Abstract

Avobenzone (*) is by far the most widely used UVA filter in the world and has been used in sunscreens for more than 30 years. Its complex photophysics and photochemistry continues to be a subject of study by researchers from around the world. This article draws on the work of researchers as published in the literature and new, previously unpublished studies sponsored by the HallStar Company and conducted by one of us (Jockusch) to paint as clear a picture as possible of the photophysical and photochemical properties of this fascinating and important compound. Using data from the new studies, we show that the photostabilizer Ethylhexyl methoxycrylene (SolaStay S1, HallStar) quenches the triplet excited state of Avobenzone’s diketo tautomer with a sufficiently high rate constant to prevent unwanted unimolecular chemical reactions, and so is able to contribute safely to photoprotection by stabilization of Avobenzone

INTRODUCTION

The first thing to know about Avobenzone is that, in its ground state, it is not one but two species called tautomers. There’s an enol tautomer, and a diketo tautomer, often called simply the keto form (Figure 1).
The enol tautomer features a hydrogen atom bonded to two oxygen atoms and for that reason is often referred to as a chelated enol. The two tautomers exist in an equilibrium that is solvent dependent. In all solvents, the enol form predominates. For example in deuterated cyclohexane the ratio is 96.5% enol and 3.5% diketo. In a protic solvent such as isopropanol, the diketo tautomer may not be detectable (1). In sunscreen oil phases, expect there to be some diketo form present. That’s really a key point: under conditions of actual use, there is always some diketo tautomer present. It has been known for about 20 years that photodegradation of Avobenzone depends greatly on the presence of the diketo tautomer (2). That makes the diketo tautomer a worthy subject for further study.
The second thing to know about Avobenzone is that the two tautomers can interconvert; that is, the enol tautomer can be induced to conver ...