Tracing Sunscreen’s Reaction Dynamics

The most common UV-B filter used in sunscreens is the cinnamic acid derivative 2-ethylhexyl-4-methoxycinnamate (EHMC), which is supposed to protect skin by converting the sun’s harmful UV radiation to heat. But UV-B absorption by EHMC can initiate unwanted chemical reactions, create harmful by-products, and generate reactive oxygen species—defeating the compound’s protective purpose. Now, Wybren J. Buma of the University of Amsterdam and colleagues have used gas-phase spectroscopy to follow the excited-state dynamics of EHMC and a simpler analog of EHMC, methyl-4-methoxycinnamate. They found that the electronically excited chromophores do not immediately return to the ground state. Instead, they take an energetic pathway to another excited state, which slows down energy dissipation by a factor of more than 10,000 (J. Phys. Chem. Lett. 2014, DOI: 10.1021/jz501140b). But the authors have also found a potentially easy fix: add water. A polar environment interferes with the energetics of the chromophore’s various pathways, making a return to ground state more favorable. “One might develop much better sunscreens if one simply puts the active component in a watery environment,” Buma tells C&EN.