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.

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Ultrafast time-resolved ion yield (TR-IY) and velocity map imaging spectroscopies are employed to reveal the relaxation dynamics after photoexcitation in ethyl 4-hydroxy-3-methoxycinnamate (ethyl ferulate, EF), an active ingredient in commercially available sunscreens. In keeping with a bottom-up strategy, the building blocks of EF, 2-methoxy-4-vinylphenol (MVP) and 4-hydroxy-3-methoxycinnamyl alcohol (coniferyl alcohol, ConA), were also studied to assist in our understanding of the dynamics of EF as we build up in molecular complexity. In contrast to the excited state dynamics of MVP and ConA, which are described by a single time constant (>900 ps), the dynamics of EF are described by three time constants (15 ± 4 ps, 148 ± 47 ps, and >900 ps). A mechanism is proposed involving internal conversion (IC) between the initially excited S(1ππ*) and S(1nπ*) states followed by intramolecular vibrational redistribution (IVR) on both states, in competition with intersystem crossing onto neighbouring triplet states (15 ± 4 ps). IVR and IC within the triplet manifold then ensues (148 ± 47 ps) to populate a low-lying triplet state (>900 ps). Importantly, the fluorescence spectrum of EF at the S origin, along with the associated lifetime (6.9 ± 0.1 ns), suggests that population is trapped, during initial IVR, on the S(1ππ*) state. This serves to demonstrate the complex, competing dynamics in this sunscreen filter molecule.

To explore the effects of ring substitution on dissociation dynamics, the primary photochemistry of 2-ethylpyrrole was explored using ultrafast ion imaging techniques. Photoexcitation to the S state, a πσ* state, in the range from 238 to 265 nm results in cleavage of the N-H bond with an H atom appearance lifetime of ca. 70 fs. The insensitivity of this lifetime to photon energy, combined with a small kinetic isotope effect, suggests that tunneling does not play a major role in N-H bond cleavage. Total kinetic energy release spectra reveal modest vibrational excitation in the radical counter-fragment, increasing with photon energy. At wavelengths less than or equal to 248 nm, an additional low kinetic energy H atom loss mechanism becomes available with an appearance lifetime of ∼1.5 ps, possibly due to the population of higher-lying ππ* states.

The mechanism for interconversion between the nuclear spin isomers (NSI) of HO remains shrouded in uncertainties. The temperature dependence displayed by NSI interconversion rates for HO isolated in an argon matrix provides evidence that confinement effects are responsible for the dramatic increase in their kinetics with respect to the gas phase, providing new pathways for o-HO↔p-HO conversion in endohedral compounds. This reveals intramolecular aspects of the interconversion mechanism which may improve methodologies for the separation and storage of NSI en route to applications ranging from magnetic resonance spectroscopy and imaging to interpretations of spin temperatures in the interstellar medium.

PbTiO (PT) and PbZrO (PZ) are the two primary blocks of the solid solution PbZr Ti O (PZT). They can be modelled in different ways; but, in order to do comparable DFT calculations on PZT, with different values of x, one must find a unique method that can be used for both PT and PZ. In particular, we want to evaluate their vibrational properties to compare them with experimental data. Density functional theory (DFT) is used to perform structure geometry optimizations and electronic structure calculations, both on low- and high-temperature phase. Then, harmonic vibrational frequencies of their low-temperature phase are determined for transverse and longitudinal optical (TO & LO) phonons. Moreover, a detailed study of the eigenvectors shows that accurate calculations are necessary to correctly interpret and understand the IR spectra. In the end, the comparison of our theoretical results with previous experimental and theoretical data confirm the strong potential of the SOGGA (second-order generalized gradient approximation) functional to correctly describe PT, PZ and, hopefully, PZT; especially their structural and vibrational properties.

The perovskites lead zirconium titanate, PbZr1–x Ti x O3 (0 < x < 1), known as PZT, are solid solutions widely exploited for their strong piezoelectric properties. The utmost technological importance of this class of materials led to considerable activity on piezoelectric films from the experimental and simulation side. In a solid solution like PZT, the distribution of Zr and Ti atoms has no long-range ordering. Thus, these materials are highly challenging to model theoretically but also to investigate experimentally. In this study, we combine infrared (IR) absorption spectroscopy, with a Density Functional Theory method adapted for the calculation of solid solutions. The complexity of PZT material is reproduced through a combination of 2 × 2 × 2 supercells. Using such combination procedure, we show here that ab initio calculations shed light on the interpretation of IR measured absorption spectra for thin films of 5, 10, 45, 90 nm with composition x = 0.75 in PbZr1–x Ti x O3, as well as for pure PbZrO3 and PbTiO3. Furthermore, the simulation on the supercell structure was also performed for multiple compositions 0.5 ≤ x ≤ 1 in both tetragonal and cubic structures, allowing the understanding of the evolution of spectra with temperature (during the tetragonal to cubic phase transition) and with doping. This temperature study reveals the first experimental evidence of polar modes associated with the loss of spontaneous polarization in nanometer size PZT films, deposited on a silicon wafer. This combination of experimental and theoretical methods opens the way for the investigation of other solid solution materials.