Broadband ultrafast photoprotection by oxybenzone across the UVB and UVC spectral regions

Abstract

Recent studies have shed light on the energy dissipation mechanism of oxybenzone, a common ingredient in commercial sunscreens. After UVA photoexcitation, the dissipation mechanism may be understood in terms of an initial ultrafast excited state enol → keto tautomerisation, followed by nonadiabatic transfer to the ground electronic state and subsequent collisional relaxation to the starting enol tautomer. We expand on these studies using femtosecond transient electronic absorption spectroscopy to understand the non-radiative relaxation pathways of oxybenzone in cyclohexane and in methanol after UVB and UVC excitation. We find that the relaxation pathway may be understood in the same way as when exciting in the UVA region, concluding that oxybenzone displays proficient broadband non-radiative photoprotection, and thus photophysically justifying its inclusion in sunscreen mixtures.

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

“…Minimal dynamics are seen, all of which occur within the temporal resolution of the experimental instrument response function (∼100 fs, [17]), except for when using 25mM-TiO 2 -M where a strong negative signal is observed which does not fully recover by the maximum available pump-probe delay time of 2 ns. Comparing the spectral location of the negative signal to the UV-visible spectrum of TiO 2 , this signal can be attributed to a ground state bleach (GSB), which we return to discuss.…”

“…All global fits include probe spectral regions between 355-415 nm which encompasses the main absorption feature of OB, and begin at a large enough ∆t so that convolution with an instrument response function is not necessary (see SI). This procedure closely follows that used in previous studies on OB solutions alone [16,17].…”

“…Figure 1 shows the overall relaxation mechanism of oxybenzone. Multi-molecular compositions have already been shown to display photodynamics different to that of the individual constituents, for example, octocrylene is known to behave as a photostabiliser to avobenzone when in solution to- Figure 1: Overall relaxation mechanism of UV photoexcited oxybenzone as has been previously proposed [16,17]. (i) Initial UV photoexcitation populates the S 2 (1 1 ππ * ) state (ii).…”

“…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.…”