1 Basic Properties and Protective Wavelength Range of Avobenzone

1.1 Basic Chemical Information

The International Nomenclature Cosmetic Ingredient name of avobenzone is Butyl Methoxydibenzoylmethane. Its Chinese chemical name is butyl methoxydibenzoylmethane. It is a dibenzoylmethane derivative and a β-diketone compound.[1]

The diketo skeleton of avobenzone can be represented in simplified form as:

Ar¹—CO—CH₂—CO—Ar²

where:

① Ar¹ represents an aromatic ring bearing a tert-butyl substituent;

② Ar² represents an aromatic ring bearing a methoxy substituent.

The two aromatic rings and the central β-dicarbonyl structure together determine its electron distribution, oil solubility, tautomerism, and ultraviolet absorption characteristics.

1.2 Which Region of Ultraviolet Radiation Does Avobenzone Primarily Absorb?

Ultraviolet radiation reaching the Earth’s surface and closely associated with sunscreen evaluation is commonly divided into:

① Ultraviolet B (UVB): approximately 290–320 nm;

② Ultraviolet A2 (UVA2): approximately 320–340 nm;

③ Ultraviolet A1 (UVA1): approximately 340–400 nm.

The absorption band of avobenzone covers UVA2 and extends into UVA1, with its maximum absorption generally located at approximately 357 nm. Its principal role in sunscreen formulations is therefore to supplement long-wave UVA absorption above 340 nm.[2]

The absorption of avobenzone gradually decreases as the wavelength approaches 400 nm. It also cannot provide complete UVB protection when used alone and therefore usually needs to be combined with UVB filters and other UVA filters.

2 Molecular Basis of UVA Absorption by Avobenzone

2.1 Keto–Enol Tautomerism

Avobenzone contains a β-diketone structure and exists in tautomeric equilibrium between the diketo and enol forms:

Diketo form:

Ar¹—CO—CH₂—CO—Ar²

Enol form:

Ar¹—CO—CH=C(OH)—Ar²

Tautomerization involves not only a change in the position of the hydrogen atom but also a rearrangement of the carbonyl group and carbon–carbon double bond.

In many organic solvents and sunscreen oil phases, ground-state avobenzone exists predominantly in the chelated enol form. The enolic hydroxyl group can form an intramolecular hydrogen bond with the adjacent carbonyl oxygen:

Ar¹—CO···H—O—C=CH—Ar²

This intramolecular hydrogen bond forms a chelated structure resembling a six-membered ring, which helps maintain the planarity of the central molecular framework and stabilizes the enol form.[2][3]

2.2 Why Can the Chelated Enol Form Absorb Long-Wave UVA?

The relationship between photon energy and wavelength is expressed as follows:

E = hc/λ

where:

• E is the photon energy;

• h is Planck’s constant;

• c is the speed of light;

• λ is the wavelength of light.

The longer the wavelength, the lower the energy of an individual photon. For a molecule to absorb long-wave UVA, the energy gap between its electronic ground state and excited state must correspond to the energy of these lower-energy photons.

In the chelated enol form, the aromatic rings, carbon–carbon double bond, and carbonyl group form a relatively continuous π-electron conjugated system. The π electrons can be delocalized over a relatively large region, reducing the energy difference between the relevant molecular orbitals and thereby enabling the absorption of longer-wavelength UVA photons.

The electron-donating effect of the methoxy group, the substitution patterns of the two aromatic rings, and the degree of molecular planarity also influence electron delocalization and the position of the absorption maximum. Changes in solvent polarity, hydrogen bonding, and the local microenvironment can therefore alter both the absorption maximum and absorption intensity of avobenzone.[2]

Its light-absorption process can be simplified as:

AB-E (S₀) + hν → AB-E* (S₁)

where:

• AB-E represents avobenzone in the enol form;

• S₀ represents the electronic ground state;

• S₁ represents the first excited singlet state;

• hν represents the energy of a UVA photon.

The fundamental basis of UVA absorption by avobenzone is the transfer of energy from the incident photon to the molecular electronic system, promoting an electron from the ground state to an excited state.

2.3 Why Does the Diketo Form Lack UVA1 Absorption of Comparable Intensity?

The central structure of the diketo form is:

Ar¹—CO—CH₂—CO—Ar²

The two carbonyl groups are separated by an approximately saturated methylene group, making it difficult to form a continuous conjugated system comparable to that of the chelated enol form. The energy required for electronic transitions is therefore higher, and the absorption spectrum shifts toward shorter wavelengths.

The diketo form is therefore not completely incapable of absorbing ultraviolet radiation, but it lacks the strong long-wave UVA absorption characteristic of the chelated enol form. When irradiation reduces the proportion of the chelated enol form or causes the molecule to adopt a non-chelated, nonplanar structure, absorbance in the UVA1 region decreases.

3 Energy Changes After Avobenzone Absorbs Photons

3.1 Ideal Photophysical Deactivation Process

After absorbing UVA, avobenzone enters an excited state. Because the excited state has a relatively high energy, the molecule must release energy to return to the ground state.

For explanatory purposes, an idealized nonradiative deactivation process can be represented in simplified form as:

AB-E (S₀) + hν → AB-E* (S₁)

AB-E* (S₁) → AB-E (S₀) + heat

During this process, the excited molecule transfers energy to the surrounding medium through nonradiative pathways such as vibrational relaxation and internal conversion. After returning to its original ground-state structure, the molecule can continue to absorb subsequent UVA photons.

This cycle of “light absorption–deactivation–recovery” is the basis for the continuous function of organic ultraviolet filters.

3.2 Three Major Fates of the Excited State

These three processes may compete with one another or occur sequentially. The photostability of avobenzone depends on how much of the excitation energy is released through non-destructive deactivation pathways and how much enters structural-transformation and chemical-degradation pathways.

3.3 Intersystem Crossing and the Triplet Excited State

Some excited singlet-state avobenzone molecules can enter a triplet excited state through intersystem crossing (ISC):

AB* (S₁) → AB* (T₁)

The triplet excited state generally has a longer lifetime, giving the molecule more opportunities to collide with oxygen, oils, other UV filters, or impurities.

Possible pathways of the triplet state include:

① Returning to the ground state;

② Transferring energy to other molecules;

③ Undergoing energy transfer or electron transfer with oxygen;

④ Entering molecular-rearrangement or chemical-bond-cleavage pathways.

The triplet state does not itself mean that avobenzone has already degraded, but its lifetime and reactivity affect the probability of subsequent photochemical reactions.[3]

4 Photoinduced Structural Transformation and Irreversible Photodegradation

4.1 Photoinduced Structural Transformation

Photoinduced structural transformation refers to the conversion of a molecule, after light absorption, into different geometric isomers, conformations, or tautomers while the main molecular skeleton may remain intact.

Possible structural changes in avobenzone include:

① Conversion of the chelated enol form into a non-chelated enol form;

② Changes in the geometric configuration of the double bond in the enol structure;

③ Disruption of the intramolecular hydrogen bond;

④ Twisting between the aromatic rings and the central conjugated framework;

⑤ Shifts in the keto–enol equilibrium.

These changes can reduce molecular planarity, shorten the effective conjugation length, or alter the energy difference between electronic orbitals, thereby weakening UVA1 absorption.

Some structures can reconvert into the chelated enol form after irradiation is discontinued, resulting in a certain degree of dark recovery. Dark recovery indicates that the decrease in absorption does not arise entirely from molecular fragmentation; part of it may result from reversible structural transformation.

4.2 Photolytic Cleavage Reactions

Photolytic cleavage refers to the breaking of covalent bonds after avobenzone absorbs light, producing compounds that differ from the original molecule.

The process can be represented in simplified form as:

Avobenzone + hν → excited-state avobenzone → radicals or highly reactive intermediates → aromatic carbonyl compounds and other degradation products

The β-dicarbonyl region of avobenzone is an important site of photochemical reactions. Depending on the conditions, carbon–carbon bond cleavage at the α-position of a carbonyl group, oxidation, rearrangement, and subsequent radical reactions may occur.

The differences between photoinduced structural transformation and photolytic cleavage are shown below:

4.3 Photooxidation and Oil-Phase Radical Chain Reactions

After absorbing UVA, avobenzone enters an excited state. Most excited molecules can return to the ground state through internal conversion and vibrational relaxation, whereas some may undergo structural transformation, intersystem crossing, or photolysis. Under specific solvent, oxygen-concentration, and formulation conditions, excited-state avobenzone and its primary photolytic intermediates may participate in electron-transfer, hydrogen-transfer, or energy-transfer reactions, producing radical intermediates or reactive oxygen species.

AB + hν → AB*

AB* → R· or other photoreactive intermediates

Here, AB represents avobenzone, AB* represents its excited state, and R· represents a primary radical generated through the photochemistry of avobenzone or other photooxidation processes within the system.

When the formulation contains readily oxidizable lipids, primary radicals may abstract hydrogen atoms from lipid molecules and initiate a lipid-peroxidation chain. The following equations represent one possible pathway of secondary lipid oxidation:

LH + R· → L· + RH

L· + O₂ → LOO·

LOO· + LH → LOOH + L·

where:

• LH represents an oxidizable lipid;

• L· represents a lipid radical;

• LOO· represents a lipid peroxyl radical;

• LOOH represents a lipid hydroperoxide.

The degree of unsaturation of the oils, the initial peroxide value of the raw materials, oxygen diffusion, trace metals, antioxidants, and the structure of the sunscreen film can all affect the progression of lipid peroxidation. Packaging oxygen permeability mainly affects oxidative stability during product storage, whereas after application, oxidation is primarily influenced by contact between the sunscreen film and air and by local diffusion conditions.

5 Photodegradation Products and Cutaneous Photoreactions

5.1 Degradation Products Under Different Experimental Conditions

The degradation products of avobenzone are strongly dependent on the experimental conditions. Light-source wavelength, irradiation dose, solvent, oxygen concentration, water content, pH, metal ions, and residual chlorine can all alter the reaction pathway.

Studies conducted in common organic solvents, oxygen-containing systems, or chlorinated water under ultraviolet irradiation have detected the following types of products:

① Substituted aromatic aldehydes;

② Substituted aromatic acids;

③ Phenolic compounds;

④ Acetophenone-type compounds;

⑤ Chlorinated aromatic derivatives.

Chlorinated products are mainly associated with swimming-pool water or chlorinated-water models. Such experiments reflect reactions caused by the combined effects of ultraviolet radiation, residual chlorine, and the aqueous environment and cannot directly represent the product profile generated by sunscreen under normal conditions on the skin surface.[6]

The oil phase, film-forming agents, antioxidants, sebum, water content, and oxygen-diffusion conditions in a sunscreen film differ from those in dilute solutions or chlorinated-water models. The actual degradation behavior must therefore be evaluated through irradiation testing of the complete finished product.

5.2 Are Arylglyoxals Confirmed Major Products?

Arylglyoxal compounds are α-dicarbonyl compounds with a certain degree of electrophilic reactivity and can react with nucleophilic groups in proteins under suitable conditions.

From the perspective of organic reaction mechanisms, certain oxidation or cleavage pathways of avobenzone may potentially generate reactive carbonyl intermediates. However, current studies have not established arylglyoxals as major photodegradation products that are commonly present under routine conditions of use on the skin.

Published studies more commonly report aromatic aldehydes, aromatic acids, phenolic compounds, and acetophenone-type products, and some of these findings were obtained in aqueous-solution or chlorinated-water systems.[6]

5.3 Protein Binding and Photoallergy

Compounds containing aldehyde groups, α-dicarbonyl groups, or other electrophilic groups may form adducts with lysine, arginine, or cysteine residues in proteins under suitable chemical-reaction conditions.

From the perspective of general chemical sensitization mechanisms, if avobenzone or its photoproducts participate in photocontact sensitization, the following process may theoretically occur:

Photoproduct formation → penetration into the stratum corneum or viable epidermis → reaction with skin proteins → formation of complexes recognizable by the immune system → sensitization → immune response after subsequent exposure

Avobenzone itself and other sunscreen agents can cause contact or photocontact allergy in a small number of individuals. The specific sensitizing source may also involve fragrances, preservatives, other UV filters, and photodegradation products.

6 How Formulations Can Improve the Photostability of Avobenzone

Stabilization strategies for avobenzone mainly focus on four objectives:

① Maintaining uniform dissolution or dispersion of avobenzone in the oil phase and sunscreen film to reduce crystallization;

② Shortening the effective lifetime of highly reactive excited states;

③ Reducing the probability of contact between the excited state and oxygen, metal ions, or incompatible ingredients;

④ Terminating radical chain reactions that have already formed.

6.1 Combination with Octocrylene

The International Nomenclature Cosmetic Ingredient name of octocrylene is Octocrylene. It primarily absorbs UVB and also covers part of the short-wave UVA region. When combined with avobenzone, it may improve system performance in three ways.

① Spectral supplementation

Octocrylene contributes to absorption in the UVB and part of the UVA2 region, whereas avobenzone is primarily responsible for UVA1. Their combination helps produce a more continuous ultraviolet absorption profile.

② Improvement of oil-phase solubility

Octocrylene is an oily, lipophilic ingredient that can improve the oil-phase dissolution of some crystalline UV filters and may reduce the risk of avobenzone crystallization during storage or after film formation.

③ Influence on excited-state processes

Some formulation studies have shown that octocrylene can improve the retention of avobenzone absorbance after irradiation. Its effects may be related to spectral sharing, the oil-phase environment, and intermolecular interactions, although the specific mechanism is formulation-dependent.

6.2 Microencapsulation and Inclusion Technologies

Carrier technologies used for avobenzone include polymeric microcapsules, cyclodextrin inclusion complexes, lipid carriers, and porous inorganic carriers.

These technologies may function through the following mechanisms:

① Reducing direct contact between avobenzone and oxygen, metal ions, or incompatible ingredients;

② Restricting molecular migration and collisions;

③ Modifying the local polarity and hydrogen-bonding environment;

④ Reducing the proportion of free avobenzone;

⑤ Improving the uniformity of its dispersion within the sunscreen film.

Cyclodextrins have a hydrophilic exterior and a relatively hydrophobic internal cavity and can form host–guest inclusion complexes with avobenzone. Some studies have shown that hydroxypropyl-β-cyclodextrin can reduce the photoinduced degradation of avobenzone and alter its transdermal migration behavior.[5]

Encapsulation does not completely isolate avobenzone from light. The carrier must still allow UVA to reach avobenzone while enabling the molecule to release energy effectively after light absorption. Encapsulation efficiency, particle size, carrier transparency, release behavior, and distribution within the sunscreen film can all influence the actual outcome.

6.3 Vitamin E and Antioxidant Systems

Tocopherol in vitamin E can donate a hydrogen atom to lipid peroxyl radicals:

LOO· + Toc-OH → LOOH + Toc-O·

where:

• Toc-OH represents tocopherol;

• Toc-O· represents the tocopheroxyl radical.

The tocopheroxyl radical is less reactive than the lipid peroxyl radical, so this process helps slow radical chain reactions in the oil phase. The primary role of vitamin E is to control secondary oxidation that may be initiated after avobenzone is exposed to light.

Antioxidant efficacy is also influenced by the following factors:

① The specific form and concentration of tocopherol;

② The initial peroxide value of the oil-based raw materials;

③ The oxygen content of the formulation;

④ Whether other antioxidants can participate in regeneration;

⑤ The levels of trace metals and photosensitive impurities.

Some antioxidants have been shown in studies to slow the photodegradation of avobenzone, although their effects are formulation-dependent.[4]

6.4 Control of Metal Ions and Inorganic Particles

Under certain conditions, the β-dicarbonyl structure of avobenzone can coordinate with metal ions such as iron, aluminum, magnesium, and zinc, potentially causing color changes, precipitation, or alterations in photochemical properties.

Metal complexation and metal-catalyzed oxidation can be reduced by controlling metal impurities in raw materials, using appropriate chelating agents, and avoiding incompatible metal salts.

When avobenzone is used together with titanium dioxide or zinc oxide, the surface treatment of the inorganic particles must also be considered. Uncoated particles with relatively high photocatalytic activity may promote the formation of reactive oxygen species, whereas suitable surface coatings can help reduce this interaction.

7 Maximum Permitted Concentrations in Major Markets

The following values represent the maximum permitted concentration of avobenzone in the finished product and are not equivalent to recommended use levels.

Regulatory limits represent the maximum concentration permitted in the finished product and do not directly determine the UVA protection factor. The actual use level must also take the following factors into account:

① The solubility of avobenzone in the oil phase;

② Spectral compatibility with other UV filters;

③ Stability after irradiation;

④ Uniformity of film formation;

⑤ The target Sun Protection Factor (SPF) and Ultraviolet A Protection Factor (UVA-PF);

⑥ Current regulations in the target market.

8 Classification Tables of Chemicals Related to Avobenzone UV Absorption, Photochemical Instability, and Formulation Stability

Table 1. UV Filters and Spectral Compatibility Materials

Table 2. Oil-Phase Solvents, Inclusion Carriers, and Particle Surface-Treatment Materials

Table 3. Antioxidants, Analytical Reference Materials, and Metal-Ion Control Agents

Table 4. Compounds for Molecular-Structure, Candidate-Degradation-Product, and Reaction-Mechanism Studies

Note: The products listed above are representative Aladdin products related to scientific research and formulation studies. Additional product specifications, grades, and certificate of analysis information can be searched on the Aladdin website using the product name, CAS number, or catalog number.

References

[1] NATIONAL CENTER FOR BIOTECHNOLOGY INFORMATION. PubChem Compound Summary for CID 51040, Avobenzone[DB/OL]. Bethesda (MD): National Library of Medicine (US).

[2] MTURI G J, MARTINCIGH B S. Photostability of the sunscreening agent 4-tert-butyl-4-methoxydibenzoylmethane (avobenzone) in solvents of different polarity and proticity[J]. Journal of Photochemistry and Photobiology A: Chemistry, 2008, 200(2–3): 410–420. DOI: 10.1016/j.jphotochem.2008.09.007.

[3] PARIS C, LHIAUBET-VALLET V, JIMÉNEZ O, TRULLAS C, MIRANDA M A. A blocked diketo form of avobenzone: photostability, photosensitizing properties and triplet quenching by a triazine-derived UVB filter[J]. Photochemistry and Photobiology, 2009, 85(1): 178–184. DOI: 10.1111/j.1751-1097.2008.00414.x.

[4] AFONSO S, HORITA K, SOUSA E SILVA J P, ALMEIDA I F, AMARAL M H, LOBÃO P A, COSTA P C, MIRANDA M S, ESTEVES DA SILVA J C G, SOUSA LOBO J M. Photodegradation of avobenzone: stabilization effect of antioxidants[J]. Journal of Photochemistry and Photobiology B: Biology, 2014, 140: 36–40. DOI: 10.1016/j.jphotobiol.2014.07.004.

[5] YANG J, WILEY C J, GODWIN D A, FELTON L A. Influence of hydroxypropyl-β-cyclodextrin on transdermal penetration and photostability of avobenzone[J]. European Journal of Pharmaceutics and Biopharmaceutics, 2008, 69(2): 605–612. DOI: 10.1016/j.ejpb.2007.12.015.

[6] TREBŠE P, POLYAKOVA O V, BARANOVA M, BAVCON KRALJ M, DOLENC D, SARAKHA M, KUTIN A, LEBEDEV A T. Transformation of avobenzone in conditions of aquatic chlorination and UV-irradiation[J]. Water Research, 2016, 101: 95–102. DOI: 10.1016/j.watres.2016.05.067.

[7] COMMISSION OF THE EUROPEAN COMMUNITIES. Commission Recommendation of 22 September 2006 on the efficacy of sunscreen products and the claims made relating thereto (2006/647/EC)[Z]. Official Journal of the European Union, 2006, L 265: 39–43.

[8] U.S. FOOD AND DRUG ADMINISTRATION. Labeling and Effectiveness Testing; Sunscreen Drug Products for Over-the-Counter Human Use: Final Rule[Z]. Federal Register, 2011, 76(117): 35620–35665.

[9] EUROPEAN PARLIAMENT AND COUNCIL OF THE EUROPEAN UNION. Regulation (EC) No 1223/2009 of 30 November 2009 on cosmetic products (recast)[Z/OL]. Official Journal of the European Union, 2009, L 342: 59–209. Current consolidated version of 1 May 2026, Annex VI, entry 8.

[10] U.S. FOOD AND DRUG ADMINISTRATION. Final Administrative Order (OTC000039): Amending Over-the-Counter Monograph M020: Sunscreen Drug Products for Over-the-Counter Human Use, and Related Information[Z/OL]. Issued June 10, 2026. OTC Monograph M020, § M020.10(b).

[11] HEALTH CANADA, HEALTH PRODUCTS AND FOOD BRANCH. Primary Sunscreen Monograph[EB/OL]. Ottawa: Health Canada, 2026-02-27. Table 2: NPD Medicinal Ingredients.

[12] AUSTRALIAN GOVERNMENT, THERAPEUTIC GOODS ADMINISTRATION. Understanding the Regulation of Therapeutic Sunscreens[EB/OL]. Version 4.0. Canberra: Therapeutic Goods Administration, August 2025. Appendix 3, Table 9: Active Ingredients Restricted for Use in Therapeutic Sunscreens, p. 43.

[13] CHINA FOOD AND DRUG ADMINISTRATION. Announcement on the Issuance of the Safety and Technical Standards for Cosmetics (2015 Edition) (Announcement No. 268 of 2015)[EB/OL]. 2015-12-23. Attachment: Safety and Technical Standards for Cosmetics (2015 Edition), Chapter 2, Table 5, “Permitted Sunscreen Agents.”

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