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.
Highlights
The incidence of melanoma skin cancer has reached epidemic proportions globally, with cases predicted to continue rising by 2.5% year on year
The expected geometric forms of protonated and deprotonated oxybenzone predict that the keto-enol site is the protonation/deprotonation location, so it is entirely unsurprising to find that the ultrafast decay mechanism is significantly perturbed in alkaline or acidic media. This is the key finding from the series of deprotonated and protonated sunscreen molecules we have studied via Laser-interfaced mass spectrometry (LIMS), which include avobenzone, 2-phenylbenzimidazole-5-sulfonic acid, and benzophenone-4 (Wong et al, 2019a; Berenbeim et al, 2020b; Wong et al, 2021a)
Given that protonation can impact on the function of an organic sunscreen at the molecular level, it is clearly important to understand if cation binding could produce a similar effect
Summary
The incidence of melanoma skin cancer has reached epidemic proportions globally, with cases predicted to continue rising by 2.5% year on year. Laser spectroscopy techniques have been applied to better understand how intermolecular interactions (e.g., with solvent molecules) and the local environment (pH) affect sunscreen photochemistry. High-level computational studies of the ground and excited state potential energy surfaces of these sunscreen molecules confirm the pathways outlined here, with the oxybenzone system having been studied by Domcke and coworkers (Karsili et al, 2014), and the cinnamates by Cui and coworkers and Ebata and co-workers (Chang et al, 2015; Kinoshita et al, 2021).
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