Picosecond Dynamics of Avobenzone in Solution

Avobenzone, a dibenzoylmethane compound commonly found in sunscreens, can photoisomerize after exposure to near-ultraviolet light. At equilibrium, avobenzone exists as a chelated enol characterized by a strong intramolecular hydrogen bond. Many nanosecond- to microsecond-scale experiments have shown that the photoisomerization involves several nonchelated enol (NCE) isomers and reaction paths, including some that reduce avobenzone's efficacy as a sunscreen. Because some of the NCE isomers are unstable, these experiments do not directly measure their spectroscopic signatures. Here, we report the dynamics of avobenzone on the picosecond time scale. We excite avobenzone at 350 nm and observe the formation and relaxation of new isomers and vibrationally excited species with broadband visible probe pulses and 266 nm probe pulses. Our results show the first direct evidence of two unstable NCE isomers and establish the lifetimes of and the branching ratio between these isomers.