A difluoroboron complex of avobenzone undergoes the fairly novel process of reversible mechanochromic luminescence at room temperature. Scratch it under UV light and it changes color from green-blue to yellow, the yellow then reverts to blue given time, although the reverse process occurs more quickly on heating. The complex is a simple derivative of a sunscreen product and responds to the lightest touch, leading to a red-shift in the color of the material under ultraviolet excitation. The writable "scratch the surface" ink could have many potential commercial applications as well as being yet another technique for old school cloak and dagger spies to master.
Spying for chemists