A Reevaluation of the Photolytic Properties of 2‐Hydroxybenzophenone‐Based UV Sunscreens: Are Chemical Sunscreens Inoffensive?

Abstract

The excited states of a set of popular sunscreen agents (2-hydroxybenzophenone, oxybenzone, and sulisobenzone) are studied by using femto- and nanosecond time-resolved spectroscopy. Upon excitation, the compounds undergo an ultrafast excited-state intramolecular proton transfer (ESIPT) reaction as the major energy-wasting process and the rate constant of this reaction is k=2×10(12) s(-1) . The ESIPT yields a keto conformer that undergoes a fast, picosecond internal conversion decay. However, a photodegradative pathway is a monophotonic HO bond breakage that subsequently leads to trace yields of phenoxyl radicals. Because potentially harmful phenoxyl radicals are formed upon irradiation of sunscreen agents, care should be taken about their reactivity towards biologically relevant compounds.

Observing and Understanding the Ultrafast Photochemistry in Small Molecules: Applications to Sunscreens

“…The conclusion of these calculations and subsequent analysis points to the likely photoproduct being a long-lived keto-isomer which has undergone significant excess rotation about the aliphatic C−C bond, referred to as a trans keto-isomer, vide infra. Similar studies have also observed the generation of a photoproduct and have attributed it to the formation of a phenoxyl radical after O−H bond fission 26 . Thus TVAS measurements have not only complemented TEAS measurements, where the timescales of identified processes are in very good agreement, each has provided something unique.…”

“…Oxybenzone has been shown to be an effective sunscreen with respect to its ability in absorbing UVR across the UV-A and UV-B regions dissipating it on ultrafast timescales 24;25 . Contrary to this, oxybenzone has been suggested to form a potentially adverse photoproduct after being excited by UVR 26 . Furthermore, there is evidence to suggest that oxybenzone may behave as an endocrine disrupter and a photoallergen 23;27 .…”

Ultrafast photophysical studies of a multicomponent sunscreen: Oxybenzone–titanium dioxide mixtures

“…(ii ) Formation of a long-lived photoproduct, most likely through extended C−C bond rotation forming a non-chelated, trans keto tautomer as previously reported [16]. However, a possibility remains that the photoproduct might be a phenoxyl radical [18]. The first of these routes appears dominant, given that the majority of the GSB recovers on the time scale of the experiment; the incomplete recovery is therefore attributed to the formation of a photoproduct which does not recover by the maximum time delay available of 2 ns (selected transients are given in the SI).…”

“…Organic filters are typically aromatic molecules which absorb UVR and dissipate it through ultrafast pathways. Examples include avobenzone [14,15], oxybenzone [16][17][18][19] and octocrylene [20]. Inorganic scatterers, in addition to absorption, also operate by scattering UV radiation away.…”

“…Most studies of the photoprotection properties of these sunscreen components have focused on individual filters in isolation, but commercial sunscreen products typically contain tens of components in order to produce an efficient, broadband photoprotective and aesthetically pleasing product. In this study we focus on oxybenzone and titanium dioxide, two sunscreen constituents which are often combined to provide broad spectral coverage and efficient photoprotection, to understand if the previously observed photodynamics in oxybenzone are changed [16][17][18] by the presence of the inorganic scatterer. Figure 1 shows the overall relaxation mechanism of oxybenzone.…”