Bottom-up excited state dynamics of two cinnamate-based sunscreen filter molecules

Abstract

Methyl-E-4-methoxycinnamate (E-MMC) is a model chromophore of the commonly used commercial sunscreen agent, 2-ethylhexyl-E-4-methoxycinnamate (E-EHMC). In an effort to garner a molecular-level understanding of the photoprotection mechanisms in operation with E-EHMC, we have used time-resolved pump-probe spectroscopy to explore E-MMC's and E-EHMC's excited state dynamics upon UV-B photoexcitation to the S (1ππ*) state in both the gas- and solution-phase. In the gas-phase, our studies suggest that the excited state dynamics are driven by non-radiative decay from the 1ππ* to the S (1nπ*) state, followed by de-excitation from the 1nπ* to the ground electronic state (S). Using both a non-polar-aprotic solvent, cyclohexane, and a polar-protic solvent, methanol, we investigated E-MMC and E-EHMC's photochemistry in a more realistic, 'closer-to-shelf' environment. A stark change to the excited state dynamics in the gas-phase is observed in the solution-phase suggesting that the dynamics are now driven by efficient E/Z isomerisation from the initially photoexcited 1ππ* state to S.

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

“…Differences in the UV/vis absorption spectra of samples illuminated using, for example, an arc lamp and non-illuminated samples can provide information on photochemical stability, unwanted photoinduced production of singlet oxygen, and/or signatures of photodegradation products, 17,122 with NMR spectroscopy offering another route to product identification. 123 Such 'classical' photochemical studies have been crucial in the development of the many commercial sunscreens used today but are not discussed further since they do not report directly on the excited state photophysics that is the primary focus of this Review.…”

“…201,202 These studies served to encourage higher time-resolution pump-probe transient electronic absorption measurements that provide further insights into the early time dynamics following photoexcitation at the wavelength of peak absorption (l max ) of E-OMC, in both cyclohexane and in methanol. 123 Specifically, the E-OMC ( 1 pp*) molecules prepared by photoexcitation are deduced to distort along the E -Z isomerisation coordinate and couple to a region of CI with the S 0 state with a 1-2 ps time constant; a much longer (4ns) transient feature was assigned to the longlived Z-OMC isomer. Again, we note the obvious similarities between the excited state decay mechanisms deduced for this synthetic chemical filter and for the natural sunscreen sinapoyl malate.…”

“…NMR and GC-MS have shown success in identifying products after photoexcitation. 123,218 Identification alone does not ensure adequate evaluation of the safety of the component in a sunscreen product, however. Any full assessment also requires that we investigate the fates of the (potentially reactive) excited states formed when these photoproducts themselves interact with UV light.…”

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 a subsequent study, the trans–cis isomerization in the ππ* state was probed by matrix isolation Fourier transform infrared (FTIR) spectroscopy . The nonradiative pathway mediated by the nπ* state has also been clearly resolved by femtosecond pump‐probe ionization spectroscopy of ( E )‐methyl‐4‐methoxycinnamate ( E isomers of p ‐MMC) and ( E )‐2‐ethylhexyl‐4‐methoxycinnamate ( E ‐EHMC) in the jet . It was probed that a three‐step pathway is followed, involving excited state intramolecular vibrational redistribution (IVR), IC via ππ*/nπ* CI, and subsequent decay to S 0 .…”