A Perspective on Femtosecond Pump-Probe Spectroscopy in the Development of Future Sunscreens

Abstract

Given the negative impacts of overexposure to ultraviolet radiation (UVR) on humans, sunscreens have become a widely used product. Certain ingredients within sunscreens are responsible for photoprotection and these are known, collectively herein, as ultraviolet (UV) filters. Generally speaking, organic UV filters work by absorbing the potentially harmful UVR and dissipating this energy as harmless heat. This process happens on picosecond time scales and so femtosecond pump-probe spectroscopy (FPPS) is an ideal technique for tracking this energy conversion in real time. Coupling FPPS with complementary techniques, including steady-state spectroscopy and computational methods, can provide a detailed mechanistic picture of how UV filters provide photoprotection. As such, FPPS is crucial in aiding the future design of UV filters. This Perspective sheds light on the advancements made over the past two years on both approved and nature-inspired UV filters. Moreover, we suggest where FPPS can be further utilized within sunscreen applications for future considerations.

Figure 1

Schematic of a typical transient electronic absorption spectroscopy (TEAS) setup. Reprinted with permission from ref (38). Copyright 2020 MDPI.

Figure 2

Molecular structure of (a) DHHB in its enol form, (b) DHHB in its keto form, and (c) oxybenzone in its enol form.

Figure 3

Transient electronic absorption spectra of DHHB in (a) methanol (MeOH), (b) acetonitrile (ACN), (c) dimethyl sulfoxide (DMSO), and (d) cyclohexane (CYCH) solutions. The color code indicates the spectra obtained at different pump–probe time delays. The insets in each panel show longer time delay spectra corresponding to the triplet states. Reproduced with permission from ref (32). Copyright 2021 American Chemical Society.

Figure 4

Schematic of the PES for relaxation of DHHB along the ESHT and C–C bond torsion from the S₁ state following UV photoexcitation. Reprinted with permission from ref (32). Copyright 2021 American Chemical Society.

Figure 5

Structure of (a) shinorine and (b) porphyra-334.

Figure 6

Schematic of the photoprotection mechanism of the MAAs shinorine and porphyra-334. Reprinted with permission from ref (33). Copyright 2021 American Chemical Society.

Figure 7

UV–visible spectra before and after 5 h of irradiation with a solar simulator for (a) shinorine and (b) porphyra-334. Reprinted with permission from ref (33). Copyright 2021 American Chemical Society.

Figure 8

Structure of (a) coumaryl Meldrum and (b) sinapyl Meldrum.

Figure 9

Schematic of the PES for relaxation of (a) CMe and (b) SMe. Reprinted with permission from ref (34). Copyright 2020 The Authors.