Illuminating Sunscreens: Probing the Intrinsic Absorption Properties of Ionic UV Filters via Laser Photodissociation Action Spectroscopy

Few studies have explored the fundamental mechanisms that govern how sunscreens function at the molecular level until recently. Here, we apply the novel approach of UV laser photodissociation action spectroscopy to measure how the intrinsic absorption properties of commercially-available organic sunscreen molecules are affected at the molecular level by pH (i.e., protonation or deprotonation). In several systems, we observe that protonation state has a substantial effect on the UV absorption profile of common sunscreens. Deprotonated oxybenzone, for instance, displays a remarkably modified absorption spectrum and photogenerates both electrons and free radicals. Likewise, deprotonated 2-phenylbenzimidazole-5-sulfonic acid yields anionic and neutral free radicals via all photodissociation routes. Importantly, these experiments allow us to characterize their photophysical behavior, through analysis of the photofragments generated and comparison of these to the fragment ions produced upon the thermal breakdown of the ground electronic state molecule. We further report, for the first time in a study of an anionic UV filter, high-level ab initio potential energy surfaces for the popular sunscreen benzophenone-4 in conjunction with brand-new results from gas-phase laser photodissociation and higher-energy collisional dissociation studies for the deprotonated species, which would be present under alkaline conditions. The ab initio calculations confirm the implied photophysics that we deduced in the earlier studies on oxybenzone and 2-phenylbenzimidazole-5-sulfonic acid. Using a series of new solution-phase irradiation setups which couple home-built photolysis cells with electrospray ionization mass spectrometry, we use a model system (i.e., riboflavin) to exemplify how gas-phase photofragmentation of this UV chromophore is mirrored by its solution-phase behavior. Broader application of this approach to identifying photoproducts of other photoactive molecules, i.e., sunscreens, is discussed. These findings, at the molecular level, address the issues surrounding the suitability of existing sunscreens and demonstrate the utility of laser-interfaced mass spectrometry for fundamental studies for sunscreen photochemistry.