New Generation UV-A Filters: Understanding Their Photodynamics on a Human Skin Mimic

The sparsity of efficient commercial ultraviolet-A (UV-A) filters is a major challenge towards 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 non-radiative 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 towards 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.