Towards elucidating the photochemistry of the sunscreen filter ethyl ferulate using time-resolved gas-phase spectroscopy

Abstract

Ultrafast time-resolved ion yield (TR-IY) and velocity map imaging spectroscopies are employed to reveal the relaxation dynamics after photoexcitation in ethyl 4-hydroxy-3-methoxycinnamate (ethyl ferulate, EF), an active ingredient in commercially available sunscreens. In keeping with a bottom-up strategy, the building blocks of EF, 2-methoxy-4-vinylphenol (MVP) and 4-hydroxy-3-methoxycinnamyl alcohol (coniferyl alcohol, ConA), were also studied to assist in our understanding of the dynamics of EF as we build up in molecular complexity. In contrast to the excited state dynamics of MVP and ConA, which are described by a single time constant (>900 ps), the dynamics of EF are described by three time constants (15 ± 4 ps, 148 ± 47 ps, and >900 ps). A mechanism is proposed involving internal conversion (IC) between the initially excited S(1ππ*) and S(1nπ*) states followed by intramolecular vibrational redistribution (IVR) on both states, in competition with intersystem crossing onto neighbouring triplet states (15 ± 4 ps). IVR and IC within the triplet manifold then ensues (148 ± 47 ps) to populate a low-lying triplet state (>900 ps). Importantly, the fluorescence spectrum of EF at the S origin, along with the associated lifetime (6.9 ± 0.1 ns), suggests that population is trapped, during initial IVR, on the S(1ππ*) state. This serves to demonstrate the complex, competing dynamics in this sunscreen filter molecule.

The role of symmetric functionalisation on photoisomerisation of a UV commercial chemical filter

“…Several reports have suggested the presence of 1 np* states playing a role within relaxation of similar systems. 8,28,45 Fluorescence has also been reported for similar systems 7,20,46 and was again observed here with the quantum yield reported as 0.039 AE 0.002 in cyclohexane (see ESI, † Fig. S17), hence a relatively minor pathway.…”

“…[1][2][3][4][5] Chemical filters, found in commercial formulations, have been designed to allay such overexposure. 6 Chemical filters based on cinnamates 7,8 are currently being used as a class of UVR absorbing molecules in commercial formulations due to their strong absorption of UVR and potent antioxidant properties. [9][10][11][12] Investigations into their photochemistry has shown a photoisomerisation pathway as a main route of deactivation, irrespective of solvent environment, [13][14][15][16][17] and starting geometric isomer (either E or Z); 14,18 although a change in lifetimes is noted.…”

Conservation of ultrafast photoprotective mechanisms with increasing molecular complexity in sinapoyl malate derivatives

“…This relaxation mechanism, being mediated by the trans-cis isomerisation coordinate, is particularly interesting since it is readily observed in the solution phase and involves ππ* dynamics, whereas analogous gas-phase experiments involve significant nπ* dynamics and the different isomers are not readily distinguished. [15,[17][18][19]21,22] It is on this solution phase mechanism where this work focusses. We present transient absorption measurements of sinapoyl malate derivatives (Figure 1(A)) whose carboxyl groups are augmented to include increasingly large alkane chains to observe any changes to the trans-cis mediated relaxation pathway.…”

Gas‐Solution Phase Transient Absorption Study of the Plant Sunscreen Derivative Methyl Sinapate

“…One class of chemical filters which have been the subject of recent studies are cinnamate derivatives, such as sinapoyl malate, the naturally occurring sunscreen molecule deposited in the leaves of Arabidopsis thaliana (see Figure ) . Studies of sinapoyl malate, and the subunit ethyl ferulate, show distinct photophysical relaxation mechanisms depending on the phase of the sample. For example, in gas‐phase ethyl ferulate, the initially excited population relaxes to a long‐lived triplet state over ∼150 ps .…”

“…Studies of sinapoyl malate, and the subunit ethyl ferulate, show distinct photophysical relaxation mechanisms depending on the phase of the sample. For example, in gas‐phase ethyl ferulate, the initially excited population relaxes to a long‐lived triplet state over ∼150 ps . In the solution‐phase, even under the weakly perturbing influence of cyclohexane, the relaxation occurs on a timescale of 10’s of ps, mediated by a trans ‐ cis isomerisation about ethyl ferulate's aliphatic C=C bond.…”