New Generation UV-A Filters: Understanding Their Photodynamics on a Human Skin Mimic
Temitope T. Abiola, Natércia d. N. Rodrigues, Casey Ho, Daniel J. L. Coxon, Michael D. Horbury, Josene M. Toldo, Mariana T. do Casal, Benjamin Rioux, Cédric Peyrot, Matthieu M. Mention, Patrick Balaguer, Mario Barbatti, Florent Allais, Vasilios G. Stavros
Abstract
The sparsity of efficient commercial ultraviolet-A (UV-A) filters is a major challenge toward developing effective broadband sunscreens with minimal human- and eco-toxicity. To combat this, we have designed a new class of Meldrum-based phenolic UV-A filters. We explore the ultrafast photodynamics of coumaryl Meldrum, CMe, and sinapyl Meldrum (SMe), both in an industry-standard emollient and on a synthetic skin mimic, using femtosecond transient electronic and vibrational absorption spectroscopies and computational simulations. Upon photoexcitation to the lowest excited singlet state (S1), these Meldrum-based phenolics undergo fast and efficient nonradiative decay to repopulate the electronic ground state (S0). We propose an initial ultrafast twisted intramolecular charge-transfer mechanism as these systems evolve out of the Franck–Condon region toward an S1/S0 conical intersection, followed by internal conversion to S0 and subsequent vibrational cooling. Importantly, we correlate these findings to their long-term photostability upon irradiation with a solar simulator and conclude that these molecules surpass the basic requirements of an industry-standard UV filter.