Determining the photostability of avobenzone in sunscreen formulation models using ultrafast spectroscopy.

Emily L Holt,Natércia D N Rodrigues,Juan Cebrián,Vasilios G Stavros

doi:10.1039/d1cp03610f

Abstract

Avobenzone is an ultraviolet (UV) filter that is often included in sunscreen formulations despite its lack of photostability. Its inclusion is necessary due to few existing alternatives for photoprotection in the UVA region (320-400 nm). To better understand and predict the photostability of avobenzone, ultrafast transient electronic absorption spectroscopy (TEAS) has been used to study the effects of solvent (including emollients), concentration and skin surface temperature on its excited-state relaxation mechanism, following photoexcitation with UVA radiation (∼350 nm). Subtle differences between the excited-state lifetimes were found between the systems, but the TEAS spectral features were qualitatively the same for all solution and temperature combinations. Alongside TEAS measurements, UV filter/emollient blends containing avobenzone were irradiated using simulated solar light and their degradation tracked using steady-state UV-visible spectroscopy. Sun protection factor (SPF) and UVA protection factor (UVA-PF) assessments were also carried out on representative oil phases (higher concentration blends), which could be used to formulate oil-in-water sunscreens. It was found that there was an apparent concentration dependence on the long-term photoprotective efficacy of these mixtures, which could be linked to the ultrafast photodynamics by the presence of a ground-state bleach offset. This combination of techniques shows potential for correlating long-term behaviours (minutes to hours) of avobenzone with its ultrafast photophysics (femtoseconds to nanoseconds), bridging the gap between fundamental photophysics/photochemistry and commercial sunscreen design.

Highlights

Avobenzone is a UVA (320–400 nm) filter that is commonly included in sunscreen formulations to protect the skin from the detrimental effects of solar UV radiation.[1]
The photostability and photoprotection afforded by avobenzone were assessed using transient electronic absorption spectroscopy (TEAS) at skin surface temperature to reflect real-world conditions of use, by heating the sample at the interaction region
A review of the literature, which is included in the Supplementary Discussion in the Electronic supplementary information (ESI),† concludes that any temperature within the range 32–38 1C is justifiable as a skin surface temperature for investigations relating to sunscreens; the experiments reported here were performed at 35 1C as the midpoint of this range

Summary

Introduction

Avobenzone ( known as butyl methoxydibenzoylmethane) is a UVA (320–400 nm) filter that is commonly included in sunscreen formulations to protect the skin from the detrimental effects of solar UV radiation.[1].

Results

Conclusion