Insight into the photodynamics of photostabilizer molecules

Solar exposure of avobenzone, one of the most widely used commercial UVA filters on the market, is known to cause its significant degradation. This finding has fueled research into developing photostabilizers. In an effort to provide further insight into their mode of action as well as their own photoprotection capacity as UV boosters, 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 non-radiative decay within a few picoseconds involving a twisted intramolecular charge transfer in the excited state, 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 to the end user in the form of a UV filter booster and, more generally, applications as molecular photon-to-heat converters.