Sunscreen spies

Sunscreen and boron can work together to make a compound that changes colour when touched under ultraviolet light. The compound changes from blue-green to yellow with the gentlest of rubs and then reverts quickly to blue-green when gently warmed, although the process is reversible at room temperature. Intriguing commercial application awaits.

Chemists Guoqing Zhang, Michal Sabat, and Cassandra Fraser of the University of Virginia, Charlottesville, working with materials scientist Jiwei Lu, have developed difluoroboron avobenzone (BF₂AVB), a relatively simple boron compound of a commercially available sunscreen product. The compound displays morphology-dependent emission and mechanochromic luminescence in the solid state.

"When scratched, smeared, or even gently touched, the emission color of BF₂AVB films is significantly red-shifted under UV excitation," the researchers say, "In the rubbed regions, the fluorescence recovers slowly at room temperature or much faster with heating, resulting in a simple rewritable 'scratch the surface' ink of potential commercial use."

The researchers explain that underpinning their findings is the fact that the solid-state fluorescence of an organic molecule depends strongly on its structure and any intermolecular forces that affect different morphologies. It is possible to produce emission of different frequency, and so colour, with the same molecule by producing different polymorphic forms of the solid. One way to alter the structure, other than fine-tuning the crystallization process, is to apply an appropriate force so that this induces a change in morphology that is reflected by a shift in emission frequency, so-called mechanochromic luminescence. One such force is applied in the grinding of mechanoresponsive gold complexes, for instance, which can change colour after processing in this way.

The team adds that mechanochromic luminescence can also be induced in a solid by disrupting the hydrogen bonding network among the molecules so that a less ordered polymorph is formed. Some researchers have used a shear force and ultraviolet irradiation to switch on a fluorescence shift in a cyanostilbene system.

Fraser and colleagues hope to gain a clearer understanding at the molecular level of the mechanochromic phenomenon and the various mechanisms that give rise to it. One way to obtain new insights would be to produce a mechanochromic substance that changes reversibly without solvent or thermal processing.

 The potential commercial applications of such a reversible perhaps lie with the manufacturers of toys, games, and displays as well as in engineering applications for pressure testing equipment or providing a visual warning of imminent structural failure; just don't tell the cloak and dagger or the lemonjuice and wax brigade about this...