In Light of Exposure: Understanding Avobenzone Part I, Characterization

Editor's note: This first in a two-part series reviews the inherent characteristics and behavior of avobenzone in solvents; part two will consider its behavior with other sunscreens and in formulas.

1-4-(Tert-butylphenyl)-3-(4-methoxyphenyl) propane-1,3 dione is a member of the class of dibenzonyl methane molecules and is widely used as an active UV filter in sunscreen and cosmetic products. Indeed, in a 2014 review of cosmetic products in the United Kingdom, UV filters were present in a wide variety of cosmetics; this one in particular appeared in 48.7% of the 4,447 products examined.⁶

This molecule is also referred to in the literature as butylmethoxy dibenzoylmethane (BMDM), avobenzone and the trade names Escalol 517, Eusolex 9020, Parsol 1789 and NeoHeliopan 357. The present paper will refer to it as avobenzone.

Avobenzone has the empirical formula C₂₀H₂₂O₃ and a molecular weight of 310.39 g/mol. It is a yellow powder with a weak characteristic odor. While it is soluble in a variety of polar and non-polar solvents, it exhibits low solubility in water; i.e., 0.01 mg/L at 20°C.

Within the UV region of the electromagnetic spectrum, the portion of radiation that reaches the earth’s surface is divided into UVB (290–320 nm) and UVA (320–400 nm)—and avobenzone is one of the most effective absorbers in the UVA range, which explains
its popularity.

Considering the extensive use of sunscreens and cosmetic products, the implication is that a significant fraction of consumers around the world is applying avobenzone to their bodies. Many studies have focused on the photochemistry, photodegradation, photostabilization and interaction of this material both with biological and nonbiological systems; recent reviews are available.^1–5 In this paper, we review the literature on the photochemistry of avobenzone, with particular emphasis on how its unique molecular structure and related photochemistry determine its end use.

Avobenzone Structure

Solutions of avobenzone consist of an equilibrium mixture of a chelated enol form and the corresponding keto form, as shown in Figure 1. Two isomeric forms of the enolic form coexist in solution. Calculations suggest the O-H group on the benzene with the t-butyl group is slightly energetically favored (see structure A in Figure 1; ΔE (intramolecular hydrogen bond) = 69.8 kJ/mol) versus the other enol (see structure B in Figure 1; ΔE (intramolecular hydrogen bond) = 71.7 kJ/mol).⁷

The enol structure is planar while the keto form has a butterfly-like structure. The methoxy group promotes extended delocalization of the enolic form. The H-bonding in the chelated enol form is a resonance-assisted, strong H-bond arising from a synergistic effect of π-delocalization and H-bond formation.

The equilibrium favors the enol over the keto form, with the exact ratio of enol to keto being solvent-dependent. The enolic form is favored in non-polar solvents, e.g., the equilibrium constant in dimethyl sulfoxide (DMSO) and cyclohexane are 10 and 46, respectively, favoring the enol form.⁸ The enol is converted to the keto form upon photolysis in certain solvents and will revert slowly to the enol form in the dark. The rate constant varies between solvents: for conversion of keto to enol in the dark, it is 1.8 × 10^-3s^-1, 8.6 × 10^-4s^-1 and 7.0 × 10^-5s^-1 in ethyl acetate, DMSO and cyclohexane, respectively.⁸

Absorption Spectra

Figure 2 shows the absorption spectrum of 2 × 10^-5 M of avobenzone in cyclohexane. The band at 355 nm (ε = 32400 M^-1cm^-1) is due to the enol, and the band at 265 nm (ε = 28400 M^-1cm^-1) is due to the keto form. The enol band shifts to higher wavelengths in more polar solvents: ethyl acetate = 356 nm; DMSO = 363 nm; and methanol = 358 nm.⁹ The singlet excited state of the enol (¹S) is reached via ππ* transition and that of the keto form via a nπ* transition. In the nonpolar solvent cyclohexane, the ¹S lifetime (420 ± 40 fs) of the enol is one-third of that observed in methanol (1.4 ± 0.2 ps), indicating a charge localized excited state.¹⁰

Fluorescence Spectra

The enol form exhibits fluorescence with significant vibronic structure between 400–450 nm (the energy of the 0-0 singlet excited state is 390 nm) and weak phosphorescence with peaks at 490 nm and 530 nm; all measurements were in ethanol at 77K.¹¹ The lack of fluorescence at room temperature indicates efficient nonradiative relaxation and/or a fast photochemical reaction. The lifetime of the phosphorescent state of the enol is 30 ms. The electron paramagnetic resonance (EPR) spectrum of the triplet state at 77K suggests the two unpaired electrons are not localized on the phenyl or carbonyl fragment.¹¹ In another study, room temperature fluorescence was observed for avobenzone dissolved in ethyl acetate, with the fluorescence maxima at 405 nm, a quantum yield of 0.01 and fluorescence lifetime of 13 ps.¹²

The keto form is only weakly fluorescent but does exhibit strong phosphorescence at 410–450 nm, and the lifetime of the triplet state is ~190 ms. The triplet state does not exhibit an EPR signal.¹¹ For the triplet state, in both keto and enol forms, there is more 3ππ* character. The methoxy and tert-butyl groups influence the mixing of the 3nπ* and 3ππ* state, increasing the lifetime of the triplet state.¹¹ This long-lived triplet state of the keto form is relevant for the photo instability of avobenzone.

Transient Spectroscopy

Several transient spectroscopy studies have unraveled the fate of photoexcited avobenzone^{9, 10, 13, 14} and related compounds.¹⁵ In relation, a review of the ultrafast photochemistry of molecules used as sunscreens was recently publishes.¹⁶ Figure 3 summarizes the results from the transient spectroscopy studies, and is discussed below.

Transient absorption spectra of avobenzone in different solvents showed that photoexcited enol (absorbing at 300 nm) decays with varying lifetimes in different solvents: 0.08 ms in CCl₄, 1.21 ms in cyclohexane and to 24.4 ms in acetonitrile.¹⁴ To explain this solvent-dependence, different degrees of exciplex formation with the various solvents was proposed.

In acetonitrile, excitation with ns 355 nm pulses resulted in a photoexcited species with λ_max at 300 nm, with a quantum yield of 0.25.¹³ This species, originating from the enol form, was assigned to a mixture of the isomers formed by rotation around the C₂-C₃ single bond (NCE2) and the isomeric species formed by isomerization around the C₁-C₂ double bond (NCE1), as shown in Figure 3.¹³ It was proposed that the isomer formed via rotation around the single bond (NCE2) quickly reverts to the chelated enol, with the primary species responsible for the ns transient spectra being the non-chelated isomer NCE1 formed via double bond isomerization.

Considering the extensive use of sunscreens and cosmetic products, a significant fraction of consumers uses avobenzone.

The kinetics of decay of this NCE1 isomer exhibited both first and second-order decay, with first order decay more evident in polar solvents. The lifetime of the NCE1 state was solvent-dependent, being faster in protic solvents, and varied from 159 ms in acetonitrile to 12 ms in cyclohexane and 0.7 ms in butanol. NCE1 can follow two solvent-dependent pathways: reformation of the chelated enol, or shifting to the keto form via hydrogen transfer.

With 266 nm excitation of avobenzone in acetonitrile, a broad transient spectrum (300–500 nm) with a lifetime of 500 ns was observed.¹³ These features are characteristic of alkylated avobenzone, which can only exist in the keto form.¹⁷ Also, this species formed in avobenzone was quenched rapidly with oxygen (5 x10⁹ M^-1s^-1), and is most likely the triplet state of the keto form. Permanent loss of avobenzone occurred upon 266 nm excitation, indicating photodegradation via the excited keto form is occurring.¹³

Another ns laser photolysis in acetonitrile and ethanol gave similar observations but for the transient excited state; besides the NCE1 isomer, a non-chelated enol formed by the rotation around the C-OH bond (breaking of H-bond, NCE3, Figure 3) was proposed.⁹ The decay pathways for the isomer involves relaxation to the chelated enol via isomerization around the C₁-C₂ double bond, promoted in polar solvents via H-bonding; or shifting to the keto form through a 1,3-H atom shift, as observed in acetonitrile, which through rotation (around C₂-C₃) can relax back to the regular keto form, as shown in Figure 4.

This study estimated a quantum yield for the keto formation of 0.014 ± 0.002 in acetonitrile and was independent of dissolved oxygen, suggesting the excited singlet state of the enol was responsible for the formation of the keto form.⁹ Transient photolysis at 266 nm of the keto form resulted in a triplet excited state (390 nm) but showed no evidence for tautomerization to the enol form—requiring a 1,3-H atom shift in the excited state—though this reaction takes place in the dark; demonstrating a lifetime of 5.1 hr at 295K in acetonitrile.

A recent picosecond transient photolysis study with 350 nm excitation of avobenzone in acetonitrile, methanol and cyclohexane has been reported.¹⁰ The transient absorption signals monitored at 266 nm were believed to arise from the relaxation of three isomers including: NCE1 or the keto form (this could not be distinguished but was most likely NCE1), NCE2 and the non-chelated isomer NCE3 formed via rotation around the C-OH bond.¹⁰

The NCE3 form was solvent-insensitive and returned to the chelated enol with a lifetime of 1.3 ps. The NCE2 form reverted back to the chelated enol with a lifetime of 30 ± 10 ps in methanol, 59 ± 8 ps in cyclohexane and 80 ± 20 ps in acetonitrile. NCE1 dynamics were slow on the picosecond time scale. It is interesting to note that the quantum yield of the NCE1 form was similar for methanol and acetonitrile, though the steady-state photochemistry of avobenzone is quite different in these solvents, as detailed below.

Figure 5 summarizes the following observations from the transient spectroscopy studies:

• There are at least three intermediate structures in the excited state, accessible by excitation of the enolic form.
• These structures result from rotation around single bonds, disruptions of the H-bond or isomerization around the double bond.
• Of particular interest is the longer-lived isomer formed by isomerization since it can convert to the keto form.
• Upon excitation of the keto isomer, a long-lived triplet state is formed and is relevant since it can take part in further photochemistry.

Steady State Photolysis

The photolysis of avobenzone is both solvent- and wavelength-dependent. There is considerable variation in the literature in this area, primarily due to different irradiation conditions, both flux and wavelength, and the concentrations used. In general, there is agreement that in nonpolar solvents there are more changes in the absorption spectra of avobenzone than in polar solvents. The structural change that takes place in avobenzone is the phototautomerization of the enol to the keto form. The keto form will revert back to enol in the dark. There can also be further photodegradation and it appears this happens primarily in the keto form.

Water: Solar simulated photolysis (250 W/m² for 4 min, repeated 5×) of avobenzone in water shows considerable degradation.¹⁸ Considering the low solubility of avobenzone in water, these were dilute solutions. Water appears to be different from other protic solvents, which stabilize the enolic form.

Methanol: In methanol, 2.6 × 10^-5 M avobenzone is relatively photostable, with λ_excitation > 300 nm (high-pressure Hg lamp, 150 min illumination; flux not reported). In methanol, the H-exchange of the photoexcited transient nonchelated enol and the solvent was proposed to promote the reformation of ground state enol. Presence or absence of dissolved oxygen does not influence the photolysis.⁸

Ethyl acetate: There is a marked decrease in the enolic form by about 60%, with λ_excitation > 300 nm, in air-equilibrated 2.4 × 10^-5 M avobenzone in ethyl acetate, along with an increase in the keto form, with a clear isosbestic point (high-pressure Hg lamp, 30 min illumination; flux not reported). Photodegradation was also noted by HPLC analysis. Removing oxygen did not significantly impact the photostability.⁸ In another report, with the solar simulated photolysis (250 W/m² for 4 min, repeated 5×) of a dilute solution of avobenzone in ethyl acetate, no change in the absorption spectra of avobenzone was observed.¹⁸

Various theoretical methods have examined the structural and photochemical aspects of avobenzone.

Cyclohexane and other hydrocarbon solvents: In air-equilibrated cyclohexane, considerable loss of the enolic form occurs within an hour upon photoexcitation at λ_excitation > 300 nm of 1.0 × 10^-5 M avobenzone, with formation of the keto form (high-pressure Hg lamp; flux not reported). The keto form undergoes photodegradation within 30 min of photoirradiation. The presence of oxygen accelerated the photoconversion.⁸ Another study also noted the enol to keto phototautomerization with solar-simulated photolysis (250 W/m² for 4 min, repeated 5×) for a dilute solution of avobenzone and in the dark, the enolic form of avobenzone was fully recovered. Similar behavior was noted in hexane and heptane.¹⁸

Acetonitrile: In neat acetonitrile, the spectrum changed significantly with disappearance of the enol band (355 nm) and appearance of the keto form (265 nm).^{9, 14} Removal of dissolved oxygen did not influence this process. With avobenzone in acetonitrile, upon 80 mW of 355 nm illumination for 3 hr, the formation of photoproducts was noted, being different for oxygen and argon saturated samples.¹⁹ One report observed little change in absorbance upon solar simulated light irradiation (250 W/m² for 4 min, repeated 5×) of diluted avobenzone in acetonitrile.¹⁸ In support of the above observations, Figure 6 demonstrates the partial disappearance of avobenzone in acetonitrile upon photolysis with > 280 nm (~80 mW/cm²) for an hour and reappearance with an hour in the dark.

Dimethyl sulfoxide: In air-equilibrated DMSO, almost a complete conversion of the enol to the keto form occurs within 40 min, with λ_excitation > 300 nm, with a clear isosbestic point (1.5 × 10^-5 M solution). With deaerated DMSO, the enol form is stable, indicating that oxygen plays a role in the photochemical conversion of enol to the keto form.

It has been proposed that the key structural feature responsible for the photoisomerization of the enol to keto is the ability of DMSO to H-bond with the OH group of the enol, disrupting the chelated enol in the ground state. The photoexcited keto form, with its tail of absorption extending beyond 300 nm, can react with oxygen to form singlet oxygen. The singlet oxygen was proposed to promote the photoisomerization to the keto form.⁸ It is unclear why minimal photodegradation via the keto form was observed in DMSO, compared with ethyl acetate, cyclohexane and acetonitrile.

The important aspects gleaned from photolysis studies are as follows:

• The excited state of the enol formed due to the isomerization around the C=C bond (NCE1) is the most relevant to photolysis;
• This isomer can revert to the ground-state enolic form or can convert to the keto form;
• The chemistry of the isomer is solvent-dependent; in particular, the H-bonding property of the solvent;
• Nonpolar solvents promote conversion to the keto form; and
• The keto form does not photochemically transform to the enol form but will do so in the dark, slowly.

Photodecomposition

Several studies have examined the photodegradation of avobenzone.^8, 18–24 The possible pathways of photodecomposition and resulting products that have been reported are shown in Figure 7 and Figure 8, and discussed below.

The total photodecomposition of avobenzone in cyclohexane was observed upon irradiation by a mercury lamp (185–4000 nm) for 100 hr.²² Three degradation products—including p-methoxybenzoic acid (see E in Figure 8), p-tertbutylbenzoic acid (see C in Figure 8) and t-butylbenzene (see I in Figure 8)—were observed by GC and MS. These severe photochemical conditions decomposed some of the primary photoproducts that were formed. A mechanism involving the fission of either one of the two C-C bonds adjacent to the C=O of the keto form, as shown in Figure 7, could explain the formation of benzoic acids. With this type of symmetric bond breakage, it was surprising that methoxybenzene was not observed.²²

In another study,²³ a solar simulator with two cutoff filters showed the photolysis of a 3.5 mM solution of avobenzone in cyclohexane: filter A, with λ > 260 nm 12.4 mW/cm² of UVA and 0.54 mW/cm² of UVB; and filter B, with λ > 320 nm, 11.1 mW/cm² of UVA and 0 mW/cm² of UVB; incident on the samples for up to 8 hr.

As expected, photodegradation with the light source including UVB was more pronounced, although even with UVA only, photodegradation was still observed.²³ Twelve products, including benzaldehydes (see B and E in Figure 8), benzoic acids (see C and F), phenylglyoxals (see D and G), acetophenones (see A), benzils (see J, K and L) dibenzoylmethane (see N) and dibenzoylethane (see M and O), were identified by HPLC and GC/MS. These photoproducts primarily absorb around 250 nm, in the UVC range.

The photolysis of avobenzone is both solvent and wavelength-dependent.

In water (250 W/m² for 4 min, repeated 5× with a solar simulator) several photoproducts were identified by HPLC-MS, including E, F, K, O, P and Q shown in Figure 8. In addition, a hydroperoxide product was also identified (see N).¹⁸ In ethyl acetate, two photodegradation products, 4-t-butyl-4'-methoxybenzyl (J, K or L in Figure 4) and 4-t-butyl phenylglyoxal (D or G in Figure 4) were identified.⁸

In non-volatile industrial solvents, mineral oil, alkyl tartrate, capric/caprylic triglyceride, isostearyl isostearate, alkyl lactate and glycerol, 495 kJ/m² of illumination using a solar simulator showed a significant loss of avobenzone; up to 80%.¹⁸ LC/MS analysis showed a complex mixture of photodegraded products, some of which are shown in Figure 8, but there were other peaks whose structures were not analyzed.¹⁸

The photodegradation is proposed to occur in the keto form, which can undergo α-cleavage reactions from the singlet state to form benzoyl and phenacyl free radicals, as shown in Figure 7. Support of the formation of carbon-centered radicals upon photodecomposition is provided by EPR studies of mixtures of avobenzone and piperidine nitroxide or indolinic nitroxide radical. The EPR signal of the nitroxide radicals decreases with photolysis time, and suggests the carbon-centered radicals from avobenzone couple with the nitroxide radical to form a spin-paired adduct.²⁵

The photochemistry of the methylated form of avobenzone that restricts the molecule to only the keto form (CH₃ group at position 2 in Figure 1) also demonstrates the importance of the triplet state.¹⁷ Upon photoexcitation of this blocked diketo form at 308 nm, a broad absorption band centered at 380 nm was observed. This band has been previously assigned to the triplet state of the keto form of avobenzone.¹³ In keeping with the triplet assignment, the excited state was quenched by oxygen and β-carotene. Direct photoexcitation of the keto form of avobenzone also leads to a long-lived (~ 500 ns) triplet state that is quenched by oxygen to form singlet oxygen with rate constant of 2.2 x 10⁹ M^-1s^-1 (in acetonitrile). A quantum yield of 0.3 was estimated for singlet oxygen formation;¹⁷ and singlet oxygen can cause further photodegradation.

Modifications to the functional groups of avobenzone have an influence on its photostability. Replacement of tert-butyl group with isopropyl makes the molecule more susceptible to UVA photodegradation. The presence of an –OH group adjacent to the C=O (4-t-butyl 2¹ hydroxyl 4¹ methoxy dibenzoyl methane) shows a 5% loss upon photolysis, as compared with a 36% loss with avobenzone.²¹

These photodecomposition experiments indicate that:

• The photoexcited keto form is responsible for photodegradation;
• In the triplet state of the keto form, there is C-C bond fission, and the Norrish Type I process is initiated;
• Photodecomposition is complex and generates many species that absorb primarily at wavelengths below 260 nm;
• The triplet keto form can also generate singlet oxygen, which can react with ground-state enol to form oxygenated products; and
• The enol form can be completely destroyed in photolysis.

The following scheme summarizes the sections above:

Here, AB is the enolic form of avobenzone and K is the keto form.

Behavioral Theories

Various theoretical methods have examined the structural and photochemical aspects of avobenzone. Coupled cluster theory suggests the solvent-dependent photolability of avobenzone arises due to the change in relative ordering of the lowest triplet states of the ππ* and nπ* states of the keto isomer.²⁶ Density Functional Theory (DFT) calculations suggest the enol form is more stable due to the resonance-assisted hydrogen bond and that photodegradation occurs from the triplet state of the keto form; where breakage of the C₂-C₃ bond (see Figure 7) is slightly more favorable.⁷ Another DFT study explained the difference in keto and enol electronic spectra based on their nonplanar and planar conformations, respectively; the latter facilitating electronic resonance across the molecule.²⁷

How will this behavior translate to formulations, in the presence of other chemistries? Part II in this series will explore this facet of avobenzone.

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