Sunscreens are aimed at providing protection from solar UV radiation. However, the same mechanism that underlies this protection (absorption of UV radiation) is also responsible for their light-induced adverse effects. Here, high-resolution spectroscopic methods are applied to one of the most commonly used sunscreen chromophores to study the excited-state dynamics that determine the delicate balance between favorable and adverse effects. In contrast to common belief, we find that excitation to the "bright" ππ* state does not directly lead to repopulation of the electronic ground state. Instead, internal conversion to another electronically excited state identified as the "dark" nπ* state is a major decay pathway that impedes fast energy dissipation. Microsolvation studies of sunscreen chromophores with water demonstrate that under such conditions, this bottleneck is no longer present. These observations could be a first step toward the development of sunscreens with improved photochemical properties.

Azobenzene, a versatile and polymorphic molecule, has been extensively and successfully used for photoswitching applications. The debate over its photoisomerization mechanism leveraged on the computational scrutiny with ever-increasing levels of theory. However, the most resolved absorption spectrum for the transition to S1(nπ*) has not followed the computational advances and is more than half a century old. Here, using jet-cooled molecular beam and multiphoton ionization techniques we report the first high-resolution spectra of S1(nπ*) and S2(ππ*). The photophysical characterization reveals directly the structural changes upon excitation and the timescales of dynamical processes. For S1(nπ*), we find that changes in the hybridization of the nitrogen atoms are the driving force that triggers isomerization. In combination with quantum chemical calculations we conclude that photoisomerization occurs along an inversion-assisted torsional pathway with a barrier of ~2 kcal mol−1. This methodology can be extended to photoresponsive molecular systems so far deemed non-accessible to high-resolution spectroscopy.

General rights It is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), other than for strictly personal, individual use, unless the work is under an open content license (like Creative Commons). Disclaimer/Complaints regulationsIf you believe that digital publication of certain material infringes any of your rights or (privacy) interests, please let the Library know, stating your reasons. In case of a legitimate complaint, the Library will make the material inaccessible and/or remove it from the website. Please Ask the Library: http://uba.uva.nl/en/contact, or a letter to: Library of the University of Amsterdam, Secretariat, Singel 425, 1012 WP Amsterdam, The Netherlands. You will be contacted as soon as possible. We report on experimental high-resolution spectroscopic studies in combination with advanced theoretical calculations that focus on the excited-state dynamics of various forms of the chromophore of the Photoactive Yellow Protein (PYP), and the dependence of these dynamics on conformational and isosteric structure, as well as the biological environment. Three-colour nanosecond multiphoton ionization pump-probe studies confirm and extend previous conclusions that the dominant decay channel of the lowest excited pp* state (the so-called V 0 state) of methyl-4-hydroxycinnamate is picosecond internal conversion to the adiabatically lower np* state, and enable us to resolve apparent contradictions with picosecond pump-probe studies. Comparison of multiphoton ionization and laser induced fluorescence excitation spectra leads to the assignment of the hitherto elusive excitation spectrum of the V(pp*) state.Complexation of methyl-4-hydroxycinnamate with water radically changes the excitedstate dynamics; internal conversion to the np* state is absent, and bond isomerization channels instead play a prominent role. Excited states of the thio-ester compound, the form in which the chromophore is present in PYP, have till the present study remained out of reach of gas-phase studies. The excitation spectra obtained here show a broad, almost structureless band system, giving evidence for enhanced nonradiative decay channels. The gas-phase results will be discussed in the context of results from ultrafast studies on these two chromophores in solution.

High-resolution Resonance Enhanced MultiPhoton Ionization (REMPI) and Laser Induced Fluorescence (LIF) excitation spectra of jet-cooled methyl-4-hydroxycinnamate, methyl-4-OD-cinnamate, and of their water clusters have been recorded. Whereas water complexation leads to significant linewidth narrowing, isotopic substitution does for all practical purposes not influence the excited-state dynamics. In this light, we evaluate two previously proposed decay channels of the photoexcited ππ* state involving the dissociative πσ* state (analogous to phenol) and involving the optically dark nπ* state (as concluded for para-coumaric acid). To come to an unambiguous interpretation of the REMPI studies, it has been necessary to determine ionization thresholds. For methyl-4-hydroxycinnamate and its water cluster values of 8.078 and 7.636 eV have been found. Apart from the electronic excitation studies, IR absorption studies have been performed as well. These studies provide important vibrational markers for the assignment of the various conformations that are present under molecular beam conditions, and offer a direct measure of the influence of hydrogen bonding on the properties of the hydroxyl group.

UV excitation and IR absorption spectroscopy on jet-cooled molecules is used to study the conformational heterogeneity of methyl 4-hydroxycinnamate, a model chromophore of the Photoactive Yellow Protein (PYP), and to determine the spectroscopic properties of the various conformers. UV-UV depletion spectroscopy identifies four different species with distinct electronic excitation spectra. Quantum chemical calculations argue that these species are associated with different conformers involving the s-cis/s-trans configuration of the ester with respect to the propenyl C-C single bond and the syn/anti orientation of the phenolic OH group. IR-UV hole-burning spectroscopy is used to record their IR absorption spectra in the fingerprint region. Comparison with IR absorption spectra predicted by quantum chemical calculations provides vibrational markers for each of the conformers, on the basis of which each of the species observed with UV-UV depletion spectroscopy is assigned. Although both DFT and wave function methods reproduce experimental frequencies, we find that calculations at the MP2 level are necessary to obtain agreement with experimentally observed intensities. To elucidate the role of the environment, we compare the IR spectra of the isolated conformers with IR spectra of methyl 4-hydroxycinnamate-water clusters, and with IR spectra of methyl 4-hydroxycinnamate in solution.