Modern Molecular Photochemistry of Organic Molecules Nicholas J. Turro COLUMBIA UNIVERSITY V. Ramamurthy UNIVERSITY OF MIAMI J. C. Scaiano UNIVERSITY OFOTTAWA TECHNISCHE INFORM A HO N SB i i.-,L IOTH EK UNIVERSITATS8IBLIOTHEK HANNOVER University Science Books Sausalito, California <^
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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
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