Avobenzone

History

Avobenzone was patented in 1973 and was approved in the EU in 1978. It was approved by the FDA in 1988. Its use is approved worldwide.

Properties

Pure avobenzone is a whitish to yellowish crystalline powder with a weak odor, dissolving in isopropanol, decyl oleate, capric acid/caprylic acid triglycerides and other oils.

Avobenzone is a dibenzoylmethane derivative. Avobenzone exists in the ground state as a mixture of the enol and keto forms, favoring the chelated enol. This enol form is stabilized by intramolecular hydrogen-bonding within the β-diketone. Its ability to absorb ultraviolet light over a wider range of wavelengths than many other sunscreen agents has led to its use in many commercial preparations marketed as "broad spectrum" sunscreens. Avobenzone has an absorption maximum of 357 nm.

It is subject to keto-enol tautomerism and exists predominantly enol:

Stability

Avobenzone is sensitive to the properties of the solvent, being relatively stable in polar protic solvents and unstable in nonpolar environments. Also, when it is irradiated with UVA light, it generates a triplet excited state in the keto form which can either cause the avobenzone to degrade or it can transfer energy to biological targets and cause deleterious effects.

Avobenzone has been shown to degrade significantly in light, resulting in less protection over time. The UV-A light in a day of sunlight in a temperate climate is sufficient to break down most of the compound. Data presented to the Food and Drug Administration by the Cosmetic, Toiletry and Fragrance Association indicates a -36% change in avobenzone's UV absorbance following one hour of exposure to sunlight. This degradation can be reduced by using a photostabilizer, like octocrylene. Other photostabilizers include:

Complexing avobenzone with cyclodextrins may also increase its photostability. Formulations of avobenzone with hydroxypropyl-beta-cyclodextrin have shown significant reduction in photo-induced degradation, as well as increased transdermal penetration of the UV absorber.

According to some studies, "the most effective sunscreens contain avobenzone and titanium dioxide." Avobenzone can degrade faster in light in combination with mineral UV absorbers like zinc oxide and titanium dioxide, though with the right coating of the mineral particles this reaction can be reduced. A manganese doped titanium dioxide may be better than undoped titanium dioxide to improve avobenzone's stability.

Various

As an enolate, avobenzone forms with heavy metal ions (such as Fe³⁺) colored complexes, and chelating agents can be added to suppress them. Stearates, aluminum, magnesium and zinc salts can lead to poorly soluble precipitates. Manufacturers also recommend to avoid the inclusion of iron and ferric salts, heavy metals, formaldehyde donors and PABA and PABA esters. The makers of Coppertone advise that avobenzone binds iron and can cause staining of clothes washed in iron-rich water.

Avobenzone also reacts with boron trifluoride to form a stable crystalline complex that is highly fluorescent under UV irradiation. More interestingly, the emission color of the crystals relies on the molecular packing of the boron avobenzone complex. The photoluminescence may also be altered by mechanical force in the solid state, resulting in a phenomenon called "mechanochromic luminescence". The altered emission color recovers itself slowly at room temperature or more swiftly at higher temperatures.

Preparation

The compound is prepared by reacting 4-tert-butylbenzoic methyl ester (from 4-tert-butylbenzoic acid by esterification with methanol) with 4-methoxyacetophenone in toluene in the presence of sodium amide via Claisen condensation.

According to a recent patent application, yields of up to 95% are obtained with the same starting materials in toluene in the presence of potassium methoxide.