Excited States and Photochemistry of Organic Molecules

281 Pages·1995·27.149 MB·English

by  Martin Klessinger, Josef Michl

Excited States and
Photochemistry
of
Organic Molecules
Martin Klessinger
WestBlische Wilhelms-Universitat Monster
Josef Michl
University of Colorado
+
VCH
Martin Klessinaer Josef Michl
Organisch-~hemischesI nstitut Department of Chemistry and
WestfMische Wilhelms-UnivenitBt Biochemistry
P48149 Monster University of Colorado
Germany Boulder, CO 80309-02 15
Library of Congress CaWoging-in-Publiestion Data
Klessinger. Martin.
Excited states and photochemistry of organic molecules I Martin
Klessinger, Josef Michl.
p. cm.
Includes index.
ISBN 1-56081-588-4
1. Chemistry, Physical organic. 2. Photochemistry. 3. Excited
state chemistry. I. Michl, Josef, 1939- . 11. Title.
QD476.K53 1994
547.1'354~20 92-46464
CIP
To our teachers
WOLFGANLGU TTKE
RUDOLZF AHRADN~K
AND
8 1995 VCH Publishers, Inc.
This work is subject to copyright.
All rights reserved, whether the whole or part of the material is concerned, specifically
those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by
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Registered names, trademarks, etc., used in this book, even when not specifically marked
as such, are not to be considered unprotected by law.
Printed in the United States of America
ISBN 1-56081-588-4 VCH Publishers, Inc.
Printing History:
10987654321
Published jointly by
VCH Publishers, Inc. VCH Verlagsgesellschaft mbH VCH Publishers (UK) Ltd.
220 East 23rd Street P.O. Box 10 11 61 8 Wellington Court
New York, New York 10010 D-69451 Weinheim Cambridge CBI IHZ
Federal Republic of Germany United Kingdom
Preface
This graduate textbook is meant primarily for those interested in physical
organic chemistry and in organic photochemistry. It is a significantly up-
dated translation of Lichtabsorption und Photochemie organischer Mole-
kule, published by VCH in 1989. It provides a qualitative description of elec-
tronic excitation in organic molecules and of the associated spectroscopy,
photophysics, and photochemistry. The text is nonmathematical and only
assumes the knowledge of basic organic chemistry and spectroscopy, and
rudimentary knowledge of quantum chemistry, particularly molecular or-
bital theory. A suitable introduction to quantum chemistry for a German-
reading neophyte is Elektronenstruktur organischer Molekule, by Martin
Klessinger, published by VCH in 1982 as a volume of the series, Physikal-
ische Organische Chemie. The present textbook emphasizes the use of sim-
ple qualitative models for developing an intuitive feeling for the course of
photophysical and photochemical processes in terms of potential energy hy-
persurfaces. Special attention is paid to recent developments, particularly
to the role of conical intersections. In emphasizing the qualitative aspects of
photochemical theory, the present text is complementary to the more math-
ematical specialized monograph by Josef Michl and Vlasta BonaCiC-Kou-
teckl, Electronic Aspects of Organic Photochemistry, published by Wiley in
1990.
Chapter 1 describes the basics of electronic spectroscopy at a level suit-
able for nonspecialists. Specialized topics such as the use of polarized light
are mentioned only briefly and the reader is referred to the monograph by
Josef Michl and Erik Thulstrup, Spectroscopy with Polarized Light, pub-
viii PREFACE
PREFACE ix
lished by VCH in 1986 and reprinted as a paperback in 1995. Spectra of the
and weeks spent with the manuscript. Last but not least, we wish to ac-
most important classes of organic molecules are discussed in Chapter 2. A
knowledge the many years of generous support for our work in photochem-
unified view of the electronic states of cyclic n-electron systems is based on
istry that has been provided by the Deutsche Forschungsgemeinschaft and
the classic perimeter model, which is formulated in simple terms. Chapter 3
the U.S. National Science Foundation.
completes the discussion of spectroscopy by examining the interaction of
Many fine books on organic excited states, photophysics, and photo-
circularly polarized light with chiral molecules (i.e., natural optical activity),
chemistry are already available. Ours attempts to offer a different perspec-
and with molecules held in a magnetic field (i.e., magnetic optical activity).
tive by placing primary emphasis on qualitative theoretical concepts in a way
An understanding of the perimeter model for aromatics comes in very handy
that we hope will be useful to students of physical organic chemistry.
for the latter.
Chapter 4 introduces the fundamental concepts needed for a discussion
Miinster
of photophysical and photochemical phenomena. Here, the section on bi-
Boulder
radicals and biradicaloids has been particularly expanded relative to the Ger-
March 1995
man original. The last three chapters deal with the physical and chemical
transformations of excited states. The photophysical processes of radiative
and radiationless deactivation, as well as energy and electron transfer, are
treated in Chapter 5. A qualitative model for the description of photochem-
ical reactions in condensed media is described in Chapter 6, and then used
in Chapter 7 to examine numerous examples of phototransformations of or-
ganic molecules. All of these chapters incorporate the recent advances in
the understanding of the role of conical intersections ("funnels") in singlet
photochemical reactions.
Worked examples are provided throughout the text, mostly from the
recent literature, and these are meant to illustrate the practical applica-
tion of theory. Although they can be skipped during a first reading of a
chapter, it is strongly recommended that the reader work them through in
full detail sooner or later. The textbook is meant to be self-contained, but
provides numerous references to original literature at the end. Moreover,
each chapter concludes with a list of additional recommended reading.
We are grateful to several friends who offered helpful comments upon
reading sections of the book: Professors E Bernardi, R. A. Caldwell, C. E.
Doubleday, M. Olivucci, M. A. Robb, J. C. Scaiano, P. J. Wagner, M. C.
Zerner, and the late G. L. Closs. The criticism of the German version pro-
vided by Professors W. Adam, G. Hohlneicher and W. Rettig was very help
ful and guided us in the preparation of the updated translation. We thank Dr.
Edeline Wentrup-Byme for editing the translation of the German original
prepared by one of us (M. K.), and to Ms. Ingrid Denker for a superb typing
job and for drawing numerous chemical structures for the English version.
It was a pleasure to work with Dr. Barbara Goldman of VCH and her edi-
torial staff, and we appreciate very much their cooperation and willingness
to follow our suggestions. Much of the work of one of us (J. M.) was done
during the tenure of a BASF professorship at the University of Kaiserslau-
tern; thanks are due to Professor H.-G. Kuball for his outstanding hospital-
ity. We are much indebted to our respective families for patient support and
understanding during what must have seemed to be interminable hours, days
Acknowledgments
The authors wish to thank the following for permission to use their figures
in this book.
Academic Press, Orlando (USA)
Figures 2.9, 2.10, 7.4, 7.5 and 7.53
Academic Press, London (UK)
Figure 7.15
American Chemical Society, Washington (USA)
Figures 2.11, 2.37, 3.7, 3.9, 3.10, 3.13, 3.17, 3.21, 4.8, 5.14, 5.17, 5.34,
5.39, 5.40, 6.1, 6.16, 6.17, 6.21, 6.27, 7.2, 7.18, 7.24, 7.34, 7.38,
7.39, 7.42 and 7.43
American Institut of Physics, New York (USA)
Figures 2.6,2.17 and 2.18
The BenjaminlCummings Publishing Company, Menlo Park (USA)
Figures 1.11, 5.4, 5.11, 7.19 and 7.57
Bunsengesellschafff ur Physikalische Chemie, Darmstadt (G)
Figures 1.15 and 1.17
Elsevier Science Publishers B.V., Amsterdam (NL)
Figures 1.20, 5.32, 5.33, 7.3, 7.6, and 7.13
Gordon and Breach Science Publishers, Yverdon (CH)
Figure 6.8
Hevetica Chimica Acra, Basel (CH)
Figures 2.35 and 7.50
International Union of Pure and Applied Chemistry, Oxford (UK)
Figures 2.30,3.15, 3.16, 5.24 and 5.25
Kluwer Academic Publishers, Dordrecht (NL)
Figures 1.25,4.12,4.13,7.20 and 7.21
R. Oldenbourg Verlag GmbH, Msinchen (G)
Figure 5.38
Pergamon Press, Oxford (UK)
Figures 2.15,2.29,3.11,3.14, 3.18,3.19,4.27,4.28,5.15,5.16,5.30
and 5.36
Plenum Publishing Corp, New York (USA)
Figure 5.10
Royal Society of Chemistry, Cambridge (UK)
Figure 2.28
ACKNOWLEDGMENTS
The Royal Society, London (UK)
Figure 1.14
Springer-Verlag, Heidelberg (G)
Figures 1.23, 1.24, 6.3, 6.20 and 7.8
VCH Publishers, Inc., New York (USA)
Contents
Figure 1.16
VCH Verlagsgesellschaft mbH, Weinheim (G)
Figures 1.8, 2.7, 2.25, 2.27,2.34,2.38,2.42, 3.3, 3.6,4.21, 5.9,5.18, 5.19,
5.20,6.5,6.9, 6.13, 6.23, 6.25, 7.28,7.33 and 7.51
Weizmann Science Press of Israel, Jerusalem
Figure 7.22
John Wiley & Sons, Znc., New York (USA)
Figures 1.3, 2.2,2.3,2.45,4.5,4.6,4.10,4.11, 4.16,4.20,4.22,4.23,4.24,
6.19 and 6.28
John Wiley & Sons, Ltd., West Sussex (UK)
Figures 7.12 and 7.14
Notation xix
1. Spectroscopy in the Visible and UV Regions 1
1.1 Introduction and Theoretical Background 1
1.1.1 Electromagnetic Radiation 1
1.1.2 Light Absorption 5
1.2 MO Models of Electronic Excitation 9
1.2.1 Energy Levels and Molecular Spectra 9
1.2.2 MO Models for the Description of Light Absorption 11
1.2.3 One-Electron MO Models 13
1.2.4 Electronic Configurations and States 16
1.2.5 Notation Schemes for Electronic Transitions 20
1.3 Intensity and Band Shape 21
1.3.1 Intensity of Electronic Transitions 21
1.3.2 Selection Rules 27
1.3.3 The Franck-Condon Principle 34
1.3.4 Vibronically Induced Transitions 36
1.3.5 Polarization of Electronic Transitions 38
1.3.6 lko-Photon Absorption Spectroscopy 40
1.4 Properties of Molecules in Excited States 44
1.4.1 Excited-State Geometries 44
1.4.2 Dipole Moments of Excited-State Molecules 47
1.4.3 Acidity and Basicity of Molecules in Excited States 48
xiv CONTENTS CONTENTS
1.5 Quantum Chemical Calculations of Electronic Excitation 52 3.3 Magnetic Circular Dichroism (MCD) 154
1.5.1 Semiempirical Calculations of Excitation Energies 53 3.3.1 General Introduction 154
1. 5.2 Computation of Transition Moments 56 3.3.2 Theory 160
+
1.5.3 Ab Initio Calculations of Electronic Absorption 3.3.3 Cyclic n Systems with a (4N 2)-Electron
Spectra 58 Perimeter 164
Supplemental Reading 60 3.3.4 Cyclic n Systems with a 4N-Electron Perimeter 167
3.3.5 The Mirror-Image Theorem for Alternant Systems 170
jc
3.3.6 Applications 171
2. Absorption Spectra of Oqjanic Molecules 63
Supplemental Reading 177
2.1 Linear Conjugated n Systems 63
2.1.1 Ethylene 64
4. Potential Energy Surfaces: Barriers, Minima, and
2.1.2 Polyenes 65
2.2 Cyclic Conjugated n Systems 71 Funnels 179
2.2.1 The Spectra of Aromatic Hydrocarbons 7 1
4.1 Potential Energy Surfaces 179
2.2.2 The Perimeter Model 76
4.1.1 Potential Energy Surfaces for Ground and Excited
2.2.3 The Generalization of the Perimeter Model for Systems with
+ States 179
4N 2 n Electrons 81
4.1.2 Funnels: True and Weakly Avoided Conical
2.2.4 Systems with Charged Perimeters 85
Intersections 182
2.2.5 Applications of the PMO Method Within the Extended
4.1.3 Spectroscopic and Reactive Minima in Excited-State
Perimeter Model 87
Surfaces 186
2.2.6 Polyacenes 92
4.2 Correlation Diagrams 193
2.2.7 Systems with a 4N n-Electron Perimeter 96
4.2.1 Orbital Symmetry Conservation 193
2.3 Radicals and Radical Ions of Alternant Hydrocarbons 101
4.2.2 Intended and Natural Orbital Correlations 197
2.4 Substituent Effects 104
4.2.3 State Correlation Diagrams 200
2.4.1 Inductive Substituents and Heteroatoms 104
4.3 Biradicals and Biradicaloids 205
2.4.2 Mesomeric Substituents 109
4.3.1 A Simple Model for the Description of Biradicals 205
2.5 Molecules with n+n* Transitions 118
4.3.2 Perfect Biradicals 208
2.5.1 Carbonyl Compounds 11 9
4.3.3 Biradicaloids 210
2.5.2 Nitrogen Heterocycles 122
4.3.4 Intersystem Crossing in Biradicals and Biradicaloids 219
2.6 Systems with CT Transitions 123
4.4 Pericyclic Funnels (Minima) 229
2.7 Steric Effects and Solvent Effects 126 +
4.4.1 The Potential Energy Surfaces of Photochemical [2, 2,]
2.7.1 Steric Effects 126 +
and x[2, 2,l Processes 230
2.7.2 Solvent Effects 129
4.4.2 Spectroscopic Nature of the States Involved in Pericyclic
Supplemental Reading 135
Reactions 238
Supplemental Reading 239
3. Optical Activity 139
3.1 Fundamentals 139
5. Photophysical Processes 243
3.1.1 Circularly and Elliptically Polarized Light 139
3.1.2 Chiroptical Measurements 141 5.1 Unimolecular Deactivation Processes 243
3.2 Natural Circular Dichroism (CD) 143 5.1.1 The Jablonski Diagram 243
3.2.1 General Introduction 143 5.1.2 The Rate of Unimolecular Processes 245
3.2.2 Theory 145 5.1.3 Quantum Yield and Efficiency 247
3.2.3 CD Spectra of Single Chromophore Systems 147 5.1.4 Kinetics of Unimolecular Photophysical Processes 250
3.2.4 lko-Chromophore Systems 152 5.1.5 State Diagrams 25 1
CONTENTS CONTENTS
5.2 Radiationless Deactivation 252 7. Oqanic Photochemistry 361
5.2.1 Internal Conversion 252
7.1 Cis-trans Isomerization of Double Bonds 362
5.2.2 Intersystem Crossing 254
7.1.1 Mechanisms of cis-trans Isomerization 362
5.2.3 Theory of Radiationless Transitions 257
7.1.2 Olefins 364
5.3 Emission 260
7.1.3 Dienes and Trienes 366
5.3.1 Fluorescence of Organic Molecules 260
7.1.4 Stilbene 369
5.3.2 Phosphorescence 266
7.1.5 Heteroatom, Substituent, and Solvent Effects 372
5.3.3 Luminescence Polarization 272
7.1.6 Azomethines 374
5.4 Bimolecular Deactivation Processes 276
7.1.7 Azo Compounds 376
5.4.1 Quenching of Excited States 277
7.2 Photodissociations 378
5.4.2 Excimers 278
7.2.1 cx Cleavage of Carbonyl Compounds (Norrish vpe I
5.4.3 Exciplexes 281
Reaction) 380
5.4.4 Electron-Transfer and Heavy-Atom Quenching 283
7.2.2 N, Elimination from Azo Compounds 387
5.4.5 Electronic Energy Transfer 287
7.2.3 Photofragmentation of Oligosilanes and Polysilanes 392
5.4.6 Kinetics of Bimolecular Photophysical Processes 297
7.3 Hydrogen Abstraction Reactions 395
5.5 Environmental Effects 301
7.3.1 Photoreductions 395
5.5.1 Photophysical Processes in Gases and in Condensed
7.3.2 The Norrish vpe I1 Reaction 399
Phases 301
' 7.4 Cycloadditions 404
5.5.2 Temperature Dependence of Photophysical Processes 302
7.4.1 Photodimerization of Olefins 404
5.5.3 Solvent Effects 303
7.4.2 Regiochemistry of Cycloaddition Reactions 41 1
Supplemental Reading 306
7.4.3 Cycloaddition Reactions of Aromatic Compounds 417
7.4.4 Photocycloadditions of the Carbonyl Group 424
7.4.5 Photocycloaddition Reactions of a,f iunsaturated Carbonyl
Compounds 433
6. Photochemical Reaction Models 309
7.5 Rearrangements 434
7.5.1 Electrocyclic Reactions 434
6.1 A Qualitative Physical Model for Photochemical Reactions
7.5.2 Sigmatropic Shifts 445
in Solution 309
7.5.3 Photoisomerization of Benzene 448
6.1.1 Electronic Excitation and Photophysical Processes 310
7.5.4 Di-n-methane Rearrangement 453
6.1.2 Reactions with and without Intermediates 313
7.5.5 Rearrangements of Unsaturated Carbonyl
6.1.3 "Hot" Reactions 320
Compounds 460
6.1.4 Diabatic and Adiabatic Reactions 322
6.1.5 Photochemical Variables 324 7.6 Miscellaneous Photoreactions 464
7.6.1 Electron-Transfer Reactions 464
6.2 Pericyclic Reactions 332
6.2.1 Tho Examples of Pericyclic Funnels 332 7.6.2 Photosubstitutions 474
6.2.2 Minima at Tight and Loose Geometries 339 7.6.3 Photooxidations with Singlet Oxygen 476
6.2.3 Exciplex Minima and Barriers 341 7.6.4 Chemiluminescence 480
6.2.4 Normal and Abnormal Orbital Crossings 344 Supplemental Reading 485
6.3 Nonconcerted Photoreactions 349
6.3.1 Potential Energy Surfaces for Nonconcerted Epilogue 491
Reactions 349 References 493
6.3.2 Salem Diagrams 355
Index 517
6.3.3 Topicity 356
Supplemental Reading 359
Notation
Operators
One-electron
Many-electron
vectors
Matrices
Wave functions
Electronic configuration
Electronic state
Nuclear
Vibronic
Orbitals
General
Atomic
Molecular
Spin orbital
Universal Constants
c, = 2.9979 x loi0c m/s speed of light in vacuum
e = -1.6022 x 10-I9C electron charge