On-the-Fly Nonadiabatic Dynamics of Caffeic Acid Sunscreen Compound

Abstract

As a widely-used sunscreen compound, the caffeic acid (CA) shows the strong UV absorption, while the photoinduced reaction mechanisms behind its photoprotection ability are not fully understood. We try to investigate the photoinduced internal conversion dynamics of CA in order to explore the photoprotectiodn mechanism. The most stable CA isomer is selected to examine its nonadiabatic dynamics using the on-the-fly surface hopping simulations at the semi-empirical level of electronic-structure theory. The dynamics starting from different electronic states are simulated to explore the dependence of the photoinduced reaction channels on the excitation wavelengths. Several S1/S0 conical intersections, driven by the H-atom detachments and the ring deformations, have been found to be responsible for the nonadiabatic decay of the CA. The simulation results show that the branching ratio towards these intersections are modified by the light with different excitation energies. This provides the valuable information for the understanding of the photoprotection mechanism of the CA compound.

Characteristic Photoprotective Molecules from the Sphagnum World: A Solution-Phase Ultrafast Study of Sphagnic Acid

“…For example, stronger interactions may impact large-amplitude nuclear rearrangement (such as an isomerisation) which could noticeably impact dynamics. Photophysical relaxation mechanisms hypothesised in previous studies of cinnamates, including ring deformation to reach a CI, are not apparent, but may provide an alternative NRD method for SphA [34,39]. It is not clear whether any photoisomerisation of SphA would result from propagation through this different CI.…”

“…Photophysical relaxation mechanisms hypothesised in previous studies of cinnamates, including ring deformation to reach a CI, are not apparent, but may provide an alternative NRD method for SphA [ 34 , 39 ]. It is not clear whether any photoisomerisation of SphA would result from propagation through this different CI.…”

“…It is not clear whether any photoisomerisation of SphA would result from propagation through this different CI. Surface-hopping and non-adiabatic dynamics simulations are required to generate a more complete image of the potential relaxation mechanisms available to SphA [ 39 ]. The observed pre- and post-irradiation ratios of trans : cis isomers of SphA do, however, demonstrate that photo-induced isomerisation is occurring to an appreciable extent, and so this should lend credence to some understanding of the excited state dynamics of the molecule.…”