Applications of ultrafast spectroscopy to sunscreen development, from first principles to complex mixtures

Spectroscopy and Excited-State Dynamics of Methyl Ferulate in Molecular Beams

The spectroscopic and dynamic properties of methyl ferulate─a naturally occurring ultraviolet-protecting filter─and microsolvated methyl ferulate have been studied under molecular beam conditions using resonance-enhanced multiphoton ionization spectroscopy in combination with quantum chemical calculations. We demonstrate and rationalize how the phenyl substitution pattern affects the state ordering of the lower excited singlet state manifold and what the underlying reason is for the conformation-dependent Franck-Condon (FC) activity in the UV-excitation spectra. Studies on microsolvated methyl ferulate reveal potential coordination sites and the influence of such coordination on the spectroscopic properties. Our quantum chemical studies also enable us to obtain a quantitative understanding of the dominant excited-state decay routes of the photoexcited ππ* state involving a ∼3 ns intersystem crossing pathway to the triplet manifold─which is much slower than found for coumarates─and a relatively fast intersystem crossing back to the ground state (∼30 ns). We show that a common T1/S0 crossing can very well explain the observation that T1 lifetimes are quasi-independent of the phenyl substitution pattern.

Evaluating the Fluorescence Quenching of Troxerutin for Commercial UV Sunscreen Filters

2-Phenylbenzimidazole-5-sulfonic acid (PBSA) and disodium phenyl dibenzimidazole tetrasulfonate (DPDT) are commercially available ultraviolet (UV) sunscreen filters that are known to undergo radiative relaxation following the absorption of UV light. The release of high-energy photons from this relaxation can be detrimental to human health; therefore, fluorescence quenchers need to be incorporated in commercial sunscreen formulations containing PBSA or DPDT. Troxerutin is a fluorescence quencher utilized for DPDT commercially. Here, its ability to quench the fluorescence of both PBSA and DPDT is evaluated using a dual-pronged approach by breaking down the multicomponent problem into its constituent parts. First, PBSA and DPDT's femtosecond to nanosecond photodynamics are uncovered in solution and on the surface of a human skin mimic to ascertain a benchmark. Second, these results are compared to their photodynamics in the presence of troxerutin. A significant reduction in the fluorescence lifetime is observed for both PBSA and DPDT on a human skin mimic with the addition of troxerutin, which is attributed to a Dexter energy transfer (DET) or Förster resonance energy transfer (FRET) quenching mechanism. This finding demonstrates the hitherto unseen fluorescence quenching mechanism of troxerutin on a human skin mimic and its role in quenching the fluorescence of commercial UV sunscreen filters through a DET or FRET mechanism.

Urocanic acid as a novel scaffold for next-gen nature-inspired sunscreens: I. electronic laser spectroscopy under isolated conditions

Urocanic acid is a naturally occurring UV-A and UV-B absorbing compound found in the skin. Its use in artificial sunscreens has been abandoned because of health risks associated with the cis isomer. Here we report laser spectroscopic studies on urocanic acid and various substituted derivatives under supersonically cooled conditions. We find that the spectroscopy and excited-state dynamics of urocanic acid are dominantly determined by the nearly degenerate 1nπ* and 1ππ* electronically excited states. These properties are only affected to a minor extent by esterification of the carboxylic acid group or NH alkylation of the N3H tautomer. Tautomerization, on the other hand, has a much more profound influence and leads-from a photoprotective point of view-to more favorable excited-state dynamics. The approach presented here paves the way to tailoring the photoactive properties of urocanic acid for specific applications amongst which their use as safe UV filters.

New theoretical insights on the nonradiative relaxation mechanism of the core structure of mycosporines: The amino-cyclohexenone central template

We present a comprehensive computational study describing the excited state dynamics and consequent photostability of amino-cyclohexenone (ACyO), the central template of mycosporine systems, widely recognized for their photoprotection of aquatic species. Photoexcitation to the first excited electronic state (S1, 1nπ*) of ACyO is considered an optically dark transition, while photoexcitation to the second excited electronic state (S21ππ*) is an optically bright 1ππ* transition and largely responsible for UV absorption properties of this molecule. We show that following initial photoexcitation to S2, ACyO relaxes via two competing deactivation mechanisms, each mediated by an S1/S0 conical intersection, which directs the excited state population to the electronic ground state (S0). Our ab initio computational results are supported with nonadiabatic dynamics simulation results, yielding an excited state lifetime of ∼280 fs for this system in vacuo. These results explain the inherent photostability of this core structure, commonplace in a wide range of microorganisms.

Valence photoelectron imaging of molecular oxybenzone

An oxybenzone molecule in the gas phase was characterized by mass spectrometry and angle-resolved photoelectron spectroscopy, using both single and multiphoton ionization schemes. A tabletop high harmonic generation source with a monochromator was used for single-photon ionization of oxybenzone with photon energies of up to 35.7 eV. From this, vertical ionization and appearance energies, as well as energy-dependent anisotropy parameters were retrieved and compared with the results from DFT calculations. For two-photon ionization using 4.7 eV light, we found a higher appearance energy than in the extreme ultraviolet (EUV) case, highlighting the possible influence of an intermediate state on the photoionization process. We found no differences in the mass spectra when ionizing oxybenzone by single-photons between 17.2 and 35.7 eV. However, for the multiphoton ionization, the fragmentation process was found to be sensitive to the photoionization order and laser intensity. The "softest" method was found to be two-photon ionization using 4.7 eV light, which led to no measurable fragmentation up to an intensity of 5 × 1012 W cm-2.

Resonance-Enhanced Multiphoton Ionization Studies of the Lower Electronically Excited States of Flavone

The spectroscopic and dynamics properties of flavone─the core chromophore of a wide variety of naturally occurring ultraviolet protecting filters─have been studied under supersonic beam conditions using (1 + 1') resonance-enhanced two-photon ionization spectroscopic techniques. Excitation spectra recorded under such conditions are found to differ significantly from previously reported spectra. Pump-probe studies find that intersystem crossing is the dominant decay pathway of the excited singlet manifold, in agreement with previous solution phase studies and quantum chemical predictions for the isolated molecule. Microsolvation studies on flavone-water clusters reveal that the addition of one and two water molecules leads to considerable shifts in excitation energies but that further complexation does not result in further noticeable shifts. The relaxation pathways of the electronically excited states, on the other hand, do not appear to be influenced by interactions with the solvent molecules. Finally, photoionization spectra have enabled the accurate determination of the adiabatic ionization energy to the ground state of the molecular ion─key to the antioxidant properties of flavone─as 65,415 cm^-1 (8.110 eV).

Excited-State Barrier Controls E → Z Photoisomerization in p-Hydroxycinnamate Biochromophores

Molecules based on the deprotonated p-hydroxycinnamate moiety are widespread in nature, including serving as UV filters in the leaves of plants and as the biochromophore in photoactive yellow protein. The photophysical behavior of these chromophores is centered around a rapid E → Z photoisomerization by passage through a conical intersection seam. Here, we use photoisomerization and photodissociation action spectroscopies with deprotonated 4-hydroxybenzal acetone (pCK^-) to characterize a wavelength-dependent bifurcation between electron autodetachment (spontaneous ejection of an electron from the S₁ state because it is situated in the detachment continuum) and E → Z photoisomerization. While autodetachment occurs across the entire S₁(ππ*) band (370-480 nm), E → Z photoisomerization occurs only over a blue portion of the band (370-430 nm). No E → Z photoisomerization is observed when the ketone functional group in pCK^- is replaced with an ester or carboxylic acid. The wavelength-dependent bifurcation is consistent with potential energy surface calculations showing that a barrier separates the Franck-Condon region from the E → Z isomerizing conical intersection. The barrier height, which is substantially higher in the gas phase than in solution, depends on the functional group and governs whether E → Z photoisomerization occurs more rapidly than autodetachment.

From Biomass-Derived p-Hydroxycinnamic Acids to Novel Sustainable and Non-Toxic Phenolics-Based UV-Filters: A Multidisciplinary Journey

Although organic UV-filters are extensively used in cosmetics to protect consumers from the deleterious effects of solar UV radiation-exposure, they suffer from some major drawbacks such as their fossil origin and their toxicity to both humans and the environment. Thus, finding sustainable and non-toxic UV-filters is becoming a topic of great interest for the cosmetic industry. A few years ago, sinapoyl malate was shown to be a powerful naturally occurring UV-filter. Building on these findings, we decided to design and optimize an entire value chain that goes from biomass to innovative biobased and non-toxic lignin-derived UV-filters. This multidisciplinary approach relies on: 1) The production of phenolic synthons using either metabolite extraction from biomass or their bioproduction through synthetic biology/fermentation/in stream product recovery; 2) their functionalization using green chemistry to access sinapoyl malate and analogues; 3) the study of their UV-filtering activity, their photostability, their biological properties; and 4) their photodynamics. This mini-review aims at demonstrating that combining biotechnology, green chemistry, downstream process and photochemistry is a powerful approach to transform biomass and, in particular lignins, into high value-added innovative UV-filters.

Quantum mechanics/molecular mechanics studies on the mechanistic photophysics of sunscreen oxybenzone in methanol solution

Herein, we have employed the QM(CASPT2//CASSCF)/MM method to explore the photophysical and photochemical mechanism of oxybenzone (OB) in methanol solution. Based on the optimized minima, conical intersections and crossing points, and minimum-energy reaction paths related to excited-state intramolecular proton transfer (ESIPT) and excited-state decay paths in the ¹ππ*, ¹nπ*, ³ππ*, ³nπ*, and S₀ states, we have identified several feasible excited-state relaxation pathways for the initially populated S₂(¹ππ*) state to decay to the initial enol isomer' S₀ state. The major one is the singlet-mediated and stretch-torsion coupled ESIPT pathway, in which the system first undergoes an essentially barrierless ¹ππ* ESIPT process to generate the ¹ππ* keto species, and finally realizes its ground state recovery through the subsequent carbonyl stretch-torsion facilitating S₁ → S₀ internal conversion (IC) and the reverse ground-state intramolecular proton transfer (GSIPT) process. The minor ones are related to intersystem crossing (ISC) processes. At the S₂(¹ππ*) minimum, an S₂(¹ππ*)/S₁(¹nπ*)/T₂(³nπ*) three-state intersection region helps the S₂ system branch into the T₁ state through a S₂ → S₁ → T₁ or S₂ → T₂ → T₁ process. Once it has reached the T₁ state, the system may relax to the S₀ state via direct ISC or via subsequent nearly barrierless ³ππ* ESIPT to yield the T₁ keto tautomer and ISC. The resultant S₀ keto species significantly undergoes reverse GSIPT and only a small fraction yields the trans-keto form that relaxes back more slowly. However, due to small spin-orbit couplings at T₁/S₀ crossing points, the ISC to S₀ state occurs very slowly. The present work rationalizes not only the ultrafast excited-state decay dynamics of OB but also its phosphorescence emission at low temperature.

A Perspective on Femtosecond Pump-Probe Spectroscopy in the Development of Future Sunscreens

Given the negative impacts of overexposure to ultraviolet radiation (UVR) on humans, sunscreens have become a widely used product. Certain ingredients within sunscreens are responsible for photoprotection and these are known, collectively herein, as ultraviolet (UV) filters. Generally speaking, organic UV filters work by absorbing the potentially harmful UVR and dissipating this energy as harmless heat. This process happens on picosecond time scales and so femtosecond pump–probe spectroscopy (FPPS) is an ideal technique for tracking this energy conversion in real time. Coupling FPPS with complementary techniques, including steady-state spectroscopy and computational methods, can provide a detailed mechanistic picture of how UV filters provide photoprotection. As such, FPPS is crucial in aiding the future design of UV filters. This Perspective sheds light on the advancements made over the past two years on both approved and nature-inspired UV filters. Moreover, we suggest where FPPS can be further utilized within sunscreen applications for future considerations.

Elucidating the photoprotective properties of natural UV screening agents: ZEKE-PFI spectroscopy of methyl sinapate

As a prominent derivative of a natural sunscreen, methyl sinapate is an ideal candidate to provide fundamental insight into strategies on how to come to a rational design of artificial sunscreen filters with improved photoprotective properties. Here, static and time-resolved Zero Kinetic Energy-Pulsed Field Ionization (ZEKE-PFI) photoelectron spectroscopy has been used to study the spectroscopy and decay pathways of its electronically excited states. We find that different conformers are subject to distinct structural changes upon electronic excitation, and trace the structural changes that occur upon excitation back to the character of the LUMO. Ionization efficiency spectra in combination with pump-probe ZEKE-PFI spectra are consistent with the conclusion that the long-lived electronically excited state observed in the decay of the lowest excited singlet state is the lowest excited triplet state. Concurrently with providing information on the electronically excited states, the studies allow for a detailed characterization of the spectroscopic properties of the ground state of the radical ion, which is important in the context of the use of cinnamates in nature as antioxidants. Our studies determine the adiabatic ionization energies of the syn/cis, anti/cis and anti/trans conformers as 60 291.1 ± 0.5, 60 366.9 ± 0.5 and 60 503.9 ± 1.0 cm^-1, respectively, and provide accurate vibrational fequencies of low-frequency modes of the molecular ion in its electronic ground state. Finally, the studies emphasize the important role of vibrational and electronic autoionization processes that start to dominate the ionization dynamics in non-rigid molecules of the present size.

Exploring the photoexcited electron transfer dynamics in artificial sunscreen PBSA-coupled biocompatible ZnO quantum dots

Frequent exposure to ultraviolet (UV) radiation without any protection turns out to be a fatal threat leading to skin cancer, necessitating the use of sunscreen cosmetic product with enhanced efficiency to dissipate the UV absorbed 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.

Determining the photostability of avobenzone in sunscreen formulation models using ultrafast spectroscopy

Avobenzone is an ultraviolet (UV) filter that is often included in sunscreen formulations despite its lack of photostability. Its inclusion is necessary due to few existing alternatives for photoprotection in the UVA region (320-400 nm). To better understand and predict the photostability of avobenzone, ultrafast transient electronic absorption spectroscopy (TEAS) has been used to study the effects of solvent (including emollients), concentration and skin surface temperature on its excited-state relaxation mechanism, following photoexcitation with UVA radiation (∼350 nm). Subtle differences between the excited-state lifetimes were found between the systems, but the TEAS spectral features were qualitatively the same for all solution and temperature combinations. Alongside TEAS measurements, UV filter/emollient blends containing avobenzone were irradiated using simulated solar light and their degradation tracked using steady-state UV-visible spectroscopy. Sun protection factor (SPF) and UVA protection factor (UVA-PF) assessments were also carried out on representative oil phases (higher concentration blends), which could be used to formulate oil-in-water sunscreens. It was found that there was an apparent concentration dependence on the long-term photoprotective efficacy of these mixtures, which could be linked to the ultrafast photodynamics by the presence of a ground-state bleach offset. This combination of techniques shows potential for correlating long-term behaviours (minutes to hours) of avobenzone with its ultrafast photophysics (femtoseconds to nanoseconds), bridging the gap between fundamental photophysics/photochemistry and commercial sunscreen design.

A post-HF approach to the sunscreen octyl methoxycinnamate

Octyl methoxycinnamate (2-ethylhexyl 4-methoxycinnamate, OMC) is a commercial sunscreen known as octinoxate with excellent UVB filter properties. However, it is known to undergo a series of photodegradation processes that decrease its effectiveness as a UVB filter. In particular, the trans (E) form-which is considered so far as the most stable isomer-converts to the cis (Z) form under the effect of light. In this work, by using post-Hartree-Fock approaches [CCSD, CCSD(t), and CCSD + T(CCSD)] on ground state OMC geometries optimized at the MP2 level, we show that the cis and trans forms of the gas-phase OMC molecule have comparable stability. Test calculations on the same structures with a series of dispersion-corrected density functional theory-based approaches including the B2PLYP double hybrid predict the trans structures to be energetically favored, missing the subtle stabilization of cis-OMC. Our results suggest that the cis form is stabilized by intra-molecular dispersion interactions, leading to a folded more compact structure than the trans isomer.

Linking Electronic Relaxation Dynamics and Ionic Photofragmentation Patterns for the Deprotonated UV Filter Benzophenone-4

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 S₁ and S₃ 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.

Suppression of menthyl anthranilate (UV-A sunscreen)-sensitized singlet oxygen generation by Trolox and α-tocopherol

Menthyl anthranilate (MA, tradename meradimate) is a UV-A absorber. The interactions of ground-state molecular oxygen with the long-lived triplet state of MA produce singlet oxygen through energy transfer. The quantum yield of singlet oxygen generation is 0.12 in air-saturated ethanol. Kinetic traces of the near-IR phosphorescence of singlet oxygen generated by MA-photosensitization have been measured in the absence and presence of Trolox (a water-soluble analogue of vitamin E and a quencher of singlet oxygen) and α-tocopherol (vitamin E, a natural antioxidant) in ethanol. Fluorescence and transient absorption measurements suggest that Trolox and α-tocopherol quench the lowest excited singlet and triplet states of MA. As a result, Trolox and α-tocopherol suppress MA-photosensitized singlet oxygen generation. Not only the quenching of singlet oxygen but also the suppression of singlet oxygen generation is the mechanism of antioxidant properties of Trolox and α-tocopherol for MA. The ability of α-tocopherol to suppress the MA-photosensitized singlet oxygen generation in isododecane, used as a solvent for an oil-soluble UV absorber, is close to that in ethanol. Suppression of sunscreen-photosensitized singlet oxygen generation is an important method for the formulation of safe cosmetic sunscreens.

Computational and experimental characterization of novel ultraviolet filters

Many current ultraviolet filters present potential hazards both to humans and to the natural environment. As such there is a new impetus to develop, through intimate characterisation, ultraviolet filters for use in cosmeceuticals. Here we report a new class of organic molecules which have a strong absorption band across the ultraviolet-A and -B regions of the electromagnetic spectrum and high photostability. We have performed ultrafast transient electronic absorption spectroscopy and steady-state spectroscopies, alongside computational studies to track and manipulate photoprotection mechanisms. Our results present a potentially new generation of ultraviolet filters for use in commercial formulations.

Sodium cationization can disrupt the intramolecular hydrogen bond that mediates the sunscreen activity of oxybenzone

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 architecture have the potential to block ultrafast decay processes, and hence promote direct excited-state molecular dissociation, profoundly affecting the efficiency of an organic sunscreen. Herein, we investigate the binding of alkali metal cations to a prototype organic sunscreen molecule, oxybenzone, using IR characterization. Mass-selective IR action spectroscopy was conducted at the free electron laser for infrared experiments, FELIX (600-1800 cm-1), on complexes of Na+, K+ and Rb+ bound to oxybenzone. The IR spectra reveal that K+ and Rb+ adopt binding positions away from the key OH intermolecular hydrogen bond, while the smaller Na+ cation binds directly between the keto and enol oxygens, thus breaking the intramolecular hydrogen bond. UV laser photodissociation spectroscopy was also performed on the series of complexes, with the Na+ complex displaying a distinctive electronic spectrum compared to those of K+ and Rb+, in line with the IR spectroscopy results. TD-DFT calculations reveal that the origin of the changes in the electronic spectra can be linked to rupture of the intramolecular bond in the sodium cationized complex. The implications of our results for the performance of sunscreens in mixtures and environments with high concentrations of metal cations are discussed.

Unravelling the Photoprotective Mechanisms of Nature-Inspired Ultraviolet Filters Using Ultrafast Spectroscopy

There are several drawbacks with the current commercially available ultraviolet (UV) filters used in sunscreen formulations, namely deleterious human and ecotoxic effects. As a result of the drawbacks, a current research interest is in identifying and designing new UV filters. One approach that has been explored in recent years is to use nature as inspiration, which is the focus of this review. Both plants and microorganisms have adapted to synthesize their own photoprotective molecules to guard their DNA from potentially harmful UV radiation. The relaxation mechanism of a molecule after it has been photoexcited can be unravelled by several techniques, the ones of most interest for this review being ultrafast spectroscopy and computational methods. Within the literature, both techniques have been implemented on plant-, and microbial-inspired UV filters to better understand their photoprotective roles in nature. This review aims to explore these findings for both families of nature-inspired UV filters in the hope of guiding the future design of sunscreens.

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.

Effects of solvent and micellar encapsulation on the photostability of avobenzone

Avobenzone, the only UVA-absorbing molecule approved for use in sunscreens by the FDA, degrades to its diketone structure under UV light. We found that this photoisomerization is effectively prevented when avobenzone is sequestered in micelles.