Illuminating the Effect of the Local Environment on the Performance of Organic Sunscreens: Insights From Laser Spectroscopy of Isolated Molecules and Complexes

Abstract

Sunscreens are essential for protecting the skin from UV radiation, but significant questions remain about the fundamental molecular-level processes by which they operate. In this mini review, we provide an overview of recent advanced laser spectroscopic studies that have probed how the local, chemical environment of an organic sunscreen affects its performance. We highlight experiments where UV laser spectroscopy has been performed on isolated gas-phase sunscreen molecules and complexes. These experiments reveal how pH, alkali metal cation binding, and solvation perturb the geometric and hence electronic structures of sunscreen molecules, and hence their non-radiative decay pathways. A better understanding of how these interactions impact on the performance of individual sunscreens will inform the rational design of future sunscreens and their optimum formulations.

Photostability of the deprotonated forms of the UV filters homosalate and octyl salicylate: molecular dissociation versus electron detachment following UV excitation

“…This situation is highly unusual, with [HS À H] À or [OS À H] À representing the first two molecules we have studied by LIMS which do not produce ionic photofragments following photoexcitation. 1,3,4,41 To further explore the possible ionic breakdown pathways for these ions, we proceeded to study them using collision-induced dissociation (CID).…”

“…The intrinsic photophysics of organic sunscreen molecules has been studied intensively over recent years via advanced laser spectroscopy and photochemistry experiments which aim to provide a more detailed picture of how sunscreens perform at a molecular level. 1,2 One example of such studies is the application of laser interfaced mass spectrometry (LIMS) to probe how the behavior of protonated and deprotonated sunscreens differ, and hence to probe the effect of pH on sunscreens at the molecular level. 1 These studies have revealed that protonation and deprotonation can alter the UV absorption profile of a sunscreen, as well as differentially affect the available decay pathways.…”

“…1,2 One example of such studies is the application of laser interfaced mass spectrometry (LIMS) to probe how the behavior of protonated and deprotonated sunscreens differ, and hence to probe the effect of pH on sunscreens at the molecular level. 1 These studies have revealed that protonation and deprotonation can alter the UV absorption profile of a sunscreen, as well as differentially affect the available decay pathways. 3,4 Indeed, we have found that protonation/deprotonation can remove the ability of certain sunscreens to act effectively by perturbing the excited state decay pathways, rendering the molecules photo-unstable and prone to molecular dissociation.…”

“…8,18 Deprotonation of a salicylate molecule at the hydroxyl hydrogen therefore has the potential to strongly perturb its ability to undergo ESIPT, and in turn, lower their capacity to absorb in the UV-B, and perform effectively within a bulk sunscreen formulation. 1 For that reason, it is important to examine the influence of protonation state on the intrinsic photochemical behaviors of HS and OS.…”

Targeted small molecule identification using heartcutting liquid chromatography–infrared ion spectroscopy

“…In both applications, we demonstrated identification of multiple analytes from a single LC injection, including a partial bottom-up structural elucidation via the recording of an IR spectrum of a fragment ion. Our set-up for IRIS employs a free-electron laser, but the heartcutting LC-IRIS method is also directly suitable for experiments with other IR lasers, such as table-top optical parametric oscillators (OPOs) 54 or other types of MS-integrated spectroscopy experiments 55,56 . Using additional and/or different types of switching valves, the setup can as well be expanded with additional sample loops to allow the analysis of more than two analytes per LC injection.…”