From Chromic Phenomena: Technological Applications of Colour Chemistry

4.1 Introduction

The excited state produced by the interaction of a molecule with light can lose or transfer its energy in various ways.1 Return to the ground state can be accompanied by the emission of light (fluorescence) or by non-radiative deactivation and output of heat energy as successively lower vibrational levels are occupied. The excited molecule can alternatively undergo intersystem crossing to the triplet state before returning to the ground state with emission of light (phosphorescence) or by non-radiative deactivation and output of heat energy, both at a much slower rate than decay from the singlet state. The excited state (either singlet or triplet) can also undergo unimolecular or bimolecular photochemical reactions to give different products or it can transfer its excitation to another ground state molecule (electronic excitation transfer). Because the triplet excited state is longer lived than the singlet excited state it is often more important in photochemical reactions. These reaction pathways are shown schematically in Figure 4.1.

Figure 4.1: Photophysical and photochemical pathways for excited molecules.

The photoluminescent processes, fluorescence and phosphorescence, together with the products exhibiting this phenomenon and its applications, are covered in Chapter 3 of this book. A unimolecular photochemical process, involving internal rearrangement, namely photochromism, is covered in Chapter 1 under colour change phenomena. The present chapter will deal with those phenomena involving photochemical reactions of coloured molecules, e.g. sensitisation of oxygen in photodynamic therapy, and the use of others that can transfer electromagnetic energy, often from a laser, into another useful...

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Topics of Interest

5.1 Introduction This last chapter covers those phenomena that use materials to manipulate light in a variety of ways in order to produce useful effects. In some of these phenomena light is...

3.2 Luminescent Pigments As stated above, there is no intention to provide any detailed discussions of the theory of luminescent phenomena; however, a basic understanding is essential. A simplified...

3.6 Chemiluminescence Chemiluminescence is the production of light by chemical substances without the help from other energy sources; in effect it is the conversion of energy stored in chemical bonds...

2.1 Photochemical Primary Processes In dynamic spin chemistry, photochemical reactions in solution have mostly been studied. Thus, we will start with a review of photochemical primary processes in...

3.5 Fluorescence Fluorescence from organic materials is a very important phenomenon that is exploited in a broad range of application areas. These include their use in the dyeing of synthetic textile...

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