Chapter 26 Photochemical and photophysical properties of sunscreens

Abstract

Abstract Topical sunscreens provide a means of sun protection and function by the absorption of light effecting an electronic excitation of the sunscreen molecule (from its ground state to a first excited singlet state). Effective sunscreens are those that absorb strongly in the appropriate UV region, display good photostability and exhibit minor spectral modifications upon exposure to UV radiation and thermally dissipate the absorbed energy harmlessly. The most effective deactivation routes are internal conversion, vibrational relaxation and photoisomerisation. However, organic UV absorbers have the potential to form relatively long-lived triplet states, which can stimulate singlet oxygen production, effect transformations in biological substrates, such as thymine or in constituents of sunscreen formulations. This chapter is concerned with a critical assessment of the photochemical and photophysical properties of a range of commercially available organic (UVA and UVB) and inorganic sunscreens with respect to their efficacy and suitability as common sunscreen ingredients. We focus on the positive and negative attributes of the photochemical properties of sunscreens and consider the impact of published findings on sunscreen manufacturers and the development of new sunscreen agents. We conclude that many organic sunscreens function by photoisomerisation, where isomeric mixtures serve as the major sun protection components. However, a number of sunscreens also form triplet states and singlet oxygen. Furthermore, inorganic sunscreens such as titanium dioxide and zinc oxide are known to degrade organic materials and produce hydroxyl radicals, although this is somewhat overcome by the use of surface treatment. Photoactivity testing of inorganic sunscreens is at an early stage of development and further work is required to achieve suitable protocols.