Modern Molecular. Photochemistry. Nicholas J. Turro COLUMBIA UNIVERSITY. V. Ramamurthy UNIVERSITY OF MIAMI. J. C. Scaiano UNIVERSITY OF OTTAWA

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1 Modern Molecular Photochemistry of Organic Molecules Nicholas J. Turro COLUMBIA UNIVERSITY V. Ramamurthy UNIVERSITY OF MIAMI J. C. Scaiano UNIVERSITY OF OTTAWA TECHNISCHE INFORM A HON SB i i.-,l IOTHEK UNIVERSITATS8IBLIOTHEK HANNOVER University Science Books Sausalito, California <^

2 Contents Preface xxxi chapter 1 Molecular Photochemistry of Organic Compounds: An Overview \ 1.1 What Is Molecular Organic Photochemistry? Learning Molecular Organic Photochemistry through the Visualization of Molecular Structures and the Dynamics of Their Transformations Why Study Molecular Organic Photochemistry? The Value of Pictorial Representations and Visualization of Scientific Concepts Scientific Paradigms of Molecular Organic Photochemistry Exemplars as Guides to the Experimental Study and Understanding of Molecular Organic Photochemistry The Paradigms of Molecular Organic Photochemistry Paradigms as Guides for Proceeding from the Possible to the Plausible to the Probable Photochemical Processes Some Important Questions that Will Be Answered by the Paradigms of Molecular Organic Photochemistry From a Global Paradigm to the Everyday Working Paradigm Singlet States, Triplet States, Diradicals, and Zwitterions: Key Structures Along a Photochemical Pathway from *R to P State Energy Diagrams: Electronic and Spin Isomers An Energy Surface Description of Molecular Photochemistry Structure, Energy, and Time: Molecular-Level Benchmarks and Calibration Points of Photochemical Processes 25

3 x Contents 1.15 Calibration Points and Numerical Benchmarks for Molecular Energetics Counting Photons Computing the Energy of a Mole of Photons for Light of Wavelength A, and Frequency v The Range of Photon Energies in the Electromagnetic Spectrum Calibration Points and Numerical Benchmarks for Molecular Dimensions and Time Scales Plan of the Text 35 References 38 chapter 2 Electronic, Vibrational, and Spin Configurations of Electronically Excited States Visualization of the Electronically Excited Structures through the Paradigms of Molecular Organic Photochemistry Molecular Wave Functions and Molecular Structure The Born-Oppenheimer Approximation: A Starting Point for Approximate Molecular Wave Functions and Energies Important Qualitative Characteristics of Approximate Wave Functions From Postulates of Quantum Mechanics to Observations of Molecular Structure: Expectation Values and Matrix Elements The Spirit of the Use of Quantum Mechanical Wave Functions, Operators, and Matrix Elements From Atomic Orbitals, to Molecular Orbitals, Configurations, to Electronic States Ground and Excited Electronic Configurations 52 to Electronic 2.9 The Construction of Electronic States from Electronic Configurations Construction of Excited Singlet and Triplet States from Electronically Excited Configurations and the Pauli Principle Characteristic Configurations of Singlet and Triplet States: A Shorthand Notation Electronic Energy Difference between Molecular Singlet and Triplet States of *R: Electron Correlation and the Electron Exchange Energy Evaluation of the Relative Singlet and Triplet Energies and Singlet- Triplet Energy Gaps for Electronically Excited States (*R) of the Same Electronic Configuration Exemplars for the Singlet-Triplet Splittings Systems 63 in Molecular

4 Contents xi 2.15 Electronic Energy Difference between Singlet and Triplet States of Diradical Reactive Intermediates: Radical Pairs, I(RP), and Biradicals, 1(BR) A Model for Vibrational Wave Functions: The Classical Harmonic Oscillator The Quantum Mechanical Version of the Classical Harmonic Oscillator The Vibrational Levels of a Quantum Mechanical Harmonic Oscillator The Vibrational Wave Functions for a Quantum Mechanical Harmonic Oscillator: Visualization of the Wave Functions for Diatomic Molecules A First-Order Approximation of the Harmonic-Oscillator Model: The Anharmonic Oscillator Building Quantum Intuition for Using Wave Functions Electron Spin: A Model for Visualizing Spin Wave Functions A Vector Model of Electron Spin Important Properties of Vectors Vector Representation of Electron Spin Spin Multiplicities: Allowed Orientations of Electron Spins Vector Model of Two Coupled Electron Spins: Singlet and Triplet States The Uncertainty Principle and Cones of Possible Orientations for Electron Spin 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 Making a Connection between Spin Angular Momentum and Magnetic Moments Due to Spin Angular Momentum The Connection between Angular Momentum and Magnetic Moments: A Physical Model for an Electron with Angular Momentum The Magnetic Moment of an Electron in a Bohr Orbit The Connection between Magnetic Moment and Electron Spin Magnetic Energy Levels in an Applied Magnetic Field for a Classical Magnet Quantum Magnets in the Absence of Coupling Magnetic Fields Quantum Mechanical Magnets in a Magnetic Field: Constructing a Magnetic State Energy Diagram for Spins in an Applied Magnetic Field Magnetic Energy Diagram for a Single Electron Spin and for Two Coupled Electron Spins 103

5 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 Transitions between States A Starting Point for Modeling Transitions between States Classical Chemical Dynamics: Some Preliminary Comments Quantum Dynamics: Transitions between States Perturbation Theory The Spirit of Selection Rules for Transition Probabilities 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 The Effect of Vibrations on Transitions between Electronic States: The Franck-Condon Principle A Classical and Semiclassical Harmonic Oscillator Model of the Franck-Condon Principle for Radiative Transitions (R + hv *R and *R R + ftv) A Quantum Mechanical Interpretation of the Franck-Condon Principle and Radiative Transitions The Franck-Condon Principle and Radiationless Transitions (*R -» R + heat) Radiationless and Radiative Transitions between Spin States of Different Multiplicity Spin Dynamics: Classical Precession of the Angular Momentum Vector Precession of a Quantum Mechanical Magnet in the Cones of Possible Orientations Important Characteristics of Spin Precession Some Quantitative Benchmark Relationships between the Strength of a Coupled Magnetic Field and Precessional Rates Transitions between Spin States: Magnetic Energies and Interactions The Role of Electron Exchange (J) in Coupling Electron Spins Couplings of a Spin with a Magnetic Field: Visualization of Spin Transitions and Intersystem Crossing Vector Model for Transitions between Magnetic States 148

6 Contents xiil 3.21 Spin-Orbit Coupling: A Dominant Mechanism for Inducing Spin Changes in Organic Molecules Coupling of Two Spins with a Third Spin: T+ -» S and T_ -> S Transitions Coupling Involving Two Correlated Spins: Tq -» S Transitions Intersystem Crossing in Diradicals, 1(D): Radical Pairs, I(RP), and Biradicals, I(BR) Spin-Orbit Coupling in 1(D): The Role of Relative Orbital Orientation Intersystem Crossing in Flexible Biradicals What All Transitions between States Have in Common 166 References 167 chapter 4 Radiative Transitions between Electronic States The Absorption and Emission of Light by Organic Molecules The Nature of Light: A Series of Paradigm Shifts Black-Body Radiation and the "Ultraviolet Catastrophe" and Planck's Quantization of Light Energy: The Energy Quantum Is Postulated The "Photoelectric Effect" and Einstein's Quantization of Light The Quantum of Light: Photons If Light Waves Have the Properties of Particles, Do Particles Have the Properties of Waves? de Broglie Integrates Matter and Light Absorption and Emission Spectra of Organic Molecules: The State Energy Diagram as a Paradigm for Molecular Photophysics Some Examples of Experimental Absorption and Emission Spectra of Organic Molecules: Benchmarks The Nature of Light: From Particles to Waves to Wave Particles A Pictorial Representation of the Absorption of Light The Interaction of Electrons with the Electric and Magnetic Forces of Light A Mechanistic View of the Interaction of Light with Molecules: Light as a Wave An Exemplar of the Interaction of Light with Matter: The Hydrogen Atom From the Classical Representation to a Quantum Mechanical Representation of Light Absorption by a Hydrogen Atom and a Hydrogen Molecule Photons as Massless Reagents Relationship of Experimental Spectroscopic Quantities to Theoretical Quantities The Oscillator Strength Concept 195

7 xiv Contents 4.17 The Relationship between the Classical Concept of Oscillator Strength and the Quantum Mechanical Transition Dipole Moment Examples of the Relationships of e, k e, t, < ^\\P\^2 >> and / Experimental Tests of the Quantitative Theory Relating Emission and Absorption to Spectroscopic Quantities The Shapes of Absorption and Emission Spectra The Franck-Condon Principle and Absorption Spectra of Organic Molecules The Franck-Condon Principle and Emission Spectra The Effect of Orbital Configuration Mixing and Multiplicity Mixing on Radiative Transitions Experimental Exemplars of the Absorption and Emission of Light by Organic Molecules Absorption, Emission, and Excitation Spectra Order of Magnitude Estimates of Radiative Transition Parameters Quantum Yields for Emission (*R -> R + hv) Experimental Examples of Fluorescence Quantum Yields Determination of "State Energies" Es and Er from Emission Spectra Spin-Orbit Coupling and Spin-Forbidden Radiative Transitions Radiative Transitions Involving a Change in Multiplicity: S0 T(n,7r*) and S0 (jt.jt*) Transitions as Exemplars Experimental Exemplars of Spin-Forbidden Radiative Transitions: S0 -* T, Absorption and Tj -> S0 Phosphorescence Quantum Yields of Phosphorescence, $P: The Ti -> S0 + hv Process Phosphorescence in Fluid Solution at Room Temperature Absorption Spectra of Electronically Excited States Radiative Transitions Involving Two Molecules: Absorption Complexes and Exciplexes Examples of Ground-State Charge-Transfer Absorption Complexes Excimers and Exciplexes Exemplars of Excimers: Pyrene and Aromatic Compounds Exciplexes and Exciplex Emission Twisted Intramolecular Charge-Transfer States Emission from "Upper" Excited Singlets and Triples: The Azulene Anomaly 260 References 262

8 Contents xv chapter 5 Photophysical Radiationless Transitions Photophysical Radiationless Transitions As a Form of Electronic Relaxation Radiationless Electronic Transitions as the Motion of a Representative Point on Electronic Energy Surfaces Wave Mechanical Interpretation of Radiationless Transitions between States Radiationless Transitions and the Breakdown of the Born- Oppenheimer Approximation An Essential Difference between Strongly Avoiding and Matching Surfaces Conical Intersections Near Zero-Order Surface Crossings Formulation of a Parameterized Model of Radiationless Transitions Visualization of Radiationless Transitions Promoted by Vibrational Motion; Vibronic Mixing Intersystem Crossing: Visualization of Radiationless Transitions Promoted by Spin-Orbit Coupling Selection Rules for Intersystem Crossing in Molecules The Relationship of Rates and Efficiencies of Radiationless Transitions to Molecular Structure: Stretching and Twisting as Mechanisms for Inducing Electronic Radiationless Transitions The "Loose Bolt" and "Free-Rotor" Effects: Promoter and Acceptor Vibrations Radiationless Transitions between "Matching" Surfaces Separated by Large Energies Factors That Influence the Rate of Vibrational Relaxation The Evaluation of Rate Constants for Radiationless Processes from Quantitative Emission Parameters Examples of the Estimation of Rates of Photophysical Processes from Spectroscopic Emission Data Internal Conversion (Sn^S1,S1^S0,Tn->T1) The Relationship of Internal Conversion to the Excited-State Structure of *R The Energy Gap Law for Internal Conversion (S1 S0) The Deuterium Isotope Test for Internal Conversion Examples of Unusually Slow S St Internal Conversion Intersystem Crossing from St -> Tt The Relationship Between Sj T! Intersystem Crossing to Molecular Structure Temperature Dependence of Sl T Intersystem Crossing Intersystem Crossing (T, -> S0) 309

9 xvi Contents «5.26 The Relationship between Tj -> S0 Intersystem Crossing and Molecular Structure The Energy Gap Law for Tt-> S0 Intersystem Crossing: Deuterium Isotope Effects on Interstate Crossings Perturbation of Spin-Forbidden Radiationless Transitions Internal Perturbation of Intersystem Crossing by the Heavy-Atom Effect External Perturbation of Intersystem Crossing The Relationship between Photophysical Radiationless Transitions and Photochemical Processes 314 References 315 chapter 6 A Theory of Molecular Organic Photochemistry Introduction to a Theory of Organic Photoreactions Potential Energy Curves and Surfaces Movement of a Classical Representative Point on a Surface The Influence "of Collisions and Vibrations on the Motion of the Representative Point on an Energy Surface Radiationless Transitions on PE Surfaces: Surface Maxima, Surface Minima, ana Funnels on the Way from *R to P A Global Paradigm for Organic Photochemical Reactions Toward a General Theory of Organic Photochemical Reactions Based on Potential Energy Surfaces Determining Plausible Molecular Structures and Plausible Reaction Pathways of Photochemical Reactions 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 From 2D PE Curves to 3D PE Surfaces: The "Jump" from Two Dimensions to Three Dimensions The Nature of Funnels Corresponding to Surface Avoidings and Surface Touchings Involved in Primary Photochemical Processes "The Noncrossing Rule" and Its Violations: Conical Intersections and Their Visualization Some Important and Unique Properties of Conical Intersections Diradicaloid Structures and Diradicaloid Geometries Diradicaloid Structures Produced from Stretching Twisting n Bonds 344 a Bonds and

10 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 An Exemplar for Diradicaloid Geometries Produced by jr-bond. Twisting and Breaking: Twisting of the jt Bond of Ethylene Frontier Orbital Interactions As a Guide to the Lowest-Energy Pathways and Energy Barriers on Energy Surfaces The Principle of Maximum Positive Orbital Overlap for Frontier Orbitals Stabilization by Orbital Interactions: Selection Rules Based on Maximum Positive Overlap and Minimum Energy Gap Commonly Encountered Orbital Interactions in Organic Photoreactions 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 Electronic Orbital and State Correlation Diagrams An Exemplar for Photochemical Concerted Pericyclic Reactions: The Electrocyclic Ring Opening of Cyclobutene and Ring Closure of 1,3-Butadiene Frontier Orbital Interactions Involving Radicals as Models for Half-Filled Molecular Orbitals Orbital and State Correlation Diagrams The Construction of Electron Orbital and State Correlation Diagrams for a Selected Reaction Coordinate Typical State Correlation Diagrams for Concerted Photochemical Pericyclic Reactions Classification of Orbitals and States for the Electrocyclic Reactions of Cyclobutene and 1,3-Butadiene: An Exemplar Concerted Reaction Concerted Photochemical Pericyclic Reactions and Conical Intersections Typical State Correlation Diagrams for Nonconcerted Photoreactions: Reactions Involving Intermediates (Diradicals and Zwitterions) Natural Orbital Correlation Diagrams The Role of Small Barriers in Determining the Efficiencies of Photochemical Processes An Exemplar for the Photochemical Reactions of n,?r* States The Symmetry Plane Assumption: Salem Diagrams An Exemplar State Correlation Diagram for n-orbital Initiated Reaction of n,jr* States: Hydrogen Abstraction via a Coplanar Reaction Coordinate 372

11 xviii Contents 6.37 Extension of an Exemplar State Correlation Diagram to New Situations State Correlation Diagrams for a-cleavage of Ketones A Standard Set of Plausible Primary Photoreactions for tt,tt* and n,7t* States The Characteristic Plausible Primary Photochemistry Processes of tt,tc* States The Characteristic Plausible Primary Photochemical Processes of n,;r* States Summary: Energy Surfaces as Reaction Graphs or Maps 381 References 382 chapter 7 Energy Transfer and Electron Transfer Introduction to Energy and Electron Transfer The Electron Exchange Interaction for Energy and Electron Transfer "Trivial" Mechanisms for Energy and Electron Transfer Energy Transfer Mechanisms Visualization of Energy Transfer by Dipole-Dipole Interactions: A Transmitter-Antenna Receiver-Antenna Mechanism Quantitative Aspects of the Forster Theory of Dipole-Dipole Energy Transfer The Relationship of ket to Energy-Transfer Efficiency and Separation of Donor and Acceptor RDA Experimental Tests for Dipole-Dipole Energy Transfer Electron Exchange Processes: Energy Transfer Resulting from Collisions and Overlap of Electron Clouds Electron Exchange: An Orbital Overlap or Collision Mechanism of Energy Transfer Electron-Transfer Processes Leading to Excited States Triplet-Triplet Annihilation (TTA): A Special Case of Energy Transfer via Electron Exchange Interactions Electron Transfer: Mechanisms and Energetics Marcus Theory of Electron Transfer A Closer Look at the Reaction Coordinate for Electron Transfer Experimental Verification of the Marcus Inverted Region for Photoinduced Electron Transfer Examples of Photoinduced Electron Transfer That Demonstrate the Marcus Theory Long-Distance Electron Transfer Mechanisms of Long-Distance Electron Transfer: Through-Space and Through-Bond Interactions 442

12 Contents 7.20 A Quantitative Comparison of Triplet-Triplet Energy and Electron Transfer A Connection between Intramolecular Electron, Hole, and Triplet Transfer Photoinduced Electron Transfer between Donor and Acceptor Moieties Connected by a Flexible Spacer Experimental Observation of the Marcus Inversion Region for Freely Diffusing Species in Solution Control of the Rate and Efficiency of Electron-Transfer Separation by Controlling Changes in the Driving Force for Electron Transfer Application of Marcus Theory Distributions 451 to the Control of Product 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 Comparison between Exciplexes and Contact Radical Ion Pairs Energy and Electron-Transfer Equilibria Energy-Transfer Equilibria Electron-Transfer Equilibria in the Ground State Excited-State Electron-Transfer Equilibria Excited-State Formation Resulting from Electron-Transfer Reactions: Chemiluminescent Reactions Role of Molecular Diffusion in Energy and Electron-Transfer Processes in Solution An Exemplar Involving Energy Transfer Controlled by Diffusion Estimation ofrate Constants for Diffusion Controlled Processes Examples of Near-Diffusion-Controlled Reactions: Reversible Formation of Collision Complexes The Cage Effect Distance-Time Relationships for Diffusion Diffusion Control in Systems Involving Charged Species Summary 479 References 479 chapter 8 Mechanistic Organic Photochemistry Photochemical Reaction Mechanisms Some Philosophical Comments Concerning the Fundamental Nature of Reaction Mechanisms Creation of a Standard Mechanistic Set Use of Kinetic Plausibility in Quantitative Mechanistic Analyses 493

13 xx Contents 8.5 Introduction to the Reactions of Free Radicals and Biradicals The Use of Structural Criteria for Mechanistic Analysis: The Role of Reaction Intermediates (*R, I) Correlations 513 in Structure-Reactivity 8.7 The Use of Reaction Types and Structural Relationships in Mechanistic Analyses An Exemplar of the Use of Structural Relationships in Mechanistic Analysis Rules for Proceeding from Rate Laws to Photochemical Reaction Mechanisms Rules for Proceeding from Quantum Yields and Efficiency Laws to Kinetic Information on Photochemical Reaction Mechanisms Experimental Methods for Determining Rate Constants of Photoreactions Pulsed Excitation of R to Produce *R Techniques for Monitoring Upper Electronic States, **R Low-Temperature Matrix Isolation Techniques Two-Laser (Two-Color) Flash Photolysis The Laser Jet Technique Stern-Volmer Analysis of Photochemical Kinetics: Competition between Unimolecular and Bimolecular Deactivation of *R Stern-Volmer Quenching: Rate Constants from Efficiency versus Concentration Measurements Stern-Volmer Analysis Based on Data from Time-Resolved Measurements Using Gated Detection Experimental Exemplars of the Measurements of Photochemical Rate Constants Measurement of Absolute Efficiencies in Determining Kinetic Parameters Kinetics of Reactions Involving More Than One Excited State The Probe Method for Detecting Spectroscopically "Invisible" Transients Experimental Measurement of the Efficiency Processes: The Photoacoustic Method 555 of Radiationless 8.25 Reactive Intermediates: Experimental Detection and Characterization of *R and I 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 Investigation of the Structure of *R by Time-Resolved Infrared Spectroscopy (TRIR) Investigation of the a-cleavage *R I(RP) Process by TRIR 564

14 Contents xxi 8.29 Time-Resolved Electron Paramagnetic Resonance and CIDEP Electron Spin Polarization: Deviations from the Boltzmann Distribution of Spins and Its Effect on the Intensities of Magnetic Resonance Signals Investigation of the Structure of *R(T1) and the Mechanism of the S] *R(T1) ISC by TR EPR Investigation of the Photochemical Primary Process *R -> I Process bytrepr The Direct Observation of I(RP)gem and I (BR) by TR EPR Experimental Tests for the Involvement of Electronically Excited States *R: Qualitative Aspects. Deciding between *R(Sj) and *R(Ti) Experimental Tests for the Involvement of Electronically Excited States (*R): Quantitative Aspects The Use of Kinetic Methods to Detect and to Identify Reaction Intermediates, *R and I Reactions Involving Biradical Intermediates Spin Chemistry: Spin Selection Rules for Chemical Reactions Magnetic Effects on Reactions of I(RP) and I(BR) Kinetic Basis for Magnetic Field Effects (MFE), Magnetic Isotope Effects (MIE) and Chemically Induced Dynamic Nuclear Polarization (CIDNP) Magnetic Field Effects on the Reactivity and Products of 3I(RP) and 3I(BR) Magnetic Isotope Effects on the Reactivity and Products of 3I(RP) and 3I(BR) Chemically Induced Dynamic Nuclear Polarization of Radical Pairs: The Nuclear Spin Orientation Dependence of Chemical Reactivity of 3I(RP)gem CIDNP of Conformational^ Flexible Biradicals Chemical Spectroscopy: The Use of Photochemical Reactions to Measure Excited-State Energetics and Dynamics Advances in Modern Mechanistic Organic Photochemistry: Ultrafast Reactions and Laser Coherent Photochemistry Femtosecond Photochemistry Single-Molecule Spectroscopy Coherent Laser Photochemistry Multiphoton Microscopy Some Exemplar State Energy Parameters Ketones Alkenes and Polyenes Conjugated Enones and Dienones 622

15 xxii Contents 8.55 Aromatic Hydrocarbons Summary 623 References 624 chapter 9 Photochemistry of Carbonyl Compounds Introduction to the Photochemistry of Carbonyl Compounds Molecular Orbital description of the *R(n,7r*): Primary Carbonyl Compounds 630 Processes of 9.3 The *R(n,7r*) ~> I Primary Photochemical Processes Based on Frontier Orbital Interactions The I -> P Secondary Thermal Processes Based on Radical Pair, Free Radical, and Biradical Reactions The Alkoxy Radical: A Close Analogue of the Reactive n,7r* Carbonyl Chromophore State Energy Diagrams for Ketones The *R(n,7r*) > P Processes of Ketones and Aldehydes An Exemplar of an n <- HO Initiated *R(n,jr *) I Process: The Primary Process of Intermolecular Hydrogen Abstraction "Invisible Transients" in Radical-Radical Combination Reactions: Transients Formation by Radical-Radical Combination that Revert Back to Starting Materials The Primary Process of Intermolecular Electron Transfer: Reaction of n,7r* States With Amines Structure-Reactivity Relationships in Intermolecular Hydrogen Abstraction The Primary Process of Electron Abstraction: Reactive T^tt.tt*) State Competition between Hydrogen and Electron Abstraction: Effect of Variation for Orbital Structure and Hydrogen (Electron) Donor Structure The Primary Photochemical Process of Intramolecular Hydrogen Abstraction: Norrish Type II Reactions Reactivity and Efficiency Relationships in Type II Reactions The Product Forming I(BR) -> P Step in Type II Reactions: A Paradigm for the Behavior of a 1,4-Biradical Geometry of y-hydrogen Abstraction and Its Consequence on Competing Primary Photochemical Processes The role of Intersystem Crossing in Determining the Products of Biradicals Produced by y -Hydrogen Abstraction Beyond y-hydrogen Abstraction: Intramolecular l,n-hydrogen Abstraction 664

16 Contents xxiit 9.20 The Primary Process of a-cleavage of n,7t* States: Acyclic Ketones The Primary Process of a-cleavage from n,;r* States: Cyclic Ketones Reactions of Primary Radical Pair Produced from a-cleavage Photochemistry of Cyclobutanones: A Special Case of a-cleavage The Primary Process of a-cleavage of n,7r* states. Structure- Reactivity Relationships An Orbital Model for ar-cleavage The Primary Process for Addition of n,7r* States to Electron-Rich C=C Bonds The Primary Process of Addition of n,n* States to Electron-Rich C=C: Reaction Intermediates Evidence for a Biradical Intermediate Endo-Exo Selectivity During Photoaddition of Excited Carbonyls to Olefins 683 for n,7r* States to Electron-Poor 9.30 Examples of [2 + 2] Cycloaddition Ethylenes: An Example of a n* -» tt* Interaction Stereoselectivity of the [2 + 2] Cycloaddition of n,;r* States to Ethylenes Intramolecular [2 -f 2] Photocycloaddition Examples of Photorearrangements Initiated by /3-Cleavage Followed by Combination and Disproportionation Photochemical Fragmentations Initiated by ^-Cleavage Synthetic Applications of the Photoreactions of Carbonyl Compounds Applications of the Photochemistry of Carbonyl Compounds in Photoimaging Applications of the Photochemistry of Carbonyl Compounds in Designing "Phototriggers" and "Photoprotecting Groups" Summary: The Photochemistry of Carbonyl Compounds 701 References 702 chapter Photochemistry of Olefins Introduction to the Photochemistry of Olefins 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

17 xxiv Contents 10.6 The cis-trans Isomerization of Acyclic and Cyclic Alkenes The cis-trans Isomerization of Conjugated Polyenes: The Nonequilibrating Excited Rotomers Principle The cis-trans Isomerization of Aryl-Substituted Alkenes A Case Study of cis-trans Isomerization of Stilbene Adiabatic cis-trans Isomerization in S, (n, n *): Examples of *R *P Processes Trapping of Strained /rans-cycloalkenes Cycloalkenes 726 from cis The cis-trans Isomerization through Conical Intersections Intramolecular Pericyclic Reactions of the S^Trjjr*) States of Alkenes: Examples of the Si(tt,tv*) F > P Processes Electrocyclic Ring Openings and Ring Closures Involving 1,3-dienes Electrocyclic Ring Openings of 1,3-Cyclohexadienes and the Ring Closures of 1,3,5-Hexatrienes Other Electrocyclic Reactions of Trienes Electrocyclic Ring Closures of Stilbenes and Related Systems Sigmatropic Rearrangements of the S^n^rt*) States of Alkenes The Di-7r-methane (Zimmerman) Reactions: A Sigmatropic Reaction of Wide Scope Di-^-methane Reactions: Acyclic 1,4-dienes Di-jr-methane Reactions: Rigid Cyclic 1,4-Dienes Compounds The [n + m] Photocycloaddition Reactions 751 and Related The [2 + 2] Photocycloaddition Reactions: Alkenes The [2 + 2] and [4 + 2] Photocycloaddition Reactions of 1,3-Dienes Intramolecular Photocycloadditions of Alkenes and Polyenes The [2 + 2] Photocycloaddition Reactions: Aryl Alkenes Proton-Transfer Reactions from S^tt,^*): Zwitterionic Photoaddition Reactions 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 ys-cleavage Reactions a-cleavage Reactions Photoinduced Electron-Transfer Reactions Involving Alkenes: Examples of *R -» I(D*+, A'-) Processes Structure and Reactivity of Radical Cations and Anions Pathways to Radical Cations and Anions of Alkenes 770

18 Contents xxv Reactions of Alkene Radical Ion Pairs: Addition of Amines Generation of Alkene Radical Cations Choice of Electron-Transfer Sensitizers Generation of Alkene Cation Radicals: Maximizing the Yield of Radical Ion Pair Formation Reactions of Alkene Cation Radicals: Geometric Isomerization Reactions of Alkene Cation Radicals: Addition to Nucleophiles Reactions of Alkene Cation Radicals: Dimerization Reactions of Alkene Cation Radicals: Intramolecular Cyclization Applications of Photoinduced cis-trans Isomerization in Biological Systems The cis-trans Isomerization as a Photoswitch The cis-trans Isomerization as a Photoregulator of Phase Transitions and Packing Arrangements in Liquid Crystals, Langmuir-Blodgett Films, and Solgels Controlling Ion Transport through Membranes through cis-trans Isomerization Application of cis-trans Isomerization in Laboratory and Industrial Syntheses Application of Photoinduced Pericyclic Reactions Summary 796 References 797 chapter 11 Photochemistry of Enones and Dienones Introduction to the Photochemistry of Enones and Dienones Molecular Orbital Description of the *R(n,7T*) and *R(;r,7r*) States of Enones: Primary Processes of Enones and Dienones The I ~> P Secondary Processes ofenones and Dienones Exemplar State Energy Diagrams for Enones and Related Structures The Photochemistry of f3, y -Enones: Exemplars of the Photochemistry of Enones With Isolated But Proximate C=0 and C=C Bonds Photochemistry of the n,7r* States of /3,y-Enones Competition between the Reactions of n,7r* and tc,ti* States of Enones Competitive Reactions from the T^n^*) States of y6,y-enones: Oxa-di-nr -methane Rearrangement and cis-trans Isomerization Introduction to the Photochemistry of a,/3-enones Photochemistry of a.^-enones Originating in the Tjfn.Tr*) State: Analogies with the Primary Processes of the n,7r* Carbonyls 815 States of

19 xxvi Contents Photochemistry of a,/s-enones Originating in the Tx{n,n*) State: Analogies to the Primary Processes of the Tt,n* States of Alkenes The Sigmatropic Rearrangement of Cyclohexenones: Type A and B Rearrangements Role of Geometric Isomerization in the Type A Reaction of 2-Cyclohexenones Type B Rearrangement of 2-Cyclohexenones: The [1,2] Aryl and [1,2] Vinyl Migrations Starting from a T^n.Tr*) State The [2 + 2] Cycloaddition Reactions of Cyclic a,/?-enones Sigmatropic Rearrangements of Cross-Conjugated Dienones Photochemistry of Linear Conjugated Cyclohexadienones: the [6e] Electrocyclic Ring Opening and [1,2] Sigmatropic Rearrangement Synthetic Applications of Enone and Dienone Photochemistry Developing Useful Synthetic Methodologies for Construction of Diastereoselective and Enantioselective Cyclobutane Rings Photocycloaddition Reactions of Coumarin and Psoralen. Psoralen Ultraviolet A Treatment Photocycloaddition Reactions of Nucleic Acid-Base Pairs and Skin Cancer Summary 843 References 844 chapter 12 Photochemistry of Aromatic Molecules Introduction to the Photochemistry of Aromatic Molecules Molecular Orbital Description of the *R(7r,7r*) Primary Photochemical Processes of Aromatic Molecules The Primary Photochemical Processes of Aromatic Molecules Exemplar State Energy Diagrams of Aromatic Molecules Pericyclic Photochemical Reactions: Electrocyclic and Related Reactions of Aromatic Nuclei Pericyclic Photochemical Reactions: [6e] Electrocyclization Aryl-Vinyl Di-7r-methane Rearrangement Photocycloaddition of Aromatic Molecules: Photocyclodimerization Photocycloaddition Reactions of Benzene and Its Derivatives Photocycloaddition Reactions of Benzene and Its Derivatives: ortho or [2 + 2] Cycloadditions Photocycloaddition Reactions of B enzene and Its Derivatives: meta or [2 + 3] Cycloaddition 867

20 Contents XXVII Photocycloaddition Reactions of Benzene and Its Derivatives: Competition between [2 + 2] and [2 + 3] Photocycloaddition Photocycloaddition of Polycondensed Aromatic Molecules: Addition to Olefins Homolytic /J-Cleavage of the C O Bond of Aryl Esters and Related Compounds: The Photo-Fries and Related Rearrangements Homolytic ^-Cleavage of the C C Bond of Small Rings Heterolytic /9-Cleavage: Photosolvolysis and Related Reactions Excited-State Acidity and Basicity: Base-Assisted 3-Cleavage (Ar-O-H) Homolytic a-cleavage of Aryl Halides: Aryl-Aryl Coupling Electron-Transfer Reactions: Addition to Amines Aromatic Molecules as Electron-Transfer Photosensitizers of Radical Cation Formation Photochemical Electrophilic Aromatic Substitution: Proton-Transfer Reactions of Aromatic Molecules Photoinduced Nucleophilic Aromatic Substitution via a Photoinduced Electron-Transfer Process Photoinduced Nucleophilic Aromatic Substitution Involving Direct Attack of a Nucleophile on *R: The SNAr* Mechanism (Substitution, Nucleophilic, Excited State) Photoinduced Nucleophilic Aromatic Substitution Involving Electron Transfer from Nucleophile to *R: The SN(et)Ar* Mechanism (Substitution, Nucleophilic, Electron Transfer, Excited State) Nucleophilic Substitution via SNR-Ar* Mechanism (Substitution, Radical Anion, Nucleophilic, Excited State) Photoinduced Nucleophilic Aromatic Substitution Triggered by Photoionization: The SNR+Ar* Mechanism (Substitution, Nucleophilic, Radical Cation, Excited State) Summary of Photoinduced Nucleophilic Substitution Reactions Synthetic Applications of the Photochemistry of Aromatics Potential Applications of the Luminescence Properties of Aromatic Molecules: Molecular Luminescence Probes Polarity Probes Based on the Ham Effect Polarity Probes Based on the Twisted Intramolecular Charge-Transfer Phenomenon Viscosity Probes Viscosity Probes Based on the TICT Phenomenon Fluorescence Thermometers Fluorescence Thermometers Based on a Temperature-Dependent Radiationless Process 916

21 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 CHAPTER 13 Supramolecular Organic Photochemistry: The Control of Organic Photochemistry and Photophysics through Intermolecular Interactions The Current and Emerging Paradigm of Supramolecular Organic Chemistry A Paradigm of Supramolecular Organic Chemistry: guest@host Complexes Toward a Paradigm for Supramolecular Organic Photochemistry An Enzyme as an Exemplar Supramolecular Host for guest@host Complexes. Control of Activation Parameters and Competitive Reaction Rates through Supramolecular Effects Extending Some of the Key Structural and Dynamic Features of guest enzyme Complex to Organic guest@host Complexes. The Host Reaction Cavity Concept Some Exemplar Organic Hosts for Aqueous Solution Supramolecular Photochemistry: Supercages, Cavitands, and Capsules Some Exemplar Hosts of Supramolecular Photochemistry in the Solid State: Crystals and Porous Solids The Role of Time Scale and Dynamics in Supramolecular Organic Photochemistry. The Transient and Persistent Supramolecular Complex Concept. Hemicarceplexes and Carceplexes Supramolecular Control of Photochemical and Photophysical Processes: General Principles Supramolecular Control of Unimolecular Photophysical Processes by Preorganization of guest@host Complexes: Enhancement of Room Temperature Phosphorescence Supramolecular Control of Bimolecular Photophysical Processes by Preorganization of guest@host Complexes: Enhancement of Excimer Formation of *R Supramolecular Control of Triplet-Triplet Energy Transfer through the Walls of a Carcerand Host Supramolecular Control of Unimolecular Photochemical Processes by Preorganization in guest@host Complexes: Supramolecular Selectivity of the Reactive State 964

22 Contents xxix Supramolecular Control of Unimolecular Photochemical Processes by Preorganization of Complexes: Supramolecular Selectivity of the *R I Processes Supramolecular Chiral Effects on Two Competing Primary Processes of *R Involving Biradical Intermediates: Preorganization in guest@host Assemblies Supramolecular Effects on Bimolecular Primary Processes: Preorganization through Orientational Effects in guest/coguest@host Supramolecular Assemblies Supramolecular Effects on *R in the Solid State: Preorganization through Conformational and Orientational Control in the Solid State Supramolecular Effects on *R: Templated Photodimerization in the Solid State Supramolecular Chiral Effects on *R in Concerted Reactions and Reactions Involving Funnels: Preorganization in guest@host Assemblies Supramolecular Effects on Reaction Intermediates I: Mobility Control on I@host Assemblies Time-Dependent Supramolecular Effects on Reaction Intermediates (I) Supramolecular Effects on Products (P@carcerand): Stabilization of Reactive Product Molecules (P) Supramolecular Effects on Reactive Intermediates (I@carcerand): Making Transient Intermediates (I) Persistent through Incarceration Summary 996 References 997 chapter 14 Molecular Oxygen and Organic Photochemistry The Role of Molecular Oxygen in Organic Photochemistry The Electronic Structure of the Oxygen Excited States 1003 Molecule: Ground and 14.3 Thermodynamic and Electrochemical Properties of Oxygen and Oxygen-Related Species Interaction of Oxygen with the Ground States of Organic Molecules Interaction of Ground-State Oxygen with Electronically Excited Singlet States, *R (S {), of Organic Molecules Quenching ofexcited Triplet States (Tj) by Oxygen: Energy-Transfer Processes Mechanism of Triplet Photosensitization of Singlet Oxygen Generation Charge-Transfer Interactions in the Triplet Quenching Process 1020

23 xxx Contents 14.9 Efficiency of Singlet Oxygen, 02(1A), Generation: Selecting a Good Singlet Oxygen Sensitizer Spectroscopy and Dynamics of Singlet Molecular Oxygen: Dynamics of Radiative and Radiationless Processes in Singlet Oxygen Physical and Chemical Quenching of Singlet Oxygen Intermolecular Interactions Leading to the Radiationless Deactivation of Singlet Oxygen (Physical Quenching) Intermolecular Interactions Leading to Chemical Transformations (Chemical Quenching of ^ The Reversible [4 -f 2] Cycloaddition Reaction of '02 to 1,4-Dienes and Aromatic Systems The ene Reaction: An Important Tool in Organic Synthesis Chemical Quenching of Excited Triplet States by Oxygen Reaction of Oxygen with Reaction Intermediates, 1(D) + 02: Mechanisms and Kinetics Free Radical Scavenging by Oxygen: I(FR) > Peroxides Biradical Scavenging by Oxygen: I(BR) * Products Reactions of Carbenes with Oxygen Molecular Oxygen and Other Reaction Intermediates Molecular Oxygen in Biology Is Evidence for Oxygen Quenching of a Reaction Good Evidence for Triplet Involvement? Summary 1040 References 1040 chapter 15 A Generalization of the Photochemistry of Organic Molecules A Paradigm and Strategy for Understanding the Photochemistry of Organic Functional Groups Some Examples of the Extension of the Paradigms "Other *R" and "Other I" 1046 of Scheme 15.1 to 15.3 Photochemistry of the Nitro (R N02) Functional Group The Azo ( N=N ) Functional Group The Diazo (R2CN2) Chromophore The Thioketone (R2C=S) Group Summary 1053 References 1053 Index 1055