chapter  17
30 Pages

The Photostability of Organic Sunscreen Actives: A Review

Introduction 322

Photostability as a Sunscreen Industry Concern 323

Photochemistry Review 323

Background 323

The Nature of Photon Absorption 324

Photochemical Reactions 324

Energy Transfer 325

Solvent Polarity and Electron Transfer Theory 327

Photostability of Individual Sunscreen Active Ingredients 328

Avobenzone (Butyl Methoxydibenzoylmethane) 329

Octinoxate (Octyl Methoxycinnamate) 334

Other UV Filters 335

UV Filter Combinations 338

Photostability of Sunscreen Formulations 339

Photostabilization Strategies 341

Formulation Strategies 341

Molecular Strategies 344

Conclusions 345

Acknowledgments 346

References 346

INTRODUCTION

Organic ultraviolet (UV) filters, such as those used in sunscreens, convert the

energy in UV radiation into electronic excitation energy (1). At a molecular

level, the physical reality of this conversion is a sudden expansion of an area

of the electron cloud surrounding the molecule (2,3). This happens so rapidly

(on the order of 10215 s) (1: p.6) that the nuclei of the molecule at first remain

in their original positions. In effect, an electronic isomer of the original molecule

has been created. If we were to isolate and observe the molecule, we might in one

common scenario see the distorted electron cloud collapse almost immediately to

its original shape. Simultaneously, we might see a flash of light emerge from the

molecule. Close examination would reveal that the molecule is now indistin-

guishable from its preabsorbance condition. In another common scenario, we

might see that the distortion of the electron cloud persists and exerts a force

that causes bonds to stretch and nuclei to move to accommodate the new

shape. We might see that the bond stretching and nuclear motion dissipates the

electronic excitation energy until the electron cloud returns to its preabsorbance

shape and the nuclei return to their previous positions relative to each other.

Again, the molecule would be indistinguishable from its preabsorbance con-

dition. Therefore, it can repeat the cycle of absorbance, electronic isomerization,

and energy dissipation. It is, in effect, photostable.