Laser photodissociation experiments on gas-phase protonated and deprotonated oxybenzone reveal how the absorption properties and photodegradation products are significantly affected by pH.

The common sunscreen molecule 2-phenylbenzimidazole-5sulfonic acid (PBSA) is studied in its gas-phase deprotonated form ([PBSA-H] À ) for the first time as an important step in achieving a better understanding of its behavior as a photosensitizer. UV laser-interfaced mass spectrometry is employed, revealing that [PBSA-H] À photofragments into three ionic products (m/z 208, 193, and 80) with distinctive wavelengthdependent production profiles. Both the m/z 208 and 80 channels produce associated neutral free radical species.showing that its hot ground-state dissociates only into m/z 193 (statistical fragment). Therefore, the m/z 208 and 80 fragments which are produced strongly through the UVA/UVB are characterized as non-statistical photofragments associated with non-ergodic excited-state decay. Our observation of nonstatistical photofragments reveal that [PBSA-H] À is not behaving as a model sunscreen molecule. Further, our results indicate that the T 1 state, associated with photosensitization, decays with direct free radical production.[a] N.

Avobenzone (AB) is a widely used UVA filter known to undergo irreversible photodegradation. Here, we investigate the detailed pathways by which AB photodegrades by applying UV laser-interfaced mass spectrometry to protonated AB ions. Gas-phase infrared multiple-photon dissociation (IRMPD) spectra obtained with the free electron laser for infrared experiments, FELIX, (600−1800 cm −1 ) are also presented to confirm the geometric structures. The UV gas-phase absorption spectrum (2.5−5 eV) of protonated AB contains bands that correspond to selective excitation of either the enol or diketo forms, allowing us to probe the resulting, tautomer-dependent photochemistry. Numerous photofragments (i.e., photodegradants) are directly identified for the first time, with m/z 135 and 161 dominating, and m/z 146 and 177 also appearing prominently. Analysis of the production spectra of these photofragments reveals that that strong enol to keto photoisomerism is occurring, and that protonation significantly disrupts the stability of the enol (UVA active) tautomer. Close comparison of fragment ion yields with the TD-DFT-calculated absorption spectra give detailed information on the location and identity of the dissociative excited state surfaces, and thus provide new insight into the photodegradation pathways of avobenzone, and photoisomerization of the wider class of β-diketone containing molecules.

Understanding how deprotonation impacts the photophysics of UV filters is critical to better characterize how they behave in key alkaline environments including surface waters and coral reefs. Using anion photodissociation spectroscopy, we have measured the intrinsic absorption electronic spectroscopy (400–214 nm) and numerous accompanying ionic photofragmentation pathways of the benzophenone-4 anion ([BP4–H] − ). Relative ion yield plots reveal the locations of the bright S 1 and S 3 excited states. For the first time for an ionic UV filter, ab initio potential energy surfaces are presented to provide new insight into how the photofragment identity maps the relaxation pathways. These calculations reveal that [BP4–H] − undergoes excited-state decay consistent with a statistical fragmentation process where the anion breaks down on the ground state after nonradiative relaxation. The broader relevance of the results in providing a basis for interpreting the relaxation dynamics of a wide range of gas-phase ionic systems is discussed.

Sunscreens are essential for protecting the skin from UV radiation, but significant questions remain about the fundamental molecular-level processes by which they operate. In this mini review, we provide an overview of recent advanced laser spectroscopic studies that have probed how the local, chemical environment of an organic sunscreen affects its performance. We highlight experiments where UV laser spectroscopy has been performed on isolated gas-phase sunscreen molecules and complexes. These experiments reveal how pH, alkali metal cation binding, and solvation perturb the geometric and hence electronic structures of sunscreen molecules, and hence their non-radiative decay pathways. A better understanding of how these interactions impact on the performance of individual sunscreens will inform the rational design of future sunscreens and their optimum formulations.

We present the first study to measure the dissociative photochemistry of 2-thiouracil (2-TU), an important nucleobase analogue with applications in molecular biology and pharmacology. Laser photodissociation spectroscopy is applied to the deprotonated and protonated forms of 2-TU, which are produced in the gas-phase using electrospray ionization mass spectrometry. Our results show that the deprotonated form of 2-thiouracil ([2-TU-H]−) decays predominantly by electron ejection and hence concomitant production of the [2-TU-H]· free-radical species, following photoexcitation across the UVA-UVC region. Thiocyanate (SCN−) and a m/z 93 fragment ion are also observed as photodecay products of [2-TU-H]− but at very low intensities. Photoexcitation of protonated 2-thiouracil ([2-TU·H]+) across the same UVA-UVC spectral region produces the m/z 96 cationic fragment as the major photofragment. This ion corresponds to ejection of an HS· radical from the precursor ion and is determined to be a product of direct excited state decay. Fragment ions associated with decay of the hot ground state (i.e., the ions we would expect to observe if 2-thiouracil was behaving like UV-dissipating uracil) are observed as much more minor products. This behaviour is consistent with enhanced intersystem crossing to triplet excited states compared to internal conversion back to the ground state. These are the first experiments to probe the effect of protonation/deprotonation on thionucleobase photochemistry, and hence explore the effect of pH at a molecular level on their photophysical properties.

A key decay pathway by which organic sunscreen molecules dissipate harmful UV energy involves excited-state hydrogen atom transfer between proximal enol and keto functional groups. Structural modifications of this molecular...

The solution and gas-phase dissociative photochemistry of two ruthenium half-sandwich complexes are analysed with electrospray ionisation mass spectrometry in a novel instrument.