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

Understudied and underestimated impacts of organic UV filters on terrestrial ecosystems

Organic UV filters (OUVFs) are vital components in various personal care products (PCPs) and commercial goods, with the annual consumption estimated at 10,000 tons. Consequently, the unavoidable use of OUVFs in PCPs and other unregulated commercial applications could present a considerable risk to human and environmental health. These chemical entities enter terrestrial ecosystems through wastewater discharge, agriculture, atmospheric deposition, and recreational activities. Compared to aqueous ecosystems, the effects of OUVFs on terrestrial environments should be more studied and potentially underestimated. The present review addresses the abovementioned gap by summarizing 189 studies conducted between 2006 and 2024, focusing on the analytical measures, occurrence, and ecotoxicological effects of OUVFs on terrestrial ecosystems. These studies underscore the harmful effects of certain OUVFs on the development, reproduction, and endocrine systems of terrestrial organisms, highlighting the necessity for comprehensive toxicological assessments to understand their impacts on non-target species in terrestrial ecosystems. Besides, by underscoring the ecological effects of OUVFs, this review aims to guide future research and inform regulatory measures to mitigate the risks posed by these widespread contaminants. Meanwhile, interdisciplinary research is essential, integrating environmental science, toxicology, ecology, and chemistry to tackle OUVF challenges in terrestrial ecosystems.

Natural AIEgens as Ultraviolet Sunscreens and Photosynergists for Solar Fuel Production

Bio-nano hybrids (BNH), combining semiconductors and microorganisms, have shown great promise for effective solar-to-fuel energy conversion. However, the high-energy ultraviolet (UV) photons in the solar spectrum can cause severe photocorrosion of semiconductors and irreversible photodamage to microorganisms within BNH. Here, we developed an encapsulation strategy using natural luminogens with aggregation-induced emission characteristics (AIEgens) to construct a protective layer for BNH, effectively shielding them against high-energy UV photons. We incorporated natural berberine (BBR) into the BNH composed of Methanosarcina barkeri and polymeric carbon nitrides (CNx). The self-assembled BNH-BBR system displayed a 2.75-fold higher CH4 yield than BNH under simulated solar irradiation. Mechanism analysis revealed that BBR acted as a UV sunscreen for BNH by converting high-energy short wavelengths into low-energy long wavelengths, thereby reducing the accumulation of reactive oxygen species and alleviating the photocorrosion of CNx. Furthermore, BBR functioned as a photosynergist for BNH by regulating photoelectron production and utilization, enhancing the intracellular energy formation in M. barkeri for growth and metabolism. This work provides important insights into the effective and scalable conversion of CO2 into valuable biofuels with BNH under light illumination containing high-energy photons.

How to fight acute sun damage? Current skin care strategies

Excessive exposure to sunlight can contribute for skin photo-damage, such as sunburn, dryness, wrinkles, hyperpigmentation, immunosuppressive events and skin sensitization reactions. The use of aftersun products is an effective strategy to reduce the visible signs and symptoms of acute photodamage in the skin. Aiming to unveil the active ingredients able to offset acute sun damage, this work focuses on the characterization of the aftersun products market. A total of 84 after-sun formulations from 41 international brands currently marketed in Portugal were analyzed concerning the composition described on the product label, identifying natural and synthetic/semi-synthetic ingredients with the ability to mitigate solar-induced effects. The majority of aftersun formulations contained ingredients derived from terrestrial and marine sources (> 80%). An in-depth examination of these compounds is also offered, revealing the top of the most used natural and synthetic/semi-synthetic ingredients present in aftersun products, as well as their mechanism of action. A critical appraisal of the scientific data was made aiming to highlight the scientific evidence of ingredients able to mitigate skin photodamage. Amino acids and peptides, and A. barbadensis extract were tested for their in vivo efficacy. Nevertheless, all the ingredients were analyzed with in vitro studies as preliminary screening before in vivo, ex vivo and/or clinical studies. In summary, this study provides an overview of the use of active ingredients in commercial aftersun products to understand better the benefits associated with their use in cosmetic formulations and identify opportunities for innovation.

A Multipronged Bioengineering, Spectroscopic and Theoretical Approach in Unravelling the Excited-State Dynamics of the Archetype Mycosporine Amino Acid

Mycosporine glycine (MyG) was produced by the fermentation of a purposely engineered bacterial strain and isolated from this sustainable source. The ultrafast spectroscopy of MyG was then investigated in its native, zwitterionic form (MyGzwitter), via femtosecond transient electronic absorption spectroscopy. Complementary nonadiabatic (NAD) simulations suggest that, upon photoexcitation to the lowest excited singlet state (S1), MyGzwitter undergoes efficient nonradiative decay to repopulate the electronic ground state (S0). We propose an initial ultrafast ring-twisting mechanism toward an S1/S0 conical intersection, followed by internal conversion to S0 and subsequent vibrational cooling. This study illuminates the workings of the archetype mycosporine, providing photoprotection, in the UV-B range, to organisms such as corals, macroalgae, and cyanobacteria. This study also contributes to our growing understanding of the photoprotection mechanisms of life.

Cosmetic Products with Potential Photoprotective Effects Based on Natural Compounds Extracted from Waste of the Winemaking Industry

Grape marc is a by-product resulting from the winemaking industry that still contains beneficial compounds that can be valorized. Thus, we report here the possibility of using polyphenolic extracts of grape marc origin to obtain sun protection creams. The extractions were performed in ethanol and acetone solutions using pomace from different grape varieties (Merlot, Bläufrankisch, Fetească Neagră, Isabella) as a raw material. The obtained extracts were analyzed in order to determine the total phenolic content, the antioxidant activity, and the sun protection factor (SPF) via Mansur spectrophotometric assay. The best results were achieved using 70% ethanol in water as a solvent. The extracts with the highest potential photoprotective effects are from the Merlot variety (SPFspectrophotometric = 7.83 ± 0.76). The sunscreens were prepared using the 70% ethanolic extract of the Merlot variety evaporated to dryness, redissolved in either distilled water or ethanol. The SPF estimated in vitro via the COLIPA method showed values of 14.07 ± 1.50 and 11.46 ± 1.32 for the aqueous and ethanolic extracts, respectively, when working with a cream to polyphenolic extract a ratio of 1/1 (w/w). At the same time, the use of aqueous polyphenolic extracts ensures the better stability of creams compared with the ethanolic ones.

Ring buckling and C=N isomerization pathways for efficient photoprotection in two nature-inspired UVA sunscreens revealed through ultrafast dynamics and high-level calculations

Sunscreens provide a frontline defense for our DNA against the damage caused by ultraviolet (UV) radiation. The active ingredients in topically applied sunscreens that provide this defense are UV filters, which preferentially absorb or reflect UV radiation before it penetrates the skin and interacts with photosensitive nucleic acids. However, there are concerns related to human and environmental toxicity of current UV filters, and consequently a shift toward nature-inspired, particularly microbial, UV filters. In this paper, new physical insight is provided into the fundamental mechanisms of photoprotection in two synthetic analogs of mycosporine-like amino acid-type UV filters, demonstrating new methods of protection that are distinct from those of current commercial sunscreens, extending previous work in this area. Transient absorption measurements (both transient electronic absorption spectroscopy and transient vibrational absorption spectroscopy) are combined with steady-state studies and high-level computational results to aid our mapping of the experimentally derived lifetimes to real-time photodynamic processes. The conclusions reached here pave the way toward developing new and more efficient biomimetic DNA photoprotectant materials.

Ultrafast non-adiabatic dynamics of stilbene-based plant-derived sunscreens with cis-trans isomerization structures

In this work, we investigated the potential UV protection mechanism of the natural compounds hydroxy resveratrol and pterostilbene by combining theoretical calculations and femtosecond transient absorption spectra (FTAS). The UV absorption spectra showed that the two compounds exhibited strong absorption properties and high photostability. We found two molecules will reach the S₁ state or an even higher excited state after UV exposure and molecules in S₁ will cross a lower energy barrier to reach the conical intersection. The adiabatic trans-cis isomerization process happened and finally return to the ground. Meanwhile, FTAS clarified the time scale of trans-cis isomerization of two molecules was ∼ 10 ps, which also met the requirement of fast energy relaxation. This work also provides theoretical guidance for developing new sunscreen molecules from natural stilbene.

Understanding the Impact of Symmetrical Substitution on the Photodynamics of Sinapate Esters Using Gas-Phase Ultrafast Spectroscopy

Two model biomimetic systems, ethyl sinapate (ES) and its symmetrical analogue, diethyl 2-(4-hydroxy-3,5-dimethoxybenzylidene)malonate (or diethyl sinapate, DES), are stripped to their core fundamentals through gas-phase spectroscopy to understand the underlying photophysics of photothermal materials. Following photoexcitation to the optically bright S1(ππ*) state, DES is found to repopulate the electronic ground state over 3 orders of magnitude quicker than its nonsymmetrical counterpart, ES. Our XMS-CASPT2 calculations shed light on the experimental results, revealing crucial differences in the potential energy surfaces and conical intersection topography between ES and DES. From this work, a peaked conical intersection, seen for DES, shows vital importance for the nonradiative ground-state recovery of photothermal materials. This fundamental comparative study highlights the potential impact that symmetrical substitution can have on the photodynamics of sinapate esters, providing a blueprint for future advancement in photothermal technology.

Characteristic Photoprotective Molecules from the Sphagnum World: A Solution-Phase Ultrafast Study of Sphagnic Acid

A natural UV-absorbing chromophore extracted from sphagnum mosses, sphagnic acid, is proposed as a new natural support to chemical UV filters for use in cosmetic applications. Sphagnic acid is structurally related to the cinnamate family of molecules, known for their strong UV absorption, efficient non-radiative decay, and antioxidant properties. In this study, transient electronic absorption spectroscopy is used, in conjunction with steady-state techniques, to model the photodynamics following photoexcitation of sphagnic acid in different solvent systems. Sphagnic acid was found in each system to relax with lifetimes of ~200 fs and ~1.5 ps before generating a cis-isomer photoproduct. This study helps to elucidate the photoprotective mechanism of a new potential natural support to sunscreens, from a unique plant source.

Synthesis and characterisation of novel composite sunscreens containing both avobenzone and octocrylene motifs

Avobenzone and octocrylene are popular sunscreen active ingredients. Experiments that probe the stability of avobenzone in binary mixtures with octocrylene are presented, together with the synthesis of a class of novel composite sunscreens that were designed by covalently linking avobenzone and octocrylene groups. Spectroscopy, both steady-state and time-resolved, of the fused molecules was performed to investigate the stability of the new molecules and their potential function as ultraviolet filters. Computational results are detailed for truncated versions of a subset of the molecules to reveal the energy states underlying the absorption processes of this new class of sunscreen. The results indicate that the combination of elements of the two sunscreen molecules into one molecule creates a derivative with good stability to UV light in ethanol and in which the main degradation pathway of the avobenzone component in acetonitrile is reduced. Derivatives containing p-chloro substituents are particularly stable to UV light.

Indole-5,6-quinones display hallmark properties of eumelanin

Melanins are ubiquitous biopolymers produced from phenols and catechols by oxidation. They provide photoprotection, pigmentation and redox activity to most life forms, and inspire synthetic materials with desirable optical, electronic and mechanical properties. The chemical structures of melanins remain elusive, however, creating uncertainty about their roles, and preventing the design of synthetic mimics with tailored properties. Indole-5,6-quinone (IQ) has been implicated as a biosynthetic intermediate and structural subunit of mammalian eumelanin pigments, but its instability has prevented its isolation and unambiguous characterization. Here we use steric shielding to stabilize IQ and show that 'blocked' derivatives exhibit eumelanin's characteristic ultrafast nonradiative decay and its ability to absorb light from the ultraviolet to the near-infrared. These new compounds are also redox-active and a source of paramagnetism, emulating eumelanin's unique electronic properties, which include persistent radicals. Blocked IQs are atomistically precise and tailorable molecules that can offer a bottom-up understanding of emergent properties in eumelanin and have the potential to advance the rational design of melanin-inspired materials.

Synergic photoprotection of phenolic compounds present in tomato fruit cuticle: a spectroscopic investigation in solution

Coumaric acids and flavonoids play pivotal roles in protecting plants against ultraviolet radiation (UVR) exposure. In this work, we focus our photoprotection studies on p-coumaric acid and the flavonoid naringenin chalcone. Photoprotection is well-understood in p-coumaric acid; in contrast, information surrounding photoprotection in naringenin chalcone is lacking. Additionally, and vitally, how these two species work in unison to provide photoprotection across the UV-B and UV-A is unknown. Herein, we employ transient absorption spectroscopy together with steady-state irradiation studies to unravel the photoprotection mechanism of a solution of p-coumaric acid and naringenin chalcone. We find that the excited state dynamics of p-coumaric acid are significantly altered in the presence of naringenin chalcone. This finding concurs with quenching of the p-coumaric acid fluorescence with increasing concentration of naringenin chalcone. We propose a Förster energy transfer mechanism is operative via the formation of dipole-dipole interactions between p-coumaric acid and naringenin chalcone. To our knowledge, this is the first demonstration in plants of a synergic effect between two classes of phenolics to bypass the potentially damaging effects of UVR.

Examining the substituent effect on mycosporine-inspired ultraviolet filters

Current organic ultraviolet (UV) filters found in sunscreen formulations suffer a number of drawbacks. In this work, we have synthesised four biomimetic molecules built on the mycosporine molecular scaffold (a natural UV filter) with varying substituents at one of the carbons on the ring and investigated their photoprotective properties. From our findings, we infer design guidelines which may have a direct result on the production of next generation UV filters.

Structural and UV-blocking properties of carboxymethyl cellulose sodium/CuO nanocomposite films

Nanoparticles have made a substantial contribution to the field of skincare products with UV filters in preserving human skin from sun damage. The current study aims to create new polymer nanocomposite filters for the efficient block of UV light that results from the stratospheric ozone layer loss. The casting approach was used to add various mass fractions of copper oxide nanoparticles (CuO-NPs) to a solution of carboxymethyl cellulose (CMC). The amorphous nature of CMC was revealed by XRD analysis, with the intensity of the typical peak of virgin polymer in the nanocomposite spectrum decreasing dramatically as the doping amount was increased. The FTIR spectra revealed the functional groups of CMC and the good interaction between the CMC chain and CuO-NPs. Optical experiments revealed that the optical transmittance of pure CMC was over 80%, whereas it dropped to 1% when CuO-NPs content was increased to 8 wt.%. Surprisingly, the inclusion of CuO-NPs considerably improved the UV blocking property of the films extended from the UV region (both UV-A: 320-400 nm and UV-B: 280-320 nm) to the visible region. Optical band gap of CMC decreased sharply with increasing CuO concentration. The tunable optical characteristics can be utilized in UV- blocking filters and various optoelectronics applications.

Direct structural observation of ultrafast photoisomerization dynamics in sinapate esters

Sinapate esters have been extensively studied for their potential application in 'nature-inspired' photoprotection. There is general consensus that the relaxation mechanism of sinapate esters following photoexcitation with ultraviolet radiation is mediated by geometric isomerization. This has been largely inferred through indirect studies involving transient electronic absorption spectroscopy in conjunction with steady-state spectroscopies. However, to-date, there is no direct experimental evidence tracking the formation of the photoisomer in real-time. Using transient vibrational absorption spectroscopy, we report on the direct structural changes that occur upon photoexcitation, resulting in the photoisomer formation. Our mechanistic analysis predicts that, from the photoprepared ππ* state, internal conversion takes place through a conical intersection (CI) near the geometry of the initial isomer. Our calculations suggest that different CI topographies at relevant points on the seam of intersection may influence the isomerization yield. Altogether, we provide compelling evidence suggesting that a sinapate ester's geometric isomerization can be a more complex dynamical process than originally thought.

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.

Molecular Design Learned from the Natural Product Porphyra-334: Molecular Generation via Chemical Variational Autoencoder versus Database Mining via Similarity Search, A Comparative Study

A comparative study is presented. The method via chemical variational autoencoder (VAE) and the method via similarity search are compared, focusing on their generation ability for new functional molecular design. Focusing on the natural porphyra-334 as a model molecule, we generated three groups: molecules of mycosporine-like amino acids (MAAs) as seeds (G SEEDS ), molecules generated via chemical VAE (G VAE ) and molecules gathered via similarity search (G SIM ). The number of molecules that satisfy the condition for the light absorption ability of porphyra-334 in G SEEDS , G VAE , and G SIM are 52, 138, and 6, respectively. The method via chemical VAE shows a promising potential for future molecular design. By using quantum chemistry wave function properties for chemical VAE, we find new molecules that are comparable to porphyra-334, including some with unexpected geometries. At the end, we show a group of molecules found with this method.

UV Filters: Challenges and Prospects

The use of sunscreens is an established and recommended practice to protect skin from solar-induced damage. Around 30 UV filters can be used in sunscreen products in the European Union, which ought to follow the requirements of the regulation 1223/2009 to ensure their efficacy and safety for humans. Nevertheless, low photostability and putative toxicity for humans and environment have been reported for some UV filters. Particularly, the negative impact in marine organisms has recently raised concern on the scientific community. Therefore, it is important to develop new UV filters with improved safety profile and photostability. Over the last two decades, nearly 200 new compounds have revealed promising photoprotection properties. The explored compounds were obtained through different approaches, including exploration of natural sources, synthetic pathways, and nanotechnology. Almost 50 natural products and around 140 synthetic derivatives, such as benzimidazoles, benzotriazoles, hydroxycinnamic acids, xanthones, triazines, among others, have been studied aiming the discovery of novel, effective, and safer future photoprotective agents. Herein, we provide the reader with an overview about UV filters’ challenges and prospects, offering a forward-looking to the next-generation of UV filters.

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.

Unravelling the Photoprotection Properties of Garden Cress Sprout Extract

Plants, as with humans, require photoprotection against the potentially damaging effects of overexposure to ultraviolet (UV) radiation. Previously, sinapoyl malate (SM) was identified as the photoprotective agent in thale cress. Here, we seek to identify the photoprotective agent in a similar plant, garden cress, which is currently used in the skincare product Detoxophane nc. To achieve this, we explore the photodynamics of both the garden cress sprout extract and Detoxophane nc with femtosecond transient electronic absorption spectroscopy. With the assistance of liquid chromatography-mass spectrometry, we determine that the main UV-absorbing compound in garden cress sprout extract is SM. Importantly, our studies reveal that the photoprotection properties of the SM in the garden cress sprout extract present in Detoxophane nc are not compromised by the formulation environment. The result suggests that Detoxophane nc containing the garden cress sprout extract may offer additional photoprotection to the end user in the form of a UV filter booster.

Mechanistic Aspects of the Photophysics of UVA Filters Based on Meldrum Derivatives

Skin photoprotection against UVA radiation is crucial, but it is hindered by the sparsity of approved commercial UVA filters. Sinapoyl malate (SM) derivatives are promising candidates for a new class of UVA filters. They have been previously identified as an efficient photoprotective sunscreen in plants due to their fast nonradiative energy dissipation. Combining experimental and computational results, in our previous letter (J. Phys. Chem. Lett. 2021, 12, 337-344) we showed that coumaryl Meldrum (CMe) and sinapoyl Meldrum (SMe) are outstanding candidates for UVA filters in sunscreen formulations. Here, we deliver a comprehensive computational characterization of the excited-state dynamics of these molecules. Using reaction pathways and excited-state dynamics simulations, we could elucidate the photodeactivation mechanism of these molecules. Upon photoexcitation, they follow a two-step logistic decay. First, an ultrafast and efficient relaxation stabilizes the excited state alongside a 90° twisting around the allylic double bond, giving rise to a minimum with a twisted intramolecular excited-state (TICT) character. From this minimum, internal conversion to the ground state occurs after overcoming a 0.2 eV barrier. Minor differences in the nonradiative decay and fluorescence of CMe and SMe are associated with an additional minimum present only in the latter.

Exploring the Blueprint of Photoprotection in Mycosporine-like Amino Acids

Microorganisms require protection against the potentially damaging effects of ultraviolet radiation exposure. Photoprotection is, in part, provided by mycosporine-like amino acids (MAAs). Previous reports have proposed that nonradiative decay mediates the impressive photoprotection abilities of MAAs. In this letter, we present the first ultrafast dynamics study of two MAAs, shinorine and porphyra-334. We demonstrate that, in aqueous solution, these MAAs relax along their S₁ coordinates toward the S₁/S₀ conical intersection within a few hundred femtoseconds after photoexcitation and then traverse the conical intersection and vibrationally cool in approximately 1 ps through heat transfer to the solvent. This new insight allows a quintessential component of microbial life to be unraveled and informs the development of molecular photon-to-heat converters for a myriad of applications.

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.

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.