Elucidating nuclear motions in a plant sunscreen during photoisomerization through solvent viscosity effects

Ultrafast restricted intramolecular rotation in molecules with aggregation induced emission

In this work, the ultrafast intramolecular rotation behavior of 1,1,2,3,4,5-hexaphenylsilole has been investigated in several solutions with different viscosities using femtosecond transient absorption spectroscopy combined with density functional theory and time-dependent density functional theory calculations. It is demonstrated that the nonradiative process, which competes with radiative decay, involves two main stages, namely the restricted intramolecular rotation and internal conversion processes. The intramolecular rotation depends on viscosity and presents a significant restriction. The restricted rotational rate is determined to be dozens of picoseconds. The following nonradiative process is strongly dominated by intramolecular rotation. The nonradiative decay rate will decrease with the increase in viscosity, leading to a rise in the radiative probability and photoluminous yield. These results have borne out the mechanism of ultrafast restricted intramolecular rotation of aggregation induced emission and provided a detailed photophysical picture of nonradiative processes.

An expeditive and green chemo-enzymatic route to diester sinapoyl-l-malate analogues: sustainable bioinspired and biosourced UV filters and molecular heaters

Sinapoyl malate, naturally present in plants, has proved to be an exceptional UV filter and molecular heater for plants. Although there are nowadays industrially relevant sustainable synthetic routes to sinapoyl malate, its incorporation into certain cosmetic formulations, as well as its adsorption on plant leaves, is limited by its hydrophilicity. To overcome these obstacles, it is important to find a way to effectively control the hydrophilic-lipophilic balance of sinapoyl malate to make it readily compatible with the cosmetic formulations and stick on the waxy cuticle of leaves. To this end, herein, we describe a highly regioselective chemo-enzymatic synthesis of sinapoyl malate analogues possessing fatty aliphatic chains of variable length, enabling the lipophilicity of the compounds to be modulated. The potential toxicity (i.e., mutagenicity, carcinogenicity, endocrine disruption, acute and repeated-dose toxicity), bioaccumulation, persistence and biodegradability potential of these new analogues were evaluated in silico, along with the study of their transient absorption spectroscopy, their photostability as well as their photodegradation products.

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.

Spectroscopic insight on impact of environment on natural photoprotectants

Biomimicry has become a key player in researching new materials for a whole range of applications. In this study, we have taken a crude extract from the red algae Palmaria palmata containing mycosporine-like amino acids - a photoprotective family of molecules. We have applied the crude extract onto a surface to assess if photoprotection, and more broadly, light-to-heat conversion, is retained; we found it is. Considering sunscreens as a specific application, we have performed transmission and reflection terahertz spectroscopy of the extract and glycerol to demonstrate how one can monitor stability in real-world 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.

Radiationless mechanism of UV deactivation by cuticle phenolics in plants

Hydroxycinnamic acids present in plant cuticles, the interphase and the main protective barrier between the plant and the environment, exhibit singular photochemical properties that could allow them to act as a UV shield. Here, we employ transient absorption spectroscopy on isolated cuticles and leaf epidermises to study in situ the photodynamics of these molecules in the excited state. Based on quantum chemical calculations on p-coumaric acid, the main phenolic acid present in the cuticle, we propose a model in which cuticle phenolics display a photoprotective mechanism based in an ultrafast and non-radiative excited state deactivation combined with fluorescence emission. As such, the cuticle can be regarded as the first and foremost protective barrier against UV radiation. This photostable and photodynamic mechanism seems to be universal in land plants giving a special role and function to the presence of different aromatic domains in plant cuticles and epidermises.

Phenolics are abundant in plant cuticles. Here, via transient absorption spectroscopy and quantum chemical calculations, the authors propose a model by which cuticle phenolics provide photoprotection due to ultrafast and non-radiative excited state deactivation combined with fluorescence emission.

Impacts of Solvent and Alkyl Chain Length on the Lifetime of Singlet Cyclopentane-1,3-diyl Diradicaloids with π-Single Bonding

The singlet 2,2-dialkoxycyclopentane-1,3-diyl diradicaloids are not only the important key intermediates in the process of bond homolysis but are also attracting attention as π-single bonding compounds. In the present study, the effects of solvent viscosity η (0.24-125.4 mPa s) and polarity π* (-0.11 to 1.00 kcal mol^-1) on the reactivity of localized singlet diradicaloids were thoroughly investigated using 18 different solvents including binary mixed solvent systems containing ionic liquids. In low-η solvents (η < 1 mPa s), the lifetimes of singlet diradicaloids, which are determined by the rate constant for the isomerization of π-single-bonded singlet diradicaloids to the σ-bonded isomer, were substantially dependent on π*. Slower isomerization was observed in more polar solvents. In high-η solvents (η > 2 mPa s), the rate of isomerization was largely influenced by η in addition to π*. Slower isomerization was observed in more viscous solvents. Experimental results demonstrated the crucial roles of both solvent polarity and viscosity in the reactivity of singlet diradicaloids and thus clarified the characters of singlet diradicaloids and molecular motions during the chemical transformation. The dynamic solvent effect was further proved by a long alkyl chain introduced at a remote position of the reaction site.

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.

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.

Substitution effect on the nonradiative decay and trans → cis photoisomerization route: a guideline to develop efficient cinnamate-based sunscreens

Cinnamate derivatives are very useful as UV protectors in nature and as sunscreen reagents in daily life. They convert harmful UV energy to thermal energy through effective nonradiative decay (NRD) including trans → cis photoisomerization. However, the mechanism is not simple because different photoisomeirzation routes have been observed for different substituted cinnamates. Here, we theoretically examined the substitution effects at the phenyl ring of methylcinnamate (MC), a non-substituted cinnamate, on the electronic structure and the NRD route involving trans → cis isomerization based on time-dependent density functional theory. A systematic reaction pathway search using the single-component artificial force-induced reaction method shows that the very efficient photoisomerization route of MC can be essentially described as "1ππ* (trans) → 1nπ* → T1 (3ππ*) → S0 (trans or cis)". We found that for efficient 1ππ* (trans) → 1nπ* internal conversion (IC), MC should have the substituent at the appropriate position of the phenyl ring to stabilize the highest occupied π orbital. Substitution at the para position of MC slightly lowers the 1ππ* state energy and photoisomerization occurs via a slightly less efficient "1ππ* (trans) → 3nπ* → T1 (3ππ*) → S0 (trans or cis)" pathway. Substitution at the meta or ortho positions of MC significantly lowers the 1ππ* state energy so that the energy barrier of IC (1ππ* → 1nπ*) becomes very high. This substitution leads to a much longer 1ππ* state lifetime than that of MC and para-substituted MC, and a change in the dominant photoisomerization route to "1ππ* (trans) → C[double bond, length as m-dash]C bond twisting on 1ππ* → S0 (trans or cis)". As a whole, the "1ππ* → 1nπ*" IC observed in MC is the most important initial step for the rapid change of UV energy to thermal energy. We also found that the stabilization of the π orbital (i) minimizes the energy gap between 1ππ* and 1nπ* at the 1ππ* minimum and (ii) makes the 0-0 level of 1ππ* higher than 1nπ* as observed in MC. These MC-like relationships between the 1ππ* and 1nπ* energies should be ideal to maximize the "1ππ* → 1nπ*" IC rate constant according to Marcus theory.

New Generation UV-A Filters: Understanding Their Photodynamics on a Human Skin Mimic

The sparsity of efficient commercial ultraviolet-A (UV-A) filters is a major challenge toward developing effective broadband sunscreens with minimal human- and eco-toxicity. To combat this, we have designed a new class of Meldrum-based phenolic UV-A filters. We explore the ultrafast photodynamics of coumaryl Meldrum, CMe, and sinapyl Meldrum (SMe), both in an industry-standard emollient and on a synthetic skin mimic, using femtosecond transient electronic and vibrational absorption spectroscopies and computational simulations. Upon photoexcitation to the lowest excited singlet state (S₁), these Meldrum-based phenolics undergo fast and efficient nonradiative decay to repopulate the electronic ground state (S₀). We propose an initial ultrafast twisted intramolecular charge-transfer mechanism as these systems evolve out of the Franck-Condon region toward an S₁/S₀ conical intersection, followed by internal conversion to S₀ and subsequent vibrational cooling. Importantly, we correlate these findings to their long-term photostability upon irradiation with a solar simulator and conclude that these molecules surpass the basic requirements of an industry-standard UV filter.

Reinvestigating the Photoprotection Properties of a Mycosporine Amino Acid Motif

With the growing concern regarding commercially available ultraviolet (UV) filters damaging the environment, there is an urgent need to discover new UV filters. A family of molecules called mycosporines and mycosporine-like amino acids (referred to as MAAs collectively) are synthesized by cyanobacteria, fungi and algae and act as the natural UV filters for these organisms. Mycosporines are formed of a cyclohexenone core structure while mycosporine-like amino acids are formed of a cyclohexenimine core structure. To better understand the photoprotection properties of MAAs, we implement a bottom-up approach by first studying a simple analog of an MAA, 3-aminocyclohex-2-en-1-one (ACyO). Previous experimental studies on ACyO using transient electronic absorption spectroscopy (TEAS) suggest that upon photoexcitation, ACyO becomes trapped in the minimum of an S₁ state, which persists for extended time delays (>2.5 ns). However, these studies were unable to establish the extent of electronic ground state recovery of ACyO within 2.5 ns due to experimental constraints. In the present studies, we have implemented transient vibrational absorption spectroscopy (as well as complementary TEAS) with Fourier transform infrared spectroscopy and density functional theory to establish the extent of electronic ground state recovery of ACyO within this time window. We show that by 1.8 ns, there is >75% electronic ground state recovery of ACyO, with the remaining percentage likely persisting in the electronic excited state. Long-term irradiation studies on ACyO have shown that a small percentage degrades after 2 h of irradiation, plausibly due to some of the aforementioned trapped ACyO going on to form a photoproduct. Collectively, these studies imply that a base building block of MAAs already displays characteristics of an effective UV filter.

Conservation of ultrafast photoprotective mechanisms with increasing molecular complexity in sinapoyl malate derivatives

Sinapoyl malate is a natural plant sunscreen molecule which protects leaves from harmful ultraviolet radiation. Here, the ultrafast dynamics of three sinapoyl malate derivatives, sinapoyl L-dimethyl malate, sinapoyl L-diethyl malate and sinapoyl L-di-t-butyl malate, have been studied using transient electronic absorption spectroscopy, in a dioxane and methanol solvent environment to investigate how well preserved these dynamics remain with increasing molecular complexity. In all cases it was found that, upon photoexcitation, deactivation occurs via a trans-cis isomerisation pathway within ∼20-30 ps. This cis-photoproduct, formed during photodeactivation, is stable and longed-lived for all molecules in both solvents. The incredible levels of conservation of the isomerisation pathway with increased molecular complexity demonstrate the efficacy of these molecules as ultraviolet photoprotectors, even in strongly perturbing solvents. As such, we suggest these molecules might be well-suited for augmentations to further improve their photoprotective efficacy or chemical compatibility with other components of sunscreen mixtures, whilst conserving their underlying photodynamic properties.

Exploring the Photochemistry of an Ethyl Sinapate Dimer: An Attempt Toward a Better Ultraviolet Filter

The photochemistry and photostability of a potential ultraviolet (UV) radiation filter, dehydrodiethylsinapate, with a broad absorption in the UVA region, is explored utilizing a combination of femtosecond time-resolved spectroscopy and steady-state irradiation studies. The time-resolved measurements show that this UV filter candidate undergoes excited state relaxation after UV absorption on a timescale of ~10 picoseconds, suggesting efficient relaxation. However, steady-state irradiation measurements show degradation under prolonged UV exposure. From a photochemical standpoint, this highlights the importance of considering both the ultrafast and “ultraslow” timescales when designing new potential UV filters.

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.

Electronic States and Nonradiative Decay of Cold Gas-Phase Cinnamic Acid Derivatives Studied by Laser Spectroscopy with a Laser-Ablation Technique

We performed UV spectroscopy for p-coumaric acid (pCA), ferulic acid (FA), and caffeic acid (CafA) under jet-cooled gas-phase conditions by using a laser-ablation source. These molecules showed the S₁(¹ππ*)-S₀ absorption in the 31 500-33 500 cm^-1 region. Both pCA and FA exhibited sharp vibronic bands, while CafA showed only a broad feature. The decay time profile of the ¹ππ* state was measured by picosecond pump-probe spectroscopy, and the transient state produced through the nonradiative decay (NRD) from ¹ππ* and its time profile were measured by nanosecond UV-deep UV pump-probe spectroscopy. The transient state was observed for pCA and FA and assigned to the T₁ state, and we concluded that the NRD process of ¹ππ* is S₁(¹ππ*) → ¹nπ* → T₁(³ππ*), similar to those of methyl cinnamate and para-substituted cinnamates such as p-hydroxy and p-methoxy methyl cinnamate. On the other hand, the transient T₁ state was not detected in CafA, and its NRD route is suggested to be S₁(¹ππ*) → ¹πσ* → H atom elimination, similar to those of phenol and catechol. The effect of a hydrogen bond on the electronic state and NRD process was investigated, and it was found that the H-bonding lowers the ¹ππ* energy and suppresses the NRD process for all the species.

Towards symmetry driven and nature inspired UV filter design

In plants, sinapate esters offer crucial protection from the deleterious effects of ultraviolet radiation exposure. These esters are a promising foundation for designing UV filters, particularly for the UVA region (400 – 315 nm), where adequate photoprotection is currently lacking. Whilst sinapate esters are highly photostable due to a cis-trans (and vice versa) photoisomerization, the cis-isomer can display increased genotoxicity; an alarming concern for current cinnamate ester-based human sunscreens. To eliminate this potentiality, here we synthesize a sinapate ester with equivalent cis- and trans-isomers. We investigate its photostability through innovative ultrafast spectroscopy on a skin mimic, thus modelling the as close to true environment of sunscreen formulas. These studies are complemented by assessing endocrine disruption activity and antioxidant potential. We contest, from our results, that symmetrically functionalized sinapate esters may show exceptional promise as nature-inspired UV filters in next generation sunscreen formulations.

Sinapate esters are promising nature-inspired sunscreen and antioxidant agents but their photoisomerization may lead to ineffective or harmful species. Here the authors propose a symmetric ester with indistinguishable trans and cis isomers and prove its effectiveness by optical spectroscopies on a skin mimic.

Examining solvent effects on the ultrafast dynamics of catechol

We consider the effect of a polar, hydrogen bond accepting, solvent environment on the excited state decay of catechol following excitation to its first excited singlet state (S₁). A comparison of Fourier transform infrared spectroscopy and explicit-solvent ab initio frequency prediction suggests that 5 mM catechol in acetonitrile is both nonaggregated and in its "closed" conformation, contrary to what has been previously proposed. Using ultrafast transient absorption spectroscopy, we then demonstrate the effects of aggregation on the photoexcited S₁ lifetime: at 5 mM catechol (nonaggregated) in acetonitrile, the S₁ lifetime is 713 ps. In contrast at 75 mM catechol in acetonitrile, the S₁ lifetime increases to 1700 ps. We attribute this difference to aggregation effects on the excited-state landscape. This work has shown that explicit-solvent methodology is key when calculating the vibrational frequencies of molecules in a strongly interacting solvent. Combining this with highly complementary steady-state and transient absorption spectroscopy enables us to gain key dynamical insights into how a prominent eumelanin building block behaves when in polar, hydrogen bond accepting solvents both as a monomer and as an aggregated species.

New Insight into the Photoprotection Mechanism of Plant Sunscreens: Adiabatic Relaxation Competing with Nonadiabatic Relaxation in the cis → trans Photoisomerization of Methyl Sinapate

A great deal of thermally instable cis form photoisomerization products will be formed from the thermally stable trans form of the plant sunscreens sinapate esters upon ultraviolet radiation. To reveal the photoisomerization mechanism of the cis-isomer, we explore the photodynamics of a model plant sunscreen methyl sinapate (MS) in the cis form in organic solution. The high photoisomerization quantum yield of the cis-isomer results in the relatively higher photostability of trans-MS. By utilizing femtosecond transient absorption spectroscopy and quantum chemical calculation, we propose that an adiabatic relaxation competes with nonadiabatic relaxation for the excited-state cis form of methyl sinapate. These results suggest that the photoprotection mechanism of the cis form of sinapate esters is significantly different from that of the trans form of sinapate esters and plays an important role in the overall photoprotection effect.

The role of symmetric functionalisation on photoisomerisation of a UV commercial chemical filter

Photoisomerisation has been shown to be an efficient excited-state relaxation mechanism for a variety of nature-based and artificial-based molecular systems.

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.

Investigating isomer specific photoprotection in a model plant sunscreen

Sinapate esters are used throughout the plant kingdom, for example in photoprotection from ultraviolet radiation.