Making safe explosives is a balancing act. An energetic material needs to be stable enough to handle but not so inert that the intended blast is stymied. And changing a material’s real-time sensitivity in the field has remained aspirational.
Now researchers in Adam J. Matzger’s laboratory at the University of Michigan have demonstrated that altering an explosive’s sensitivity could be as easy as flicking a light switch (J. Am. Chem. Soc. 2026, DOI: 10.1021/jacs.6c09505). The method relies on a class of neutral molecules that can be snapped into their disruptive zwitterionic isomers—and vice versa—using light.
The origin of the project “is a story as old as time,” Matzger says. “Well, as old as academic time.” He had circuitously ended up in a career finding ways to change the characteristics of explosives by tweaking their crystal structure when the US Defense Advanced Research Projects Agency (DARPA) put out a call for “energetic materials that you can switch on and off,” he says. He assembled a team to tackle the problem.
The solution didn’t come immediately. The scientists first considered photostrictive polymers. “These are the kinds of things that are used in artificial muscles,” Matzger explains. “You shine light on a polymer, and it changes shape.” Those results were interesting, he says, but the samples required too much processing to be compatible with coating explosives.
The researchers then considered using piezoelectrics—materials that develop charge separation under pressure. “It works beautifully in one direction, but you can’t turn it back and forth,” he says. Once an energetic material is destabilized in this way, it stays destabilized.
Their final idea—the one that worked—came as they parsed research tying the inherent charge on an explosive material’s surface to its sensitivity. A photosensitive coating on the material’s surface “could perturb its electrostatics and then make it either more or less sensitive,” Matzger thought.
Benzospiropyran derivatives offered a way to test the theory. Under ultraviolet (UV) light, neutral benzospiropyrans rapidly isomerize into merocyanines that carry both a positive and a negative charge. And when those zwitterions are exposed to enough visible light, they isomerize back into their electrostatically neutral parent.
As a coating, the neutral molecule should, in theory, provide stability to an energetic material. At the flip of a light switch, the zwitterion should disrupt the material’s electrostatic surface, destabilizing the explosive. As a bonus, the solid zwitterion is vividly colored, making it easy to see when the material has been irradiated.
When researchers coat granules of a common explosive in a benzospiropyran derivative and expose them to ultraviolet light, the coating undergoes rapid photoisomerization into its merocyanine form, destabilizing the explosive. The coating also turns purple. The change is reversible with visible light. Credit:
Adam J. Matzger/University of Michigan
To test the idea, Matzger’s team of researchers synthesized two different benzospiropyran derivatives to use as coatings on granules of HMX, an explosive that is used in a variety of real-world contexts. In addition to coating pure HMX granules, the scientists also tested granules that were stabilized with polymer binders; the explosive is most commonly transported and used in this form.
In both cases, the electrostatically neutral coatings did little to change the explosive’s stability but, after a flash of UV light, measurably destabilized the material. And Matzger’s team coated plenty of material, although the researchers usually worked on the smallest scale possible to ensure safety: “It was totally traumatic when we had to make 10 grams of this stuff or 15 grams of the stuff and send it off,” he says.
“The authors ought to be congratulated on their work,” energetic materials chemist Thomas M. Klapötke of Ludwig Maximilian University Munich writes in an email. It’s a clear proof of concept showing that spiropyran molecules can be used to change the sensitivity of an explosive using light. But the difference in stability between the coated HMX’s safe state and sensitized state “is still significantly smaller than necessary for military application,” he says.
“It’s one of those research efforts that plants the seed for people to expand upon,” Los Alamos National Laboratory explosives chemist David E. Chavez tells C&EN. He’s especially interested in the efforts that might be sparked outside of academia.
Researchers need a lot of extra infrastructure to thoroughly study explosive materials and develop new ideas into real-world applications at scale. Few academics have labs equipped for such endeavors (although Matzger certainly has more explosive safety gear than many academic chemists). Overall, “it’s just a very novel concept,” Chavez adds.
