Abstract
A key decay pathway by which organic sunscreen molecules dissipate harmful UV energy involves excited-state hydrogen atom transfer between proximal enol and keto functional groups. Structural modifications of this molecular architecture have the potential to block ultrafast decay processes, and hence promote direct excited-state molecular dissociation, profoundly affecting the efficiency of an organic sunscreen. Herein, we investigate the binding of alkali metal cations to a prototype organic sunscreen molecule, oxybenzone, using IR characterization. Mass-selective IR action spectroscopy was conducted at the free electron laser for infrared experiments, FELIX (600-1800 cm-1), on complexes of Na+, K+ and Rb+ bound to oxybenzone. The IR spectra reveal that K+ and Rb+ adopt binding positions away from the key OH intermolecular hydrogen bond, while the smaller Na+ cation binds directly between the keto and enol oxygens, thus breaking the intramolecular hydrogen bond. UV laser photodissociation spectroscopy was also performed on the series of complexes, with the Na+ complex displaying a distinctive electronic spectrum compared to those of K+ and Rb+, in line with the IR spectroscopy results. TD-DFT calculations reveal that the origin of the changes in the electronic spectra can be linked to rupture of the intramolecular bond in the sodium cationized complex. The implications of our results for the performance of sunscreens in mixtures and environments with high concentrations of metal cations are discussed.
Highlights
Skin exposure to excess ultraviolet radiation has the potential to trigger skin cancers, with commercial sunscreens having been widely used to filter light at the skin’s surface
In this work the gas-phase UV laser photodissociation absorption spectra of the M+ÁOB (M+ = Na+, K+, Rb+) complexes were measured for the first time
The electronic spectrum associated with the Na+ÁOB complex is strikingly different from those of K+ÁOB and Rb+ÁOB
Summary
Skin exposure to excess ultraviolet radiation has the potential to trigger skin cancers, with commercial sunscreens having been widely used to filter light at the skin’s surface. Recent work from our group has demonstrated how pH can impact at the molecular level on the function of an organic sunscreen.[8,9,10] Upon protonation, both oxybenzone and avobenzone were observed to photodissociate directly from the excited state surface, i.e., undergo non-statistical dissociation, which is an undesirable photodegradation process. A similar effect was observed for oxybenzone and 2-phenylbenzimidazole-5-sulfonic acid upon deprotonation These experiments were conducted in the gas-phase, with mass selection being used to unambiguously isolate and characterize the different protonation states. We use gas-phase IR photodissociation spectroscopy (600–1800 cmÀ1) of mass-selected molecular complexes, M+ÁOB, where M+ = Na+, K+ and Rb+, supported by ab initio calculations to probe directly the binding motifs of this series of alkali–metal cations to oxybenzone. The interaction of an organic sunscreen such as OB with alkali metal cations is of practical interest since sunscreen mixtures include such counterions (e.g., coupled to pH buffer anions),[21] and since they are commonly encountered in the environments where sunscreens are intensively used, including oceans and swimming pools, as well as on human skin.[22,23]
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