From its origin over a century ago, organic photochemistry has undergone a transformation from an area of science populated by a few specialized organic and physical chemists to a field that now attracts the interest of members of the broad synthetic organic chemistry community. Along the way, the basic chemical and physical foundations of the science were developed and the full synthetic potential of photochemical reactions of organic substrates has been realized. The science of organic photochemistry can be traced back to observations made in the nineteenth century, which showed that ultraviolet irradiation of certain organic substances leads to formation of products that have unique and sometimes highly trained structures. An example of this is found in studies in the early 1800s, which demonstrated that irradiation of the naturally occurring, cross-conjugated cyclohexadienone, a-santonin, in the crystal state induces a deep-seated, multistepped rearrangement reaction. It is fair to conclude that at that time observations like this could only have been attributed to the magic of Nature, since little if anything was known about the fundamental principles of the light absorption process and the relationships between structures and decay pathways of electronic excited states. The science of organic photochemistry experienced a significant transformation in the middle part of the twentieth century when it began to attract the interest of organic chemists, who were skilled in the use of valence bond theory, and physical chemists, who were able to probe and theoretically analyze the properties of electronic excited states. These efforts led to a basic mechanistic framework for understanding and predicting how electronic excited states of organic substrates undergo reactions to form products. Clear examples of the insight provided by organic chemists during this era are found in ground-breaking investigations performed independently by Zimmerman and Chapman that probed the photochemistry of simple, cross-conjugated cyclohexadienones. The realization that these processes could be described by utilizing Lewis electron-dot-line structures of excited states and reactive intermediates brought organic photochemistry into the intellectual sphere of organic chemists, who already had learned the benefits of writing arrow-pushing mechanisms for ground-state reactions.
Another important contribution to the field of organic photochemistry arose from investigations of excited state redox processes in the latter part of the twentieth century. These efforts showed that when the oxidation and reduction potentials and excited state energies of interacting electron donors and acceptors are appropriate, thermodynamically and kinetically favorable excited state single electron transfer (SET) will take place to produce ion radical intermediates. This phenomenon expanded the vista of organic photochemistry, since it enabled the unique and predictable reactivity profiles of charged radicals to be included in the concept library used to design new photochemical transformations. Many examples of the exceptional impact that SET has had on the field of organic photochemistry came from the pioneering work of Arnold and a cadre of other organic chemists who developed synthetic applicable SET photochemical processes. It is clear that studies in the area of organic photochemistry have led to the discovery of a large number of novel reactions, and that some of these processes meet the high standards needed for use as preparative methodologies. The compilation in this Handbook, which begins with a useful chapter describing practical experimental methods used in photochemistry, reviews several of the more synthetically prominent photochemical reactions of organic substrates.
There is no doubt that the field of organic photochemistry was subjected to intense scrutiny in the latter half of the twentieth century, and that efforts during this period led to a firm understanding of basic photochemical principles and to the discovery of a wealth of highly unique chemical reactions. Moreover, during this period members of the synthetic organic chemistry community recognized that several photochemical processes could be applied as key steps in routes for the construction of complex target molecules. It is likely that activity in the area of organic photochemistry will not diminish in the twenty first century where it will used in finding matchless solutions to challenging chemical problems. Thus, rather than being caused by the need to prepare sophisticated organic substances made by Nature, problems in the new century are likely to revolve about the search for green methods for promoting chemical reactions and for processes that can be performed in confined spaces (e.g., cells), defined patterns (e.g., lithography), and precisely controlled time domains (e.g., triggers). Organic photochemistry is uniquely applicable to these types of challenges and, as a result, it should continue to be an interesting area in which to work.
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