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28: 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.

  • 28.1: Prelude to Photochemistry
    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.
  • 28.2: Light Absorption, Flourescence, and Phosphorescence
    When a molecule absorbs sufficient radiant energy to cause electronic excitation, the spin of the excited electron remains unchanged in the transition. That is to say, ground-state molecules with paired electrons give excited states with paired electrons, not triplet states. The excited state can return to the ground state with emission of radiation; this is known as fluorescence, the wavelength of fluorescence being different from that of the original light absorbed. Other processes compete.
  • 28.3: Organic Photochemistry
    Compounds have very different chemical behavior in their excited states compared to their ground states. Not only is the energy much higher, but the molecular geometry and electronic configurations are different. Intuitively, we expect that excited states of molecules, in which two electrons occupy separate unfilled orbitals, would have substantial diradical character. This is the case, especially for triplet states, as we shall see.
  • 28.4: Chemiluminescence
    Many reactions are known that generate products in electronically excited states, and this is especially evident when the electronically excited products go to the ground state by the emission of visible light. This behavior is known as chemiluminescence and is transduction of chemical energy into radiant energy. Chemiluminescence is possible only when the ΔH of the reaction is sufficiently large to allow for production of at least one of the products in an electronically excited state.
  • 28.5: Color and Constitution
    The color perceived of a compound depends on the shape of the electronic spectral curve of the absorbing substance, which depends on the chemical structure. Shifting the absorption from absorbing blue to red light corresponds to a decrease in the energy of the electronic transitions. This trend is associated with increasing conjugation of multiple bonds with the more extended a planar system of conjugated bonds is, the smaller is the energy difference between the ground and excited states.
  • 28.6: The Sensation of Color
    The sensation of color can be achieved in different ways. We could recognize a given color, say yellow, by direct perception of light encompassing a narrow band of wavelengths around 580nm, or by subtraction of blue light ( 435 - 480nm ) from white light. A third way of producing color is by an additive process. In fact, a wide range of colors can be achieved by the addition of three colors - red, green, and blue.
  • 28.7: Color Photography
    Photography is a popular activity for many, but relatively few have an understanding of the chemistry involved, particularly in color photography. This is unfortunate because color photography represents an interesting combination of photochemistry (energy transfer), organic chemistry (dye formation), optics, psychology and physiology (color perception), and engineering (production and development of film).
  • 28.8: Chemistry of Vision
    Vision is a process in which light is absorbed by a pigment in a photoreceptor cell (by a dye in the eye) and the photochemistry that ensues ultimately produces a transient electrical signal that is transmitted to the brain and interpreted as a visual image. There is much that is not fully understood about this process, but we shall discuss briefly the chemistry involved.
  • 28.E: Photochemistry (Exercises)
    These are the homework exercises to accompany Chapter 28 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).

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."


This page titled 28: Photochemistry is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by John D. Roberts and Marjorie C. Caserio.

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