thermosalient behavpowders, which masks the ior of oxitropium bromide in salient effects because very 2010 (J. Am. Chem. Soc., DOI: fine particles barely jump, and 10.1021/ja105508b). All these Crystals of this Y-shaped naphthalene compound (stacked their motions are hard to obefforts helped revive interest head to tail in the space-filling models, bottom) respond to in jumping crystals. temperature changes with a tweezer-like motion that triggers a serve and track. Naumov also suspects researchers may have Since then, researchers reversible phase transition, converting one polymorph (left) to occasionally witnessed crystal working in this area have catthe other (right). C = gray, H = white, O = red, and F = green. jumping but didn’t report it aloged a small but growing and certainly did not delve into it, perhaps causing self-propulsion. “It’s a very fast number of organic, organometallic, and because they felt the behavior was quirky transformation of energy,” Naumov says. metal coordination compounds—about and inexplicable or irrelevant. “That’s what caught our attention.” 30 so far—that exhibit the thermosalient To the field’s aficionados, the puzzling These crystals often shatter or disineffect. They have also uncovered additionbehavior isn’t a research deterrent; it’s an tegrate as a result of the energetic phase al compounds that jump, spin, and flip invitation to dig for answers about these change, causing fragments to fly through in response to light and, in some cases, crystals. Digging into oxitropium bromide the air. “That’s not the same as a chemmechanical agitation, meaning they leap to kick off their collaboration, Bernstein ical explosion,” Naumov stresses. When when you poke them. and Naumov found that gently heating the a crystal like potassium permanganate In all these cases, the salient crystals pharmaceutical compound causes its unit explodes, the crystal-shattering reaction accumulate strain in their framework, cell to expand along one axis and shrink generates oxygen, nitrogen, or other gases or lattice, when stimulated. When the and is irreversible. Thermosalient crystals, along another. As the strain builds, the strain builds to a certain limit, the crysmolecule, made of two rigid cyclic fragon the other hand, can undergo configuratals undergo a rapid structural change, tion changes reversibly, upon both heating ments connected by a flexible ester link, converting from one form to another. The responds like a spring. It releases suddenly transformation likely begins at a defect—a and cooling, and the fragments often conand forcefully, shifting its molecular packtinue jumping. For instance, as a crystal point in the crystal lattice where one moling to a high-temperature configuration jumps away from a hot spot, it cools down ecule’s environment differs from that of that alleviates the strain. and then may heat up again depending on its neighbors. That molecule is the first to Working with Boldyreva, Naumov where it lands, causing it to flip back and succumb to the strain and change its conuncovered another salient compound’s forth between polymorphic forms. figuration. The motion triggers a molecumechanism. The simple structure of “It’s apparent that this behavior is lar domino effect that can propagate at a [Co(NH3)5(NO2)]Cl(NO3) gives no hint rate of over 1 billion molecules per second. more common than people thought,” Naumov says of the ever-growing list of The sudden solid-state phase transition that crystals of the material have the poheat-, light-, and touch-activated crysreleases the mechanical strain in a way tential to jump. But as the pair learned, tals that scientists are discovering. He that imparts momentum to the crystal, 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 Tetrabromobenzene is thermosalient (left, C = gray, H = white, and Br = gold). Crystals of the modes of crystal splitting compound jump when they undergo a heat-induced reversible phase change that alters their and disintegration. molecular packing (unit cell shown, right) from a low-temperature polymorph (blue) to a highAs a result of the analytemperature one (red). JANUARY 21, 2019 | CEN.ACS.ORG | C&EN
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“It’s apparent that this behavior is more common than people thought.” —Panče Naumov, chemistry professor, New York University Abu Dhabi sis, 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. ◾
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