ABSTRACT
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.
