ELECTROCHEMISTRY
OF FUNCTIONAL
SUPRAMOLECULAR
SYSTEMS
Edited by

Paola Ceroni
Alberto Credi
Margherita Venturi

The Wiley Series on Electrocatalysis and Electrochemistry

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ELECTROCHEMISTRY
OF FUNCTIONAL
SUPRAMOLECULAR
SYSTEMS

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WILEY SERIES ON ELECTROCATALYSIS
AND ELECTROCHEMISTRY
Andrzej Wieckowski, Series Editor

Fuel Cell Catalysis: A Surface Science Approach, Edited by Marc T. M. Koper
Electrochemistry of Functional Supramolecular Systems, Paola Ceroni, Alberto
Credi, and Margherita Venturi
Catalysis in Electrochemistry: From Fundamentals to Strategies for Fuel Cell
Development, Elizabeth Santos and Wolfgang Schmickler
Fuel Cell Science: Theory, Fundamentals, and Biocatalysis, Andrzej Wieckowski
and Jens Norskov

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ELECTROCHEMISTRY
OF FUNCTIONAL
SUPRAMOLECULAR
SYSTEMS
Edited by

Paola Ceroni
Alberto Credi
Margherita Venturi

The Wiley Series on Electrocatalysis and Electrochemistry

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Copyright Ó 2010 by John Wiley & Sons, Inc. All rights reserved.
Published by John Wiley & Sons, Inc., Hoboken, New Jersey
Published simultaneously in Canada
No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by

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Library of Congress Cataloging-in-Publication Data:
ISBN 9780470255575
Printed in the United States of America
10 9 8

7 6 5 4

3 2 1

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CONTENTS

Preface to the Wiley Series on Electrocatalysis
and Electrochemistry

vii

Foreword

ix

Preface

xi

Contributors

xiii

1. Electrochemically Controlled H-Bonding

1

Diane K. Smith

2. Molecular Motions Driven by Transition Metal
Redox Couples: Ion Translocation and Assembling–
Disassembling of Dinuclear Double-Strand Helicates


33

Valeria Amendola and Luigi Fabbrizzi

3. Molecular Encapsulation of Redox-Active Guests

59

Angel E. Kaifer

4. Dendritic Encapsulation of Redox-Active Units

87

Christopher B. Gorman

5. Redox-Active Metal–Polypyridine Dendrimers
as Light-Harvesting Antennae

121

Fausto Puntoriero, Scolastica Serroni, Francesco Nastasi, and
Sebastiano Campagna

6. Dendrimers as Multielectron Storage Devices

145

Paola Ceroni and Margherita Venturi


7. Self-assembled Monolayers and Multilayers
of Electroactive Thiols

185

Ibrahim Yildiz, Fran¸cisco M. Raymo and Massimiliano Lamberto

v

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vi

CONTENTS

8. Electrochemistry of Carbon Nanoparticles

201

Luis Echegoyen, Amit Palkar, and Frederic Melin

9. Molecular Devices Based on Fullerenes and Carbon Nanotubes

229

Matteo Iurlo, Demis Paolucci, Massimo Marcaccio, and Francesco Paolucci

10. Functional Electroactive Biomolecules


261

Xiaomin Bin, Piotr Michal Diakowski, Kagan Kerman, Heinz-Bernhard Kraatz

11. Functional Nanoparticles as Catalysts and Sensors

301

Brian J. Jordan, Chandramouleeswaran Subramani, and Vincent M. Rotello

12. Biohybrid Electrochemical Devices

333

Ran Tel-Vered, Bilha Willner, and Itamar Willner

13. Electroactive Rotaxanes and Catenanes

377

Alberto Credi and Margherita Venturi

14. Electrochemically Driven Molecular Machines Based
on Transition-metal Complexed Catenanes and Rotaxanes

425

Jean-Paul Collin, Fabien Durola, and Jean-Pierre Sauvage


15. Electroactive Molecules and Supramolecules
for Information Processing and Storage

447

Guanxin Zhang, Deqing Zhang, and Daoben Zhu

16. Electrochemiluminescent Systems as Devices and Sensors

477

Andrzej Kapturkiewicz

17. Recent Developments in the Design of Dye-Sensitized Solar
Cell Components

523

Stefano Caramori and Carlo Alberto Bignozzi

Index

581

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PREFACE to the Wiley Series on Electrocatalysis
and Electrochemistry


This series covers recent advances in electrocatalysis and electrochemistry and
depicts prospects for their contribution to the present and future of the industrial
world. It illustrates the transition of electrochemical sciences from a solid chapter of
physical electrochemistry (covering mainly electron transfer reactions, concepts of
electrode potentials, and structure of the electrical double layer) to the field in which
electrochemical reactivity is shown as a unique chapter of heterogeneous catalysis, is
supported by high-level theory, connects to other areas of science, and includes focus
on electrode surface structure, reaction environment, and interfacial spectroscopy.
The scope of this series ranges from electrocatalysis (practice, theory, relevance to
fuel cell science and technology) to electrochemical charge transfer reactions,
biocatalysis, and photoelectrochemistry. While individual volumes may look quite
diverse, the series promises updated and overall synergistic reports on insights into
further the understanding of properties of electrified solid/liquid systems. Readers of
the series will also find strong reference to theoretical approaches for predicting
electrocatalytic reactivity by such high-level theories as DFT. Beyond the theoretical
perspective, further vehicles for growth are the sound experimental background and
demonstration of significance of such topics as energy storage, syntheses of catalytic
materials via rational design, nanometer-scale technologies, prospects in electrosynthesis, new instrumentation, surface modifications in basic research on charge
transfer, and related interfacial reactivity. In this context, readers will notice that new
methods that are being developed for a specific field may be readily adapted for
application in others.
Electrochemistry has benefited from numerous monographs and review articles
due to its unique character and significance in the practical world (including
electroanalysis). Electrocatalysis has also been the subject of individual reviews and
compilations. The Wiley Series on Electrocatalysis and Electrochemistry is dedicated
to the current activity by focusing each volume on a specific topic of choice. The
chapters also demonstrate electrochemistry’s connections to other areas of chemistry
and physics, such as biochemistry, chemical engineering, quantum mechanics,
chemical physics, surface science, and biology, and illustrate the wide range of
literature that each topic contains. While the title of each volume informs of the

specific focus chosen by the volume editors and chapter authors, the integral outcome
offers a broad-based analysis of the total development of the field. The progress of the
series will provide a global definition of what electrocatalysis and electrochemistry
are concerned with now and how they evolve with time. The purpose is manifold,
vii

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viii

PREFACE TO THE WILEY SERIES ON ELECTROCATALYSIS AND ELECTROCHEMISTRY

mainly to provide a modern reference for graduate instruction and for active
researchers in the two disciplines, as well as to document that electrocatalysis and
electrochemistry are dynamic fields that expand rapidly and likewise rapidly change
in their scientific profiles.
Creation of each volume required the editor’s involvement, vision, enthusiasm, and
time. The Series Editor thanks all Volume Editors who graciously accepted his
invitations. Special thanks are for Ms. Anita Lekhwani, the Series Acquisition Editor,
who extended the invitation to the Series Editor and is a wonderful help in the Series
assembling process.
ANDRZEJ WIECKOWSKI
Series Editor

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FOREWORD


Like the currently popular area, called “nanoscience”, the field of “supramolecular
chemistry” has rather hazy boundaries. Indeed, both areas now share much common
ground in terms of the types of systems that are considered. From the beginning,
electrochemistry, which provides a powerful complement to spectroscopic techniques, has played an important role in characterizing such systems and this very useful
book goes considerably beyond the volume on this same topic by Kaifer and Go´mezKaifer that was published about 10 years ago. Some of the “classic” supramolecular
chemistry topics such as rotaxanes, catenanes, host–guest interactions, dendrimers,
and self-assembled monolayers remain, but now with important extensions into the
realms of fullerenes, carbon nanotubes, and biomolecules, like DNA.
These topics lead to considerations of supramolecular devices, for example for use
as sensors, and to molecular machines. Not only is electrochemistry an excellent way
of characterizing such systems, for example, via cyclic voltammetry, but in the world
of molecular machines, it is also the most straightforward approach to providing the
energy to power such devices. These topics then naturally lead to consideration of the
conversion of electricity to light (electrochemiluminescence) and light to electricity
(dye-sensitized solar cells) via electrochemical devices. While the latter are not
fundamentally supramolecular systems, their design could certainly benefit from the
considerations in the very detailed and authoritative treatments in this volume. The
idea of integrated chemical systems, based on nanoscience and nanotechnology, was
proposed a little over 20 years ago and was the subject of my 1994 monograph, but, so
far, few such systems have reached widespread practical utilization. Nevertheless, the
principles of such systems, for example for synthesis, analysis, and perhaps computation remain of interest, and supramolecular electrochemistry can play a major role in
their development. I hope this important volume will go a long way toward
introducing such principles to a wide audience, especially to the young people who
are less burdened by impressions of what is impossible.
The University of Texas at Austin

Allen J. Bard

ix


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PREFACE

Supramolecular chemistry is a highly interdisciplinary field that has been developed
at an astonishingly fast rate during the last three decades. In a historical perspective, as
pointed out by Jean-Marie Lehn, supramolecular chemistry originated from Paul
Ehrlich’s receptor idea, Alfred Werner’s coordination chemistry, and Emil Fischer’s
lock-and-key image. It was only after 1970, however, that fundamental concepts such
as molecular recognition, preorganization, self-assembly, and self-organization were
introduced to chemistry; supramolecular chemistry then began to emerge as a welldefined discipline and was consecrated by the award of the Nobel Prize in Chemistry
to Charles Pedersen, Donald Cram, and Jean-Marie Lehn in 1987.
Supramolecular chemistry, according to its most popular definition, is “the
chemistry beyond the molecule, bearing on organized entities of higher complexity
that result from the association of two or more chemical species held together by
intermolecular forces.” As the field developed, it became evident that a definition
strictly based on the nature of the bond that links the components would be limiting.
Many scientists, therefore, started to distinguish between what is molecular and what
is supramolecular based on the degree of intercomponent interactions. In a general
sense, one can say that with supramolecular chemistry there has been a shift in focus
from molecules to molecular assemblies or multicomponent structures driven by the
emergence of new functions.
In the frame of research on supramolecular systems, the idea began to arise in a few
laboratories that the concepts of “device” and “machine” could be applied at the
molecular level. In other words, molecules might be used as building blocks for the
assembly of multicomponent structures exhibiting novel and complex functions that

arise from the cooperation of simpler functions performed by each component. This
strategy, encouraged by a better understanding of biomolecular devices, has been
implemented on a wide variety of chemical systems, leading to highly interesting
results. As a matter of fact, the molecular bottom-up construction of nanoscale devices
and machines has become one of the most stimulating challenges of nanoscience.
Such achievements have been made possible because of the substantial progresses
obtained in other areas of chemistry and physics—particularly concerning the
synthesis and characterization of complex chemical systems, and the study of surfaces
and interfaces. In this perspective, electrochemistry is a very powerful tool not only
for characterizing a supramolecular system, but also for operating the device. Indeed,
molecular devices, as their macroscopic counterparts, need energy to operate and
signals to communicate with the operator. Electrochemistry can be an interesting
xi

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xii

PREFACE

answer to this dual requirement: it can be used to supply the energy needed to make the
system work, and, by means of the various electrochemical techniques (e.g.,
voltammetry), it can also be used to “read” the state of the system, controlling and
monitoring the operation performed by the device. Furthermore, electrodes represent
one of the best ways to interface molecular-level systems to the macroscopic world, a
feature that is important for future applications. Hence, it is not surprising that the
marriage of electrochemistry and supramolecular chemistry has produced a wealth of
very interesting devices and functions, thereby generating new scientific knowledge
and raising expectations for practical applications in energy conversion, information

and communication technologies, advanced materials, diagnostics, and medicine.
Our aim with this book is to provide the reader with an overview of current
electrochemical research applied to multicomponent chemical systems, with particular attention to properties and functions, and to strengthen the contacts between the
electrochemical community and the researchers engaged in the field of nanoscience.
Although the text covers a wide range of topics with contributions from leading
authorities in their respective fields, it does not even attempt to be a comprehensive
book on supramolecular electrochemistry. Rather, we would like to give the reader a
flavor of the level of creativity and ingenuity reached by scientists working in this
area. We hope that the book will be useful as a reference not only for experienced
researchers, but also for graduate students and postdoctoral fellows who are interested
in exploring electrochemistry at its frontiers with supramolecular chemistry, materials
science, and biochemistry. It may also be a useful complement for students attending
nanoscience and nanotechnology courses.
The 17 chapters of the book are not grouped in sections but they are somehow
logically ordered. The initial contributions, describing basic science investigations on
systems in solution, are followed by chapters dealing with less conventional multicomponent architectures and/or environments. The final part contains contributions
on devices and systems of high complexity and/or applicative interest. Although the
book does not include introductory or tutorial sections, most chapters begin with a
discussion of the basic concepts that are relevant for the presented topics. We hope that
these sections will make the book comprehensible also to nonspecialists.
We would like to express our gratitude to the distinguished colleagues and friends
who contributed the chapters: their commitment is indeed a fundamental ingredient of
this initiative. We also thank people at Wiley for their assistance during the various
phases of the editorial work. Finally, we would like to thank our families because their
love and patience are an invaluable support for our professional activity.
PAOLA CERONI
ALBERTO CREDI
MARGHERITA VENTURI

Bologna, June 2009


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CONTRIBUTORS

Valeria Amendola, Dipartimento di Chimica Generale, Universita di Pavia, Pavia,
Italy
Carlo Alberto Bignozzi, Dipartimento di Chimica, Universita di Ferrara, Ferrara,
Italy
Xiaomin Bin, Department of Chemistry, The University of Western Ontario,
London, Ontario, Canada
Sebastiano Campagna, Dipartimento di Chimica Inorganica, Chimica Analitica e
Chimica Fisica, Universit
a di Messina, Messina, Italy
Stefano Caramori, Dipartimento di Chimica, Universita di Ferrara, Ferrara, Italy
Paola Ceroni, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum
Universit
a di Bologna, Bologna, Italy
Jean-Paul Collin, Laboratoire de Chimie Organo-Minerale, UMR 7177 du CNRS,
Faculte de Chimie, Universite de Strasbourg, Strasbourg Cedex, France
Alberto Credi, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum,
Universit
a di Bologna, Bologna, Italy
Piotr Michal Diakowski, Department of Chemistry, The University of Western
Ontario, London, Ontario, Canada
Fabien Durola, Laboratoire de Chimie Organo-Minerale, UMR 7177 du CNRS,
Faculte de Chimie, Universite de Strasbourg, Strasbourg Cedex, France
Luis Echegoyen, Department of Chemistry, Clemson University, Clemson, SC,
USA

Luigi Fabbrizzi, Dipartimento di Chimica Generale, Universita di Pavia, Pavia,
Italy
Christopher B. Gorman, Department of Chemistry, North Carolina State
University, Raleigh, NC, USA
Matteo Iurlo, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum,
Universit
a di Bologna, Bologna, Italy
Brian J. Jordan, Department of Chemistry, University of Massachusetts, Amherst,
MA, USA
xiii

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xiv

CONTRIBUTORS

Angel E. Kaifer, Center for Supramolecular Science, Department of Chemistry,
University of Miami, Coral Gables, FL, USA
Andrzej Kapturkiewicz, Institute of Physical Chemistry, Polish Academy of
Sciences, Warsaw, Poland and Institute of Chemistry, University of Podlasie,
Siedlce, Poland
Kagan Kerman, Department of Chemistry, The University of Western Ontario,
London, Ontario, Canada
Heinz-Bernhard Kraatz, Department of Chemistry, The University of Western
Ontario, London, Ontario, Canada
Massimiliano Lamberto, Department of Chemistry, Medical Technology and
Physics, Monmouth University, West Long Branch, NJ, USA
Massimo Marcaccio, Dipartimento di Chimica “G. Ciamician”, Alma Mater

Studiorum, Universit
a di Bologna, Bologna, Italy
Frederic Melin, Department of Chemistry, Clemson University, Clemson, SC, USA
Francesco Nastasi, Dipartimento di Chimica Inorganica, Chimica Analitica e
Chimica Fisica, Universit
a di Messina, Messina, Italy
Amit Palkar, Department of Chemistry, Clemson University, Clemson, SC, USA
Demis Paolucci, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum,
Universit
a di Bologna, Bologna, Italy
Francesco Paolucci, Dipartimento di Chimica “G. Ciamician”, Alma Mater Studiorum, Universit
a di Bologna, Bologna, Italy
Fausto Puntoriero, Dipartimento di Chimica Inorganica, Chimica Analitica e
Chimica Fisica, Universit
a di Messina, Messina, Italy
Fran¸cisco M. Raymo, Department of Chemistry, University of Miami, Coral
Gables, FL, USA
Vincent M. Rotello, Department of Chemistry, University of Massachusetts,
Amherst, MA, USA
Jean-Pierre Sauvage, Laboratoire de Chimie Organo-Minerale, UMR 7177
du CNRS, Faculte de Chimie, Universite de Strasbourg, Strasbourg Cedex,
France
Scolastica Serroni, Dipartimento di Chimica Inorganica, Chimica Analitica e
Chimica Fisica, Universit
a di Messina, Messina, Italy
Diane K. Smith, Department of Chemistry and Biochemistry, San Diego State
University, San Diego, CA, USA
Chandramouleeswaran Subramani, Department of Chemistry, University of
Massachusetts, Amherst, MA, USA


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CONTRIBUTORS

xv

Margherita Venturi, Dipartimento di Chimica “G. Ciamician”, Alma Mater
Studiorum, Universit
a di Bologna, Bologna, Italy
Ran Tel-Vered, Institute of Chemistry, The Hebrew University of Jerusalem,
Jerusalem, Israel
Bilha Willner, Institute of Chemistry, The Hebrew University of Jerusalem,
Jerusalem, Israel
Itamar Willner, Institute of Chemistry, The Hebrew University of Jerusalem,
Jerusalem, Israel
Ibrahim Yildiz, Department of Chemistry, University of Miami, Coral Gables,
FL, USA
Deqing Zhang, Beijing National Laboratory for Molecular Sciences, Organic Solids
Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China
Guanxin Zhang, Beijing National Laboratory for Molecular Sciences, Organic
Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing,
China
Daoben Zhu, Beijing National Laboratory for Molecular Sciences, Organic Solids
Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China

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CHAPTER 1

Electrochemically Controlled
H-Bonding
DIANE K. SMITH
Department of Chemistry and Biochemistry, San Diego State University,
San Diego, CA, USA

1.1

INTRODUCTION

Due to their strength and directionality, hydrogen bonds are one of the most important
and useful types of intermolecular interactions available for the construction of
supramolecular complexes. The iconic examples of DNA base pairing and the
formation of secondary structure in proteins provide ample proof of their utility for
the assembly of well-defined, functional structures. Examples1 of the use of hydrogen
bonds in synthetic, solution-phase supramolecular chemistry range from H-bonded
dimers2 held together by up to 6 H-bonds3 to large “rosette” assemblies constructed
from up to 15 components and 72 H-bonds.4 A wide variety of open H-bonded
structures have also been prepared, including those that self-assemble into capsules of
various sizes and shapes5,6 and cyclic peptides that assemble into hollow tubes.7
Although, from a purely chemical point of view, learning how to create these
complicated supramolecular structures has its own value, there are plenty of more
practical reasons to investigate this chemistry. In the short term, these include
catalysis and sensor applications, and in the long term, molecular electronics and
molecular machines. With perhaps the exception of catalysis, all these applications
will require some sort of signal transduction to allow for communication with the

supramolecular device. This, of course, is one of the main reasons that electrochemistry is useful for supramolecular chemistry. Electron transfer provides a wellunderstood and very sensitive method to both communicate with supramolecular
assemblies and control their structure.8
However, although electrochemistry can be used for the above, it will do so only if
this functionality is designed into the structure. At minimum, two requirements
Electrochemistry of Functional Supramolecular Systems. Edited by Paola Ceroni, Alberto Credi,
and Margherita Venturi
Copyright Ó 2010 John Wiley & Sons, Inc.

1

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2

ELECTROCHEMICALLY CONTROLLED H-BONDING

must be met. First, a reversible redox couple must be present as part of the
structure. Reversible in this context means that both oxidation states are chemically
stable under the experimental conditions used and that the electron transfer kinetics
are reasonably fast. Second, reduction or oxidation of the redox couple must
significantly perturb the strength of important binding interactions holding
the assembly together. The most straightforward type of interaction to perturb
electrochemically are ion–ion interactions, but the electrostatic nature of a hydrogen
bond makes it a close second, while also allowing for neutral molecules as binding
partners.
In this chapter, the basic principles behind electrochemically controlled
H-bonding will first be described, along with some simple, illustrative examples
and a brief discussion of the use of cyclic voltammetry to characterize such systems.
Next, some general considerations regarding the design of these systems are

discussed: the properties of the redox couple, the structures of host and guest, the
choice of solvent and electrolyte, and the possibility for proton transfer. Finally, a
selection of electrochemically controlled H-bonding systems will be described,
organized by the nature of the binding partner and the type of redox couple.

1.2

BASIC PRINCIPLES

A H-bond is a favorable interaction formed between a relatively positively charged
hydrogen atom in a polar bond, XÀH, and either a lone pair on a relatively negatively
charged atom, Y (Eq. 1.1), or a highly polarizable pi-bonding electron pair (Eq. 1.2).9
As this description implies, there is a high degree of electrostatic character associated
with H-bonding, although only the very weakest H-bonds are purely electrostatic.
Strong H-bonds range in strength from 15 to 40 kcal/mol and are considered to be
mainly covalent in character. These bonds are characterized by close to linear
XÀHÀY bond angles and XÀY distances that are substantially smaller than the
sum of the van der Waals radii. These are typically formed from ionic species, for
example, N ỵ H or O ỵ H as the H-donor and/or FÀ, OÀ, or NÀ as the H-accepting
atom. Moderate strength H-bonds range in strength from 4 to 15 kcal/mol and are
mostly electrostatic in character. They can show a greater range in bond angles, from
130 to 180 , and will have bond lengths that are slightly smaller than the sum of the
van der Waals radii. Typical examples are H-bonds formed from neutral oxygen and
nitrogen functional groups, for example, NÀH and OÀH as the donor group and
uncharged N and O as the accepting atoms. Weak H-bonds, those less than 4 kcal/mol
in strength, are almost purely electrostatic in character and are characterized by a
range of bond angles and XÀY distances that may be greater than the sum of the van
der Waals radii. These are formed from the weaker H-donors such as CÀH and/or
weaker acceptors such as pi electron pairs.
+


X H

+

+

Y R

X H

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Y R

ð1:1Þ


1.2

+

+

X H

1.2.1

+


+

R Y Y R

X H

3

BASIC PRINCIPLES

R
Y
Y
R

ð1:2Þ

Direct Perturbation of Hydrogen Bonds

Although, from the above discussion, H-bonds are generally not purely electrostatic,
a favorable electrostatic interaction plays an important role in all types of H-bonds.
This provides a very simple way to think about how to use electron transfer to directly
perturb the strength of H-bonds. There are two main ways to do this as shown in
Scheme 1.1. The first is to make a H-acceptor a better acceptor by using a reduction
reaction to increase the negative charge on the H-accepting atom (Scheme 1.1a).
The second is to make the H-donor a better donor by using oxidation to increase
the positive charge on a H-donating functional group (Scheme 1.1b). Alternatively,
electron transfer can be used to weaken H-bonds through the opposite effects, making
a H-donor a weaker donor through reduction or making a H-acceptor a weaker
acceptor through oxidation.

A simple example of reduction-based, electrochemically controlled H-bonding is
provided by nitrobenzenes.10 Despite the standard Lewis structure that places a
formal negative charge on one of the oxygens and a positive charge on the nitrogen,
nitro groups are generally weak H-acceptors in solution because the NÀO bond is
not very polar. However, reduction of an aromatic nitro compound to its radical anion
greatly increases the negative charge on the oxygens, resulting in much stronger
H-bonding to a H-donating guest. In the case of nitroaniline, 1 (X ¼ NH2), with
1,3-diphenylurea, 2 (Eq. 1.3), the equilibrium constant for H-bonding in 0.1 M
NBu4PF6/DMF goes from <1 in the zero state to 8 Â 104 MÀ1 in the radical anion
state.

O
N
O

X

O
N
O

+ e–
X

O
N
O

+2
X


1

Ph
H N
O
H N
2 Ph

ð1:3Þ

Y
(a)

(b)

+
X H

+

+ e–

Y

– e–

X H

HX


HX

YR

YR

Scheme 1.1 Methods to increase H-bond strength using redox reactions.

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4

ELECTROCHEMICALLY CONTROLLED H-BONDING

Oxidation reactions can also be used to control H-bonding. These have mainly been
successful with anionic guests, but a recent example involving the dimethylaminophenylurea 3 gives a large binding enhancement with the cyclic diamide 4 (Eq. 1.4).11 In this
case, reversible oxidation of the dimethylaminophenyl group increases the positive
charge on one of the urea NHs, greatly increasing its H-donating ability to a good
H-accepting guest such as 4. This results in the binding constant in 0.1 M NBu4B(C6F5)4/
CH2Cl2 increasing from 60 MÀ1 in the reduced state to 2 Â 105 MÀ1 in the oxidized state.
Me
Me N

Me
Me N

N H
O


N H

– e–

+4

O
Ph
3

N H

O

N H

O

Me
N

ð1:4Þ

O

N H

1.2.2


Me
Me N

N H
Ph

N
Me

Ph
4

Indirect Perturbation of Hydrogen Bonds

An alternative strategy to control H-bonding electrochemically does not rely on the
electroactive unit playing a direct role in the H-bonding, but rather uses it to create
additional favorable or unfavorable interactions that either strengthen or weaken an
assembly held together through H-bonds. For example, electron transfer can be used
to create charged sites that break apart a H-bonded dimer due to electrostatic
repulsion. While the focus of this chapter will be on the direct perturbation methods,
a well-characterized example of the indirect method will be described later on.

1.3 DETECTION AND CHARACTERIZATION OF
ELECTROCHEMICALLY CONTROLLED H-BONDING
The most important and useful technique for studying electrochemically controlled
H-bonding and other forms of redox-dependent binding is cyclic voltammetry (CV).
By observing how the voltammetry of the electroactive component changes in the
presence of possible binding partners, one can readily determine whether redoxdependent binding is occurring and which oxidation state binds the strongest. More
careful analysis of this type of data can yield the equilibrium constants for binding
and possibly kinetic data as well.

Typically in supramolecular chemistry, the term “host” refers to the larger and more
structurally complex of two binding partners, while the term “guest” refers to the smaller,
less complex binding partner. However, for the purpose of this discussion, the term “host”
will be used to refer to the electroactive binding partner and “guest” to refer to the
nonelectroactive binding partner, irrespective of their size or structural complexity.

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1.3

DETECTION AND CHARACTERIZATION

5

+ e–
H

Kox

H

EH
+G

Kred

+G

+ e–

HG

Scheme 1.2

EHG

HG

Equilibria involved in redox-dependent host–guest binding.

With the above definitions, the equilibria involved in the redox-dependent
formation of a 1:1 host–guest complex can be described by the square shown in
Scheme 1.2. In this representation, the electron transfer reaction of the host by itself is
shown on the top horizontal axis and that of the host–guest complex on the bottom
axis. Binding of the guest to the oxidized host is on the left vertical axis and binding to
the reduced host is on the right. EH is the standard electrode potential for the host by
itself and EHG is that of the host–guest complex. Kox is the binding constant of the
guest to the host in its oxidized form and Kred is that in the reduced form.
Qualitatively, if a guest binds more strongly to the oxidized form of the host, the
oxidized host will be stabilized and it will be harder to reduce the host in the presence
of the guest. This means that EHG is negative of EH. On the other hand, if the guest
binds more strongly to the reduced form, it will be easier to reduce the host in the
presence of the guest and EHG is positive of EH.
Quantitatively, it is straightforward to show that if electron transfer and binding are
fast and reversible, the four-membered square behaves as a one-electron redox couple
with an E that depends on the true E values and the K values (Eq. 1.5). Note that in this
equation [G] ¼ the concentration of the free (unbound) guest, which is not equal to the
added guest if binding occurs. However, if [G] is greater than 10 times [H], it is
reasonable to assume that [G] is approximately equal to the added guest concentration. When Kox/red[G] ) 1 (large [G] and/or large K), Equation 1.5 reduces to the more
often seen Equation 1.6, which relates the maximum change in E to the ratio of

binding constants or binding enhancement. A particularly handy form of this equation
can be derived by switching to regular logarithms and filling in the constants to give
Equation 1.7, which says that at 25 C each 60 mV shift in E corresponds to a 10-fold
difference in binding strength between oxidation states.


RT
1 ỵ Kred ẵG
1:5ị
ln
Eobs ẳ EH ỵ
F
1 ỵ Kox ẵG


RT
Kred
DEmax ẳ EHG EH ẳ
1:6ị
ln
Kox
F
10DEmax =60 mV ẳ

Kred
Kox

at 25 C

1:7ị


In general, there are two types of limiting voltammetric behavior observed
for redox-dependent receptors.12 First, if there is strong binding in both oxidation

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6

ELECTROCHEMICALLY CONTROLLED H-BONDING

states (Kox and Kred are both large), then addition of a half equivalent of the guest
results in the current for the original CV wave decreasing by half and the appearance
of a new CV wave of approximately equal height. The half-wave potential, E1/2, for
the original CV wave should be approximately equal to EH, and that of the new wave
approximately equal to EHG. Under these circumstances, after addition of 1 equivalent
of the guest, only the new CV wave remains, now with a height equal to that of the
original host-only wave. Further addition of guest produces no additional changes,
unless greater than 1:1 guest–host binding is possible.
In contrast, if the binding constants are small or there is only strong binding in one
oxidation state, then generally one CV wave is observed even with less than 1
equivalent of guest. This is the behavior commonly observed for redox-dependent
H-bonding systems. As the guest concentration is increased, the E1/2 of the CV wave
will shift from EH toward EHG.
An example of this typical behavior is shown in Fig. 1.1 with CVs of nitroaniline, 1
(X ¼ NH2), in the presence of increasing amounts of diphenylurea, 2.10 As discussed
previously, diphenylurea is expected to bind more strongly to the reduced nitroaniline
(Eq. 1.3), making it easier to reduce and resulting in a positive shift in the E1/2.
This is indeed what is observed. Scan (a) is that of nitroaniline by itself. Addition of
half equivalent of diphenylurea, scan (b), results in a broad wave, with a new shoulder

appearing at more positive potentials. However, unlike the case in which strong
binding is observed in both oxidation states, the position of the new shoulder does not
correspond to the maximum shift. With 1 equivalent of diphenylurea, scan (c), the

Figure 1.1 CVs of p-nitroaniline in 0.1 M NBu4PF6/DMF in the presence of different
amounts of 1,3-diphenylurea: (a) 0 mM urea, (b) 0.5 mM urea, (c) 1 mM urea, and (d) 10 mM
urea. 500 mV/s scan rate.10

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