Direct Structural Observation of Ultrafast Photoisomerization Dynamics in Sinapate Esters

Sinapate esters have been extensively studied for their potential application in ‘nature-inspired’ photoprotection. There is general consensus that the relaxation mechanism of sinapate esters following photoexcitation with ultraviolet radiation is mediated by geometric isomerization. This has been largely inferred through indirect studies involving transient electronic absorption spectroscopy in conjunction with steady-state spectroscopies.  However, to-date, there is no direct experimental evidence tracking the formation of the photoisomer in real-time. Using transient vibrational absorption spectroscopy, we report on the direct structural changes that occur upon photoexcitation, resulting in the photoisomer formation. Our mechanistic analysis suggests that, from the photoprepared ππ^* state, internal conversion takes place through a conical intersection (CI) near the geometry of the initial isomer. We demonstrate that the isomerization yield strongly depends on the different topographies of CI near each of the initial isomer geometries. In so doing, we provide compelling evidence suggesting that a sinapate ester's geometric isomerization can be a more complex dynamical process than originally thought.