Fundamentals of photoinduced electron transfer

185 Pages·1993·13.528 MB·English

by  George J Kavarnos

Fundamentals of
Photoinduced
Electron Transfer
by
George J. Kaoarnos
*
VCH
Dedicated to the memory
of my late brother
Preface
This book is printed on acid-free paper. @
Library of Congress Cataloging-in-Publication Data
Kavamos, George J.
Fundamentals of photoinduced electron transfer I by George J.
Kavamos.
p. cm. Although numerous review articles on photoinduced electron transfer have appeared
Includes index. in recent years, there is yet no pedagogical text expressly written for undergraduate
ISBN 0-89573-75 1-5 and graduate students. Fundamentals of Photoinduced Electron Transfer has been
1. Photochemistry. I. Title written for this readership, and is intended as an introductory text for students of
QD708.2.K38 1993
541.3'54~20 93-1291 chemistry, biology, physics, and material sciences, as well as readers with a general
CIP background in chemistry with an interest in the field. The book is meant as a primary
or secondary text in introductory and advanced photochemistry courses at the un-
dergraduate and graduate level. As a self-contained text that includes a brief intro-
O 1993 VCH Publishers, Inc. duction to the principles of photochemistry, this book can also be used in a course
This work is subject to copyright.
exclusively devoted to photoinduced electron transfer, or it can be used as a sup-
All rights reserved, whether the whole or part of the material is concerned, specifically those of
plementary source in chemistry and biology courses for advanced undergraduates
translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying
machine or similar means and storage in data banks. or beginning graduate students. Fundamentals of Photoinduced Electron Transfer
Registered names, trademarks, etc., used in this book, even when not specifically marked as such, requires, at the minimum, 2 or 3 years of college chemistry (general, organic, and
are not to be considered unprotected by law. physical). Accordingly, the material is within the grasp of junior or senior under-
graduates with no prior knowledge of photochemistry or electrochemistry.
Printed in the United States of America
ISBN 0-89573-751-5 VCH Publishers, Inc. The author has included material that should appeal to chemists, physicists, and
biologists who may want to familiarize themselves with this vital subject. The book
is not intended as a comprehensive review. For those requiring additional and
Printing History: specialized information, the review articles listed at the end of each chapter should
1 0 9 8 7 6 5 4 3 2
suffice. In writing on such a rapidly expanding field, it was virtually impossible to
include all of the excellent research that has been done. Consequently, it was found
Published jointly by
necessary to exclude several lines of research. An effort, however, was made to
VCH Publishers, Inc. VCH Verlagsgesellschaft mbH VCH Publishers (UK) Ltd. limit these omissions.
220 East 23rd Street P.O. Box 10 11 61 8 Wellington Court The material in the first few chapters is presented at a level to allow the stu-
New YO&. New York 10010 69451 w e ~ ~ i Cambridge CB 1 lfIZ
Rdcd Republic of Oamrny United Kingdom dent to become comfortable with pe subsequent subject matter. The first chapter
vi FUNDAMENTALS OF PHOTOINDUCED ELECTRON TRANSFER
introduces the reader to the basic principles and terminology of photochemistry.
Chapter 1 also introduces the student to the Weller equation and the role of ener-
getics. In Chapter 2, the properties of ion pairs and exciplexes are discussed as
well as experimental procedures to study them. Chapter 3 covers photoinduced
electron-transfer reactions of organic substrates; Chapter 4 deals with intramolecular Contents
and supramolecular photoinduced electron transfer. A number of topics are covered
in Chapter 5, including photoinduced electron transfer in organized environments,
photocatalysis with Semiconductors, solar energy capture and utilization, and pho-
toimaging. The last chapter is an introduction to classical and nonclassical theories
of electron transfer and is written at a level to help the student understand the
underpinnings of photoinduced electron transfer. The author has included a set of
problems at the end of each chapter. Answers to most of the problems can be found
in the references provided.
The author has received considerable support from several colleagues who over
the years have stimulated his interest in the subject matter of this book. Above all,
he owes a special note of gratitude to his mentor, Professor Nicholas Turro of
Columbia University, for his encouragement and inspiration. The author recalls
with appreciation that unique gift of Professor Turro to challenge his students by
asking the "right" questions. It was this "prodding" and "questioning" that helped Chapter I Introductory Concepts 1
motivate the author to think about certain issues in this field and eventually to write
1.1. Scope of Photoinduced Electron Transfer 1
this book. Warm thanks also go to Sister Claire Markham and Professor Harold
McKone of Saint Joseph College and to Professor Bruno Vittimberga of the Uni- 1.2. Review of Photochemical Principles and Definitions 2
1.3. An Overview of Photochemical and Photophysical
versity of Rhode Island, who generously gave of their time to read initial drafts of
Processes 14
the text. It is a distinct pleasure to recognize Dr. Gunter Grampp of the Institut fur
Physikalische und Theoretische Chemie of the University of Erlangen in Nuremberg 1.4. General Features of Quenching by Electron Transfer 18
for his perceptive comments. 1.5. Quantum Yields, Efficiencies, and Lifetimes 21
The author also wishes to acknowledge the assistance of several individuals who 1.6. Energetics of Photoinduced Electron Transfer 23
1.7. Photophysical and Electrochemical Properties of Electron
offered assistance during the long and tortuous ordeal of bringing this project to
Donors and Acceptors 40
fruition. The staff of VCH Publishers offered much guidance. for which the author
is most grateful. James Powers, Thomas Pantelis, Dr. Roger Richards, and the 1.8. Energy Transfer versus Electron Transfer 45
Gilardi's, both Chris and Steve, are thanked for help with the figures at a critical
time. The author extends his heartfelt thanks to his colleagues at the Naval Undersea Chapter 2 Properties of Charge-Transfer Intermediates in
Warfare Center in New London for their good-natured understanding and encour- Photoinduced Electron Transfer 53
agement. And, finally, the author acknowledges with deep gratitude the support of
2.1. The Nature of Charge-Transfer Intermediates 53
his mother and late father, who offered much to teach and educate him.
2.2. The Energetics of Exciplex and Ion-Pair Formation 61
2.3. Flash Spectroscopy 63
George J. Kavarnos
2.4. Photoconductivity 75
2.5. Pulse Radiolysis 78
New London
June 1993 2.6. Electron and Nuclear Spin 79
Chapter 3 Electron-Transfer Photochemistry 103
3.1. The Pathways of Photoinduced Electron-Transfer
Reactions 103
3.2. Examples 108
viii FUNDAMENTALS OF PHOTOINDUCED ELECTRON TRANSFER
C H A P T E R
Chapter 4 Intramolecular Photoinduced Electron
Transfer 185
4.1. Introduction 185
4.2. Through-Space and Through-Bond Coupling 187
Introductory Concepts
4.3. Photoinduced Electron Transfer in Bridged Metal-Ligand
Systems 189
4.4. Photoinduced Electron Transfer in Flexible Organic
Systems 194
4.5. Photoinduced Electron Transfer in Rigid Systems 210
4.6. Models for Photosynthesis: Photoinduced Electron Transfer
in Porphyrin-Electron Acceptor Pairs 2 13
4.7. Supramolecular Photoinduced Electron Transfer 225
4.8. Photoinduced Electron Transfer in Molecular Electronic
Devices 227
Chapter 5 Photoinduced Electron Transfer in Organized
Assemblies and the Solid State 235
5.1. Photoinduced Electron Transfer in Organized
Assemblies 235
5.2. Photoinduced Electron Transfer on Semiconductor 1.1. Scope of Photoinduced Electron Transfer
Surfaces 255
5.3. The Photocleavage of Water 268
Photoinduced electron transfer is the branch of photochemistry dealing with the
5.4. Photoinduced Electron Transfer in Imaging
property of certain photoexcited molecules to act as strong oxidizing or reducing
Applications 275
species. A photoexcited molecule is often a better electron donor or electron acceptor
than its ground state. The photoexcited species is an electron-transfer photosensitizer
Chapter 6 Theories of Photoinduced Electron in the sense that it induces or "sensitizes" permanent chemical changes in a neigh-
Transfer 287 boring ground-state molecule by an electron-transfer mechanism.
Photoinduced electron transfer plays a central role in many areas of science. The
6.1. Diffusion-Controlled and Electron-Transfer Rate
reader is undoubtedly aware of biological photosynthesis where sunlight is harnessed
Constants 288
for the growth and nourishment of plants. The early events in photosynthesis involve
6.2. Classical Theories of Electron Transfer 293
light absorption by an antenna system followed by a series of electron transfers.
6.3. The Inverted Region 3 18
These ultrafast electron-transfer processes are known as charge separations since
6.4. Dynamic Solvent Effects 322
they involve the development of a large separation of positive and negative charges
6.5. Quantum Mechanical Theories 324
within photosynthetic reaction centers. The maintenance of this charge separation
6.6 Applications of Classical and Quantum Mechanical Theories
is critical for ensuing biochemical reactions. By "mimicking" the light-harvesting
of Biological Photosynthesis 33 1
ability of green plants, chemists have attempted to duplicate the events in photo-
synthesis with model compounds. These models have been used as artificial pho-
Index 351 tosynthetic systems for harnessing solar energy. Since photoinduced electron trans-
fer is fundamental to these systems, much effort has been expended to understand
the principles of photoinduced electron transfer, with the ultimate goal of achieving
the efficiency and economy of natural light-harvesting systems.
A closely related area is the study of electron transfer between excited and
ground-state species embedded in proteins. This knowledge has contributed to
2 FUNDAMENTALS OF PHOTOINDUCED ELECTRON TRANSFER
our understanding of the key features of biological electron-transfer processes such Molecule
I
as oxidative phosphorylation. In proteins and other macromolecules such as DNA,
the transfer of electrons is often over large distances. In an effort to rationalize the
Light
factors influencing the rates of electron transfer in these systems, chemists have
attempted to modify biological macromolecules such as blue copper proteins, cy-
tochrome, and DNA by covalently attaching light-active electron acceptors and Excited Molecule
donors to these molecules. By exposing these light- and redox-active complexes to
short-lived bursts of light and analyzing the subsequent transformations, they have
learned much about the structural and environmental factors controlling the passage
of electrons over long distances.
Photoinduced electron transfer is of great interest to organic chemists concerned Decay Unimolecular Bimolecular
with the synthesis of novel organic compounds that may be difficult to synthesize
Pathways Photoreactions Photoreactions
by other routes. After being oxidized or reduced by a photosensitizer, an organic
substrate can be transformed to a reactive intermediate which may be capable of
I. Thermal I. Decompositions I. Photoadditions
undergoing further reaction. These photosensitized electron-transfer reactions often
culminate in the formation of stable products. 2. Luminescence 2. Rearrangements 2. Hydrogen abstraction
Photoinduced electron transfer also plays a key role in several emerging tech-
3. lsomerizations 3. Energy transfer
nologies. In semiconductor photocatalysis, for example, a light-exposed semicon-
I
ductor surface provides the environment for complex but often useful chemical 4. Bond cleavages 14. Electron transfer
transformations. Another area of interest concerns imaging systems such as silver
halide photography, spectral sensitization, and xerography. There have also been Figure 1.1 A classification of photochemical pathways.
proposals to design molecule-size electronic devices capable of writing and pro-
cessing information. Electron transport in molecular electronic devices might be
triggered by exposure to short-lived light pulses from a laser source. Strategies to chemical reactivity. To appreciate the role of excited states in photoinduced electron
design and synthesize such devices on the molecular scale are beginning to emerge transfer, it is desirable to review several basic photochemical concepts. This section
and hopefully will someday be successful. briefly surveys some features and properties of electronically excited states.
The ability to exploit the full potential of photoinduced electron transfer requires
an understanding of certain principles of photochemistry and electron transfer. This
1.2.1. Electronic Excitation
book is an introduction to these principles and is intended to complement courses
in photochemistry, biochemistry, and related courses. The reader of this book, it When ground-state molecules absorb visible or ultraviolet light, electrons in the
is assumed, has a background in organic and physical chemistry and has at least highest occupied orbitals undergo transitions to unoccupied orbitals lying at higher
some familiarity with basic concepts of molecular orbital theory, spectroscopy, and energies (Fig. 1.2). By absorbing a photon of light, the ground state is converted
chemical kinetics. into a higher energy state, or electronically excited state. hv is used to represent a
photon, which is the designation for a discrete quantum of light. The energy (E)
of the photon is given by
1.2. Review of Photochemical Principles and Definitions
All photochemical reactions start with the absorption of light (Fig. 1.1). Photoex- where h is Planck's constant, which is equal to 6.626 x erg-s quantum-'.
citation of ground-state species leads to formation of excited states. Excited states v, the frequency of the photon, is expressed as reciprocal seconds, or s-'. Occa-
differ in many respects from their respective ground states. To distinguish an excited sionally, the chemist needs to know the relationship between the excitation wave-
state from its ground state, chemists usually use a superscript asterisk placed to the length and the energy of the exciting photon. Equation 1.2 is a useful aid for this
left or right of the symbol for the ground state (i.e., M* is the excited state of purpose:
molecule M). Generally, excited states are more chemically reactive. In fact, the
unique property of some excited molecules to induce transitory or permanent changes
in neighboring molecules by an electron-transfer pathway is one example of their
FUNDAMENTALS OF PHOTOINDUCED ELECTRON TRANSFER mODUCTORY CONCEPTS
1.2.2. Electronic States of Organic Molecules
It is useful to consider electronic transitions within the framework of simple mo-
lecular orbital theory. Molecular orbitals can be regarded as linear combinations of
atomic orbitals of the atoms which comprise the molecule. For purposes of this
discussion, several atomic orbitals are depicted in Fig. 1.3. When two atomic
orbitals merge to form a molecular orbital, "bonding" and "antibonding" molecular
orbitals are formed. The bonding molecular orbital is formed by the combination
+ of orbitals of the same wave-like character, whereas the antibonding molecular
u
orbital is formed by two atomic orbitals of opposite wave-like character (Fig. 1.4).
Ground state Excited state
Figure 1.2 Photoexcitation results in an electronic transition.
where c is the velocity of light or 3 X 10'' cm s-', and A is the wavelength in
centimeters. The wavelength of light is usually given as nanometers (1 nm = lC7
cm); it can also be expressed by its frequency (Eq. 1.2) or by its reciprocal wave
number, ij = A-'. The energy associated with hv is usually given in units of
kilocalories or kilojoules per mole. To convert between the various units of energy,
the reader may want to refer to Appendix I.
The tightly held electrons in the lower energy orbitals, such as the "core" Is
orbitals, are normally not perturbed by the absorption of light. However, electrons
in the higher lying orbitals are susceptible to light excitation in the spectral regions
in the far ultraviolet, near ultraviolet, and visible regions of the electromagnetic
spectrum. These regions range, respectively, from about 100 to 250, 250 to 350,
and 350 to 700 nm (Table 1.1).
Table 1.1 ELECTRONIC ABSORF'TION MAXIMA OF SELECTED COMPOUNDSa
Compound Transition Am (nm)
Ethylene
lo, 2.' and 3' amines
Benzene
Naphthalene
Carbonyl compounds
Figure 1.3 The shapes of s, p and d orbitals used to construct bonding orbitals important
in photochemistry.
bpy. 2.2'-bipyridine; dicp, 1.3-diisocyampropane.
FUNDAMENTALS OF PHOTOINDUCED ELECTRON TRANSFER INTRODUCTORY CONCEPTS
C C-C C
Figure 1.5 Molecular orbital diagram of a carbon-carbon a-bond.
excited to the antibonding a*-orbital (Fig. 1.4). The promotion of the "bonding"
electron into the higher energy orbital is referred to as a a + a* transition. The
Figure 1.4 An orbital diagram showing the energy ordering of the most common types of
newly formed state is designated a a,a* excited state. Since an electron now
electronic transitions in organic molecules.
occupies an antibonding orbital, there is less "bonding" character in the a,u* excited
state. a + a* transitions require a large amount of energy, usually about 200 kcal
mol-' .
On a vertical energy scale, the antibonding orbital lies at a higher energy than the
bonding orbital. Electrons residing within the atomic orbitals will populate the In another type of electron transition, the n + T* transition, an electron from
a bonding n-orbital is promoted to an antibonding n*-orbital (Fig. 1.4). n-orbitals,
newly formed molecular orbitals, the maximum number of electrons in one mo-
which are formed by the interaction of two 2p-orbitals (Fig. 1.6), are found in
lecular orbital being two. To complete the molecular orbital representation, we
unsaturated organic molecules. In contrast to the electrons occupying a-orbitals,
must take into account the spins of the two electrons. We use directional arrows
+ 5. the electrons populating n-orbitals are more delocalized. Also, the overlap between
to designate spin. f represents 112 spin; is used for - 112 spin. The two
the 2p-orbitals is less than the overlap between the s atomic orbitals comprising
electrons populating the same orbital are depicted with two antiparallel direction
arrows, i.e., #. This is done because two electrons occupying the same orbital the a-bond. Therefore, the strength of a n-bond is less than that of a a-bond. As
compared to the a-orbitals, which are separated by a large energy gap, the gap
must have their spins oriented in opposite directions (a consequence of the Pauli
between n-orbitals is smaller since the energy of the bonding n-orbital is raised
principle).
relative to the bonding a-orbital, and the energy of the matching n*-orbital is
Let us now consider electronic transitions in simple organic molecules. In the
lowered with respect to the a*-orbital. As a result, photon excitation of an electron
formation of a carbon-carbon single bond, 2s orbitals of two carbon atoms combine
in the bonding n-orbital to the antibonding n*-orbital requires less energy than a
to form a bonding a-orbital and antibonding a*-orbital (Fig. 1.5). The two electrons
enter the a-orbital leading to a stabilized electronic configuration. The a-orbital is a + a* transition.
A third type of transition is found in organic molecules containing the carbonyl
symmetric around the molecular axis. The electron density is greatest between the
group such as ketones, aldehydes, esters, and carboxylic acids, and is known as
two nuclei. On interaction with a photon, an electron in the bonding a-orbital is
FUNDAMENTALS OF PHOTOINDUCED ELECTRON TRANSFER INTRODUCTORY CONCEPTS
Figure 1.6 Molecular orbital diagram of an unsaturated carbon-carbon bond. Figure 1.7 Molecular orbital diagram of a carbonyl group.
the n +-I T*t ransition (Fig. 1.4). The molecular orbitals of the carbonyl group are shall note in a later section that we can also describe the ability of the excited state
constructed by combinations of carbon and oxygen atomic orbitals (Fig. 1.7). In to accept or donate electrons by invoking simple orbital representations.
oxygen, the 2p- and 2s-orbitals are hybridized into two linear sp-orbitals. One
orbital combines with the carbon sp2-orbital to form a a-bond; the other orbital is
1.2.3. Electronic States of Transition Metal Complexes
directed away from oxygen colinear with the axis of the molecule. An oxygen 2p-
orbital combines with a carbon 2p-orbital to generate a w-bond. The remaining In constructing the electronic configurations of inorganic complexes, it is useful to
oxygen 2p-orbital is designated an n-orbital, containing two "lone pair," or "non- consider an octahedral complex, a prototype of many transition metal complexes.
bonding" electrons. The n-orbital does not participate in bonding. Since tbe An octahedral complex consists of a transition metal bonded to six groups or ligands
n-orbital occupies a position higher in energy than the a- and w-bonds, the n -+ T* positioned at the comers of an octahedron (Fig. 1.8). In the ground states of
transition to create an n,n* excited state usually involves less energy than the octahedral complexes, the "ligand molecular orbitals are completely occupied by
IT + IT*t ransition (Fig. 1.4), although in some carbonyl compounds, the IT-orbital electrons. In contrast, the metal orbitals may or may not be completely filled. The
may lie at a higher energy than the n-orbital. These differences in energy ordering molecular orbitals in octahedral complexes are made up of s, p, and d-orbitals of
may be due to the structure of the molecules or to the effects of solvent on stabilizing the metals and ligands. Figure 1.9 depicts the combination of these orbitals to form
the electronic structure (the reader is urged to consult textbooks on photochemistry form a- and IT-bonds [the subscripts in Fig. 1.9 signify whether the major contri-
for more discussion on this topic). bution to an orbital is from a metal (M) orbital or ligand (L) orbital]. These
A fourth type of transition involves the promotion of an n-electron to an anti- combinations are influenced by the symmetries of the orbitals. Metal d-orbitals
bonding a*-orbital. These transitions can take place in compounds containing het- (Fig. 1.3) belong to two symmetry sets known by their spectroscopic designations
eroatoms such as aliphatic arnines, alcohols, and halogens. as t2, and e,. From a comparison of the geometries of the d-orbitals in Fig. 1.3
In summary, the most common types of electronic transitions in organic mole- with the directional drawing of the octahedral complex in Fig. 1.8, we can draw
cules are represented by the formation of n,.rr*, w,w*, n,a*, and a,a* excited certain conclusions concerning the orbital combinations which result in bonding.
states. Although these transitions have been visualized here in terms of simple d,,-, d,,-, and d,-orbitals should be particularly suited for bonding with p-orbitals
molecular orbital theory, this approach conveys the spirit of orbital occupancy. We of ligands to form m-bonds; these d-orbitals belong to the t2, symmetry set. On the
FUNDAMENTALS OF PHOTOINDUCED ELECTRON TRANSFER INTRODUCTORY CONCEFTS
Figure 1.8 In an octahedral complex, a central atom is surrounded by six ligands situated
at the comers of an octahedron.
other hand, the d,z- and ds-,z-orbitals are directed along the x, y, and z axes in
octahedral complexes and combine with s-orbitals of ligands to generate u-bonds.
The interaction of negative ligands with metal d-orbitals causes the energies of the
latter to split. Since the doubly degenerate orbitals in the e, set lie along the in-
bond axis of the metal-ligand bond, these orbitals experience more destabilization
than the triply degenerate tzg orbitals.
The electronic transitions for an octahedral complex are summarized in Fig.
1.10. These transitions may simply involve the electrons in metal-centered molecular
orbitals. These are called metal-centered transitions and usually involved d + d
transitions from tzg- to eg-orbitals. Transitions taking place in molecular orbitals
with predominantly ligand character are ligand-centered transitions. Typical ex-
amples are IT -+ IT* transitions of electrons localized in the ligands. Electron
promotions from metal-centered to ligand-centered orbitals, or vice versa, are li-
gand-to-metal charge transfer (LMCT) or metal-to-ligand charge-transfer (MLCT)
i
dtr aenlesicttiroonns ,i nr eas ptzegct oivrbeiltya.l Ais dp r-o+m oITt*e dc hinatrog ea-nt raanntsifbeor ntrdainnsgi tliiogna ntdak oersb pitlaalc. e when a Metal Octahedral L i gand
Metal-centered transitions in metal complexes may also involve spin flips. If the comp l ex
excitation energy corresponds to the energy required for a spin flip, a spin reversal
may take place with subsequent spin pairing of two electrons. Figure 1.9 Molecular orbital diagram of an octahedral complex.
1.2.4. State Descriptions
A state description of any molecule can be constructed from a knowledge of its
A state description is a shorthand description containing essential information of electronic configuration. In ground-state molecules, all electrons occupy the lowest
the electronic configuration of a molecular species. A state may be described using energy molecular orbitals. The spins of the electrons in each orbital are paired (i.e.,
spectroscopic nomenclature or, in the case of complicated molecules, by valence the directional component of the spin of one electron is oriented in an antiparallel
orbitals. These descriptions provide information on the energy ranking of the mol- direction with respect to the spin of the other). This spin configuration places
ecule and the net electronic spin. the molecule at the lowest point or "ground state" of the vertical energy scale.