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28.1: Prelude to Photochemistry

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  • The role of light in effecting chemical change has been recognized for many years. Indeed, the connection between solar energy and the biosynthesis of plant carbohydrates from carbon dioxide and water was known by the early 1800's. Yet organic photochemistry was slow to develop as a well-understood and manageable science. Progress only became rapid following the development of spectroscopy and spectroscopic techniques for structure determination and the detection of transient species. For this reason photochemistry for many years was the domain of physical and theoretical chemists. Their work laid the foundation for modern organic photochemistry, which correlates the nature of excited electronic states of molecules with the reactions they undergo.

    Quite apart from the unparalleled importance of photosynthesis, photochemical reactions have a great impact on biology and technology, both good and bad. Vision in all animals is triggered by photochemical reactions. The destructive effects of ultraviolet radiation on all forms of life can be traced to photochemical reactions that alter cellular DNA, and the harmful effects of overexposure to sunlight and the resulting incidence of skin cancer are well established. The technical applications of photochemistry are manifold. The dye industry is based on the fact that many organic compounds absorb particular wavelengths of visible light, and the search for better dyes and pigments around the turn of this century was largely responsible for the development of synthetic organic chemistry. Dye chemistry has helped establish the relationship between chemical structure and color, which also is important in color printing and color photography. We cover these important applications of photochemistry only briefly in this chapter, but we hope to convey some understanding of the fundamentals involved.

    Most photochemical reactions can be considered to occur in three stages:

    1. Absorption of electromagnetic radiation to produce electronically excited states.
    2. Primary photochemical reactions involving excited electronic states.
    3. Secondary or dark reactions whereby the products of the primary photochemical reactions are converted to stable products.

    We shall begin with a closer look at electronic excitation, some aspects of which were discussed in Section 9-9. Because transfer of electronic energy from one molecule to another is a basic process in photochemistry, we will discuss energy transfer also before giving an overview of representative photochemical reactions. The closely related phenomena of chemiluminescence and bioluminescence then will be described. Finally, there will be a discussion of several important applications of photochemistry.

    Contributors and Attributions

    John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."