Chemistry and Light

By Paul Suppan

The Royal Society of Chemistry

Copyright © 1994 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-814-1

Contents

Chapter 1 Introduction, 1,
Chapter 2 Light and Matter, 11,
Chapter 3 The Energy of Light: Excited Molecules, 27,
Chapter 4 The Chemistry of Excited Molecules, 87,
Chapter 5 Light and Life, 163,
Chapter 6 Light in Industry, 186,
Chapter 7 Experiment Techniques, 216,
Chapter 8 The Frontiers of Photochemistry, 256,
Appendices, 283,
Further Reading, 289,
Subject Index, 292,

CHAPTER 1

Introduction

The chemical effects of light play a most important role in our life, although we may often not be aware of it. The photosynthesis of green plants is the basis of our whole food chain, through the combination of water and carbon dioxide to form organic matter; in this way the heat released in the combustion of wood for example is of photochemical origin. The same is true of the fossil fuels coal, oil and natural gas; these were formed long ago by the decay of organic matter and therefore represent a storage of the energy of sunlight through a photochemical process.

There are other energies which depend indirectly on sunlight, but these do not involve photochemical reactions: hydroelectric power, wind power, and the thermal energy of oceans depend on the degradation of the energy of light into heat. This is a photophysical process, as it implies no chemical change of the light-absorbing matter. The distinction between photophysical and photochemical processes will be discussed further on, as it is more than just a matter of semantics.

So far we have considered the importance of light-driven processes for the supply of energy in more or less indirect ways. The direct conversion of the energy of light into electricity or hydrogen gas does not exist in nature, but it is the aim of current research which will be described in section 6.6.

1.1 LIGHT-INDUCED PROCESSES IN EVERYDAY LIFE

In biology, vision is probably the most important photochemical process after photosynthesis. The basic photochemistry of vision is now well understood and is considered in section 5.3 but there are other biological effects of sunlight which remain rather mysterious: phototropism is the orientation of plants towards the direction of sunlight, best known in the case of sun-flowers; photomorphogenesis is the control of the growth of plants by the intensity of light, to give only a few examples.

In these processes light is used to produce a chemical change which acts as a trigger for some complex enzyme-catalysed reaction. The primary photochemical process may be relatively simple, but the following ‘dark’ reactions are often quite complex. These secondary (dark) reactions are not considered in this book, since they are part of biochemistry rather than photochemistry.

In the processes of vision, phototropism, photomorphogenesis, etc., light acts only as a trigger and there is no permanent change either in the energy or in the composition of the chemical system. There is however one photobiological reaction in which the natural synthesis of an essential compound relies on a key photochemical step, and this is the biosynthesis of vitamin D considered in section 5.6. This leads to an important question concerning the use of photoinduced reactions in industrial synthesis. There are indeed some synthetic applications of photochemistry on an industrial scale, but these are quite small compared with the many dark reactions used in large-scale industrial synthesis. The major problem is the high cost of light as a source of energy (see section 6.3). This cost problem restricts the industrial applications of photochemical synthesis to chain reactions ( e.g. the chlorination of polymers like PVC) and the production of high-price chemicals such as pharmaceutical and cosmetic products; here the industrial syntheses of vitamin D and of ‘rose oxide’ provide examples of useful synthetic applications on small industrial scales.

1.1.1 Photodegradation Processes: The ‘Negative’ Actions of Light

So far we have been concerned with what could be called the ‘positive’ or useful aspects of photoinduced processes: light as a source of energy, light as the energy needed to build organic matter, light as information in visual and other biological processes, and light as a reactant in chemical synthesis. All these can be described as ‘positive’ within the needs and wishes of human beings. The point of view of an insect about to be killed by an ultraviolet lamp may of course be quite different, but since this book is aimed at a human readership the meanings of ‘positive’ (beneficial) or ‘negative’ (detrimental) effects of light must be seen in this context.

In photobiology these negative actions start with the processes which modify or destroy the essential molecules of life, specifically the nucleic acids and the proteins. The former carry the genetic code in a sequence of nucleotides and even a minor modification of the nucleotide sequence can have profound consequences. By the laws of chance alone these modifications and their consequences are almost always detrimental and result usually in the death of the organism ( e.g. microbes) exposed to short-wavelength ultraviolet radiation.

In industrial applications the photodegradation of synthetic polymers as well as of dyes and pigments, of agrochemicals, etc., are major causes for concern. The mechanisms of these photochemical reactions are now relatively well understood and various protective measures can be used to delay ( but never to stop totally) the complex processes of photodegradation (see section 6.2). Still, even here some photodegradation reactions can be put to good use. The design of photodegradable polymers provides one example, for plastic bags and containers made of such materials will decay to powder under the action of sunlight, instead of building up virtually stable refuse in the environment.

Many of these degradation processes involve photo-oxidations and these cannot be avoided since molecular oxygen is an essential part of our surroundings. Here also the definitions of ‘positive’ and ‘negative’ actions of light must depend on one’s point of view: in the phototherapy of cancer the malignant cells are destroyed by photo-oxidation, so from the point of view of these cells the action of light is surely highly negative, but from the point of view of the organism which will be cured of the cancer the effect is altogether positive.

1.1.2 Imaging Processes

Some of the major industrial applications of photochemistry are found in the various ‘imaging’ processes which include photography, photopolymerization/ photodepolymerization, photochromism and electrophotography — the process used in photocopying machines.

Photopolymerization is not restricted to imaging processes. However, such processes form the most spectacular applications of photochemical reactions, as they are used to make printed circuits and integrated circuits for the electronics industry.

1.2 GENERAL FEATURES OF PHOTOCHEMICAL AND PHOTOPHYSICAL PROCESSES

The distinction between ‘photophysical’ and ‘photochemical’ processes depends on the definition adopted for ‘chemical species’ and ‘chemical change’. It is often held that a chemical change must involve the breaking or making of chemical bonds; in that case it may be stated that the addition or the removal of an electron from a molecule would not be a ‘chemical’ change, so that the positive or negative ions would not be distinct chemical species but only ‘states’ of the neutral molecule. According to such a definition, the stereoisomers of a molecule would not be different ‘chemical’ species, but simply ‘states’ of the same molecule, and every chemist would recognize that this would be an absurd definition.

The concept of chemical species in terms of bonding patterns presents many problems, for even excited states of molecules differ in this respect from the ground state. To take one example, consider the valence bond structures of butadiene in its ground state (S0) and first (singlet) excited state (S1; Figure 1.1). The double bonds between carbon atoms C1 and C2, and C3 and C4, are reduced to single bonds, while the single bond between C2 and C3 becomes a double bond. According to such a definition, electronic excitation should be considered as a ‘chemical’ change.

In order to avoid such ambiguities, the definition of ‘chemical species’ will depend on the simple concept of stability. In the absence of chemical reactions, a chemical species will last indefinitely. Thus an ion is a distinct chemical species, and an electron transfer reaction must be seen as a chemical change. However, an electronic excited state of an atom or molecule must inevitably decay back to the ground state, so the processes of excitation, emission and non- radiative deactivation are photophysical processes.

1.2.1 The Pathways of ‘Dark’ Reactions and Photochemical Reactions

In the Arrhenius–Eyring model of a chemical reaction which takes place without the intervention of light, the reactant(s) R go over to the product(s) P through a transition state (X) which determines the activation barrier Ea in the rate constant equation

k = A exp (Ea/RT) (1.1)

When the molecule is excited by light to reach one of its electronically excited states (M* in Figure 1.2) it may undergo some other chemical reaction, leading to some high-energy product P’ through an activation barrier E*a. Both in the ground state (dark) and excited state (photoinduced) reactions the activation barriers (Ea and E*a respectively) must be overcome from the thermal energy of the chemical system. For this reason, it may be somewhat misleading to define ground state reactions as ‘thermal’ and excited state reactions as ‘light-induced’. The photochemical reaction is in fact the thermal reaction of the electronic excited state M* of the molecule M, while the ‘dark’ reaction of M is the thermal reaction of the ground state.

1.2.2 The Mistaken Concept of ‘Catalysis’ by Light

In a photochemical reaction light always acts as a reactant, never as a catalyst. By definition, a catalyst in a chemical reaction must be recovered unchanged, and can in principle be re-used again and again. In a photo-chemical reaction, light is absorbed and its energy is used to make electronically excited molecules; it will not come out and cannot be used after the reaction, so by definition light is not a catalyst and the concept of ‘light catalysed’ reaction is fundamentally incorrect.

The idea that light can act as a catalyst is however still rather widely held, and it comes from a consideration of the energy–reaction coordinate diagram shown in Figure 1.2. In a ground state (dark) reaction the role of a true catalyst is to lower the activation barrier Ea; it cannot change the energy levels of the reactants or products, or the free energy change of the overall reaction. Now, when light is absorbed by the reactant, the energy of the excited state (M*) is almost always largely in excess of the activation barrier Ea of the ground state reaction; so it may seem quite likely that light would have supplied the energy required to reach the transition state of the ground state reaction (the decay from M* to X providing the thermal energy). Actually this is practically never the case, as we shall see through many examples in this book. The photochemical reaction takes place on the high-energy excited state potential surface, e.g. M* [right arrow] P’ and leads in many cases to some high-energy products such as free radicals or radical ions. These may eventually react to form the final, stable products through dark reactions. In some cases these may resemble the products of the ground state (dark) reaction, but this similarity is purely coincidental.

The non-radiative decay of the excited state (M*) to the transition state (X) of the ground state reaction is so unlikely that it can be altogether forgotten. Even if there was a single example of such a process, it could not be described as ‘catalysis’ by light. The expressions ‘photoinduced’ or ‘photoactivated’ reactions are accurate; they do not imply that light acts as a catalyst, but rather as a reactant which is consumed in the chemical process.

1.2.3 The Range of Photochemical Reactions: Vibrational Photochemistry and Radiation Chemistry

The definition of a ‘photochemical’ reaction depends on the definition of ‘light’. Indeed, a photochemical reaction is a chemical reaction induced by light, a reaction in which the energy of light is used to promote molecules from their ground state to excited states. The question then arises as to the nature of these excited states, because electromagnetic radiation covers a virtually continuous spectrum of wavelengths extending from infinity to zero. The details of these excitation processes will be considered in chapter 3. For the moment, a summary of the properties of energy states of molecules will be sufficient. Figure 1.3 gives a simple picture of these various energy states, related to the wavelengths of electromagnetic radiation which can be used to bring molecules to different energy states.

The translational motions of molecules represent the lowest levels of ‘excitation’. The energy of a molecule of mass m is then

E = (1/2) mv2 (1.2)

and these energy levels form a continuum; that is to say that this energy varies nearly continuously, unlike the other forms of energy which vary in discrete steps.

Thermal energy, the energy contained in the heat of a chemical substance, is essentially the translational energy of molecules. There are two forms of kinetic energy which play an important role in photophysics: the rotational and vibrational energies of molecules.

The rotational energies represent the spinning motions of a molecule, when the entire molecule rotates around one of its inertial axes. This should not be confused with internal rotation which is the rotational motion of one part of a molecule with respect to some other part of the same molecule.

In most molecules of interest in photochemistry, these ‘external’ rotational levels are quite closely spaced and many of these states are populated even at room temperature. From Figure 1.3 it is clear that the energies of rotational states are usually below the level of the average thermal energy kT, and they can be reached by the absorption of ‘light’ (or rather, electromagnetic radiation) of very long wavelength, corresponding to the microwave region of the spectrum.

At higher energies the molecules acquire vibrational motion, the bonds between the atoms behaving like springs connecting mass centres. The spacing of vibrational energy levels is usually quite large compared with the average thermal energy kT and only the lowest ones are populated at room temperature. These vibrational levels play, however, a most important role in the photophysical processes of large molecules, and these will be discussed in Chapter 3.

At much higher energies the electronic excited states of molecules are reached; these correspond to ‘light’ in the usual sense, in the visible (VIS) , near infrared (NIR) and near ultraviolet (NUV) regions of the spectrum of electromagnetic radiation. In these electronic excited states the atomic structure of the molecule remains unchanged but one or several electrons are promoted to ‘orbitals’ of higher energy. These electronic excited states are at the basis of photochemical reactions.

Going further up the energy scale the molecule will reach its ionization limit where the impact of electromagnetic radiation is so strong that an electron is ejected and the neutral molecule ceases to exist; it becomes a positive ion separated from its departing electron

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.3)

Electromagnetic radiation of such high energy falls within the ‘vacuum ultra -violet’ (VUV), X-ray and γ-ray regions of the spectrum. They are called ‘ionizing’ radiations and their effects on matter are part of the science of radiation chemistry.

It has often been stated that photochemistry is the chemistry of electronically excited molecules. According to this definition ‘light’ is the NIR/VIS/ NUV part of the spectrum of electromagnetic radiation, in the range of wavelengths covering about 100 to 1000 nm. Within this range molecules are promoted to electronic excited states. There remains however an area which can be considered as part of photochemistry, although no ‘electronically’ excited states are involved: this is the process of infrared multiphoton absorption which can result in ‘vibrational’ photochemistry. This will be discussed in section 8.6.

With this one exception of vibrational photochemistry through multiphoton infrared light absorption, photochemistry is restricted to the chemical reactions of electronic excited states of molecules. Radiation chemistry is outside the scope of this book, so a very short section is devoted to it to conclude this introduction.

(Continues…)Excerpted from Chemistry and Light by Paul Suppan. Copyright © 1994 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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