Photochemistry

An Introduction

D. R. Arnold, N. C. Baird, J. R. Bolton

• Publisher: Academic Press
• Year: 2014
• Print ISBN: 9780120633500
• eBook ISBN: 9781483216126
• Topic: Physical Sciences
• Subtopic: Biophysics

Photochemistry: An Introduction covers topics such as industrial photochemistry, solid state photochemistry, spectroscopy and photochemistry of the solid state, industrial applications of photochemistry, and photochromism. The book discusses the application of bonding, structure, energetics, and reactivity of the ground states of molecules to describe the same properties for molecules in their electronically excited states; the electronic spectra of excited states; and how the excited states react to form chemical transients. The text also describes light sources, techniques for measuring light intensities and quantum yields, methods used to detect transient photochemical products, and some ancilliary techniques. A review of some features of typical photochemical processes conducted in the vapor state and a survey of the reactions of the urban atmosphere, are also considered. The book further tackles the mechanisms of organic photochemical reactions; the synthetic applications of organic photochemistry; and the photochemistry of the solid state. The text also looks into photochromism and the industrial applications of photochemistry. People involved in the field of photochemistry will find the book useful.

CHAPTER 1

INTRODUCTION

Publisher Summary

This chapter provides an overview of photochemical reactions. The essential of a photochemical process is that activation for reaction is provided by the absorption of a photon. Photochemical activation differs from thermal activation in that it may be more specific. Light may be absorbed by a particular chromosphere, which may be a small part of a large molecule, and this process may occur when the molecule is dissolved in vast amounts of a solvent. A photon of particular energy, corresponding to a particular wavelength, will only excite a molecule capable of absorbing at that wavelength. This is strictly true for normal light sources, but may require modification for biphotonic processes with intense sources (lasers). The only energy available for excitation is that of the photon. If E₂ is the final energy of the system and E₁ is that of the molecule in the ground state, then E₂ – E₁ = hυ, where h is Planck’s constant, and υ is the frequency of the 1ight absorbed. There is a wide variety of reactions that involve dissociation of two bonds as a primary step. In some cases, it is possible to tell the difference from the stereospecific nature of tine products. Sigmatropic reactions are reactions where the numbers of π and σ bonds remain constant but are differently distributed in the product from the starting material.

A Brief History

Photochemical processes have been intimately related to the development of Man and his environment even before his appearance on the planet. It is believed that certain stages in the generation of the building units for the macromolecules of life occurred on the primordial earth under the influence of the sun’s rays. Subsequently the evolution of the process of photosynthesis, the conversion of carbon dioxide into carbohydrate, rendered life in its present form possible. Finally the evolution of all life of an advanced form would be drastically different if the photochemical process of vision had not been developed. And the planet continues to be irradiated to the extent of 100 kcal/cm²/day …

Photochemical reactions in the laboratory have been known for almost as long as chemistry has been studied. Most of the observations were accidental and remained uninterpreted, and only at the end of the nineteenth century was any systematic approach made. Then, in Italy, largely as a result of the work of Ciamician and his collaborator Silber, and to a lesser extent that of Paterno, the organic chemist at last paid serious attention to the possibilities of the chemical action of light. The wide range of reactions discovered by Ciamician and Silber is most impressive; indeed, many of these reactions are still being studied. In view of this spectacular achievement¹, why then did the interest in photochemistry decline abruptly?

The immediate reason was the First World War, and its consequences which rendered continuation of the work difficult for Ciamician and Silber. However, a more permanent reason was the fact that with the techniques then available further work was technically difficult. They lacked adequate light sources, filter systems and, more particularly, physical means of separation of the often complex mixtures. In addition, chemical and physical theory was in no way capable of even a partial rationalization of the results obtained. A good perspective of the state of photochemistry in 1911 has been provided by Ciamician himself in an address to the Congress of Applied Chemistry in New York². At a distance of nearly sixty years his prophetic insight is truly impressive.

After the Fi...

Table of contents

• Cover image
• Title page
• Table of Contents
• Inside Front Cover
• Copyright
• PREFACE
• Chapter 1: INTRODUCTION
• Chapter 2: THEORY AND THE EXCITED STATE
• Chapter 3: ELECTRONIC SPECTRA OF EXCITED STATES
• Chapter 4: TRANSIENTS AND THEIR BEHAVIOR
• Chapter 5: EXPERIMENTAL TECHNIQUES
• Chapter 6: PHOTOCHEMICAL PROCESSES IN THE GAS PHASE
• Chapter 7: MECHANISMS OF ORGANIC PHOTOCHEMICAL REACTIONS
• Chapter 8: SYNTHETIC APPLICATIONS OF ORGANIC PHOTOCHEMISTRY
• Chapter 9: PHOTOCHEMISTRY OF THE SOLID STATE
• Chapter 10: PHOTOCHROMISM
• Chapter 11: INDUSTRIAL APPLICATIONS OF PHOTOCHEMISTRY
• SUBJECT INDEX