Triplet–triplet energy transfer between UV absorbers in solutions at room temperature

Dynamic macromolecular material design: the versatility of cucurbituril over cyclodextrin in host-guest chemistry

The keto-enol tautomerism of avobenzone (AVO) is pivotal to its photostability, influenced by microenvironmental factors, such as, the type of solvent and complexation with macrocyclic compounds. This study explores the effect of host-guest complexation on AVO photostabilization, employing cucurbit[7]uril (CB[7]) and β-cyclodextrin (β-CD) to form inclusion complexes. CB[7] exhibits a higher affinity to the keto form of AVO, a UVC radiation absorber. The complexed keto form facilitates the regeneration of the enol form, reducing skin permeation. Spectroscopic and thermal analyses confirm 1 : 1 AVO-CB[7] and AVO-β-CD complex formation. Computational and MD simulations show that host-guest complex is favored over isolated AVO and β-CD or CB[7] molecules by 95-125 kJ mol-1, depending on the presence of implicit solvent. Both macrocycles enhance AVO photostabilization in aqueous environments, with CB[7] displaying greater selectivity for the keto form, while β-CD shows ethanol concentration-dependent binding.

Environmental Fate of Organic Sunscreens during Water Disinfection Processes: The Formation of Degradation By-Products and Their Toxicological Profiles

The development of any commercial product should also be aimed at reducing the risk associated with it, according to the safe-by-design concept; that is, risk assessment should always be at the center of the design, and the impact on human and environmental health should be assessed and eliminated during the product development phase and not afterwards. Unfortunately, even today, most operators in any production sector implement the philosophy of “risk management” or rather of managing the problem when it occurs, using spot interventions instead of changing the approach. This argument is also valid in the production of solar filters, which have reached a satisfactory degree of efficiency in the face of a substantial underestimation of the risks associated with their possible environmental fate. In fact, solar filters have been found in bathing waters and their environmental fate may depend on various factors such as the pH of the water, the presence of organic material, metal ions and light, and, above all, the chemical agents used in the disinfection of the water itself. Thus, during disinfection processes, the generation of dozens of products with a lower molecular weight and generally of an aromatic nature has been tested, where some of them did not receive an exact structural definition and a precise evaluation of their precise toxicological profile. Therefore, it is interesting to draw a complete picture of organic sunscreens and of the byproducts obtained under different conditions and their related ecotoxicological profile.

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.

Influence of the Solvent Environment on the Ultrafast Relaxation Pathways of a Sunscreen Molecule Diethylamino Hydroxybenzoyl Hexyl Benzoate

The excited-state dynamics of photoexcited diethylamino hydroxybenzoyl hexyl benzoate (DHHB), a UVA absorber widely used in sunscreen formulations, are studied with transient electronic and vibrational absorption spectroscopy methods in four different solvents. In the polar solvents methanol, dimethyl sulfoxide (DMSO), and acetonitrile, strong stimulated emission (SE) is observed at early time delays after photoexcitation at a near-UV wavelength of λ_ex = 360 nm, and decays with time constants of 420 fs in methanol and 770 fs in DMSO. The majority (∼95%) of photoexcited DHHB returns to the ground state with time constants of 15 ps in methanol and 25 ps in DMSO. In the nonpolar solvent cyclohexane, ∼ 98% of DHHB photoexcited at λ_ex = 345 nm relaxes to the ground state with a ∼ 10 ps time constant, and the SE is weak. DHHB preferentially adopts an enol form in its ground S₀ state, but excited state absorption (ESA) bands seen in TEAS are assigned to both the S₁-keto and S₁-enol forms, indicating a role for ultrafast intramolecular excited state hydrogen transfer (ESHT). This ESHT is inhibited by polar solvents. The two S₁ tautomers decay with similar time scales to the observed recovery of ground state population. For molecules that avoid ESHT, torsion around a central C-C bond minimizes the S₁-enol energy, quenches the SE, and is proposed to lead to a conical intersection with the S₀ state that mediates the ground state recovery. A competing trans-enol isomeric photoproduct is observed as a minor competitor to parent recovery in polar solvents. Evidence is presented for triplet (T₁) enol production in polar solvents, and for T₁ quenching by octocrylene, a common UVB absorber sunscreen additive. The T₁ keto form is observed in cyclohexane solution.

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.

Revisiting the Role of Irradiance in the Determination of Sunscreens' Sun Protection Factor

The efficacy of a sunscreen tends to be associated with its sun protection factor (SPF) value, a figure determined in a test that relies on the independence of the SPF value to both UV radiation dose and irradiance. We probe these assumptions when photoinduced product degradation is present, and we estimate that the theoretical limit for their validity is when the sunfilter active molecule relaxation time is faster than ∼10 ns. While such threshold relaxation time should be compatible with the expected ultrafast relaxation mechanisms of sunfilter molecules (picoseconds), recent research on sunfilter photodynamics has identified the existence of much longer-lived molecular states. Such long lifetimes could compromise sunscreen performance and make the SPF value very different in natural sun irradiance conditions than in the solar simulated conditions typically used in SPF determination tests.

Investigation of different photochemical reactions of avobenzone derivatives by ultrafast transient absorption spectroscopy

Avobenzone (AB) is one of the most widely used UVA sunscreens, and it is viewed as a model compound for studying the photoisomerization process. In recent years, Miranda and co-workers studied photophysical and photochemical reactions of several AB derivatives. However, there is still a gap in the data of these compounds in the ultrafast time region. To get a better understanding of the photophysical and photochemical reaction mechanisms, selected AB derivatives of AB-Me, AB-Pr, AB-Br and AB-Cl were studied using ultrafast transient absorption spectroscopy and density functional theory calculations in the present study. It is unravelled that alkylated substituted AB compounds of AB-Me and AB-Pr exhibit an efficient intersystem crossing with the generation of the corresponding triplet state species, which further leads to the Norrish type II reaction for AB-Pr. On the other hand, AB-Br and AB-Cl prefer photochemical reactions via the singlet state surface. Based on the DFT calculations, the spin-orbit coupling constant between the singlet and triplet states, the energy difference between the singlet and triplet states and the natural transition orbital separations of the studied AB compounds were found to be the leading reasons accounting for their corresponding photochemical activities via singlet and triplet states.

Analysis of photokinetics of 2'-ethylhexyl-4-methoxycinnamate in sunscreens

2'-Ethylhexyl-4-Methoxycinnamate (EHMC), also designated as octinoxate, is an oily UV-absorber used in sunscreens for the protection of human skin against solar UV-radiation and represents one of the most employed UVB absorbers for that application. In water-in-oil emulsions EHMC was adjusted at a constant overall concentration, while changing the EHMC concentration in the oil droplets by adding a non-absorbing oil. In that way the EHMC concentration could be varied at constant optical thickness. Here we show that the kinetics of the photoreaction follows a second-order rate law, in line with the UV-induced [2 + 2]-cycloaddition reaction mechanism known for this UV absorber. The second-order rate constant decreased with higher overall EHMC concentration. This can be explained by the fact, that at higher overall concentration of the UV absorber not every EHMC molecule will absorb a photon due to the increased optical density, so that on average less photons are absorbed per molecule. On the other hand, the rate constant increases with decreasing polarity of the surrounding oil. Since the molar fraction of the trans-isomer of EHMC is augmented at lower polarity, more photons are absorbed in this case, as the strength of the absorption band of the trans-isomer is significantly higher than that of the cis-isomer. In conclusion, our experiments show that a high polarity of the oil phase and a high concentration of EHMC are advantageous for the photostability of this compound.

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.

Wavepacket insights into the photoprotection mechanism of the UV filter methyl anthranilate

Meradimate is a broad-spectrum ultraviolet absorber used as a chemical filter in commercial sunscreens. Herein, we explore the ultrafast photodynamics occurring in methyl anthranilate (precursor to Meradimate) immediately after photoexcitation with ultraviolet radiation to understand the mechanisms underpinning Meradimate photoprotection. Using time-resolved photoelectron spectroscopy, signal from the first singlet excited state of methyl anthranilate shows an oscillatory behavior, i.e., quantum beats. Our studies reveal a dependence of the observed beating frequencies on photoexcitation wavelength and photoelectron kinetic energy, unveiling the different Franck-Condon overlaps between the vibrational levels of the ground electronic, first electronic excited, and ground cationic states of methyl anthranilate. By evaluating the behavior of these beats with increasing photon energy, we find evidence for intramolecular vibrational energy redistribution on the first electronic excited state. Such energy redistribution hinders efficient relaxation of the electronic excited state, making methyl anthranilate a poor choice for an efficient, efficacious sunscreen chemical filter.

Here, the authors explore the ultrafast photodynamics of methyl anthranilate. From the quantum beat behavior, the authors find evidence for ultrafast energy redistribution processes which hinder excited state relaxation, making methyl anthranilate a poor choice for a sunscreen chemical filter.

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.

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.