Photochemical studies of new synthetic derivatives of avobenzone under sunlight using UV-spectroscopy

Abstract

Avobenzone is the most widely used chemical UV-A filter in sunscreen formulations and is prone to photodegradation under UV radiations/sunlight. Various methods have been employed to improve the photostability of avobenzone using triplet quenchers, antioxidants, and micelles. The current report has adopted a skeletal modification approach to improving the photostability of avobenzone. Electron-donating –OMe group has been replaced with electron-withdrawing –COOH/–COOMe group to yield the new derivatives, namely, avobenzone-acid/avobenzone-ester. New avobenzone derivatives were chemically synthesized and characterized using UV-spectroscopy which confirmed that they retained broad-spectrum UV-A filtering activity with additional UV-C filtering activity. Interestingly, the keto-form was present in crystals and 94:6 ratio of enol-: keto-forms were present in aprotic DMSO solvent confirming the occurrence of keto-enol tautomerization in avobenzone-acid. The photochemical properties of the new derivatives such as photostability, recovery from photodegraded enol forms in dark, and the effect of photostabilizer glutathione were accessed under sunlight using UV-spectroscopy. These studies indicate that the new derivatives compared to naïve avobenzone have improved photostability, rapid recovery from degradation products in the dark in DMSO, and enhanced response to photostabilizer glutathione. Computationally, Bond Dissociation Energy (BDE) and influence of DMSO solvent on the energetics of keto-enol tautomerizations of avobenzone and the new derivatives were calculated using density functional theory (DFT). These calculations support the experimental observations of the improved photochemical properties of the new avobenzone derivatives with retention of broad-spectrum UV-A absorbance activity. The current report has demonstrated that the skeletal modification approach has improved the intrinsic photostability of avobenzone and the new avobenzone derivatives may serve as a potential alternative to avobenzone in sunscreen lotions.