Direct structural observation of ultrafast photoisomerization dynamics in sinapate esters

Abstract

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 predicts that, from the photoprepared ππ* state, internal conversion takes place through a conical intersection (CI) near the geometry of the initial isomer. Our calculations suggest that different CI topographies at relevant points on the seam of intersection may influence the isomerization yield. Altogether, we provide compelling evidence suggesting that a sinapate ester's geometric isomerization can be a more complex dynamical process than originally thought.

Fig. 1. Structure and vibrational spectra of ethyl sinapate (ES) isomers.

a E-ES molecular structure, and b Z-ES molecular structure. c Steady-state vibrational spectra of 20 mM solution of E-ES (top panel) and Z-ES (bottom panel) in acetonitrile before (black) and after (red) 6 h irradiation with a solar simulator with irradiance equivalent to 1 sun (1000 W/m²). The overlaid vertical blue lines in c correspond to the calculated frequencies at B3LYP/6-311++G** level of theory in implicit acetonitrile; the lines’ wavenumbers have been scaled with factors of 0.9879 for E-ES and 0.9908 for Z-ES as further discussed in the supplementary information Note S2. Frequency calculations suggest that the vibrational modes centered at ~1605 cm^-1, ~1635 cm⁻¹, and ~1710 cm⁻¹ correspond to aromatic C−H bend and C=C stretch, allylic C=C stretch, and C=O stretch, respectively (see supplementary information Note S2 for details).

Fig. 2. Transient vibrational absorption (TVA) spectra and exponential fittings of E-ES and Z-ES.

a E-ES after photoexcitation with 321 nm, and b Z-ES after photoexcitation with 318 nm, both in acetonitrile. The color key in panels a and b shows the different pump-probe time delays. For both TVA spectra, the probe pulse is centered on 1710 cm⁻¹. Kinetics of the ground-state bleach (GSB) recovery band of E-ES (Z-ES), centered at ~1707 (~1713) cm⁻¹, vibrational relaxation at ~1680 (~1690) cm⁻¹, and photoisomers at 1718 (1708) cm⁻¹, obtained using the KOALA software package are shown in c for E-ES and d for Z-ES.

Fig. 3. Topography of the relaxation pathways of E-ES and Z-ES.

Schematic representation of the CI geometries and potential energy surface (PES) for the photoisomerization starting from Z and E isomers. Solid red and blue lines indicate relaxation on the S₁ surface while dashed lines indicate relaxation in the ground state.