A Fundamental Ultrafast Spectroscopic Insight into Urocanic Acid Derivatives

<i>trans</i>-Urocanic acid (UA) was once thought to be an ideal natural UV sunscreen filter because of its strong UV absorption and efficient nonradiative decay, in addition to its natural presence in human skin. However, its commercial use was abandoned following the discovery of the immunosuppressive properties of the <i>cis</i> isomer formed following photoexcitation. From the extensive literature accumulated over the past decades, UA serves as a perfect scaffold for developing next-generation nature-inspired UV filters by eliminating the immunosuppressive characteristics and retaining the favorable photophysics. Here, gas-phase time-resolved ion-yield and time-resolved photoelectron spectroscopy are combined to uncover the fundamental ultrafast photodynamics of three UA derivatives. We find that minor molecular adjustments between derivatives have considerable influence on the overall excited state behavior, leading to different relaxation mechanisms and lifetimes. These findings establish foundations for further molecular design and aid in the interpretation of these derivatives under complex environmental conditions.