Principles and Applications of Photochemistry Book

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

1.5.1 The Mercury Lamp 17

1.5.2 Lasers 18

1.6 Efficiency of Photochemical Processes: Quantum Yield 25

1.6.1 Primary Quantum Yield (φ) 25

1.6.2 Overall Quantum Yield (Φ) 26

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

3.2.1 Vibrational Relaxation 51

3.2.2 Internal Conversion 51

3.2.3 Intersystem Crossing 52

3.2.4 Fluorescence 52

3.2.5 Phosphorescence 52

3.3 Excited-state Lifetimes 53

3.3.1 Excited Singlet-state Lifetime 53

3.3.2 Excited Singlet-state Radiative Lifetime 55

3.3.3 Lifetimes of the T₁ Excited State 57

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

4.5.1 The Nature of S₁ 65

4.5.2 Molecular Rigidity 66

4.5.3 The Effect of Substituent Groups66

4.5.4 The Heavy Atom Effect 66

4.6 Molecular Fluorescence in Analytical Chemistry 67

4.7 Phosphorescence 70

4.8 Delayed Fluorescence 73

4.8.1 P-type Delayed Fluorescence (Triplet-Triplet Annihilation) 73

4.8.2 E-type Delayed Fluorescence (Thermally-activated 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

5.3.1 Case (A): Both Electronic States Have a Similar Geometry, with a Large Energy Separation between the States 80

5.3.2 Case (B): Both Electronic States Have a Similar Geometry, with a Small Energy Separation between the States 80

5.3.3 Case (C): The Electronic States Have Different Geometries, with a Large Energy Separation between the States 81

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

6.2.1 Excimer Emission in Ca ⁺ Sensing 93

6.3 Exciplexes 93

6.3.1 Exciplex Fluorescence Imaging 95

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

6.6.1 Dynamic Processes within Living Cells 101

6.6.2 Molecular Ruler 102

6.6.3 Molecular Beacons 103

6.7 Short-range Electron-exchange Energy Transfer 105

6.7.1 Triplet-Triplet Energy Transfer and Photosensitisation 106

6.7.2 Singlet Oxygen and Photodynamic Therapy for Cancer Treatment 108

6.8 Photoinduced Electron Transfer (PET) 110

6.8.1 Fluorescence Switching by PET 111

6.8.2 The Marcus Theory of Electron Transfer 112

6.8.3 Experimental Evidence Relating to the Marcus Equation 114

6.8.4 Evidence for the Inverted Region 117

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

7.3.1 Photohalogenation of Hydrocarbons 128

7.3.2 The Stratospheric Ozone Layer: Its Photochemical Formation and Degradation 129

7.3.3 Radicals in the Polluted Troposphere 132

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

7.5.1 The Photochemistry of Metal Complexes 135

7.5.2 An Aside: Redox Potentials Involved in Photoredox Reactions 140

7.5.3 Organometallic Photochemistry 141

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

8.2.1 Phototherapy 148

8.2.2 Vision 148

8.3 Photosensitised Geometrical Isomerisation of C=C Compounds 149

8.3.1 Synthesis 150

8.4 Concerted Photo reactions 151

8.4.1 Electrocyclic Reactions 152

8.4.2 Sigmatropic Shifts 155

8.5 Photocycloaddition Reactions 157

8.5.1 Solar Energy Storage 158

8.6 Photoaddition Reactions 159

8.6.1 DNA Damage by UV 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

9.6.1 Vinyl Polymerisation 170

9.6.2 Photochemical Cross-linking of Polymers 170

9.6.3 Phorodegradation of Polymers 172

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

10.5.1 An Aside: Some Basic Ideas on Reaction Kinetics 186

10.5.2 Flash Photolysis Studies in Bimolecular Electron-transfer Processes 187

10.5.3 Photochemistry of Substituted Benzoquinones in Ethanol/Water 190

10.5.4 Time-resolved Infrared Spectroscopy 192

10.5.5 Femtochemistry 193

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

11.2.1 Dye-sensitised Photovoltaic Cells 201

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

12.2.1 Micelles 215

12.2.2 Zeolites as Supramolecular Hosts for Photochemical Transformations 217

12.2.3 Cyclodextrins as Supramolecular Hosts 220

12.3 Supramolecular Photochemistry in Natural Systems 221

12.3.1 Vision 221

12.3.2 Photosynthesis 222

12.3.3 Bacterial Photosynthesis 227

12.4 Artificial Photosynthesis 229

12.5 Photochemical Supramolecular Devices 233

12.5.1 Devices for Photoinduced Energy or Electron Transfer 233

12.5.2 Devices for Information Processing based on Photochemical or Photophysical Processes 234

12.5.3 Devices Designed to Undergo Extensive Conformational Changes on Photoexcitation: Photochemically-driven Molecular Machines 235

Further Reading 238

Index 241