Some Basic Principles of Photochemistry

Before addressing the primary topics of this chapter it is essential that the reader be versed in some of the concepts of photochemistry and of the natural properties of seawater which make investigations in the marine system differ markedly from typical classical studies in organic photochemistry. The discussion here must be brief for more comprehensive treatments the reader is referred to some of the many excellent texts on organic photochemistry (Calvert and Pitts, 1966 Wayne, 1970 Cowan and Drisko, 1976) and inorganic photochemistry (Balzani and Carassiti, 1970 Adamson and Fleischauer, 1975). 

This process is known as photosensitization and should only be significant in dilute solutions of the acceptor and donor for long-lived triplet states, which for typical organic molecules have lifetimes of 10 to 10 sec versus only 10 to 10 sec for excited singlet states. Photosensitized reactions are also often used to describe non-energy transfer reactions these will be discussed in Section 4.2.1. 

The efficiency of any photophysical or photochemical process is a function of both the properties of the reaction environment and the character of the excited state species. The fundamental quantity which is used to describe the efficiency of any photo process is the quantum yield (0) it is useful in both quantifying the process and in elucidating the reaction mechanism. Quantum yield has the general definition of the number of events occurring divided by the number of photons absorbed. Therefore, for a chemical process 0 is defined as the number of moles of reactant consumed or product formed divided by the number of einsteins (an einstein is equal to 6.02 X 10 photons) absorbed. Since the absorption of light by a molecule is a one-quantum process, then the sum of the quantum yields for all primary processes occurring must be one. Where secondary reactions are involved, however, the overall quantum yield can exceed unity and for chain reactions reach values in the thousands. When values of 0 are known or can be measured for a specific photochemical reaction the rate can be determined from  

The amount of actinic radiation absorbed by the reactant (Ia) in unit volume and unit time can be calculated from the equation derived from Beer s law  

Like thermal reactions, photochemical reactions have energy thresholds below which the reaction is not energetically feasible. The energy equivalence (E, in kcal einstein ) of the wavelength or frequency of radiation can be determined from  

Light Absorption by Chemical Species Molar Extinction Coefficients 

A more particle-oriented consideration of light shows that light is quantized and is emitted, transmitted, and absorbed in discrete units, so-called photons or quanta. The energy E of a photon or quantum (the unit of light on a molecular level) is given by  

When a photon passes close to a molecule, there is an interaction between the electromagnetic field associated with the molecule and that associated with the radiation. If, and only if, the radiation is absorbed by the molecule as a result of this interaction, can the radiation be effective in producing photochemical changes (Grotthus-Draper law, see, e.g., Finlay son-Pitts and Pitts, 1986). Therefore, the first thing we need to be concerned about is the probability with which a given compound absorbs uv and visible light. This information is contained in the compounds uv/vis absorption spectrum, which is often readily available or can be easily measured with a spectrophotometer. 

Using a spectrophotometer and an appropriate solvent and reference solution (i.e., the same liquid phase as the one containing the compound so that absorption effects other than by the chemical cancel), the absorbance A of a solution of the compound  

Determining Decadic Molar Extinction Coefficients of Organic Pollutants 

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We have anticipated that the reader is familiar with the basic principles of photochemistry and with the photochemical vocabulary 374>. A short summary of some experimental photochemical procedures is intended to be helpful to the organometallic chemist who plans to initiate work in the field a). 

A molecule may absorb electromagnetic (em) radiation and, in the process, break down into its atomic or molecular components. Unstable atoms and molecular fragments may also combine to form more stable molecules, disposing of their excess energy in the form of em radiation. These chemical reactions are called photochemical, and the process by which a photochemical reaction occurs is called photolysis. Photochemical reactions play very important roles in many aspects of environmental chemistry. Therefore, this book concludes with a brief account of some of the basic principles of photochemistry, which we will then apply to ozone in the Earth s stratosphere and the problem of the stratospheric ozone hole. 

Systematic study of photochemistry is at a crossroad at this time. The past ten years have been a period of especially interesting and vigorous development. Understanding of some kinds of basic principles has been considerably clarified and the breadth of available information has increased enormously because of the imaginative study of large, complex molecules. However, I believe that the field needs clarification of concepts concerning the fundamental processes in which electronic excitation energy is used to make and break chemical bonds. 

Photophysics and photochemistry both deal with the impact of energy in the form of photons on materials. Photochemistry focuses on the chemistry involved as a material is impacted by photons, whereas photophysics deals with physical changes that result from the impact of photons. This chapter will focus on some of the basic principles related to photophysics and photochemistry followed by general examples. Finally, these principles will be related to photosynthesis. In many ways, there is a great similarity between a material s behavior when struck by photons, whether the material is small or macromolecular. Differences are related to size and the ability of polymers to transfer the effects of radiation from one site to another within the chain or macromolecular complex. 

Enormous interest shown on the photochemistry of transition metal polypyridyl complexes in fact is linked to these type of applications in the domain of photochemical conversion of solar energy. Practically every metal complex with fully or partially characterized electronically excited state has been examined as a means of generating key oxidants and/or reductants required and some have shown partial success. A number of reviews of these topics are available [63-65] and hence only the basic principles and summary of progress in these areas will be indicated. 

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