I was interested to see this paper, which goes into detail on a chemical phenomenon that many have seen but no one understands very well: triboluminescence. That word meets with a blank stare or instant recognition; there’s not much in between. It means “emission of light when a material is physically broken”, and that encompasses fracturing, scratching, rubbing, and so on.
Many people have noticed this with sugar crystals (Francis Bacon referred to this as “well known” in his Novum Organum back in 1620), and the classic modern example is with wintergreen-flavored candies such as Life Savers. In that case, it’s the sugar that’s giving off the light, and the methyl salicylate that’s fluorescing in response to it, making the whole process easier to see. Often, the light seen in these situations is from similarly excited nitrogen molecules in the air around the crystal, but the molecules of the solid itself can emit in the visible range directly. I should note that triboluminiscence doesn’t have to be in the visible or even ultraviolet range: a famous example is the small-but-detectable emission of X-rays by peeling Scotch tape (in a vacuum, mind you).
There are a lot of materials besides sugar that will do the trick, of course, quartz being perhaps the second most well-known. Some years ago, I had an aromatic amide compound in my lab that turned out to be triboluminescent, which I discovered by accident while scraping the stuff out of a round bottom flask. I wondered at the time what the structural features might be that led to the effect, and thanks to this paper I know more about that now. One thing I didn’t realize is that N-aryl phthalimides are known as a class to be prone to this, and that might have been close to what I was seeing.
The biggest factor is that the crystals have to be in a non-centrosymmetric space group. (You wouldn’t think that we’d get from wintergreen candy to space groups so quickly, but that’s the chemistry of materials for you). Of the seven crystal systems, all of them have one or more space groups with an inversion center in them, and those will not show triboluminescence or the related phenomena of piezoelectricity.There are some apparent exceptions in the literature, but those are thought to be due to impurities leading to local symmetry-breaking. When non-centrosymmetric crystal packing is broken, the newly formed surfaces can end up with notable electric charges on them, and that’s where all of this starts off. The complications come in with the various crystal-packing interactions and orientations ( through stacking, hydrogen bonding, etc.), the conformational freedom of the molecules in the lattice, the electronic structure of the compounds themselves, and so on, which give you a huge variety of possibilities, all the way from “doesn’t emit a thing because the energy all goes somewhere else” to “glows so brightly you’ll drop the flask”.
The authors of this new review note that there are a lot of potential applications for triboluminescence, but our lack of understanding of how to engineer it in a more systematic way has held it back. They say that “it would not be surprising if a sudden expansion of their potential applications in organic electronics (e.g. sensors, smart materials etc.) was to occur in the very near future“, and that would be fun to watch. On several levels!