Principles and Applications of Photochemistry

Preface xiii

1 Introductory Concepts 1

Aims and Objectives 1

1.1 The Quantum Nature of Matter and Light 2

1.2 Modelling Atoms: Atomic Orbitals 6

1.3 Modelling Molecules: Molecular Orbitals 9

1.4 Modelling Molecules: Electronic States 13

1.5 Light Sources Used in Photochemistry 16

1.6 Efficiency of Photochemical Processes: Quantum Yield 25

2 Light Absorption and Electronically-excited States 29

Aims and Objectives 29

2.1 Introduction 29

2.2 The Beer–Lambert Law 30

2.3 The Physical Basis of Light Absorption by Molecules 32

2.4 Absorption of Light by Organic Molecules 35

2.5 Linearly-conjugated Molecules 39

2.6 Some Selection Rules 42

2.7 Absorption of Light by Inorganic Complexes 43

3 The Physical Deactivation of Excited States 47

Aims and Objectives 47

3.1 Introduction 47

3.2 Jablonski Diagrams 49

3.3 Excited-state Lifetimes 53

4 Radiative Processes of Excited States 59

Aims and Objectives 59

4.1 Introduction 60

4.2 Fluorescence and Fluorescence Spectra 61

4.3 An Exception to Kasha’s Rule 63

4.4 Fluorescence Quantum Yield 64

4.5 Factors Contributing to Fluorescence Behaviour 65

4.6 Molecular Fluorescence in Analytical Chemistry 67

4.7 Phosphorescence 70

4.8 Delayed Fluorescence 73

4.9 Lanthanide Luminescence 74

5 Intramolecular Radiationless Transitions of Excited States 77

Aims and Objectives 77

5.1 Introduction 77

5.2 The Energy Gap Law 79

5.3 The Franck–Condon Factor 79

5.4 Heavy Atom Effects on Intersystem Crossing 82

5.5 El-Sayed’s Selection Rules for Intersystem Crosssing 83

6 Intermolecular Physical Processes of Excited States 87

Aims and Objectives 87

6.1 Quenching Processes 88

6.2 Excimers 90

6.3 Exciplexes 93

6.4 Intermolecular Electronic Energy Transfer 96

6.5 The Trivial or Radiative Mechanism of Energy Transfer 97

6.6 Long-range Dipole–Dipole (Coulombic) Energy Transfer 98

6.7 Short-range Electron-exchange Energy Transfer 105

6.8 Photoinduced Electron Transfer (PET) 110

7 Some Aspects of the Chemical Properties of Excited States 119

Aims and Objectives 119

7.1 The Pathway of Photochemical Reactions 120

7.2 Differences between Photochemical and Thermal Reactions 124

7.3 Photolysis 127

7.4 An Introduction to the Chemistry of Carbon-centred Radicals 133

7.5 Photochemistry of the Complexes and Organometallic Compounds of d-block Elements 135

8 The Photochemistry of Alkenes 145

Aims and Objectives 145

8.1 Excited States of Alkenes 146

8.2 Geometrical Isomerisation by Direct Irradiation of C=C Compounds 147

8.3 Photosensitised Geometrical Isomerisation of C=C Compounds 149

8.4 Concerted Photoreactions 151

8.5 Photocycloaddition Reactions 157

8.6 Photoaddition Reactions 159

9 The Photochemistry of Carbonyl Compounds 161

Aims and Objectives 161

9.1 Excited States of Carbonyl Compounds 162

9.2 α-cleavage Reactions 163

9.3 Intermolecular Hydrogen-abstraction Reactions 166

9.4 Intramolecular Hydrogen-abstraction Reactions 167

9.5 Photocyloaddition Reactions 168

9.6 The Role of Carbonyl Compounds in Polymer Chemistry 169

10 Investigating Some Aspects of Photochemical Reaction Mechanisms 173

Aims and Objectives 173

10.1 Introduction 174

10.2 Information from Electronic Spectra 174

10.3 Triplet-quenching Studies 176

10.4 Sensitisation 180

10.5 Flash Photolysis Studies 182

10.6 Low-temperature Studies 195

Further Reading 196

11 Semiconductor Photochemistry 197

Aims and Objectives 197

11.1 Introduction to Semiconductor Photochemistry 198

11.2 Solar-energy Conversion by Photovoltaic Cells 199

11.3 Semiconductors as Sensitisers for Water Splitting 204

11.4 Semiconductor Photocatalysis 208

11.5 Semiconductor-photoinduced Superhydrophilicity 211

Further Reading 212

12 An Introduction to Supramolecular Photochemistry 213

Aims and Objectives 213

12.1 Some Basic Ideas 214

12.2 Host–Guest Supramolecular Photochemistry 215

12.3 Supramolecular Photochemistry in Natural Systems 221

12.4 Artificial Photosynthesis 229

12.5 Photochemical Supramolecular Devices 233

Further Reading 238

Index 241