Theoretical insights on photoprotective mechanisms and relaxation dynamics of the sinapate family: Sinapic acid

Sinapic acid, a key natural chromophore in plant ultraviolet (UV)-protective systems, functions as an effective plant-derived sunscreen. In this work, we deliver a detailed account of its photophysical behavior and excited-state dynamics using high-level quantum chemical methods. By combining analyses of nonradiative relaxation pathways with excited-state dynamics simulations, we unravel the sinapic acid's deactivation mechanism and thus its photostability. Upon photoexcitation, the bright S1 (1ππ*) state dominates the UV absorption due to its large oscillator strength. From a photophysical standpoint, sinapic acid undergoes ultrafast internal conversion via an E/Z photoisomerization pathway, funneling from the S1(ππ*) state back to the ground (S0) state. This process features a ∼90° twist around the allylic double bond, traversing through a conical intersection with pronounced twisted intramolecular charge-transfer character. Nonadiabatic dynamics simulations corroborate this picture, revealing an S1/S0 conical intersection that drives rapid deactivation, consistent with the observed sub-picosecond lifetime (∼17 ps). These insights not only clarify transient electronic absorption spectroscopy results previously reported for sinapic acid but also point toward new design principles for next-generation plant-based sunscreen agents with superior photochemical performance.