Electronic State and Photophysics of 2-Ethylhexyl-4-methoxycinnamate as UV-B Sunscreen under Jet-Cooled Condition

Abstract

The title compound, 2-ethylhexyl-4-methoxycinnamate (2EH4MC), is known as a typical ingredient of sunscreen cosmetics that effectively converts the absorbed UV-B light to thermal energy. This energy conversion process includes the nonradiative decay (NRD): trans–cis isomerization and finally going back to the original structure with a release of thermal energy. In this study, we performed UV spectroscopy for jet-cooled 2EH4MC to investigate the electronic/geometrical structures as well as the NRD mechanism. Laser-induced-fluorescence (LIF) spectroscopy gave the well-resolved vibronic structure of the S1–S0 transition; UV–UV hole-burning (HB) spectroscopy and density functional theory (DFT) calculations revealed the presence of syn and anti isomers, where the methoxy (−OCH3) groups orient in opposite directions to each other. Picosecond UV–UV pump–probe spectroscopy revealed the NRD process from the excited singlet (S1 (1ππ*)) state occurs at a rate constant of ∼1010–1011 s–1, attributed to internal conversion (IC) to the 1nπ* state. Nanosecond UV–deep UV (DUV) pump–probe spectroscopy identified a transient triplet (T1 (3ππ*)) state, whose energy (from S0) and lifetime are 18 400 cm–1 and 20 ns, respectively. These results demonstrate that the photoisomerization of 2EH4MC includes multistep internal conversions and intersystem crossings, described as "S1 (trans, 1ππ*) → 1nπ* → T1 (3ππ*) → S0 (cis)".

Excited-state dynamics of isolated and (micro)solvated methyl sinapate: the bright and shady sides of a natural sunscreen

“…In line with the conclusion that ISC in MS is less favoured than in the cinnamates and coumarates we indeed observe (i) decay rates of V(ππ*) vibronic levels that are several orders of magnitude slower (on the order of 10 9 s −1 ) and (ii) competition with direct internal conversion pathways to the ground state. Ebata et al have shown for several coumarates and cinnamates including sinapic acid that in the lowest triplet state the molecule adopts a perpendicular geometry for the vinyl double bond [26,29,30,33,37]. In line with these results we find a similar geometry for the lowest triplet state of MS.…”

“…Since the ionisation energy of the lower-lying electronically excited states is higher than from the initially excited state, they will be more efficiently ionised when ionisation takes place with 193 nm instead of ∼ 320 nm. Such a situation is quite similar to what has been observed for cinnamates and coumarates for which it was concluded that after excitation of the bright 1 ππ* state internal conversion to a dark 1 nπ * state and intersystem crossing to the triplet manifold occurred [15,26,30,33,37,46]. We thus conclude that internal conversion pathways to other electronically excited states also play a significant role for MS.…”

“…Although these compounds have been known for a long time, highresolution laser spectroscopic studies on 2-ethylhexyl-4methoxycinnamate (EHMC) -the most common UV-B filter found in commercial sunscreens-led to the realisation that high-resolution studies in the frequency domain and time-resolved studies in the time domain can contribute significantly to the development of novel sunscreens [15][16][17]. As a result, recent years have witnessed a rapidly increasing interest in the detailed understanding of the spectroscopic properties of the electronically excited states of these compounds and their dynamics, and how to employ this knowledge to design and develop UV filters with improved properties [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33].…”

“…As a result, excitation of the bright ππ* states does not lead to fast internal conversion to the ground state as one ideally would like to have. Instead, the dominant decay pathway is internal conversion to the 1 nπ * state, giving rise to the observation of a decay on a time scale of tens of nanoseconds of an electronically excited state that has been identified by Ebata et al as the lowest excited triplet state [33,37]. Such long-lived states are clearly detrimental to the purpose of these filters, and can give rise to adverse side reactions.…”

A Post-HF Approach to the Sunscreen Octyl Methoxycinnamate

“…30 On the other hand, in solution environments (cyclohexhane and ethanol) the trans-cis isomerization process becomes of greater relevance for the 2 Post-HF Octyl Methoxycinnamate photochemistry of this molecule. 30 Very recently, Muramatsu et al 31 adopted Resonant Two-foton Ionization (R2PI) and Laser-Induced-Fluorescence (LIF) to study the electronic spectrum of jetcooled trans-octinoxate. Using the UV/UV hole-burning technique and Density Functional Theory (DFT) calculations, these authors reported the existence of at least three isomers, which should be responsible of the broad feature observed in the R2PI spectra of jet-cooled octinoxate.…”