Modern Molecular

Photochemistryof Organic Molecules

Nicholas J. TurroCOLUMBIA UNIVERSITY

V. RamamurthyUNIVERSITY OF MIAMI

J. C. ScaianoUNIVERSITY OF OTTAWA

TECHNISCHE

INFORM A HON SB i i.-,L IOTHEK

UNIVERSITATS8IBLIOTHEKHANNOVER

University Science Books

Sausalito, California

<^

Contents

Preface xxxi

chapter 1 Molecular Photochemistry of Organic Compounds:An Overview \

1.1 What Is Molecular Organic Photochemistry? 1

1.2 Learning Molecular Organic Photochemistry through the

Visualization of Molecular Structures and the Dynamics of Their

Transformations 3

1.3 Why Study Molecular Organic Photochemistry? 3

1.4 The Value of Pictorial Representations and Visualization of Scientific

Concepts 5

1.5 Scientific Paradigms of Molecular Organic Photochemistry 6

1.6 Exemplars as Guides to the Experimental Study and Understandingof Molecular Organic Photochemistry 7

1.7 The Paradigms of Molecular Organic Photochemistry 8

1.8 Paradigms as Guides for Proceeding from the Possible to the Plausible

to the Probable Photochemical Processes 8

1.9 Some Important Questions that Will Be Answered by the Paradigms

of Molecular Organic Photochemistry 10

1.10 From a Global Paradigm to the Everyday Working Paradigm 11

1.11 Singlet States, Triplet States, Diradicals, and Zwitterions: KeyStructures Along a Photochemical Pathway from *R to P 14

1.12 State Energy Diagrams: Electronic and Spin Isomers 16

1.13 An Energy Surface Description of Molecular Photochemistry 20

1.14 Structure, Energy, and Time: Molecular-Level Benchmarks and

Calibration Points of Photochemical Processes 25

x Contents

1.15 Calibration Points and Numerical Benchmarks for Molecular

Energetics 26

1.16 Counting Photons 28

1.17 Computing the Energy of a Mole of Photons for Light of Wavelength

A, and Frequency v 29

1.18 The Range of Photon Energies in the Electromagnetic Spectrum 29

1.19 Calibration Points and Numerical Benchmarks for Molecular

Dimensions and Time Scales 33

1.20 Plan of the Text 35

References 38

chapter 2 Electronic, Vibrational, and Spin Configurations of

Electronically Excited States 39

2.1 Visualization of the Electronically Excited Structures through the

Paradigms of Molecular Organic Photochemistry 39

2.2 Molecular Wave Functions and Molecular Structure 42

2.3 The Born-Oppenheimer Approximation: A Starting Point for

Approximate Molecular Wave Functions and Energies 45

2.4 Important Qualitative Characteristics of Approximate WaveFunctions 47

2.5 From Postulates of Quantum Mechanics to Observations of Molecular

Structure: Expectation Values and Matrix Elements 49

2.6 The Spirit of the Use of Quantum Mechanical Wave Functions,

Operators, and Matrix Elements 50

2.7 From Atomic Orbitals, to Molecular Orbitals, to Electronic

Configurations, to Electronic States 51

2.8 Ground and Excited Electronic Configurations 52

2.9 The Construction of Electronic States from Electronic

Configurations 56

2.10 Construction of Excited Singlet and Triplet States from ElectronicallyExcited Configurations and the Pauli Principle 56

2.11 Characteristic Configurations of Singlet and Triplet States: A

Shorthand Notation 57

2.12 Electronic Energy Difference between Molecular Singlet and TripletStates of *R: Electron Correlation and the Electron ExchangeEnergy 58

2.13 Evaluation of the Relative Singlet and Triplet Energies and Singlet-Triplet Energy Gaps for Electronically Excited States (*R) of the

Same Electronic Configuration 60

2.14 Exemplars for the Singlet-Triplet Splittings in Molecular

Systems 63

Contents xi

2.15 Electronic Energy Difference between Singlet and Triplet States

of Diradical Reactive Intermediates: Radical Pairs, I(RP), and

Biradicals, 1(BR) 66

2.16 A Model for Vibrational Wave Functions: The Classical Harmonic

Oscillator 69

2.17 The Quantum Mechanical Version of the Classical Harmonic

Oscillator 75

2.18 The Vibrational Levels of a Quantum Mechanical Harmonic

Oscillator 77

2.19 The Vibrational Wave Functions for a Quantum Mechanical

Harmonic Oscillator: Visualization of the Wave Functions for

Diatomic Molecules 78

2.20 A First-Order Approximation of the Harmonic-Oscillator Model:

The Anharmonic Oscillator 80

2.21 Building Quantum Intuition for Using Wave Functions 82

2.22 Electron Spin: A Model for Visualizing Spin Wave Functions 82

2.23 A Vector Model of Electron Spin 85

2.24 Important Properties of Vectors 85

2.25 Vector Representation of Electron Spin 86

2.26 Spin Multiplicities: Allowed Orientations of Electron Spins 87

2.27 Vector Model of Two Coupled Electron Spins: Singlet and TripletStates 89

2.28 The Uncertainty Principle and Cones of Possible Orientations for

Electron Spin 92

2.29 Cones of Possible Orientations for Two Coupled 1/2 Spins:

Singlet and Triplet Cones of Orientation as a Basis for Visualizing the

Interconversion of Spin States 93

2.30 Making a Connection between Spin Angular Momentum and

Magnetic Moments Due to Spin Angular Momentum 94

2.31 The Connection between Angular Momentum and Magnetic

Moments: A Physical Model for an Electron with AngularMomentum 94

2.32 The Magnetic Moment of an Electron in a Bohr Orbit 95

2.33 The Connection between Magnetic Moment and Electron Spin 97

2.34 Magnetic Energy Levels in an Applied Magnetic Field for a Classical

Magnet 99

2.35 Quantum Magnets in the Absence of Coupling Magnetic Fields 101

2.36 Quantum Mechanical Magnets in a Magnetic Field: Constructing a

Magnetic State Energy Diagram for Spins in an Applied MagneticField 102

2.37 Magnetic Energy Diagram for a Single Electron Spin and for Two

Coupled Electron Spins 103

i Contents

Magnetic Energy Diagrams Including the Electron Exchange

Interaction, J 104

Interactions between Two Magnetic Dipoles: Orientation and

Distance Dependence of the Energy of Magnetic Interactions 106

Summary: Structure and Energetics of Electrons, Vibrations, and

Spins 108

References 108

chapter 3 Transitions between States: Photophysical Processes 109

3.1 Transitions between States 109

3.2 A Starting Point for Modeling Transitions between States 111

3.3 Classical Chemical Dynamics: Some Preliminary Comments 112

3.4 Quantum Dynamics: Transitions between States 113

3.5 Perturbation Theory 113

3.6 The Spirit of Selection Rules for Transition Probabilities 118

3.7 Nuclear Vibrational Motion As a Trigger for Electronic Transitions.

Vibronic Coupling and Vibronic States: The Effect of Nuclear Motion

on Electronic Energy and Electronic Structure 119

3.8 The Effect of Vibrations on Transitions between Electronic States:

The Franck-Condon Principle 122

3.9 A Classical and Semiclassical Harmonic Oscillator Model of the

Franck-Condon Principle for Radiative Transitions (R + hv *R

and *R R + ftv) 124

3.10 A Quantum Mechanical Interpretation of the Franck-Condon

Principle and Radiative Transitions 128

3.11 The Franck-Condon Principle and Radiationless Transitions

(*R -» R + heat) 130

3.12 Radiationless and Radiative Transitions between Spin States of

Different Multiplicity 134

3.13 Spin Dynamics: Classical Precession of the Angular Momentum

Vector 135

3.14 Precession of a Quantum Mechanical Magnet in the Cones ofPossible

Orientations 139

3.15 Important Characteristics of Spin Precession 141

3.16 Some Quantitative Benchmark Relationships between the Strength of

a Coupled Magnetic Field and Precessional Rates 142

3.17 Transitions between Spin States: Magnetic Energies and

Interactions 144

3.18 The Role of Electron Exchange (J) in Coupling Electron Spins 144

3.19 Couplings of a Spin with a Magnetic Field: Visualization of SpinTransitions and Intersystem Crossing 146

3.20 Vector Model for Transitions between Magnetic States 148

2.38

2.39

2.40

Contents xiil

3.21 Spin-Orbit Coupling: A Dominant Mechanism for Inducing SpinChanges in Organic Molecules 149

3.22 Coupling of Two Spins with a Third Spin: T+ -» S and T_ ->• S

Transitions 157

3.23 Coupling Involving Two Correlated Spins: Tq -» S Transitions 158

3.24 Intersystem Crossing in Diradicals, 1(D): Radical Pairs, I(RP), and

Biradicals, I(BR) 159

3.25 Spin-Orbit Coupling in 1(D): The Role of Relative Orbital

Orientation 160

3.26 Intersystem Crossing in Flexible Biradicals 164

3.27 What All Transitions between States Have in Common 166

References 167

chapter 4 Radiative Transitions between Electronic States 169

4.1 The Absorption and Emission of Light by Organic Molecules 169

4.2 The Nature of Light: A Series of Paradigm Shifts 169

4.3 Black-Body Radiation and the "Ultraviolet Catastrophe" and

Planck's Quantization of Light Energy: The Energy Quantum Is

Postulated 172

4.4 The "Photoelectric Effect" and Einstein's Quantization of Light—The Quantum of Light: Photons 173

4.5 If Light Waves Have the Properties of Particles, Do Particles Have the

Properties of Waves? —de Broglie Integrates Matter and Light 176

4.6 Absorption and Emission Spectra of Organic Molecules: The State

Energy Diagram as a Paradigm for Molecular Photophysics 178

4.7 Some Examples of Experimental Absorption and Emission Spectraof Organic Molecules: Benchmarks 178

4.8 The Nature of Light: From Particles to Waves to Wave Particles 181

4.9 A Pictorial Representation of the Absorption of Light 181

4.10 The Interaction of Electrons with the Electric and Magnetic Forces of

Light 182

4.11 A Mechanistic View of the Interaction of Light with Molecules:

Light as a Wave 184

4.12 An Exemplar of the Interaction of Light with Matter: The HydrogenAtom 185

4.13 From the Classical Representation to a Quantum Mechanical

Representation of Light Absorption by a Hydrogen Atom and a

Hydrogen Molecule 188

4.14 Photons as Massless Reagents 191

4.15 Relationship of Experimental Spectroscopic Quantities to Theoretical

Quantities 194

4.16 The Oscillator Strength Concept 195

xiv Contents

4.17 The Relationship between the Classical Concept ofOscillator Strengthand the Quantum Mechanical Transition Dipole Moment 196

4.18 Examples of the Relationships of e, k°e, t°, < ^\\P\^2 >> and / 197

4.19 Experimental Tests of the Quantitative Theory Relating Emission and

Absorption to Spectroscopic Quantities 200

4.20 The Shapes of Absorption and Emission Spectra 201

4.21 The Franck-Condon Principle and Absorption Spectra of OrganicMolecules 204

4.22 The Franck-Condon Principle and Emission Spectra 208

4.23 The Effect of Orbital Configuration Mixing and Multiplicity Mixingon Radiative Transitions 210

4.24 Experimental Exemplars of the Absorption and Emission of Light by

Organic Molecules 214

4.25 Absorption, Emission, and Excitation Spectra 215

4.26 Order of Magnitude Estimates of Radiative Transition

Parameters 218

4.27 Quantum Yields for Emission (*R -> R + hv) 223

4.28 Experimental Examples of Fluorescence Quantum Yields 230

4.29 Determination of "State Energies" Es and Er from Emission

Spectra 234

4.30 Spin-Orbit Coupling and Spin-Forbidden Radiative Transitions 235

4.31 Radiative Transitions Involving a Change in Multiplicity:

S0 T(n,7r*) and S0 (jt.jt*) Transitions as Exemplars 237

4.32 Experimental Exemplars of Spin-Forbidden Radiative Transitions:

S0 -* T, Absorption and Tj -> S0 Phosphorescence 240

4.33 Quantum Yields of Phosphorescence, $P: The Ti -> S0 + hv

Process 243

4.34 Phosphorescence in Fluid Solution at Room Temperature 244

4.35 Absorption Spectra of Electronically Excited States 245

4.36 Radiative Transitions Involving Two Molecules: Absorption

Complexes and Exciplexes 247

4.37 Examples of Ground-State Charge-Transfer Absorption

Complexes 248

4.38 Excimers and Exciplexes 249

4.39 Exemplars of Excimers: Pyrene and Aromatic Compounds 253

4.40 Exciplexes and Exciplex Emission 256

4.41 Twisted Intramolecular Charge-Transfer States 257

4.42 Emission from "Upper" Excited Singlets and Triples: The Azulene

Anomaly 260

References 262

Contents xv

chapter 5 Photophysical Radiationless Transitions 265

5.1 Photophysical Radiationless Transitions As a Form of Electronic

Relaxation 265

5.2 Radiationless Electronic Transitions as the Motion of a RepresentativePoint on Electronic Energy Surfaces 266

5.3 Wave Mechanical Interpretation of Radiationless Transitions between

States 270

5.4 Radiationless Transitions and the Breakdown of the Born-

Oppenheimer Approximation 275

5.5 An Essential Difference between Strongly Avoiding and MatchingSurfaces 275

5.6 Conical Intersections Near Zero-Order Surface Crossings 275

5.7 Formulation of a Parameterized Model of Radiationless

Transitions 276

5.8 Visualization of Radiationless Transitions Promoted by Vibrational

Motion; Vibronic Mixing 277

5.9 Intersystem Crossing: Visualization of Radiationless Transitions

Promoted by Spin-Orbit Coupling 281

5.10 Selection Rules for Intersystem Crossing in Molecules 282

5.11 The Relationship of Rates and Efficiencies of Radiationless

Transitions to Molecular Structure: Stretching and Twisting as

Mechanisms for Inducing Electronic Radiationless Transitions 287

5.12 The "Loose Bolt" and "Free-Rotor" Effects: Promoter and AcceptorVibrations 288

5.13 Radiationless Transitions between "Matching" Surfaces Separated by

Large Energies 291

5.14 Factors That Influence the Rate of Vibrational Relaxation 293

5.15 The Evaluation of Rate Constants for Radiationless Processes from

Quantitative Emission Parameters 296

5.16 Examples of the Estimation of Rates of Photophysical Processes from

Spectroscopic Emission Data 298

5.17 Internal Conversion (Sn^S1,S1^S0,Tn->T1) 300

5.18 The Relationship of Internal Conversion to the Excited-State

Structure of *R 301

5.19 The Energy Gap Law for Internal Conversion (S1 S0) 303

5.20 The Deuterium Isotope Test for Internal Conversion 304

5.21 Examples of Unusually Slow S„ St Internal Conversion 305

5.22 Intersystem Crossing from St -> Tt 306

5.23 The Relationship Between Sj T! Intersystem Crossing to

Molecular Structure 307

5.24 Temperature Dependence of Sl T„ Intersystem Crossing 308

5.25 Intersystem Crossing (T, ->• S0) 309

xvi Contents «

5.26 The Relationship between Tj -> S0 Intersystem Crossing and

Molecular Structure 309

5.27 The Energy Gap Law for Tt-> S0 Intersystem Crossing: Deuterium

Isotope Effects on Interstate Crossings 310

5.28 Perturbation of Spin-Forbidden Radiationless Transitions 311

5.29 Internal Perturbation of Intersystem Crossing by the Heavy-Atom

Effect 312

5.30 External Perturbation of Intersystem Crossing 313

5.31 The Relationship between Photophysical Radiationless Transitions

and Photochemical Processes 314

References 315

chapter 6 A Theory of Molecular Organic Photochemistry 319

6.1 Introduction to a Theory of Organic Photoreactions 319

6.2 Potential Energy Curves and Surfaces 322

6.3 Movement of a Classical Representative Point on a Surface 323

6.4 The Influence "of Collisions and Vibrations on the Motion of the

Representative Point on an Energy Surface 325

6.5 Radiationless Transitions on PE Surfaces: Surface Maxima, Surface

Minima, ana Funnels on the Way from *R to P 325

6.6 A Global Paradigm for Organic Photochemical Reactions 326

6.7 Toward a General Theory of Organic Photochemical Reactions Based

on Potential Energy Surfaces 328

6.8 Determining Plausible Molecular Structures and Plausible Reaction

Pathways of Photochemical Reactions 330

6.9 The Fundamental Surface Topologies for "Funnels" from Excited

Surfaces to Ground-State Surfaces: Spectroscopic Minima, Extended

Surface Touchings, Surface Matchings, Surface Crossings, and

Surface Avoidings 330

6.10 From 2D PE Curves to 3D PE Surfaces: The "Jump" from Two

Dimensions to Three Dimensions 333

6.11 The Nature of Funnels Corresponding to Surface Avoidingsand Surface Touchings Involved in Primary Photochemical

Processes 334

6.12 "The Noncrossing Rule" and Its Violations: Conical Intersections and

Their Visualization 335

6.13 Some Important and Unique Properties of Conical Intersections 337

6.14 Diradicaloid Structures and Diradicaloid Geometries 341

6.15 Diradicaloid Structures Produced from Stretching a Bonds and

Twisting n Bonds 344

Contents XVii

6.16 An Exemplar for Diradicaloid Geometries Produced by cr-Bond

Stretching and Bond Breaking: Stretching of the a Bond of the

Hydrogen Molecule 344

6.17 An Exemplar for Diradicaloid Geometries Produced by jr-Bond.

Twisting and Breaking: Twisting of the jt Bond of Ethylene 348

6.18 Frontier Orbital Interactions As a Guide to the Lowest-Energy

Pathways and Energy Barriers on Energy Surfaces 351

6.19 The Principle of Maximum Positive Orbital Overlap for Frontier

Orbitals 353

6.20 Stabilization by Orbital Interactions: Selection Rules Based on

Maximum Positive Overlap and Minimum Energy Gap 353

6.21 Commonly Encountered Orbital Interactions in OrganicPhotoreactions 354

6.22 Selection of Reaction Coordinates from Orbital Interactions for

*R -»I or *R ->• F ->- P Reactions: Exemplars of Concerted

Photochemical Reactions and Photochemical Reactions That Involve

Diradicaloid Intermediates 357

6.23 Electronic Orbital and State Correlation Diagrams 357

6.24 An Exemplar for Photochemical Concerted Pericyclic Reactions:

The Electrocyclic Ring Opening of Cyclobutene and Ring Closure of

1,3-Butadiene 358

6.25 Frontier Orbital Interactions Involving Radicals as Models for

Half-Filled Molecular Orbitals 359

6.26 Orbital and State Correlation Diagrams 362

6.27 The Construction of Electron Orbital and State Correlation Diagramsfor a Selected Reaction Coordinate 364

6.28 Typical State Correlation Diagrams for Concerted Photochemical

Pericyclic Reactions 364

6.29 Classification of Orbitals and States for the Electrocyclic Reactions

of Cyclobutene and 1,3-Butadiene: An Exemplar Concerted

Reaction 364

6.30 Concerted Photochemical Pericyclic Reactions and Conical

Intersections 368

6.31 Typical State Correlation Diagrams for Nonconcerted Photoreactions:

Reactions Involving Intermediates (Diradicals and Zwitterions) 368

6.32 Natural Orbital Correlation Diagrams 368

6.33 The Role of Small Barriers in Determining the Efficiencies of

Photochemical Processes 369

6.34 An Exemplar for the Photochemical Reactions ofn,?r* States 370

6.35 The Symmetry Plane Assumption: Salem Diagrams 372

6.36 An Exemplar State Correlation Diagram for n-Orbital Initiated

Reaction of n,jr* States: Hydrogen Abstraction via a CoplanarReaction Coordinate 372

xviii Contents

6.37 Extension of an Exemplar State Correlation Diagram to New

Situations 375

6.38 State Correlation Diagrams for a-Cleavage of Ketones 375

6.39 A Standard Set of Plausible Primary Photoreactions for tt,tt* and

n,7T* States 378

6.40 The Characteristic Plausible Primary Photochemistry Processes of

tt,tc* States 378

6.41 The Characteristic Plausible Primary Photochemical Processes of

n,;r* States 380

6.42 Summary: Energy Surfaces as Reaction Graphs or Maps 381

References 382

chapter 7 Energy Transfer and Electron Transfer 383

7.1 Introduction to Energy and Electron Transfer 383

7.2 The Electron Exchange Interaction for Energy and Electron

Transfer 387

7.3 "Trivial" Mechanisms for Energy and Electron Transfer 391

7.4 Energy Transfer Mechanisms 396

7.5 Visualization of Energy Transfer by Dipole-Dipole Interactions:

A Transmitter-Antenna Receiver-Antenna Mechanism 399

7.6 Quantitative Aspects of the Forster Theory of Dipole-Dipole EnergyTransfer 400

7.7 The Relationship of kET to Energy-Transfer Efficiency and Separationof Donor and Acceptor RDA 404

7.8 Experimental Tests for Dipole-Dipole Energy Transfer 406

7.9 Electron Exchange Processes: Energy Transfer Resulting from

Collisions and Overlap of Electron Clouds 411

7.10 Electron Exchange: An Orbital Overlap or Collision Mechanism of

Energy Transfer 411

7.11 Electron-Transfer Processes Leading to Excited States 413

7.12 Triplet-Triplet Annihilation (TTA): A Special Case of EnergyTransfer via Electron Exchange Interactions 414

7.13 Electron Transfer: Mechanisms and Energetics 416

7.14 Marcus Theory of Electron Transfer 424

7.15 A Closer Look at the Reaction Coordinate for Electron Transfer 436

7.16 Experimental Verification of the Marcus Inverted Region for

Photoinduced Electron Transfer 438

7.17 Examples of Photoinduced Electron Transfer That Demonstrate the

Marcus Theory 441

7.18 Long-Distance Electron Transfer 441

7.19 Mechanisms of Long-Distance Electron Transfer: Through-Spaceand Through-Bond Interactions 442

Contents

7.20 A Quantitative Comparison of Triplet-Triplet Energy and Electron

Transfer 445

7.21 A Connection between Intramolecular Electron, Hole, and TripletTransfer 446

7.22 Photoinduced Electron Transfer between Donor and AcceptorMoieties Connected by a Flexible Spacer 447

7.23 Experimental Observation of the Marcus Inversion Region for Freely

Diffusing Species in Solution 448

7.24 Control of the Rate and Efficiency of Electron-Transfer Separation by

Controlling Changes in the Driving Force for Electron Transfer 449

7.25 Application of Marcus Theory to the Control of Product

Distributions 451

7.26 The Continuum of Structures from Charge Transfer to Free Ions:

Exciplexes, Contact Ion Pairs, Solvent Separated Radical Ion Pairs,

and Free Ion Pairs 454

7.27 Comparison between Exciplexes and Contact Radical Ion Pairs 458

7.28 Energy and Electron-Transfer Equilibria 461

7.29 Energy-Transfer Equilibria 461

7.30 Electron-Transfer Equilibria in the Ground State 463

7.31 Excited-State Electron-Transfer Equilibria 463

7.32 Excited-State Formation Resulting from Electron-Transfer Reactions:

Chemiluminescent Reactions 464

7.33 Role of Molecular Diffusion in Energy and Electron-Transfer

Processes in Solution 466

7.34 An Exemplar Involving Energy Transfer Controlled byDiffusion 467

7.35 Estimation ofRate Constants for Diffusion Controlled Processes 469

7.36 Examples of Near-Diffusion-Controlled Reactions: Reversible

Formation of Collision Complexes 472

7.37 The Cage Effect 474

7.38 Distance-Time Relationships for Diffusion 476

7.39 Diffusion Control in Systems Involving Charged Species 478

7.40 Summary 479

References 479

chapter 8 Mechanistic Organic Photochemistry 483

8.1 Photochemical Reaction Mechanisms 483

8.2 Some Philosophical Comments Concerning the Fundamental Nature

of Reaction Mechanisms 488

8.3 Creation of a Standard Mechanistic Set 489

8.4 Use of Kinetic Plausibility in Quantitative Mechanistic

Analyses 493

xx Contents

8.5 Introduction to the Reactions of Free Radicals and Biradicals 501

8.6 The Use of Structural Criteria for Mechanistic Analysis: The

Role of Reaction Intermediates (*R, I) in Structure-ReactivityCorrelations 513

8.7 The Use of Reaction Types and Structural Relationships in

Mechanistic Analyses 514

8.8 An Exemplar of the Use of Structural Relationships in Mechanistic

Analysis 516

8.9 Rules for Proceeding from Rate Laws to Photochemical Reaction

Mechanisms 518

8.10 Rules for Proceeding from Quantum Yields and Efficiency Laws to

Kinetic Information on Photochemical Reaction Mechanisms 524

8.11 Experimental Methods for Determining Rate Constants of

Photoreactions 527

8.12 Pulsed Excitation of R to Produce *R 528

8.13 Techniques for Monitoring Upper Electronic States, **R 529

8.14 Low-Temperature Matrix Isolation Techniques 530

8.15 Two-Laser (Two-Color) Flash Photolysis 531

8.16 The Laser Jet Technique 534

8.17 Stern-Volmer Analysis of Photochemical Kinetics: Competitionbetween Unimolecular and Bimolecular Deactivation of *R 535

8.18 Stern-Volmer Quenching: Rate Constants from Efficiency versus

Concentration Measurements 537

8.19 Stern-Volmer Analysis Based on Data from Time-Resolved

Measurements Using Gated Detection 539

8.20 Experimental Exemplars ofthe Measurements of Photochemical Rate

Constants 540

8.21 Measurement of Absolute Efficiencies in Determining Kinetic

Parameters 547

8.22 Kinetics of Reactions Involving More Than One Excited State 549

8.23 The Probe Method for Detecting Spectroscopically "Invisible"

Transients 552

8.24 Experimental Measurement of the Efficiency of Radiationless

Processes: The Photoacoustic Method 555

8.25 Reactive Intermediates: Experimental Detection and Characterization

of *R and I 557

8.26 Applications of Time-Resolved Infrared and Magnetic Resonance

Spectroscopic Methods for the Characterization of the Structure and

Dynamics of *R and I: The a-Cleavage Reaction of Ketones as an

Exemplar 560

8.27 Investigation of the Structure of *R by Time-Resolved Infrared

Spectroscopy (TRIR) 562

8.28 Investigation of the a-Cleavage *R I(RP) Process by TRIR 564

Contents xxi

8.29 Time-Resolved Electron Paramagnetic Resonance and CIDEP 564

8.30 Electron Spin Polarization: Deviations from the Boltzmann

Distribution of Spins and Its Effect on the Intensities of MagneticResonance Signals 565

8.31 Investigation of the Structure of *R(T1) and the Mechanism of the

S] *R(T1) ISC by TR EPR 567

8.32 Investigation of the Photochemical Primary Process *R -> I Process

byTREPR 570

8.33 The Direct Observation of I(RP)gem and I(BR) by TR EPR 572

8.34 Experimental Tests for the Involvement of Electronically Excited

States *R: Qualitative Aspects. Deciding between *R(Sj) and

*R(Ti) 572

8.35 Experimental Tests for the Involvement of Electronically Excited

States (*R): Quantitative Aspects 575

8.36 The Use of Kinetic Methods to Detect and to Identify Reaction

Intermediates, *R and I 579

8.37 Reactions Involving Biradical Intermediates 585

8.38 Spin Chemistry: Spin Selection Rules for Chemical Reactions 593

8.39 Magnetic Effects on Reactions of I(RP) and I(BR) 595

8.40 Kinetic Basis for Magnetic Field Effects (MFE), Magnetic IsotopeEffects (MIE) and Chemically Induced Dynamic Nuclear Polarization

(CIDNP) 596

8.41 Magnetic Field Effects on the Reactivity and Products of 3I(RP) and

3I(BR) 597

8.42 Magnetic Isotope Effects on the Reactivity and Products of 3I(RP)and 3I(BR) 602

8.43 Chemically Induced Dynamic Nuclear Polarization of Radical Pairs:

The Nuclear Spin Orientation Dependence of Chemical Reactivity of

3I(RP)gem 606

8.44 CIDNP of Conformational^ Flexible Biradicals 612

8.45 Chemical Spectroscopy: The Use of Photochemical Reactions to

Measure Excited-State Energetics and Dynamics 614

8.46 Advances in Modern Mechanistic Organic Photochemistry:Ultrafast Reactions and Laser Coherent Photochemistry 617

8.47 Femtosecond Photochemistry 618

8.48 Single-Molecule Spectroscopy 618

8.49 Coherent Laser Photochemistry 619

8.50 Multiphoton Microscopy 619

8.51 Some Exemplar State Energy Parameters 620

8.52 Ketones 620

8.53 Alkenes and Polyenes 621

8.54 Conjugated Enones and Dienones 622

xxii Contents

8.55 Aromatic Hydrocarbons 622

8.56 Summary 623

References 624

chapter 9 Photochemistry of Carbonyl Compounds 629

9.1 Introduction to the Photochemistry of Carbonyl Compounds 629

9.2 Molecular Orbital description of the *R(n,7r*): Primary Processes of

Carbonyl Compounds 630

9.3 The *R(n,7r*) ~> I Primary Photochemical Processes Based on

Frontier Orbital Interactions 632

9.4 The I -> P Secondary Thermal Processes Based on Radical Pair, Free

Radical, and Biradical Reactions 634

9.5 The Alkoxy Radical: A Close Analogue of the Reactive n,7r*

Carbonyl Chromophore 636

9.6 State Energy Diagrams for Ketones 637

9.7 The *R(n,7r*) —> P Processes of Ketones and Aldehydes 639

9.8 An Exemplar of an n <- HO Initiated *R(n,jr *) I Process:

The Primary Process of Intermolecular Hydrogen Abstraction 640

9.9 "Invisible Transients" in Radical-Radical Combination Reactions:

Transients Formation by Radical-Radical Combination that Revert

Back to Starting Materials 641

9.10 The Primary Process of Intermolecular Electron Transfer: Reaction

of n,7r* States With Amines 643

9.11 Structure-Reactivity Relationships in Intermolecular HydrogenAbstraction 646

9.12 The Primary Process of Electron Abstraction: Reactive T^tt.tt*)State 650

9.13 Competition between Hydrogen and Electron Abstraction: Effect

of Variation for Orbital Structure and Hydrogen (Electron) Donor

Structure 650

9.14 The Primary Photochemical Process of Intramolecular HydrogenAbstraction: Norrish Type II Reactions 652

9.15 Reactivity and Efficiency Relationships in Type II Reactions 653

9.16 The Product Forming I(BR) -> P Step in Type II Reactions: A

Paradigm for the Behavior of a 1,4-Biradical 655

9.17 Geometry of y-Hydrogen Abstraction and Its Consequence on

Competing Primary Photochemical Processes 657

9.18 The role of Intersystem Crossing in Determining the Products of

Biradicals Produced by y -Hydrogen Abstraction 661

9.19 Beyond y-Hydrogen Abstraction: Intramolecular l,n-HydrogenAbstraction 664

Contents xxiit

9.20 The Primary Process of a-Cleavage of n,7t* States: AcyclicKetones 665

9.21 The Primary Process of a-Cleavage from n,;r* States: CyclicKetones 668

9.22 Reactions of Primary Radical Pair Produced from a-Cleavage 669

9.23 Photochemistry of Cyclobutanones: A Special Case of

a-Cleavage 672

9.24 The Primary Process of a-Cleavage of n,7r* states. Structure-

Reactivity Relationships 673

9.25 An Orbital Model for ar-Cleavage 677

9.26 The Primary Process for Addition of n,7r* States to Electron-Rich

C=C Bonds 678

9.27 The Primary Process of Addition of n,n* States to Electron-Rich

C=C: Reaction Intermediates 680

9.28 Evidence for a Biradical Intermediate 682

9.29 Endo-Exo Selectivity During Photoaddition of Excited Carbonyls to

Olefins 683

9.30 Examples of [2 + 2] Cycloaddition for n,7r* States to Electron-Poor

Ethylenes: An Example of a n* -» tt* Interaction 684

9.31 Stereoselectivity of the [2 + 2] Cycloaddition of n,;r* States to

Ethylenes 688

9.32 Intramolecular [2 -f 2] Photocycloaddition 690

9.33 Examples of Photorearrangements Initiated by /3-Cleavage Followed

by Combination and Disproportionation 691

9.34 Photochemical Fragmentations Initiated by ^-Cleavage 694

9.35 Synthetic Applications of the Photoreactions of CarbonylCompounds 696

9.36 Applications of the Photochemistry of Carbonyl Compounds in

Photoimaging 698

9.37 Applications of the Photochemistry of Carbonyl Compounds in

Designing "Phototriggers" and "Photoprotecting Groups" 700

9.38 Summary: The Photochemistry of Carbonyl Compounds 701

References 702

chapter 10 Photochemistry of Olefins

10.1

10.2

10.3

10.4

10.5

705

Introduction to the Photochemistry of Olefins 705

Molecular Orbital Description of the *R(7r,jr*) Primary Processes of

Olefins 706

The I -» P Secondary Processes of Alkenes 709

Exemplar State Energy Diagrams for Alkenes 710

The cis-trans Isomerization: A General Process for Both S^jr.jr*)and TiOr.jr*) of Alkenes 714

xxiv Contents

10.6 The cis-trans Isomerization of Acyclic and Cyclic Alkenes 715

10.7 The cis-trans Isomerization of Conjugated Polyenes: The

Nonequilibrating Excited Rotomers Principle 716

10.8 The cis-trans Isomerization of Aryl-Substituted Alkenes 721

10.9 A Case Study of cis-trans Isomerization of Stilbene 722

10.10 Adiabatic cis-trans Isomerization in S,(n, n*): Examples of *R *P

Processes 724

10.11 Trapping of Strained /rans-Cycloalkenes from cis-

Cycloalkenes 726

10.12 The cis-trans Isomerization through Conical Intersections 729

10.13 Intramolecular Pericyclic Reactions of the S^Trjjr*) States of

Alkenes: Examples of the Si(tt,tv*) F —> P Processes 730

10.14 Electrocyclic Ring Openings and Ring Closures Involving1,3-dienes 731

10.15 Electrocyclic Ring Openings of 1,3-Cyclohexadienes and the RingClosures of 1,3,5-Hexatrienes 736

10.16 Other Electrocyclic Reactions of Trienes 739

10.17 Electrocyclic Ring Closures of Stilbenes and Related Systems 740

10.18 Sigmatropic Rearrangements of the S^n^rt*) States of Alkenes 743

10.19 The Di-7r-methane (Zimmerman) Reactions: A Sigmatropic Reaction

of Wide Scope 745

10.20 Di-^-methane Reactions: Acyclic 1,4-dienes 746

10.21 Di-jr-methane Reactions: Rigid Cyclic 1,4-Dienes and Related

Compounds 748

10.22 The [n + m] Photocycloaddition Reactions 751

10.23 The [2 + 2] Photocycloaddition Reactions: Alkenes 752

10.24 The [2 + 2] and [4 + 2] Photocycloaddition Reactions of

1,3-Dienes 754

10.25 Intramolecular Photocycloadditions of Alkenes and Polyenes 757

10.26 The [2 + 2] Photocycloaddition Reactions: Aryl Alkenes 760

10.27 Proton-Transfer Reactions from S^tt,^*): Zwitterionic Photoaddition

Reactions 763

10.28 A Comparison of the n,7r* State Reactions of Carbonyls and T^.tt*)States of Alkenes: Hydrogen Abstraction Reactions of T^n.n*)States of Alkenes 764

10.29 yS-Cleavage Reactions 767

10.30 a-Cleavage Reactions 767

10.31 Photoinduced Electron-Transfer Reactions Involving Alkenes:

Examples of *R -» I(D*+, A'-) Processes 768

10.32 Structure and Reactivity of Radical Cations and Anions 769

10.33 Pathways to Radical Cations and Anions of Alkenes 770

Contents xxv

10.34 Reactions of Alkene Radical Ion Pairs: Addition of Amines 770

10.35 Generation of Alkene Radical Cations 772

10.36 Choice of Electron-Transfer Sensitizers 773

10.37 Generation of Alkene Cation Radicals: Maximizing the Yield of

Radical Ion Pair Formation 777

10.38 Reactions of Alkene Cation Radicals: Geometric Isomerization 778

10.39 Reactions of Alkene Cation Radicals: Addition to Nucleophiles 780

10.40 Reactions of Alkene Cation Radicals: Dimerization 781

10.41 Reactions of Alkene Cation Radicals: Intramolecular

Cyclization 783

10.42 Applications of Photoinduced cis-trans Isomerization in Biological

Systems 784

10.43 The cis-trans Isomerization as a Photoswitch 787

10.44 The cis-trans Isomerization as a Photoregulator of Phase Transitions

and Packing Arrangements in Liquid Crystals, Langmuir-BlodgettFilms, and Solgels 789

10.45 Controlling Ion Transport through Membranes through cis-trans

Isomerization 791

10.46 Application of cis-trans Isomerization in Laboratory and Industrial

Syntheses 792

10.47 Application of Photoinduced Pericyclic Reactions 794

10.48 Summary 796

References 797

chapter 11 Photochemistry of Enones and Dienones 801

11.1 Introduction to the Photochemistry of Enones and Dienones 801

11.2 Molecular Orbital Description of the *R(n,7T*) and *R(;r,7r*) States

of Enones: Primary Processes of Enones and Dienones 801

11.3 The I ~> P Secondary Processes ofEnones and Dienones 803

11.4 Exemplar State Energy Diagrams for Enones and Related

Structures 803

11.5 The Photochemistry of f3,y-Enones: Exemplars ofthe Photochemistryof Enones With Isolated But Proximate C=0 and C=C Bonds 805

11.6 Photochemistry of the n,7r* States of /3,y-Enones 806

11.7 Competition between the Reactions of n,7r* and tc,ti* States of

Enones 808

11.8 Competitive Reactions from the T^n^*) States of y6,y-Enones:Oxa-di-nr-methane Rearrangement and cis-trans Isomerization 810

11.9 Introduction to the Photochemistry of a,/3-Enones 814

11.10 Photochemistry of a.^-Enones Originating in the Tjfn.Tr*) State:

Analogies with the Primary Processes of the n,7r* States of

Carbonyls 815

xxvi Contents

11.11 Photochemistry of a,/S-Enones Originating in the Tx{n,n*)

State: Analogies to the Primary Processes of the Tt,n* States of

Alkenes 818

11.12 The Sigmatropic Rearrangement of Cyclohexenones: Type A and B

Rearrangements 820

11.13 Role of Geometric Isomerization in the Type A Reaction of

2-Cyclohexenones 821

11.14 Type B Rearrangement of 2-Cyclohexenones: The [1,2] Aryl and

[1,2] Vinyl Migrations Starting from a T^n.Tr*) State 823

11.15 The [2 + 2] Cycloaddition Reactions of Cyclic a,/?-Enones 824

11.16 Sigmatropic Rearrangements of Cross-Conjugated Dienones 827

11.17 Photochemistry of Linear Conjugated Cyclohexadienones:

the [6e] Electrocyclic Ring Opening and [1,2] Sigmatropic

Rearrangement 832

11.18 Synthetic Applications of Enone and Dienone Photochemistry 833

11.19 Developing Useful Synthetic Methodologies for Construction of

Diastereoselective and Enantioselective Cyclobutane Rings 838

11.20 Photocycloaddition Reactions of Coumarin and Psoralen. Psoralen

Ultraviolet A Treatment 840

11.21 Photocycloaddition Reactions ofNucleic Acid-Base Pairs and Skin

Cancer 842

11.22 Summary 843

References 844

chapter 12 Photochemistry of Aromatic Molecules 847

12.1 Introduction to the Photochemistry of Aromatic Molecules 847

12.2 Molecular Orbital Description of the *R(7r,7r*) PrimaryPhotochemical Processes of Aromatic Molecules 848

12.3 The Primary Photochemical Processes of Aromatic Molecules 850

12.4 Exemplar State Energy Diagrams of Aromatic Molecules 851

12.5 Pericyclic Photochemical Reactions: Electrocyclic and Related

Reactions of Aromatic Nuclei 853

12.6 Pericyclic Photochemical Reactions: [6e] Electrocyclization 856

12.7 Aryl-Vinyl Di-7r-methane Rearrangement 858

12.8 Photocycloaddition of Aromatic Molecules:

Photocyclodimerization 860

12.9 Photocycloaddition Reactions of Benzene and Its Derivatives 863

12.10 Photocycloaddition Reactions of Benzene and Its Derivatives: ortho

or [2 + 2] Cycloadditions 864

12.11 Photocycloaddition Reactions of Benzene and Its Derivatives:

meta or [2 + 3] Cycloaddition 867

Contents XXVII

12.12 Photocycloaddition Reactions of Benzene and Its Derivatives:

Competition between [2 + 2] and [2 + 3] Photocycloaddition 870

12.13 Photocycloaddition of Polycondensed Aromatic Molecules: Addition

to Olefins 872

12.14 Homolytic /J-Cleavage of the C—O Bond of Aryl Esters and Related

Compounds: The Photo-Fries and Related Rearrangements 875

12.15 Homolytic ^-Cleavage of the C—C Bond of Small Rings 878

12.16 Heterolytic /9-Cleavage: Photosolvolysis and Related Reactions 879

12.17 Excited-State Acidity and Basicity: Base-Assisted |3-Cleavage(Ar-O-H) 883

12.18 Homolytic a-Cleavage of Aryl Halides: Aryl-Aryl Coupling 886

12.19 Electron-Transfer Reactions: Addition to Amines 888

12.20 Aromatic Molecules as Electron-Transfer Photosensitizers of Radical

Cation Formation 890

12.21 Photochemical Electrophilic Aromatic Substitution: Proton-Transfer

Reactions of Aromatic Molecules 893

12.22 Photoinduced Nucleophilic Aromatic Substitution via a Photoinduced

Electron-Transfer Process 894

12.23 Photoinduced Nucleophilic Aromatic Substitution Involving Direct

Attack of a Nucleophile on *R: The SNAr* Mechanism (Substitution,

Nucleophilic, Excited State) 896

12.24 Photoinduced Nucleophilic Aromatic Substitution Involving Electron

Transfer from Nucleophile to *R: The SN(et)Ar* Mechanism

(Substitution, Nucleophilic, Electron Transfer, Excited State) 899

12.25 Nucleophilic Substitution via SNR-Ar* Mechanism (Substitution,Radical Anion, Nucleophilic, Excited State) 904

12.26 Photoinduced Nucleophilic Aromatic Substitution Triggeredby Photoionization: The SNR+Ar* Mechanism (Substitution,

Nucleophilic, Radical Cation, Excited State) 904

12.27 Summary of Photoinduced Nucleophilic Substitution Reactions 907

12.28 Synthetic Applications of the Photochemistry of Aromatics 907

12.29 Potential Applications of the Luminescence Properties of AromaticMolecules: Molecular Luminescence Probes 911

12.30 Polarity Probes Based on the Ham Effect 912

12.31 Polarity Probes Based on the Twisted Intramolecular Charge-TransferPhenomenon 912

12.32 Viscosity Probes 915

12.33 Viscosity Probes Based on the TICT Phenomenon 915

12.34 Fluorescence Thermometers 916

12.35 Fluorescence Thermometers Based on a Temperature-DependentRadiationless Process 916

xxviii Contents

Fluorescence Thermometers Based on Excimer and Excited

Monomer Equilibrium 917

Fluorescence Thermometers Based on the TICT Phenomenon 917

Fluorescent Chemosensors 918

Fluorescent Chemosensors Based on Electron-Transfer

Principles 919

Summary 920

References 921

Supramolecular Organic Photochemistry: The Control of

Organic Photochemistry and Photophysics throughIntermolecular Interactions 925

13.1 The Current and Emerging Paradigm of Supramolecular OrganicChemistry 925

13.2 A Paradigm of Supramolecular Organic Chemistry: guest@host

Complexes 928

13.3 Toward a Paradigm for Supramolecular Organic Photochemistry 930

13.4 An Enzyme as an Exemplar Supramolecular Host for guest@host

Complexes. Control of Activation Parameters and CompetitiveReaction Rates through Supramolecular Effects 933

13.5 Extending Some of the Key Structural and Dynamic Features of

guest©enzyme Complex to Organic guest@host Complexes. TheHost Reaction Cavity Concept 938

13.6 Some Exemplar Organic Hosts for Aqueous Solution SupramolecularPhotochemistry: Supercages, Cavitands, and Capsules 941

13.7 Some Exemplar Hosts of Supramolecular Photochemistry in the

Solid State: Crystals and Porous Solids 948

13.8 The Role of Time Scale and Dynamics in Supramolecular Organic

Photochemistry. The Transient and Persistent Supramolecular

Complex Concept. Hemicarceplexes and Carceplexes 951

13.9 Supramolecular Control of Photochemical and PhotophysicalProcesses: General Principles 954

13.10 Supramolecular Control of Unimolecular Photophysical Processes byPreorganization of guest@host Complexes: Enhancement of Room

Temperature Phosphorescence 955

13.11 Supramolecular Control of Bimolecular Photophysical Processes by

Preorganization of guest@host Complexes: Enhancement of Excimer

Formation of *R 959

13.12 Supramolecular Control of Triplet-Triplet Energy Transfer throughthe Walls of a Carcerand Host 962

13.13 Supramolecular Control of Unimolecular Photochemical Processes

by Preorganization in guest@host Complexes: Supramolecular

Selectivity of the Reactive State 964

12.36

12.37

12.38

12.39

12.40

CHAPTER 13

Contents xxix

13.14 Supramolecular Control of Unimolecular Photochemical Processes

by Preorganization of guest@host Complexes: Supramolecular

Selectivity of the *R I Processes 965

13.15 Supramolecular Chiral Effects on Two Competing Primary Processes

of *R Involving Biradical Intermediates: Preorganization in

guest@host Assemblies 970

13.16 Supramolecular Effects on Bimolecular Primary Processes:

Preorganization through Orientational Effects in guest/coguest@host

Supramolecular Assemblies 972

13.17 Supramolecular Effects on *R in the Solid State: Preorganization

through Conformational and Orientational Control in the Solid

State 976

13.18 Supramolecular Effects on *R: Templated Photodimerization in the

Solid State 977

13.19 Supramolecular Chiral Effects on *R in Concerted Reactions

and Reactions Involving Funnels: Preorganization in guest@host

Assemblies 979

13.20 Supramolecular Effects on Reaction Intermediates I:

Mobility Control on I@host Assemblies 981

13.21 Time-Dependent Supramolecular Effects on Reaction Intermediates

(I) 987

13.22 Supramolecular Effects on Products (P@carcerand): Stabilization of

Reactive Product Molecules (P) 993

13.23 Supramolecular Effects on Reactive Intermediates (I@carcerand):

Making Transient Intermediates (I) Persistent throughIncarceration 995

13.24 Summary 996

References 997

chapter 14 Molecular Oxygen and Organic Photochemistry 1001

14.1 The Role of Molecular Oxygen in Organic Photochemistry 1001

14.2 The Electronic Structure of the Oxygen Molecule: Ground and

Excited States 1003

14.3 Thermodynamic and Electrochemical Properties of Oxygen and

Oxygen-Related Species 1008

14.4 Interaction of Oxygen with the Ground States of OrganicMolecules 1012

14.5 Interaction of Ground-State Oxygen with Electronically Excited

Singlet States, *R(S {), of Organic Molecules 1012

14.6 Quenching ofExcited Triplet States (Tj) by Oxygen: Energy-TransferProcesses 1015

14.7 Mechanism of Triplet Photosensitization of Singlet OxygenGeneration 1018

14.8 Charge-Transfer Interactions in the Triplet Quenching Process 1020

xxx Contents

14.9 Efficiency of Singlet Oxygen, 02(1A), Generation: Selecting a Good

Singlet Oxygen Sensitizer 1022

14.10 Spectroscopy and Dynamics ofSinglet Molecular Oxygen: Dynamicsof Radiative and Radiationless Processes in Singlet Oxygen 1023

14.11 Physical and Chemical Quenching of Singlet Oxygen 1026

14.12 Intermolecular Interactions Leading to the Radiationless Deactivation

of Singlet Oxygen (Physical Quenching) 1026

14.13 Intermolecular Interactions Leading to Chemical Transformations

(Chemical Quenching of^ 1027

14.14 The Reversible [4 -f 2] Cycloaddition Reaction of '02 to 1,4-Dienes

and Aromatic Systems 1029

14.15 The ene Reaction: An Important Tool in Organic Synthesis 1030

14.16 Chemical Quenching of Excited Triplet States by Oxygen 1031

14.17 Reaction of Oxygen with Reaction Intermediates, 1(D) + 02:Mechanisms and Kinetics 1032

14.18 Free Radical Scavenging by Oxygen: I(FR) + 02 -> Peroxides 1033

14.19 Biradical Scavenging by Oxygen: I(BR) + 02 -* Products 1034

14.20 Reactions of Carbenes with Oxygen 1036

14.21 Molecular Oxygen and Other Reaction Intermediates 1038

14.22 Molecular Oxygen in Biology 1038

14.23 Is Evidence for Oxygen Quenching of a Reaction Good Evidence for

Triplet Involvement? 1039

14.24 Summary 1040

References 1040

chapter 15 A Generalization of the Photochemistryof Organic Molecules 1043

15.1 A Paradigm and Strategy for Understanding the Photochemistry of

Organic Functional Groups 1043

15.2 Some Examples of the Extension of the Paradigms of Scheme 15.1 to

"Other *R" and "Other I" 1046

15.3 Photochemistry of the Nitro (R—N02) Functional Group 1047

15.4 The Azo (—N=N—) Functional Group 1049

15.5 The Diazo (R2CN2) Chromophore 1050

15.6 The Thioketone (R2C=S) Group 1051

15.7 Summary 1053

References 1053

Index 1055