The direct observation of the doorway ¹nπ* state of methylcinnamate and hydrogen-bonding effects on the photochemistry of cinnamate-based sunscreens

Abstract

The electronic states and photochemistry including nonradiative decay (NRD) and trans(E) → cis(Z) isomerization of methylcinnamate (MC) and its hydrogen-bonded complex with methanol have been investigated under jet-cooled conditions.

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].…”

A systematic approach to methyl cinnamate photodynamics

“…approach by combining resonance two-photon ionisation and theoretical calculations to investigate the electronic states and the non-radiative decay route of MC and the MC-MeOH complex [34]. In this study, it was suggested that the trans-cis isomerisation of MC upon UV radiation is mediated by a triplet state (1 3 ππ*), via the pathway: 1 1 ππ* → 1 1 nπ* → 1 3 ππ* → S 0 (trans or cis); similar to other para-substituted cinnamates [35,36].…”

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.…”