Abstract
Avobenzone (AB) is a widely used UVA filter known to undergo irreversible photodegradation. Here, we investigate the detailed pathways by which AB photodegrades by applying UV laser-interfaced mass spectrometry to protonated AB ions. Gas-phase infrared multiple-photon dissociation (IRMPD) spectra obtained with the free electron laser for infrared experiments, FELIX, (600–1800 cm–1) are also presented to confirm the geometric structures. The UV gas-phase absorption spectrum (2.5–5 eV) of protonated AB contains bands that correspond to selective excitation of either the enol or diketo forms, allowing us to probe the resulting, tautomer-dependent photochemistry. Numerous photofragments (i.e., photodegradants) are directly identified for the first time, with m/z 135 and 161 dominating, and m/z 146 and 177 also appearing prominently. Analysis of the production spectra of these photofragments reveals that that strong enol to keto photoisomerism is occurring, and that protonation significantly disrupts the stability of the enol (UVA active) tautomer. Close comparison of fragment ion yields with the TD-DFT-calculated absorption spectra give detailed information on the location and identity of the dissociative excited state surfaces, and thus provide new insight into the photodegradation pathways of avobenzone, and photoisomerization of the wider class of β-diketone containing molecules.
Highlights
That the skin can be protected against radiation damage linked to sunlight exposure, considerable effort has been put into the development of effective sunscreens.[1−4] Over recent years, a number of advanced laser spectroscopic studies have been conducted on sunscreen molecules under highly controlled conditions, with the aim of improving our fundamental knowledge of the mechanisms by which molecular sunscreens function.[1,5−13] Such measurements aim to provide fundamental insights into the properties of the sunscreen molecule free from the complications of the complex environment of a real sunscreen lotion
Previous photolysis experiments performed in hexane have shown that this photodegradation of AB occurs by a Norrish type 1 mechanism.[32,33] (The key tautomeric forms of AB involved in this photoisomerization are illustrated in Scheme 1.) In the ground state, AB is mostly found in its chelated enol (CE) form due to the stabilizing intramolecular hydrogen bond, the diketo (DK) form is typically present, with the relative tautomeric ratios being strongly solvent dependent
The carbonyl group is the primary protonation site, a natural population analysis of the low-energy structures indicates that the excess charge is delocalized
Summary
That the skin can be protected against radiation damage linked to sunlight exposure, considerable effort has been put into the development of effective sunscreens.[1−4] Over recent years, a number of advanced laser spectroscopic studies have been conducted on sunscreen molecules under highly controlled conditions, with the aim of improving our fundamental knowledge of the mechanisms by which molecular sunscreens function.[1,5−13] Such measurements aim to provide fundamental insights into the properties of the sunscreen molecule free from the complications of the complex environment of a real sunscreen lotion. One group of experiments has focused on “isolated” gas-phase sunscreen molecules to provide information on how UV light absorption varies as a function of molecular structure.[11,12,14−16] A second group of studies have used transient absorption spectroscopy to probe relaxation dynamics of two-component mixtures of a single organic sunscreen molecule in a single solvent.[1,6−8,10] Experiments are typically complemented by high-level quantum chemical calculations,[17−22] which provide important insights into the mechanism by which the sunscreen molecule operates.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
