Substitution Dependent Ultrafast Ultraviolet Energy Dissipation Mechanisms of Plant Sunscreens

Abstract

An ultraviolet energy dissipation mechanism plays a critical role in the photoprotection effect of sunscreens. In this work, we discovered substitution dependent UV energy dissipation mechanisms of model plant sunscreen methyl sinapate (MS). We found that the initially populated V­(ππ*) states of MS and p-OMeMS relax to the ground state nonradiatively along an ultrafast trans–cis photoisomerization in tens of picoseconds. However, for p-HMS, an internal conversion from V­(ππ*) to a relative dark V′(ππ*) state occurs in less than 1 ps, leading to a branching of the excited-state relaxations. The V (ππ*) state still relaxes nonradiatively as in the case of MS and p-OMeMS. In contrast, the V′(ππ*) state decays to the ground state mainly by emitting photons, exhibiting a lifetime as long as 5 ns. It is the first time to definitely distinguish the dynamics between V­(ππ*) and V′(ππ*) states in the study of sinapates and cinnamates. These results indicate the anticipation of the V′(ππ*) state should be avoided when designing sunscreens.

Unravelling the Photoprotective Mechanisms of Nature-Inspired Ultraviolet Filters Using Ultrafast Spectroscopy

“…In addition to the previously reported relaxation mechanism in MS [ 104 , 109 , 110 , 111 ], the findings revealed that for the thermally stable trans -MS, the S 1 /S 0 CI is reached on the excited PES after a barrier is overcome along the allylic C=C double bond. This then facilitates the non-adiabatic relaxation to the ground state, generating either the original trans -MS or a cis -MS photoproduct.…”

“…In the work by Zhao et al [ 111 ], the effect of para substituents on the excited-state photodynamics of MS (the trans - form) including para methoxy methyl sinapate ( p -OMeMS) and para hydrogen methyl sinapate ( p -HMS) shown in Figure 6 were investigated. Briefly, the findings revealed that p -OMeMS and MS undergo similar dynamics along the trans–cis photoisomerisation coordinate, as seen in previous studies [ 104 , 109 , 110 ].…”

“…p -HMS, on the other hand, undergoes different dynamics with an internal conversion from the initial optically bright excited state (denoted V(ππ*)) to a relative dark state (denoted V’(ππ*)), which results into branching of the excited-state relaxation. While the V(ππ*) still relaxes non-radiatively, as seen in MS and p -OMeMS, the V’(ππ*) decays to the ground state via fluorescence (i.e., emission of a photon) in 5 ns [ 111 ]. The radiative decay reported for the relaxation mechanism of p -HMS in the excited state is undesirable of a UV filter; it could result in the formation of harmful photoproducts (including radicals), which implies that it is not a good candidate for inclusion in a sunscreen [ 54 ].…”

“… Chemical structures of sinapate ester derivatives including p -hydrogen methyl sinapate ( p -HMS), p -methoxy methyl sinapate ( p -OMEMS), trans -methyl sinapate ( trans -MS) and cis -methyl sinapate ( cis -MS) studied by Zhao et al [ 111 , 113 ]. Also shown are trans -ethyl sinapate ( trans -ES) and cis -ethyl sinapate ( cis -ES) studied by Horbury et al [ 114 ].…”

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

“…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].…”