Photostability of the deprotonated forms of the UV filters homosalate and octyl salicylate: molecular dissociation versus electron detachment following UV excitation

While common molecular anions show a strong propensity to undergo electron detachment upon UV excitation, this process often occurs in competition with molecular ion dissociation. The factors that affect the balance between these two major possible decay pathways have not been well understood to date. Laser photodissociation spectroscopy of the deprotonated forms of the UV filter molecules, Homosalate (HS) and Octyl Salicylate (OS), i.e. [HS - H]^- and [OS - H]^-, was used to acquire gas-phase UV absorption spectra for [HS - H]^- and [OS - H]^-via photodepletion from 3.0-5.8 eV. No photofragmentation (i.e. dissociation of the ionic molecular framework) was observed for either [HS - H]^- and [OS - H]^- following photoexcitation, revealing that electron loss entirely dominates the electronic decay pathways for these systems. High-level quantum chemical calculations were used to map out the excited states associated with [HS - H]^- and [OS - H]^-, revealing that the minimum-energy crossing points (MECPs) between the S₁ and S₀ states are located in elevated regions of the potential energy surface, making internal conversion unlikely. These results are consistent with our experimental observation that electron detachment out-competes hot ground state molecular fragmentation. More generally, our results reveal that the competition between molecular dissociation and electron detachment following anion photoexcitation can be determined by the magnitude of the energy gap between the excitation energy and the MECPs, rather than being a simple function of whether the excitation energy lies above the anion's vertical detachment energy.

Theoretical Studies on the Excited-State Decay Mechanism of Homomenthyl Salicylate in a Gas Phase and an Acetonitrile Solution

Here, we employ the CASPT2//CASSCF and QM(CASPT2//CASSCF)/MM approaches to explore the photochemical mechanism of homomenthyl salicylate (HMS) in vacuum and an acetonitrile solution. The results show that in both cases, the excited-state relaxation mainly involves a spectroscopically "bright" S₁(¹ππ*) state and the lower-lying T₁ and T₂ states. In the major relaxation pathway, the photoexcited S₁ keto system first undergoes an essentially barrierless excited-state intramolecular proton transfer (ESIPT) to generate the S₁ enol minimum, near which a favorable S₁/S₀ conical intersection decays the system to the S₀ state followed by a reverse ground-state intramolecular proton transfer (GSIPT) to repopulate the initial S₀ keto species. In the minor one, an S₁/T₂/T₁ three-state intersection in the keto region makes the T₁ state populated via direct and T₂-mediated intersystem crossing (ISC) processes. In the T₁ state, an ESIPT occurs, which is followed by ISC near a T₁/S₀ crossing point in the enol region to the S₀ state and finally back to the S₀ keto species. In addition, a T₁/S₀ crossing point near the T₁ keto minimum can also help the system decay to the S₀ keto species. However, small spin-orbit couplings between T₁ and S₀ at these T₁/S₀ crossing points make ISC to the S₀ state very slow and make the system trapped in the T₁ state for a while. The present work rationalizes not only the ultrafast excited-state decay dynamics of HMS but also its low quantum yield of phosphorescence at 77 K.

Nonadiabatic Dynamics Mechanism of Chalcone Analogue Sunscreen FPPO-HBr: Excited State Intramolecular Proton Transfer Followed by Conformation Twisting

Nowadays, traditional sunscreen molecules face many adverse problems: single energy relaxation pathway, lack of adequate UVA light protection, and therefore no longer meeting the growing demand for UVA protection. In this work, we reported a novel sunscreen molecule (E)-3-(5-bromofuran-2-yl)-1-(2-hydroxyphenyl)prop-2-en-1-one (hereinafter referred to as FPPO-HBr) which tackled adverse problems of traditional sunscreen molecules as single energy relaxation pathway, lacking effective UVA light protection. Various nonradiative pathways were proposed and verified by combining the steady-state and femtosecond transient absorption (FTA) spectroscopy and theoretical calculation. Upon UV excitation, the FPPO-HBr mainly decays via excited-state intramolecular proton transfer (ESIPT) followed by conformation twist in ultrafast manner. Importantly, ¹H NMR spectra proved that the FPPO-HBr could not undergo trans-cis photoisomerization. Additionally, excellent photostability was also observed for newly synthesized FPPO-HBr. The current work could provide new perspectives for sunscreen molecules synthesis and mechanism.

Quantum Mechanics/Molecular Mechanics Studies on the Photophysical Mechanism of Methyl Salicylate

Methyl salicylate (MS) as a subunit of larger salicylates found in commercial sunscreens has been shown to exhibit keto-enol tautomerization and dual fluorescence emission via excited-state intramolecular proton transfer (ESIPT) after the absorption of ultraviolet (UV) radiation. However, its excited-state relaxation mechanism is unclear. Herein, we have employed the quantum mechanics(CASPT2//CASSCF)/molecular mechanics method to explore the ESIPT and excited-state relaxation mechanism of MS in the lowest three electronic states, that is, S₀, S₁, and T₁ states, in a methanol solution. Based on the optimized geometric and electronic structures, conical intersections and crossing points, and minimum-energy paths combined with the computed linearly interpolated Cartesian coordinate paths, the photophysical mechanism of MS has been proposed. The S₁ state is a spectroscopically bright ¹ππ* state in the Franck-Condon region. From the initially populated S₁ state, there exist three nonradiative relaxation paths to repopulate the S₀ state. In the first one, the S₁ system (i.e., ketoB form) first undergoes an ESIPT path to generate an S₁ tautomer (i.e., enol form) that exhibits a large Stokes shift in experiments. The generated S₁ enol tautomer further evolves toward the nearby S₁/S₀ conical intersection and then hops to the S₀ state, followed by the backward ground-state intramolecular proton transfer (GSIPT) to the initial ketoB form S₀ state. In the second one, the S₁ system first hops through the S₁ → T₁ intersystem crossing (ISC) to the T₁ state, which then further decays to the S₀ state via T₁ → S₀ ISC at the T₁/S₀ crossing point. In the third path, the T₁ system that stems from the S₁ → T₁ ISC process via the S₁/T₁ crossing point first takes place a T₁ ESIPT to generate a T₁ enol tautomer, which can further decay to the S₀ state via T₁-to-S₀ ISC. Finally, the GSIPT occurs to back the system to the initial ketoB form S₀ state. Our present work could contribute to understanding the photophysics of MS and its derivatives.

Is excited state intramolecular proton transfer frustrated in 10-hydroxy-11H-benzo[b]fluoren-11-one?

Recently, Piechowska and coworker found that hydroxybenzofluorenone 10-hydroxy11H-benzo[b]fluoren-11-one (10-HHBF) does not show dual fluorescence, which is in contrast to its well-known analogue 1-hydroxy-11H-benzo[b]fluoren-11-one (1-HHBF) [Dyes Pigm. 2019, 165, 346-353.]. Based on the increased donor-acceptor distance and the lower stability of the excited state tautomer of former, they believe that different from 1-HHBF, ESIPT is not occurring in 10-HHBF. In the preset work, in order to clarify whether ESIPT would take place in 10-HHBF, we have optimized the four-state geometrical structures (ground state S₀, first singlet excited state S₁, transition state S_1-TS and after proton transfer S_1-PT), carried out the Natural Population Analysis and scanned the ground-state and excited-state potential energy curves of 1-HHBF and 10-HHBF at TD-CAM-B3LYP/6-311 + g(2d,2p)/IEFPCM (cyclohexane) theory level. It is found that ESIPT should take place in both 1-HHBF and 10-HHBF and the Gibbs free energy diagram further indicates that the ESIPT process is more favorable in 10-HHBF than in 1-HHBF.

Insights into the photoprotection mechanism of the UV filter homosalate

Homosalate has been found to exhibit favourable photophysics for inclusion in sunscreens, using a combination of spectroscopic and computational approaches.

QM/MM Studies on the Photophysical Mechanism of a Truncated Octocrylene Model

Methyl 2-cyano-3,3-diphenylacrylate (MCDPA) shares the same molecular skeleton with octocrylene (OCR) that is one of the most common molecules used in commercially available sunscreens. However, its excited-state relaxation mechanism is unclear. Herein, we have used the QM(CASPT2//CASSCF)/MM method to explore spectroscopic properties, geometric and electronic structures, relevant conical intersections and crossing points, and excited-state relaxation paths of MCDPA in methanol solution. We found that in the Franck-Condon (FC) region, the V(¹ππ*) state is energetically lower than the V'(¹ππ*) state only by 2.8 kcal/mol and is assigned to experimentally observed maximum absorption band. From these two initially populated singlet states, there exist three nonradiative relaxation paths to repopulate the S₀ state. In the first one, when the V(¹ππ*) state is populated in the FC region, the system diabatically evolves along the V(¹ππ*) state into its minimum where the internal conversion to S₀ occurs. In the second one, the V'(¹ππ*) state is populated in the FC region and the system adiabatically overcomes a barrier of ca. 3.0 kcal/mol to approach the V(¹ππ*) minimum eventually leading to a V(¹ππ*)-to-S₀ internal conversion. In the third one, the V'(¹ππ*) state first hops via the intersystem crossing to the T₂ state, which then decays through the internal conversion to the T₁ state. The T₁ state is finally converted to the S₀ state via the T₁/S₀ crossing point. Our present work contributes to understanding the photophysics of OCR and its variants.

A novel characteristic of salicylate UV absorbers: suppression of diethylhexyl 2,6-naphthalate (Corapan TQ)-photosensitized singlet oxygen generation

40% of the quantum yield of DEHN-photosensitized singlet oxygen generation is decreased by adding 32 mmol dm⁻³ EHS.

From fundamental science to product: a bottom-up approach to sunscreen development

Despite the pivotal role of ultraviolet (UV) radiation in sustaining life on Earth, overexposure to this type of radiation can have catastrophic effects, such as skin cancer. Sunscreens, the most common form of artificial protection against such harmful effects, absorb UV radiation before it reaches vulnerable skin cells. Absorption of UV radiation prompts ultrafast molecular events in sunscreen molecules which, ideally, would allow for fast and safe dissipation of the excess energy. However, our knowledge of these mechanisms remains limited. In this article, we will review recent advances in the field of ultrafast photodynamics (light induced molecular processes occurring within femtoseconds, fs, 10-15 s to picoseconds, ps, 10-12 s) of sunscreens. We follow a bottom-up approach to common sunscreen active ingredients, analysing any emerging trends from the current literature on the subject. Moreover, we will identify the main questions that remain unanswered, pinpoint some of the main challenges and finally comment on the outlook of this exciting field of research.

Photophysics of a UV-B Filter 4-Methylbenzylidene Camphor: Intersystem Crossing Plays an Important Role

4-Methylbenzylidene camphor (4MBC) is a frequently used ultraviolet (UV) filter in commercial sunscreens, which is experimentally found to undergo efficient intersystem crossing to triplet manifolds followed by predominant radiationless decay to the ground state. However, its photophysical mechanism is unclear. Herein, we have employed combined CASPT2 and CASSCF methods to study the spectroscopic properties, geometric and electronic structures, conical intersections and crossing points, and excited-state deactivation channels of 4MBC. We have found that the V(¹ ππ*) state is populated with large probability in the Franck-Condon region. Starting from this state, there are two efficient nonradiative relaxation processes to populate the ³ ππ* state. In the first one, the V(¹ ππ*) state decays to the V'(¹ ππ*) state. The resultant V'(¹ ππ*) state further jumps to the ¹ nπ* state by internal conversion at the ¹ ππ*/¹ nπ* conical intersection. Then, the ¹ nπ* state hops to the ³ ππ* state through an efficient ¹ nπ*→³ ππ* intersystem crossing process. In the second one, the V(¹ ππ*) state can diabatically relax along the photoisomerization reaction coordinate. In this process, a ¹ ππ*/³ nπ* crossing point helps the ¹ ππ* system decay to the ³ nπ* state, which further decays to the ³ ππ* state through internal conversion at the ³ nπ*/³ ππ* conical intersection. Once the ³ ππ* state is formed, a nearly barrierless relaxation path drives the ³ ππ* system to hop to the S₀ state via the ³ ππ*/S₀ crossing point. Our current work not only rationalizes recent experimental observations but also enriches our photophysical knowledge of UV filters.

Photophysical properties of diethylhexyl 2,6-naphthalate (Corapan TQ), a photostabilizer for sunscreens

60% of the absorbed UV energy is released as fluorescence, while 40% is used to produce the excited triplet state.

Photophysical properties of hexyl diethylaminohydroxybenzoylbenzoate (Uvinul A Plus), a UV-A absorber

DHHB (photostable UV-A absorber) may act as a triplet energy donor for OMC (the most widely used UV-B absorber) because the T₁energy of DHHB is higher than that of OMC.

A Perspective on the Ultrafast Photochemistry of Solution-Phase Sunscreen Molecules

Sunscreens are one of the most common ways of providing on-demand additional photoprotection to the skin. Ultrafast transient absorption spectroscopy has recently proven to be an invaluable tool in understanding how the components of commercial sunscreen products display efficient photoprotection. Important examples of how this technique has unravelled the photodynamics of common components are given in this Perspective, and some of the remaining unanswered questions are discussed.