Insight into the Photodynamics of Photostabilizer Molecules

Abstract

Solar exposure of avobenzone, one of the most widely used commercial UVA filters on the market, is known to cause significant degradation. This finding has fueled research into developing photostabilizer molecules. In an effort to provide insight into their stand-alone photoprotection properties, the excited state dynamics of the photostabilizer, 3-(3,4,5-trimethoxybenzylidene) pentane-2,4-dione (TMBP), and its phenolic derivative, 3-(4-hydroxy-3,5-dimethoxybenzylidene) pentane-2,4-dione (DMBP), were studied with ultrafast transient absorption spectroscopy. Solutions of TMPB and DMBP in ethanol and in an industry-standard emollient, as well as TMBP and DMBP deposited on synthetic skin mimic, were investigated. These experiments were allied with computational methods to aid interpretation of the experimental data. Upon photoexcitation, these photostabilizers repopulate the electronic ground state via nonradiative decay within a few picoseconds involving a twisted intramolecular charge transfer configuration in the excited state, followed by internal conversion and subsequent vibrational cooling in the ground state. This finding implies that, aside from acting as a photostabilizer to certain UV filters, TMBP and DMBP may offer additional photoprotection in a sunscreen formulation as a stand-alone UV filter. Finally, TMBP and DMBP could also find applications as molecular photon-to-heat converters.

Spectroscopic insight on impact of environment on natural photoprotectants

“…Three exemplar cases are briefly presented: (i) the intramolecular proton transfer mechanism in oxybenzone, which facilitates photoprotection, is broken when sodium cations are present; 22,23 (ii) relaxation to the electronic ground state of cinnamates is impeded by internal conversion to an optically dark state when in vacuum, but is facilitated when in water due to reverse ordering of the electronic states; 23,24 and (iii) recent work examining the impact of depositing such photoprotective molecules on surfaces mimicking skin have shown some variability to their photodynamics depending on surface. 25–27…”