Preface

It has been over three decades since the publication of Modern Molecular Photochemistry (MMP). During this period, the concepts and paradigms described in MMP have become part and parcel of modern synthetic and mechanistic photochemistry, and have been absorbed as routine intellectual tools in a wide range of fields, including physical organic chemistry, chemical biology, polymer chemistry, materials science and nanoscience. Remarkably, most of the basic paradigms of MMP remain the bed rock of current mechanistic analyses, investigations and application of photochemical reactions. However, the elaboration of these paradigms by the effect of spin and electron transfer was not covered in MMP. It was decided that a primer, which included these factors and integrated them with the general successful pedagogical philosophy of MMP, would be of use and interest to not only practicing photochemists and their students, but also to those in a variety of other fields, such as biological scientists, polymer scientists, materials scientists and nanoscientists who integrate photochemistry and photophysics in their research and teaching. 

The primer, Principles of Molecular Photochemistry: An Introduction, introduces photochemical and photophysical concepts from a small set of principles that are familiar and understood by students of chemistry and other sciences. An initial paradigm is introduced that relates the photon and a reactant molecular structure to photochemistry through the structure of electronically excited states, reactive intermediates and products. The same paradigm is readily adapted to incorporate the photon and a reactant molecular structure to photophysics. The role of electronically excited states, and electronic-vibrational and electronic-spin interactions are clearly described in pictorial terms that can be readily understood and applied to systems of interest. 

For the first time in any photochemical text, a fundamental description of electronic spin and its impact on photochemical and photophysical processes is described with an intuitive, pictorial and powerful vector model. The mysterious processes of spin-orbit coupling, intersystem crossing and magnetic effects on photochemical and photophysical processes are readily handled with this model. 

Also, for the first time in any photochemical text, the concepts of electronic energy transfer and electron transfer are treated from a common foundation and set of concepts. The tremendous progress in theoretical and experimental electron transfer is covered in depth and will be of great assistance as an introduction to these two critical aspects of molecular photochemistry. 

Level and Approach 

The goal of this primer is to familiarize both students and researchers with the critical concepts and methodology involved in the investigation of organic photochemical reactions. A simple paradigm at the start of each chapter is elaborated with exemplars that provide the student with an understanding through examples of the underlying principles. The material can be understood easily by students with the fundamental knowledge of college general chemistry, organic chemistry and physics. An important feature of the primer is avoidance of complex mathematics and the translation of all concepts into familiar visualized representations, providing a complete and unified theoretical background for an understanding of light absorption and emission, or radiationless processes and of photochemical reactions. For example, the concept of electronic energy surfaces coupled with simple molecular orbital theory is used to visualize the triggers of photochemical processes and the analogies that intellectually reduce the thousands of organic photochemical reactions to a handful of fundamental primary photochemical reactions from electronically excited states. 

This primer will be of use to students interested in a qualitative, pictorial interpretation of spectroscopic processes involving the absorption and emission of light by organic molecules and the photochemical processes that result from light absorption. In this regard, in addition to students of chemistry, the text will be useful to nonchemistry majors pursing interests in biology, biochemistry, materials science or chemical engineering. 

Acknowledgments: This text is an outgrowth of courses and lectures on Organic Photochemistry. The authors are indebted to the many students who have assisted in the development of the text through their probing and stimulating questions as they sought an understanding of molecular organic photochemistry. We are also thankful to many colleagues who allowed their “brains to be picked” and thereby enabled the authors to produce a translation of abstruse mathematical concepts into concrete representations that provide students with an understanding of the subject. Special thanks to Professor David Schuster of New York University for a careful and critical reading of the early drafts of the book. Thanks also to the following photochemists who have read and provided critical comments of chapters 9-13: R.S.H. Liu, F.D. Lewis, J.R. Scheffer, L. Johnson, D.I. Schuster, and A. Griesbeck.