Multistep Intersystem Crossing Pathways in Cinnamate-Based UV-B Sunscreens

Abstract

The nonradiative decay pathways of jet-cooled para-methoxy methylcinnamate (p-MMC) and para-methoxy ethylcinnamate (p-MEC) have been investigated by picosecond pump-probe and nanosecond UV-Deep UV pump-probe spectroscopy. The possible relaxation pathways were calculated by the (time-dependent) density functional theory. We found that p-MMC and p-MEC at low excess energy undergo multistep intersystem crossing (ISC) from the bright S (ππ*) state to the lowest triplet T (ππ*) state via two competing pathways through the T state in the time scale of 100 ps: (a) stepwise ISC followed after the internal conversion (IC) from S to the dark nπ* state; (b) direct ISC from the S to T states. These picosecond multistep ISCs result in the torsion of C═C double bond by ∼95° in the T state, whose measured adiabatic energy and lifetime are 16577 cm and ∼20 ns, respectively, for p-MMC. These results suggest that the ISC processes play an indispensable role in the photoprotecting sunscreens in natural plants.

Photoprotection: extending lessons learned from studying natural sunscreens to the design of artificial sunscreen constituents

“…Such conclusions are also consistent with those advanced to explain TR-MS gas-phase results obtained for the simpler analogue, methyl-4-methoxycinnamate, 198 though a recent combined experimental and computational (TD-DFT) study has also argued the need for a triplet state pathway to reconcile the observed dynamics. 199 Efforts to extend such measurements to the solution-phase include gas-phase microsolvation studies involving MMC. 194,197,198 TR-MS studies of MMC-H 2 O clusters show that microsolvation accelerates the excited state relaxation.…”

Photophysics of sunscreen molecules in the gas phase: a stepwise approach towards understanding and developing next-generation sunscreens

“…As a final note, it is also important to discuss a most recent study where Yamazaki et al. [134] have presented both experimental and computational data which strongly implicates that triplet states might also be involved in the photophysics of MMC. The authors propose that, upon absorption of UV radiation, and in addition to the aforementioned IC from the bright 1 ππ * state to the 1 nπ * state, MMC undergoes ISC from both the 1 ππ * and 1 nπ * states into a manifold of triplet states.…”

“…[127] could then, according to Yamazaki et al. [134], also correspond to the population trapped in these triplet states. These results highlight the role of triplet states in the photophysics of sunscreen molecules, which will be explored later in this review.…”

TDDFT and MS‐CASPT2 Study of the Excited States of Para‐Methoxymethylcinnamate

“…The core structure of 2‐ethylhexyl‐4‐methoxycinnamate is that of para ‐methoxymethylcinnamate ( p ‐MMC), which plays a crucial role as a UV filter. Therefore, numerous experimental and theoretical investigations have been performed in order to elucidate the mechanism of the UV‐blocking activity of p ‐MMC …”

“…In the same study, solution phase transient absorption spectroscopy showed that an efficient nonradiative decay along with the E–Z isomerization coordinate was activated by a destabilization of the nπ* state due to interaction with an aprotic nonpolar solvent. Furthermore, UV‐deep UV pump‐probe spectroscopy showed that there exists an additional nonradiative relaxation pathway, which is mediated by the triplet ππ* state …”