Supramolecular Electrochemistry (Wiley-Vch)

Angel Kaifer Marielle Gomez-Kaifer

Supramolecular Electrochemistry

Angel Kaifer Marielle G6mez-Kaifer

Supramolecular Electrochemistry

@WILEY-VCH Weinheim . New York . Chichester . Brisbane . Singapore . Toronto

Professor Angel E. Kaifer Dr. Marielle Gomez-Kaifer Chemistry Department University of Miami Coral Gables, FL 33124-0413 USA

This hook was carefully produced. Nevertheless, authors and publisher do not warrant the information contained therein to be free of errors. Readers are advised t o keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No. applied for A catalogue record for this hook is available from the British Library Die Deutsche Bibliothek - CIP-Einheitsaufnahme Kaifer, Angel E.: Supramolecular clcctrochemistry / Angel E. Kaifer ; Marielle Gomez-Kaifer. Weinheim ; New York ; Chichester ; Brishane : Singapore : Toronto : Wiley-VCH, 1999 ISBN 3-527-29591-6

0 WILEY-VCH Vcrlag GmbH. D-69469 Weinheim (Federal Republic of Germany), 1999

Printed on acid-free and chlorine-free paper All rights reserved (including those of translation into other languages). N o part of this hook may he reproduced in any form - by photoprinting, microfilm, or any other means nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this hook, even when not specifically marked as such are not to he considered unprotected by law. Printing: hetz-druck GmbH. D-64291 Darmstadt Bookbinding: Wilh. Osswald + C o . , D-67433 Neustadt Printed in the Federal Republic of Germany ~

To our parents

Angel, Barbara, Edward, Ellen and Emilia in thanks for their support of our curiosity and pursuit of knowledge

Preface

Supramolecular chemistry has different meanings for different people, and, perhaps, because of this ambiguity it is best to follow Professor Lehn's definition. In his words, supramolecular chemistry concerns the "chemistry beyond the covalent bond". This definition rightfully places emphasis on the importance of intermolecular forces present in supramolecular systems. From simple host-guest complexes to infinitely more complicated supramolecular assemblies described in the recent literature, intermolecular forces are at the core of all relevant supramolecular systems. Research in supramolecular chemistry already has a long and productive history and many reviews and several books have been devoted to this field of chemistry. In spite of the influence and importance that electrochemical techniques and concepts have had in the development of the field, when we started this work there were no monographs available on supramolecular electrochemistry. This book represents a modest attempt to correct this state of affairs. In launching a project such as this one, it is important to set clear goals. Our primary and foremost purpose was to provide the research community in supramolecular chemistry with an accessible and readable summary on the use of electrochemical techniques and the applications of electrochemical concepts to this new research area. A second purpose was to increase the level of interest in supramolecular systems from the electrochemical community. The book is thus intended as a tool to build bridges between these two rather separate communities and to foster some degree of cross-fertilization between the two research areas. In order to meet these goals, and due to the wide diversity of topics that we wanted to address, we could not, therefore, provide a comprehensive or thorough description of the subject matter. As is usually the case, we were forced to make many compromises concerning the selection of topics and the depth of coverage. The first seven chapters of the book are intended as an introduction to electrochemical techniques. Readers with a reasonable background in electrochemistry can probably skip these chapters. The remaining chapters address the electrochemistry of the most important types of supramolecular systems. Overall, the book should be useful to graduate students and postdocs, as well as more experienced researchers who are interested in expanding their research horizons at the frontier of electrochemistry and supramolecular chemistry. As stated above, this book does not even attempt a comprehensive coverage of the research topics presented. Therefore, literature citations were selected by the authors using very personal and, perhaps, seemingly arbitrary

VIII

Preface

criteria. We wish to apologize in advance to all those who feel that their work has not been appropriately represented here: this book is merely our personal view of the research landscape. Miami, June1999

Angel Kaifer and Marielle Gdmez-Kaifer

Acknowledgments

The authors owe their involvement in this research field to many people. First, they wish to express their gratitude to their common doctoral advisor, Professor Luis Echegoyen, who inspired them with his great enthusiasm and love for science. Both authors have been associated with the group of Professor Allen J. Bard, to whom they are indebted for his lucid teachings on electrochemistry, and his insights into the general importance of electrochemistry and the diverse ways in which it can be applied to almost any field of chemical research. We are fortunate to have been influenced by Professor Bards approach to research, which embodies all the best of collegiality and the true spirit of scientific endeavor. Over the years the authors have worked, discussed, and in many cases published, with a number of researchers in this field. Their contributions are important to this book and are reflected at many points throughout the manuscript. At the risk of missing someone, we wish to thank Jerry Atwood, Carmen Maria Casado, Alessandro Casnati, Cecil Criss, Isabel Cuadrado, Jeff Evanseck, George W. Gokel, David Gutsche, Moisks Morln, David Reinhoudt, Neil Spencer, J. Fraser Stoddart, Rocco Ungaro, and Frank van Veggel. The Kaifer group’s contribution to research in the field stems from the hard work of graduate students and postdoctoral associates. Their work cannot be overestimated. Julio Alvarez, Anna Bernardo, Richard Bissell, Claudia Cardona, Rene Castro, Emilio Cbrdova, Luis Godinez, Tim Goodnow, Mei Han, Rahimah Isnin, Jing Li, Jian Liu, Sandra Mendoza, Armen Mirzoian, Carlos Peinador, Maria Rojas, Esteban Romln, Yun Wang and Litao Zhang deserve our special thanks. The authors gratefully acknowledge the continued financial support from the U.S. National Science Foundation and NATO. Finally, the authors wish to express their sincere gratitude to their editor, Jorn Ritterbusch, for his encouragement, help, and above all else, his mfinite patience!

Contents

1Fundamentals of Electrochemical Theory

1

1.1Cell potentials and Electrochemical Reactions 1.2 Mass Transport 4 1.3Kinetics of Electrode Reactions 6 1.4 References 10

1

2 An Overview of Electrochemical Techniques

11

2.1 Faradaic and Nonfaradaic Currents 11 2.2 Classification of Electrochemical Techniques 13 2.3 Two-Electrode and Three-Electrode Cells 14 2.4 An Overview of Voltammetric Techniques 15 2.5 The Nernst Equation in Potential Controlled Experiments 2.6 Common Reversible Redox Couples 18 2.7 References 21 3 Potential Step Experiments

22

3.1The Cottrell Experiment 22 3.2 Chronoamperometry 26 3.3 Chronocoulometry 28 3.4 Bulk Electrolysis 29 3.5 References 31 4 Potential Sweep Methods

32

4.1 Linear Sweep Voltammetry 32 4.2 Cyclic Voltammetry 34 4.3 Pulsed Voltammetric Techniques 4.4 References 44

37

5 Ultramicroelectrodes and Their Applications 5.1 Characteristics of Ultramicroelectrodes 45 5.2 Scanning Electrochemical Microscopy 49 5.3 Electrochemistry of Single Molecules 51 5.4 Conclusions and Outlook 53 5.5 References 53

45

17

XI1

Contents

6 Practical Experimental Methods

55

6.1 Electrodes and Working Electrode Surfaces 6.2 Solvents and Supporting Electrolytes 64 6.3 Basic Cell Design 68 6.4 Vacuum Methods 72 6.5 References 75

7 Digital Simulation

55

77

7.1 Principles of Digital Simulation 77 7.2 Simulations of the CV Behavior of a Simple Redox Couple 7.3 Simulation of Electron Transfer Reactions Coupled to Homogeneous Chemical Processes 84 7.4 References 87

79

8 Electrochemical Considerations for Supramolecular Systems

89

8.1 Intramolecular Forces under Electrochemical Conditions 89 8.2 Self-Assembly and Fixed Association in Supramolecular Structures: Implications for Reversible Redox-Switching 93 8.3 Systems Involving Multiple Identical or Non-Identical RedoxActive Moieties 94 8.4 References 102

9 Electrochemical Switching

103

9.1 The Concept of Electrochemical Switching 103 9.2 Switchable Binding in a Redox-Active Cation Host 105 9.3 Electrochemical Switching as a Means of Controlling Molecular Devices and Other Structures 109 9.4 References 113 10 Electrochemically Switchable Cation and Anion Binding

10.1 Electrochemically-SwitchedCation-Binding Systems 10.2 Electrochemically-SwitchedAnion Binding 122 10.3 References 125

114

114

11 Redox-Switchable Cyclophanes and Other Molecular Receptors 127 11.1Early Cyclophane Studies and Metallocyclophanes 127 11.2 Redox-Active Cyclophanes as Molecular Receptors 130 11.3Viologen Based Cyclophanes- the Ideal n-Acceptor Host 132 11.4 Electroinactive Cyclophane Hosts and Their Binding of RedoxSwitchable Guests 135

Contents

XI11

11.5 Other Molecular Receptors 11.6 Conclusions 139 11.7 References 139

137

12 Electroactive Intertwined Structures

142

12.1 Electroactive Cyclodextrin-Based Rotaxanes and Pseudorotaxanes 143 12.2 Templated Metallocatenates and Metallorotaxanes 145 12.3 Catenanes Based on x-Donor and mAcceptor Interactions 150 12.4 Rotaxanes and Shuttles Based on x-Donor/ Acceptor Chemistry 155 12.5 Perspectives on the Future of Molecular Devices 160 12.6 References 161 13 Helicates, Racks Grids and Coordination Arrays 13.1Helicates 164 13.2 Molecular Racks, Grids and Coordination Arrays 13.3Conclusions 177 13.4 References 178 14 Electroactive Langmuir-Blodgett Films

164

175

180

14.1 Langmuir-Blodgett Films 180 14.2 Electron Transfer Studies in Langmuir and Langmuir-Blodgett Films 180 14.3 Other Electroactive LB Film Studies 183 14.4 References 190

15 Self-Assembled Monolayers

191

15.1 SAMs as Barriers for Electron Transfer Reactions 15.2 Electroactive Monolayers 195 15.3 Molecular Recogrution in SAMs 198 15.4 Photoswitchable SAMs 203 15.5 References 206 16 Electroactive Dendrimers

193

207

16.1 Dendrimers with Peripheral Electroactive Groups 207 16.2 Dendrimers with internal Electroactive Groups 213 16.3 References 220 17 Molecular Wires

222

17.1 The Concept of a Molecular Wire and its Electron Transfer Kinetics 17.2 Electrochemical Studies of Molecular Wires 223 17.3 References 227

222

XIV

Contents

18 Conclusions and Outlook 18.1 References

Index

233

231

228

Supramolecular Electrochemistry

Angel Kaifer, Marielle G6mez-Kaifer 0 WILEY-VCH Vcrlag GmbH. 1999

1 Fundamentals of Electrochemical Theory

Electrochemistry is a branch of science with a long and prestigious history. The theoretical foundations of electrochemistry were laid out by Faraday, Volta, Galvani and many other prominent scientists; their names are now routinely used to designate constants, units, processes, or types of cells. Electrochemistry can be defined in a very general way as the study of chemical reactions to produce electric power or, alternatively, the use of electricity to affect chemical processes or systems. The first perspective concerns the so-called galvanic cells, while the second relates to electrolytic processes. Both have tremendous practical importance, industrially as well as in everyday life. From the electrolytic preparation of chlorine to the widespread use of batteries, electrochemistry is a branch of science that has a clear and marked impact in everyone's life. While the user of a cellular phone whose battery dies in the middle of an important conversation might all too clearly perceive the limitations of electrochemical technology, it is equally true that deveIopments and advances in electrochemical science hold the key to some important technological breakthroughs. Electric cars afford the primary example for this situation because attractive operational characteristics --that will make them competitive with vehicles based on the internal combustion engine-- require batteries with higher power densities and peak power outputs. As these better batteries become available, the feasibility and popularity of electric vehicles should improve.

1.1 Cell Potentials and Electrochemical Reactions As the simplest type of chemical reactidn, electron transfer processes are at the core of electrochemistry. Electrons, the key players in these phenomena, are also the carriers of electricity in metallic and semiconductor circuits. Therefore, the connection between chemistry and electricity is obvious. The science of electrochemistry has its origins in the fact that oxidation-reduction reactions can be performed in ways that allow the direct harvesting of the free energy released in these processes. Consider, for instance, the following spontaneous reaction

While it is possible to immerse Zn metal in a solution of Cu(I1) ions and observe the oxidation (dissolution) of the Zn metal along with the simultaneous reduction.of the Cu(I1) ions (to form metallic Cu deposits), the same overall reaction can be carried out by immersing a Zn strip in a solution of Zn(I1) ions

2

1

Fundamentals of Electrochemical

and a Cu strip in a solution of Cu(I1) ions (see Fig. 1.1).To start the reaction, one only needs to establish pathways for the charges (electrons and ions) to circulate between the sites at which the Zn oxidation and Cu(I1) reduction processes take place. This is accomplished by setting up a salt bridge to establish electrical contact between the two solutions. The salt bridge allows the circulation of ions between the two solutions while preventing their mixing. Under these conditions a potential difference between the Zn and Cu strips develops. If the circuit is closed externally, that is, if a so-called electrical "load" is connected to the metal electrodes, the existing potential difference will give rise to a current, a flow of electrons moving from the Zn electrode (negative pole) to the Cu electrode (positive pole). The free energy AGO of the overall chemical reaction taking place in the cell can be readily calculated as

where n is the total number of electrons transferred in the reaction, F is Faraday's constant and Eoceu is the standard cell potential of the cell.

t

I

Cathode

Anode Salt bridge Figure 1.1: Components of a Galvanic Cell.

Electrochemical reactions are heterogeneous in nature as they take place at interfaces, usually metal-solution boundaries. These active interfaces are usually referred to as electrodes. By definition, an electrode where a reduction

2 .I

Cell Potentials and Electrodlemical Reactions

3

(uptake of electrons by a solution species) takes place is called a cathode. Conversely, an anode is an electrode where an oxidation (loss of electrons by a solution species) occurs. Applying these definitions to the electrodes of the galvanic cell in Fig. 1.1,it is straightforward to conclude that the Zn electrode is the anode and the Cu electrode serves as the cathode. A net electrochemical reaction implies transfer of charge across the corresponding metal solution boundary and the flow of current across the electrode. The current i, a basic electrical quantity, affords an instantaneous measurement of the rate of the electrochemical reaction according to equation (3)

i = nFAr

(3)

where n is the number of electrons transferred in the interfacial reduction or oxidation process, F is Faraday's constant, A is the surface area of the metal solution boundary, and r is the instantaneous reaction rate. Since current measurements are easily done with modern instrumentation, a peculiar feature of electrochemical techniques is that they provide continuous monitoring of the reaction rate. Integration of the current over a period of time affords the electrical charge, Q, which can be transformed into the amount of material in moles, N, converted in the electrochemicalreaction using Faraday's law: Q=nFN

(4)

A third quantity of fundamental importance in electrochemistry is the electrode potential, which can be considered as an adjustable driving force for the electrochemical reactions. In general terms, as the potential of an electrode is made more negative, the average energy of the electrons in the metal, which is approximately equal to its Fermi level, becomes higher, giving the electrode more reducing power. Similarly, the oxidizing power of an electrode can be increased by making its potential more positive. While these qualitative arguments are perfectly straightforward, the definition of electrode potentials is complicated by the fact that the potential of a single electrode is not an experimentally measurable quantity. This experimental inaccessibility has given rise to many theoretical attempts to obtain absolute electrode potentials. However, to the authors' knowIedge, none of these attempts has gained universal acceptance and, therefore, relative values continue to be the only way in which electrode potentials can be quoted. Simply put, this means that electrode potentials are always measured versus a second, reference electrode, whose value is arbitrarily taken as zero. The potential of the normal hydrogen electrode (NHE) is generally assigned a standard value of zero and serves thus as the primary reference for any other electrodes. For a generalized process involving the transfer of n electrons, Ox -+ ne eRed

(5)

1 Fundamentals of Electrochemical

4

where Ox and Red represent the oxidized an reduced partners of the redox couple, the thermodynamic potential, E, of the corresponding electrode is given by the well known Nernst equation, which is unquestionably one the most important equations in electrochemistry,

RT a,, E = E" +-InnF aRed where Eo is the potential under standard conditions, uox and aRed are the activities of the oxidized and reduced species, respectively, and the remaining terms have their usual meaning. Extensive tabulations of standard potential values are available. To avoid the complications associated with the use of thermodynamic activities and activity coefficients, very often activities are replaced by concentrations. In this case, the standard potential is replaced by the formal potential, Eo', which is usually dependent on medium conditions since it includes the activity coefficients. Therefore, a more practical version of the Nernst equation is as follows

E = E"' +

2.303RT nF

log

[Ox] ~

[Redl

(7)

The factor 2.303 reflects the replacement of natural by decimal logarithms. At 25OC, 2.303RT/F is equal to the familiar O.O5916V, which every freshman chemistry student ends up committing to memory. The Nernst equation is a thermodynamic equation and, thus, can only be rigorously applied to equilibrium situations (i=O). In spite of this apparent limitation, eq. 7 is successfully applied when current flows across the electrode in question, as long as the heterogeneous electron transfer process is fast (reversible in electrochemical jargon). Under these conditions, the equation is useful to calculate the concentrations at the electrode surface of Ox and Red that are generated when specific potential values are imposed to the electrode. Fast electron transfer kinetics allows the electrochemical reaction to adapt quickly to the changing potential values on the electrode surface, maintaining a pseudoequilibrium situation as well as the validity of the Nernst equation. Therefore, the term nernstian is also used when describing kinetically fast or reversible electron transfer processes. Finally, we must point out the potential of a galvanic cell, such as that represented in Fig. 1.1, can always be calculated with the following equation,

in which the cathode and anode potentials are obtained individually using eq. 7.

1.2 Mass Transport Current is simply the movement of ions and/or electrons across conducting media. In electrochemical cells, the movement of charged and neutral species is

1.2 Mass Transport

5

fundamentally important. Quite often it is the rate of these movements that determine the potentials and currents measured in the cell. No treatment of electrochemistry can thus overlook mass transport mechanisms. The three relevant mechanisms that may arise in electrochemical cells are migration, convection, and diffusion. In most electrochemical techniques, conditions are chosen so that transport of the electroactive species is affected by a single mechanism, typically diffusion. A brief discussion of each of these modes of mass transport follows. Migration is the movement of ions under the mfluence of an electric field. Therefore, uncharged species are not affected by migration. Although migrational movements can be described mathematically, in most voltammetric techniques it is desirable to remove migration contributions to the mass transport of the primary electroactive species, that is, the molecule or ion under study or analysis. This is accomplished by adding a large excess of an easily ionizable salt, which will dissociate to produce a large amount of inert anions and cations. These ions become the migration current carriers, thus releasing the electroactive species (if charged) from migration effects. The ionizable salt used for this purpose is called the supporting electrolyte. To be effective, its concentration must be about 100 times higher than that of the electroactive species. A second beneficial effect of the supporting electrolyte is to increase the conductivity of the solution, thus decreasing cell resistance effects that are very detrimental for recording accurate current responses. Convection is mass transport resulting from movements of the solution as a whole. Convection can be driven by stirring, solution flow, or by movements of the electrodes. In quiet, thermostatted electrochemical cells, convection may arise from density gradients only after rather long experiments. In fact, it is usually the onset of convection that limits the maximum duration of voltammetric or chronoamperometric experiments. In shorter experiments convection is not a factor in mass transport as long as the solution is quiescent and the electrodes are stationary. D z ~ s i o nis mass transport driven by a gradient of chemical potential. Anytime that the concentration of a molecule or ion (charge is of no concern here) is uneven throughout a solution, mass transport will take place to restore the homogeneity of the solution. In other words, transport will proceed from regions of high concentration to regions of low concentration. Diffusional phenomena are very important across many scientific and engineering disciplines. Fortunately, diffusion can be described mathematically, which facilitates the quantitative treatment of many electrochemical phenomena. The rate of diffusion of any chemical species is described by its diffusion coefficient, D, that is usually expressed in units of cm2/s. Most small organic or inorganic molecules or ions have D values in the vicinity of 10-5 cm2/s. This value decreases with molecular size. For instance, for spherical molecules the StokesEinstein equation establishes that

1 Fundamentals of Electrochemical

6

where k is the Boltzmann constant, q is the solution viscosity and a stands for the effective hydrodynamic radius of the diffusing species. This equation also reveals explicitly that D values depend on the temperature and the composition of the solution. To quantitate one-dimensional diffusion rates the concept of material fzux is very useful. The diffusional flux, J, is defined as the number of particles crossing a unit surface area perpendicular to the direction of mass transport per unit time. Fick's first law establishes that the flux is directly proportional to the concentration gradient. The proportionality constant is precisely the diffusion coefficient, that is, J = -D .

(g)

and the negative sign denotes the fact that the material flux moves against the gradient. This equation is extremely useful to calculate currents under conditions of complete conversion, i.e., whenever all the molecules or ions reaching the electrode surface undergo instantaneous electrochemical reaction. In such cases, the flux at the electrode surface is directly proportional to the resulting current. Fick's second law permits the calculation of concentration changes as a function of time. Its mathematical expression is given below

"=.($) at

Fick's laws provide a complete and detailed description of diffusional mass transport for any species subject to concentration gradients. To find the analytical solutions of the resulting differential equations, appropriate boundary conditions must be provided detailing initial and limiting concentrations and extent of electrochemical conversion at the electrode surface. Some examples will be given in later chapters.

1.3 Kinetics of Electrode Reactions In most electrochemical experiments we are interested in recording a currentpotential curve. For instance, let us assume that we apply an increasingly positive potential to an electrode (or that we make its potential increasingly positive against a reference electrode). The more positive the potential becomes, the more oxidizing power is conferred to the electrode and, at some point, one of the cell components will start to undergo an oxidation reaction. This reaction will translate into current flowing across the cell, a situation that is represented in Fig. 1.2. Notice that this curve is composed of three distinctive regions. In the first region (low potentials), there is no significant current flow, because the potential is not sufficiently positive to drive the oxidation process. In the second region (intermediate potentials), the current increases with the potential, as one

1.3

Kinetics of Electrode Reactions

7

would generally anticipate from simple kinetic arguments. A third region (high potentials) is characterized by the leveling of the current, which reaches a constant or limiting value independent from the potential. This is due to limitations imposed by the finite rate of mass transport that can be achieved in the solution.

Region I

Current

D

Potential Figure 1.2 Typical current-potential curve.

An electrode reaction is a heterogeneous process that takes place at the interface between the electrode and the solution. Therefore, the overall rate or current depends on the rates of two distinct processes: the actual heterogeneous electron transfer process and the transport of the reactant species from the solution to the electrode surface. The slowest one of the two processes determines the overall current. Fig. 1.2 illustrates this situation clearly. In the intermediate potential region the kinetics of the electrode reaction controls the current level. In this region mass transport is still sufficiently fast to be "transparent", that is, it shows no effect on the overall current. However, at higher potentials, the electrochemical reaction is driven to very fast rates, increasing the demand for electroactive species to an extent that it becomes impossible for mass transport to keep pace. Therefore, a current plateau

1 Fundamentals of Electrocllemical

8

develops as the current reaches the maximum limit that mass transport processes can provide. These ideas are mathematically expressed by the simple equation: 1

1

i

i,

- =-+,

1 1,

in which i stands for the overall current, ik is the current that can be obtained at that potential and zr is the limiting current that can be reached through mass transport. In this section we will describe the potential dependence of the current assuming no limitations from mass transport. Any theoretical formulation of electrochemical kinetics must reduce to the thermodynamic limit (Nernst equation) when equilibrium is reached. Furthermore, the empirica1 Tafel equation establishes a mathematical relation between the current and the overpotential q (difference between the applied potential and the corresponding equilibrium potential for the electrode system in question) q = a + b .log i

(13)

where a and b are constant values characteristic of the system. Let us consider a generalized heterogeneous electron transfer process between species Ox and Red (see eq. 5). Using eq. 3, we can write for the forward reaction (Ox+Red) ic rf = k f.Cox(O,t)=-nFA

in which kf is the rate constant for the forward reaction, i, is the cathodic current. Notice that the heterogeneous character of the process is manifested by the fact that the reaction rate is directly proportional to the reactant concentration at the electrode surface Cox(O,t). We can write a similar equation for the reverse or backward process (Red+Ox)

The total current i flowing through the electrode is simply the difference between the cathodic and the anodic currents,

i = i, - i a = nFA(k, .cox(0, t) - kb .CRed(0,t)]

(16)

The way this equation is written implies that we have chosen to describe cathodic currents as positive and anodic currents as negative. This is a common, albeit completely arbitrary, choice that we will maintain throughout the book. Notice also that the rate constants have units of cm/s, a reflection of their heterogeneous character, provided that they operate on concentrations expressed in mol/cm3.

I .3 Kinefics ofElectro& Reactions

9

A key distinguishing feature of electrochemistry is that the reaction rates depend on the applied electrode potential. In fact, to further develop eq. 16 we must provide mathematical expressions to describe this dependence. The Butler-Volmer formulation is the most commonly used for this purpose. The corresponding equations are as follows k - kO .e-anF(E-Eo')/RT f -

and

k - kO -e(l-a)nF(E-Eo')/RT

(18)

b -

where ko is the standard rate constant and a is the so-called transfer coefficient.[*] It is possible to derive these equations using several physical models, but we will constrain ourselves here to explore some of the implications of the ButlerVolmer formulation. At equilibrium (E=Eeq) the net current is zero. By combining eqs. 16,17 and 18 we have nFAkOCo x (0, t) . e-anF(Eeq-Eo')/RT

= nFAk°CRed(0,t) . e

(1-a)F(E,,

-ED') / RT

Under equilibrium conditions, the concentrations of Ox and Red at the electrode surface are identical to those in the bulk solution and, thus, we can write

which is identical to the Nernst equation (eq. 7). The electrochemical equilibrium, as any other type of chemical equilibrium, is not static. In fact, the forward and backward processes take place at equal rates yielding no net current. However, the electrochemical activity at equilibrium can be expressed in terms of the exchange current, io, which is identical to the level of cathodic or anodic currrent. For instance, i o = i f = i b =nFAk 0 e -unF(E,,-E'')/RT which, after some manipulation, yields

io = nFAko[Oxll-a)[Red]"

(22)

The exchange current is directly proportional to the standard rate constant for the heterogeneous electron transfer process. Both parameters are used to express quantitatively the inherent rates of heterogeneous electron transfer reactions. Outside equilibrium conditions (q#O) the Butler-Volmer formulation leads to an important equation which is generally valid to describe the kinetics

1

10

Fundunientuls of Electrocllpmicul

of electrochemical reactions in the absence of mass transport limitations. Not surprisingly, this equation is commonly referred to as the Butler-Volmer equation and is given as

The right term in the equation describes the cathodic component (forward reaction) of the current while the left term describes the anodic component (reverse reaction). Of course, the sign of the overpotential determines which one of the two terms will predominate and control the overall current. At negative overpotentials (E<Eeq) the cathodic term predominates and at positive overpotentials (E>Eeq) the anodic term controls the total current. The transfer coefficient a is related to the degree of asymmetry in the electron transfer process. Many simple, one-step electrochemical reactions exhibit values of a close to 0.5. Kmetically sluggish processes or multi-step reactions may present transfer coefficients substantially different from 0.5. The Butler-Volmer equation reduces to Tafel conditions at extreme overpotentials. For instance, if qE1/2. Normally, the potential will be linearly scanned in the negative direction and faradaic currents will be detected near, around, and beyond the half-wave value, that is, in the potential region where the conversion Ox + Red is favored. If the solution is kept quiescent (so that diffusion is the only mass transport mechanism possible) and the Ox/ Red couple is electrochemically reversible, the electrochemical conversion gives rise to a characteristic cathodic wave (Fig. 4.2), with a maximum current value given by the Randles-SevCik equation:

i,

= (2.69~10~)

at250 c

(4.1)

4.1

Linear Sweep Voltamrnety

33

where the peak current ip is given in PA, A is the projected electrode area (in cm2), Dox is the diffusion coefficient of the electroactive species expressed in cm2/s, Cox is its concentration (mM), and v is the scan rate in V/s. It is important to use the specified units as the equation contains a numeric factor that results from the evaluation of several constants. The Randles-Sevtik equation is one of the most important equations in voltammetry. Of course, it applies only when the current is diffusion controlled and hemispherical diffusion is unimportant (we are assuming that a planar

time

-

Figure 4.1: Excitation waveform used in linear sweep voltammetry (cathodic scan).

2 80 2 30

180

b

1 u

130 080 030

-0 20 000

-0 20

-040

-060

-0SO

POTENTIAL Figure 4.2 A typical linear sweep voltammogram.

El/:=

-0.500 V.

-1

00

34

4

Potential Sweep Methods

electrode of conventional size is used.) Note that the current depends on the square root of the scan rate. The implicit time dependence ( W 2 ) is identical to that expressed by the Cottrell equation for a potential step experiment. It is important to point out here that the potential of the voltammetric peak does not equal the half-wave potential of the corresponding redox couple. For reversible electrochemical couples, the cathodic peak occurs 20-30 mV more negative than the E 1 p value and its position is independent of the scan rate. The position of the peak represents the onset of diffusion control on the current. That is, beyond the peak potential the current does not depend on the potential anymore and is fully controlled by the rate of diffusion, which decreases gradually as the thickness of the diffusion layer increases. Therefore, it is necessary to go past the half-wave potential to reach the necessary Cottrell-like conditions. For slower (irreversible)electrochemicalcouples, a peak may or may not be reached. If the voltammogram exhibits a peak, the corresponding peak potential will shift cathodically as the scan rate increases.

4.2 Cyclic Voltammetry Cyclic voltammetry (CV) is based on the same principles as linear sweep voltammetry. However, in CV the potential of the working electrode is scanned back after reaching a certain value E,, the so-called switching potential (see Fig. 4.3). This figure shows a typical excitation waveform for CV. It is also possible to utilize excitation waveforms with more than two h e a r segments. In Fig. 4.3, the reverse scan is set to end at the initial potential, but this does not have to be the case in every CV experiment. It is not unusual to extend the reverse scan

time Figure 4.3 A typical potential excitation waveform used in CV.

*

4.2

Cyclic Voltmimetry

35

past the initial potentia1 and have a third linear segment to take it back to the initial value. Scan rates can also be varied for each linear segment of the waveform. The key advantage of CV over simple LSV results from the reverse scan. Reversing the scan after the electrochemical generation of a species is a direct and straightforward way to probe its stability. A stable electrogenerated species will remain in the vicinity of the electrode surface and yield a current wave of opposite polarity to that observed in the forward scan. An unstable species will react as it is formed and no current wave will be detected in the reverse scan. A typical cyclic voltammogram for the reversible reduction of Ox to Red is shown in Fig. 4.4.The electrochemical process is fast in the time scale of the experiment and the electrogenerated species Red is perfectly stable in the electrolytic solution. Under those conditions, and assuming that the solution is kept unstirred during the experiment, the ratio of the cathodic and anodic peak currents (the peak currents measured in the forward and reverse scans, respectively) should be equal to one. Deviations from unity reveal the presence of chemical reactions involving either redox partner (Ox or Red) or both partners. The average of the two peak potentials affords the half-wave potential for the corresponding couple, that is,

-2

-3

000

-020

-040

-0GU

-080

-1 00

POTENTIAL

Figure 4.4: Cyclic voltammetric response for a reversible redox couple. E i p = -0.500 V.

36

4

Potential Sweep Methods

The differential equations describing the diffusional movements of the electroactive species cannot be solved exactly along with the boundary conditions for LSV or CV experiments. Therefore, the current-potential curve cannot be described analytically. The voltammetric response can be calculated using numerical techniques or digital simulation techniques (see Chapter 7). The current-potential curves shown in Figs. 4.2 and 4.4 were simulated by the authors using the Electrochemical Simulation Package (ESP) written by Professor C. Nervi and freely available at his internet site.Pl The lack of analytical equations for the voltammetric current-potential responses makes it advisable to describe the observed response in detail as we discuss the parameters that can be derived from CV experiments. Fig. 4.4 shows that the flow of faradaic current does not start until a potential value of about 0.4 V is reached. If we were to reverse the potential scan at -0.4 V and return to the initial values, we would record a flat voltammogram having approximateIy constant levels of cathodic (in the forward scan) and anodic (in the reverse scan) current. As we already know, these currents are due to the capacitive charging of the working electrode's double layer. At any potential, the difference between the cathodic and anodic current (Ai) is given by: Ai=2vC

(4.3)

where v is the scan rate and C is the capacitance of the electrode at the potential of choice. This equation provides a simple method to determine the capacitance of the working electrode. However, electrode capacitance values obtained this way should only be considered estimates. In the forward scan the peak current is gwen by the Randles-Sevcik equation (eq. 4.1) as it is in LSV experiments. This equation is often used to analyze the behavior of a redox couple by plotting peak currents as a function of the square root of the scan rate. A linear plot is taken as evidence for the reversible character of the couple and demonstrates that the currents are controlled by planar diffusion to the electrode surface. The slope of such a plot can also be used to determine the diffusion coefficient of the electroactive species (Ox in our discussion) if A and CO, are known beforehand. This is not, however, a recommended method to determine diffusion coefficient values, as the peak currents are usually obtained with sizable error margins and the slope of the plot depends only on the square root of the diffusion coefficient. Chronocoulometry or voltammetric experiments with ultramicroelectrodes are much preferred for the determination of diffusion coefficient values. Another method to assess the reversibility of a redox couple is the evaluation of the potential difference between the peak potentials (AEp) of the anodic and cathodic peaks associated with the couple. Based on numerical solutions of the current-potential response in CV experiments,[2]a value of 57/n mV (at 25oC, first cycle voltammogram) is expected for a reversible redox couple. It is extremely important to realize that this value will only be obtained if the switching potential is at least 200 mV beyond the peak potential observed in the forward scan. The proximity of the switching potential to the voltammetric peaks leads to increased AEp values. Furthermore, the presence of

4.2

Cyclic Volturnmetnj

37

uncompensated cell resistance also leads to increased A€, values. If the researcher can insure that the levels of uncompensated resistance in the electrochemical cell are small and the switching potential is at least 200 mV beyond the forward scan peak potential, the observed deviations from the theoretical A€, value can be used to estimate the standard rate constant (ko) for the heterogeneous electron transfer process.[31 We should note that this method yields only estimates of ko values. As mentioned above the half-wave potential ( E l p ) can be readily obtained from the midpoint between the two peak potentials (eq 4.2) for a reversible or quasi-reversible redox couple. This value is characteristic of a redox couple and is typically within a few mV of the formal potential for the couple ( E o ' ) according to the following equation: RT El,, = E"'- -In 2nF

Do, ~

(eq. 4.4)

DRed

where the ratio of the diffusion coefficients Dox and DRed is usually very close to unity. The easy determination of half-wave potentials and estimation of formal potentials is an extremely attractive feature of CV.

4.3 Pulsed Voltammetric Techniques Although CV is a powerful and extremely useful electrochemical technique, capacitive charging currents set its detection limit to about 10-4 M under optimal conditions. This is inadequate for many analytical problems. From the standpoint of supramolecular chemistry, solubility limitations and/or material availability concerns would be eased by electrochemical techniques exhibiting higher sensitivity. The most successful way to accomplish this goal relies on the use of pulsed waveforms as potential excitation functions. These techniques take advantage of the sophisticated capabilities for potential control, current measurement, and timing in the millisecond domain that are accessible with modern microcomputers. In this chapter we will review the three most popular and potentially useful pulse voltammetric techniques: normal pulse voltammetry (NPV), differential pulse voltammetry (DPV), and square wave voltammetry (SWV).

4.3.1 Normal Pulse Voltammetry The potential excitation function used in NPV is illustrated in Fig. 4.5. It essentially consists of a series of short duration pulses of gradually increasing magnitude. After each pulse the potential returns to the initial value, a feature

38

4 Potentiai Sweep Methods

that is unique to this technique and gives rise to special applications as we will see later in this section.

-4

Figure 4.5 Potential excitation function for NPV experiments. The dots indicate current measuring points.

As indicated in the figure, the current is measured at the end of each pulse. Measuring current at the end of a period through which the potential remains constant is a common feature of many pulse voltammetric techniques. This is done to minimize capacitive charging currents, taking advantage of the exponentially fast decay of the charging current once the potential reaches a fixed value. Using pulse widths (tp) in the millisecond regime, the current measured at the end of each pulse is essentially faradaic in nature. The scan rate can be readily calculated by dividing the potential step size (AE,) by the period of the waveform (T). The current potential response in this technique takes the form of a sigmoidal, steady state voltammogram (Fig. 4.6) from which one can easily determine the half-wave potential and other voltammetric parameters. The relative rejection of capacitive currents results in concomitant gains in sensitivity. However, NPV does not afford the sensitivity levels provided by DPV or SWV (vide infra). As mentioned before, the single feature that makes NPV a useful technique for supramolecular chemists is the periodic return of the potential to the initial value. This is particularly useful in cases in which the electrogenerated species is insoluble in the electrolytic solution, as the cyclic return to the initial potential periodically regenerates the initial conditions, cleaning the electrode surface from insoluble deposits and leading to currentpotential curves that are relatively unaffected by the precipitation of the electrogenerated species. The authors' group has recently reported an example on this application of NPV.[41

4.3 Pulsed Volturnmetric Techniques

-5.0 0.00

-0.20

39

-0.40

-0.60

-0.80

-1.00

POTENTIAL

Figure 4.6: A typical current-potential curve obtained using NPV.

El,?

= -0.500 V.

4.3.2 Differential Pulse Voltammetry The potential excitation function used in DPV is illustrated in Fig. 4.7. The waveform is composed of a series of potentiaI pulses. After each pulse the potential returns to a value which is slightly more negative (in a cathodic scan, or more positive in an anodic scan) than the value preceding the pulse. This potential difference (AEs in the figure) is the net potential change that takes place after a full waveform cycle. As in NPV, the scan rate is given by the ratio between AEs and the period of the cycle, T. Two current samples are taken during every cycle of the excitation function. The current measuring points are indicated by the numbers 1 and 2 in the figure. The quantity of interest in DPV is the dzference between the currents measured at the end of the pulse (point 2) and immediately before the pulse (point l), 6i = iz - i ~ . The differential pulse voltammogram is simply a plot of 6i against the potential value at the beginning of the corresponding waveform cycle. The differential nature of the current measurement results in a peaked output, a key difference in comparison to the wave-like current-potential curves obtained in most other voltammetric techniques.

40

4

5-

Potential Sweep Methods

-

-

Figure 4.7:A typical excitation function for DPV. See text for symbol definitions.

0.5

0

0.4

I= 0.3

L

E u

o.2 0.1

0.0 0.00

-0.20

-0.40

-0.60

-0.80

-1.00

POTENTIAL Figure 4.8: A typical differential pulse voltammogram.

E l l 2 = -0.500 V

and AEp = -50 mV.

The shape of the DPV response can be quantitatively treated. The events during each waveform cycle correspond to those in a double potential step experiment. At the beginning of the cycle, the base potential E is enforced until the application of the pulse. After the pulse a new fixed potential E + AEp

4.3 Piilsed Voltammetric Techniques

41

(AE, is the pulse amplitude) is applied during the pulse width t,. shown that

It can be

(eq. 4.5)

where ( t 2 - t l ) is the time difference between the two current readings, and the parameters P and 0 are defined as follows: (eq. 4.6)

.=exp( nF ' AEp

]

2RT

The bracketed factor of eq. 4.6 describes the potential dependence of the differential current 6i. Its shape, that is, the shape of a typical differential pulse voltammogram is given in Fig. 4.8. At € >> Eo', P is very large and 6i is essentially zero. At E 3. The cathodic voltammetric behavior for Cob+-COO- exhibits a reversible one-electron reduction wave centered at -0.99 V vs Ag/AgCl. In the presence of (3-CD this wave shifts anodically, while the AEp value tends to increase from its theoretical 57 mV for a reversible process. These findings are consistent with the complexation of the electrogenerated cobaltocene by the p-CD host. Using electrochemical and 1H-NMR spectroscopic data, we have proposed the following E,C, mechanism for the reduction of Cob+-COO-in the presence of 0CD.[q

coo+ e

&

coo-

+

Figure 7.9 Proposed mechanism for the reduction of carboxycobaltocenium in the presence of (3-CD.

Digital simulations were utilized to validate this mechanism and to determine the equilibrium ( K ) and kinetic rate (kf)constants for the association between p-CD and cobaltocene. Good fits between the simulated and experimental voltammograms were obtained,[71yielding optimum values of K = 1,800 M-l and kf = 3.6 x lo7 M-Is-'. A good test of the accuracy of these parameters is that they yield simulated voltammograms that fit the experimental ones very well through the entire range of 0-CD concentrations surveyed (see for instance Fig. 7.10 in the next page). Notice that the proposed mechanism does not take into account the direct oxidation of the inclusion complex. As the

7.4

References

87

dissociation of the guest from the complex is fast, the electron transfer reaction takes place on the free guest. Similar results have been reported with other cyclodextrin inclusion complexes of electroactive guests.[8-101 The lack of electrochemical activity of these complexes is consistent with the thermodynamic and kinetic hindrances that we have observed on the electron transfer reactions of fully encapsulated redox centers.

t -2.01

-0.4

'

-0.6

'

'

-0.8

'

-1.0

'

'

-1.2

'

-1.4

Potential, V vs Ag/AgCI

Figure 7.10 Experimental (continuous Iine) and simuIated (circles) voltammograms for the reduction of 1.0 mM Cob+-COO-+ 10 mM p-CD in 0.1 M phosphate buffer (pH = 7 ) . Scan rate: 0.1 V/s.

In principle, similar methods can be used to analyze electrochemical data obtained with any supramolecular system in which electron transfer reactions are coupled to chemical processes. In practice, however, when the number of parameters that must be fitted increases so does the uncertainty associated with their estimation. In those cases the task is greatly facilitated if one can determine some of the pertinent equilibrium and kinetic rate constants using independent methodology.

7.4 References 1.S. W. Feldberg and C. Auerbach, Anal. Chem. 1964,36,505-509. 2. For reviews, see: (a) M. Rudolph in Physical Electrochemistry, I. Rubinstein, Ed., Marcel Dekker, New York, 1995, Chapter 3. (b) S. Feldberg in Electroanalytical Chemistry, Vol. 3, A. J. Bard, Ed.; Marcel Dekker: New York, 1969, Chapter 4. 3. M. Rudolh, D. P. Reddy and S. Feldberg, Anal. Chem. 1994,66,589A-600A.

88

7

Digital Simulations

4. Copyright by Professor Carlo Nervi. This package can be downloaded at the Internet address: http://lem.ch.unito.itlchemistrv/electrochemistry.html 5. D. J. Cram and J.M Cram, Monographs in Supramolecular Chemistry, Vol. 4: Molecular Containers and Their Guests, J. F. Stoddart, Ed.; Royal Society of Chemistry, Cambridge, 1994. 6. S. Mendoza, P. D. Davidov and A. E. Kaifer, Chem. Eur. J. 1998,4,864-870. 7. Y. Wang, S. Mendoza and A. E. Kaifer, lnorg. Chem. 1998,37,317-320. 8. T. Matsue, D. H. Evans, T. Osa and N. Kobayashi, J. Am. Chem. SOC.1985,107, 3411-3417. 9. R. Isnin, C. Salam and A. E. Kaifer, J. Org. Chem. 1991,56,35-41. 10. A. Mirzoian and A. E. Kaifer, Chem. Eur. J. 1997,3,1052-1058.

Supramolecular Electrochemistry

Angel Kaifer, Marielle G6mez-Kaifer 0 WILEY-VCH Vcrlag GmbH. 1999

8 Electrochemical Considerations for Supramolecular Systems

In this chapter we shall review several basic concepts relevant to the study of electrochemistry of supramolecular systems. The first of these is a consideration of intermolecular interactions and their relevance to electrochemical processes. The role played by these forces in host-guest chemistry and self-assembly will be discussed briefly. Considerations of molecular design and its effect on electrochemical reversibility will also be addressed. Finally, we shall consider the electrochemical behavior of systems with multiple redox active sites.

8.1 Intermolecular Forces under Electrochemical Conditions The intermolecular interactions that comprise the basis of host-guest interactions or self-assembling systems have been discussed extensively in the literature. In supramolecular systems, as in biological systems, these interactions are largely electrostatic in character, encompassing ion-ion, ion-dipole and dipole-dipole, hydrogen bonding, n--71 and cation-7t interactions. In self-assembling systems, particularly those relying on monolayer formation, Van der Waals, hydrophobic or solvophobic interactions may also be of significance. While much has been written about the nature of these interactions, the complicated interplay between these forces and their nature under electrochemical conditions has been less explored. The requirements of simple electrochemical experiments, e.g., solvents with reasonably high dielectric constants and a large excess of supporting electrolyte, in and of themselves, can significantly affect the magnitude of electrostatic interactions, particularly in the case of host-guest complexes. Thus, a cogent topic of discussion is the consideration of these fundamental interactions and how electrochemical experimental conditions may mfluence or alter them. This can further lead us to several salient points for reflection on the design of experiments and their comparison to mformation obtained by spectroscopic methods. Electrostatic forces play the most sigruficant role in supramolecular electrochemistry, since a change in the oxidation state of an electroactive species may result in changes in the interaction energy (AE) between that species and its prospective hosts or guests. The dielectric constant of the solvent medium is of obvious import, as demonstrated by consideration of the well known equation for the Coulombic interaction energy:

90

8 Electrochemical Considerations for Supramolecular Systems

AE =

Z,Z,E2

Dr12 where r12 is the distance between the two charges, ZI and 2 2 are their unit charges (a positive or negative integer), E is the unit of electronic charge and D is the dielectric constant of the solvent. In most instances, the solvent chosen for experimental work will be a compromise between one that provides the best solubility for the electroactive species (in its initial and switched oxidation states, reflecting oxidation or reduction), and one providing the highest possible dielectric constant, in order to reduce the resistance of the medium. Given the choice between employing CHKL (D = 8.93)"l or DMSO (D = 46.5)[11 as a solvent, most electrochemists would certainly prefer the latterP1 (Water, with its high dielectric constant of 78.30,['1 would be even more desirable.) From the above equation we can see that the magnitude of the coulombic interaction is reduced when the solvent dielectric is large. Furthermore, a large amount of supporting electrolyte is required to reduce the overall solution resistance and maintain diffusion controlled, rather than migratory, conditions. The high supporting electrolyte concentration may further affect the dielectric of the medium, while the electrolyte's ions may interact extensively with any charged or polar host/guest species. Clearly therefore, under the foregoing conditions, some of the electrostatic forces between an electroactive species and its prospective host or guest may be minimized, or of diminished significance. Nonetheless cooperative interactions, i.e. the summation of several low energy electrostatic contacts, or nonelectrostatic forces, can still dominate the binding process. Indeed, this point is borne out by the-innumerable examples of successful redox-switchable binding studies in the literature. In this respect we can consider two points: a molecular design of the host and guest species that maximizes these interactions, and the selection of a supporting electrolyte that is unlikely to compete with them. In the first case we can see that molecular design is of paramount importance: a thoughtful approach to optimizing the strength of the intermolecular interactions between the host-guest pair. While it is obvious that this is crucial to any host-guest system, it is even more so for a system which is to be electrochemically switched. In at least one of its redox states, the binding of a guest to a host should permit the maximum possible favorable interaction, e.g., in hydrogen-bonded systems the target atoms should have contacts at the appropriate distance and angle. The supporting electrolyte can also be chosen to lessen the degree of interaction with a host-guest complex, e.g., ions too large or too small to be tightly bound in a charged host may be employed. Both the size and the relative hardness or softness of the electrolyte species can be considered. In organic solvents with relatively low dielectric constants (e.g. CHClz, THF, and toluene, with dielectrics of 8.93, 7.20 and 2.38, respectively, at 25 C"]) electrostatic forces become more significant. However, in these solvents incomplete dissociation of supporting electrolyte ion pairs may be observed and

8.2

Intermolecular Forces under Electrochemical Conditions

91

solution resistance is markedly higher. While these solvents may favor an increase in the role of electrostatic forces in a host-guest complex, the solubility of a charged reduced or oxidized electroactive species can be quite low in such media and may result in precipitation at the electrode surface, making electrochemical study difficult or impossible.PI High supporting electrolyte concentrations are a requirement in such solvents and a charged analyte species may also exhibit a stronger interaction with the electrolyte in these solvents. We should note that the advent of ultramicroelectrodes, which permit electrochemical study in higher resistance solvents, in some instances in the absence of supporting electrolyte, shows promise for analyzing the electrochemical systems under conditions closer to those employed in traditional spectroscopic studies. Ultramicroelectrodes are discussed in Chapter 5. Although they may be employed with some success in higher resistance solvents, their behavior under such conditions is often still far from ideal. Considering hydrogen bonding as a special class of electrostatic interaction, it is obvious that the choice of solvent is also important for systems heavily dependent upon molecular recognition via this class of interaction. The higher dielectric solvents that are desirable for electrochemical work (e.g. water, alcohols, or the aprotic DMF and DMSO) are also more likely to be capable of hydrogen bonding with the guests or hosts, thus competing as hydrogen bond acceptors and/or donors. Host-guest complexes that are designed to be stabilized by hydrogen bonding are obviously most appropriately studied in nonpolar solvents such as CH2C12 or, less ideally, DMF or DMSO. Supporting electrolytes containing non-hydrogen bond-accepting anions are also a necessity. Van der Waals, or dispersion forces, which are present between any two atoms, are not influenced by the special solvent media requirements of electrochemical experiments. However, the complex processes leading to hydrophobic interaction~,[~] or the so-called solvophobic interactions, can, of course, be profoundly affected by the solvent medium. These forces have been used to advantage in the self-assembly of a number of electroactive supramolecular assemblies at interfaces. The extent of the effect of hydrophobic or solvophobic interactions on complexation and molecular recognition has been a topic of recent interest. Diederich and coworkers have studied apolar complexation and have found that apolar arene binding occurs in solvents of all polaritie~,[~] although the stabilities of such complexes formed in organic solvents, rather than water, is greatly reduced. Aromatic systems have been noted to interact via n--71 arrangements, a process that should not be perturbed by the presence of most solvents used for electrochemistry. An exception is the fullerene-based systems, which are most frequently studied in aromatic solvents such as toluene, benzene and benzonitrile, or in binary systems containing these solvents. Such solvent systems might be anticipated to interfere with the n-n interactions that would contribute to the binding of fullerenes by aromatic hosts such as cyclophanes. Cation-n interactions have been a topic of interest in recent years. This type of interaction has been theorized to be largely electrostatic, attributable

92

8 Electrochemical Considerations for Supramolecular Systems

primarily to ion-quadrupole effects, but also relying on additional contributions from polarizablities, and dispersion forces of the components.[51 In experimental terms, these interactions have largely been studied in aqueous solution[5,61and so little is known about the effect of solvent polarity and dielectric constant or of other ions (due to a supporting electrolyte) on these interactions. Since evidence suggests that aromatic molecules such as benzene can compete effectively with water for solvation of larger cations (K+, Rb'), one can speculate that moderately nonpolar, low dielectric solvents (excluding, of course, x-donor aromatic solvents like benzene and toluene) may foster the electrostatic component of these interactions, strengthening their character. Thus, such solvents may enhance the binding of cations in aromatic systems such as cyclophanes. We should note that even organic cations, such as tetramethylammonium ions and acetylcholine, have been shown to exhibit cation-x interactions. Alkylammonium ions are, of course, among the most commonly used supporting electrolyte cations for organic solvent systems. The surprisingly high estimated strength of such interaction~[~] (-9 kcal/mol per cation-n-face interaction for tetramethylammonium ion in aqueous solution) implies that the sum interaction energy might be significant enough to exert an effect on the electrochemistry of, for example, redox active cyclophanes, which can often undergo reduction to more electron rich charge states. Should this point be considered when performing electrochemical binding studies in organic solvent systems using alkylammonium ions as supporting electrolyte? Clearly the trend suggested by these authors indicates less interaction should be anticipated between bulkier cations such as tetrabutylammonium ion and aromatic species. In this instance, the magnitude of the effect should be anticipated to be quite small. However the magnitude of the interaction that is reported for the tetramethylammonium ion makes this a point of interest when the smaller alkylammonium ions are employed. In this instance we might wish to consider the possibility that the redox potentials for an aromatic electroactive receptor or guest may already display a shiftfrom its ''true" redox potential, due to interactions with the supporting electrolyte. Thus the strength of the electroactive species' binding with a positively charged target species may be underestimated. This is a question that might bear further study. Ascertaining the magnitude of any such an effect is, however, a challenging undertaking. From the foregoing points, it is clear that in order to obtain consistent and meaningful assessments of binding in redox-switchable ligands we should study the binding of initial states of any host/guest pair under conditions that correspond as closely as possible to those employed for the electrochemical switching studies. By this we mean that binding studies of the initial states of the system should preferably be performed in the same solvent and supporting electrolyte system to be employed in electrochemical experiments. In some instances this may not be possible, e.g. in some spectroscopic studies the presence of a given supporting electrolyte may interfere with the desired spectral window. However, in such cases efforts can be made to try to mimic electrochemical conditions with alternative electrolytes, if possible.

8.2

SelfAssenibly and Fixed Associntion

111

Siipranioleciilar Structures

93

8.2 Self-Assembly and Fixed Association in Supramolecular Structures: Implications for Reversible Redox-Switching Over the past two decades the goal of self-assembly of electroactive supramolecular systems has been achieved exploiting combinations of the forces detailed above. Self-assembly has no doubt been a popular route to such structures because it affords a more facile means of preparation. One avenue of development has been interfacial assemblies, e.g., the preparation of selfassembled monolayers or SAMs. Amphiphilic aggregation and thiol/ disulfide or silane attachment of amphiphiles directly onto electrode surfaces is still a rapidly expanding research area. These assemblies rely primarily on apolar and van der Waals forces to drive aggregation at the interface. Multicomponent supramolecular systems have proven capable of electrochemical interfacial molecular recognition.[7] In most iistances-however, molecular recognition will still rely on diffusion of a guest species to the immobilized host. Alternatively, some supramolecular systems have employed a host and guest that are covalently interlocked (mechanically linked) or maintained in some other type of ”fixed association” that prevents the complete dissociation (into separate solution components) of the host and guest from one another. This would appear to permit rapid interchange between their complexed and “dissociated” states. Self-assembled structures in this class have included catenanes, rotaxanes, shuttles, helicates, stacks and grid-like structures, many of which are discussed in detail in the chapters ahead. Typically these systems rely on multiple electrostatic interactions, especially x-donor and n- or metal ion acceptor type interactions. A clear advantage of systems in a “fixed association” is that many of the types of interactions that may suffer under typical electrochemical conditions, high ion concentrations and polar solvents, are compensated by the guaranteed proximity of the host and guest. In these systems, albeit on a much smaller scale, some of the same effects- proximity and exclusion of infewening solvent and electrolytes- that drive weak interactions to become dominant in proteins, may begin to become more apparent. Thus, electrostatic interactions may be somewhat more significant in such structures than might be anticipated (vide supra). The host-guest pairing may work far more efficiently for electrochemical switching. However, it is possible to envision scenarios in which this may not be the case. While fixed association guarantees the prospect of complexation, important criteria to be considered are the resulting effects on the kinetics and thermodynamics of the redox processes of interest. Changes in the dynamic association-dissociation of the host-guest complex may be expected to have significant effects on kinetic barriers and thermodynamic stability. For example, the work of Evans has shown that oxidation of ferrocene to ferrocenium in the

94

8 Electrochemical Considerations for Supramolecular Systems

presence of P-cyclodextrin takes place only when the ferrocene dissociates from the cyclodextrin.[81What then could be anticipated in the instance of a ferrocenyl moiety trapped with a host molecule? Recently, we have explored such a case for ferrocene encapsulated within a hernicarcerand.Ig1 In this instance the permanent association between host and guest alters the heterogeneous kinetics to such an extent that electrochemical conversion (switching) is significantly hindered. Achieving higher charge states in such a non-polar host environment may be difficult in host-guest systems with a fixed association. In other words, it might be reasonable to anticipate that the thermodynamic stability of the system could change, resulting in a shift in redox potentials, while a loss of electrochemical reversibility, as indicated by a larger separation (AEp) between the peak potentials for a redox couple, would attest to heterogeneous kinetic complications. Such points have been of recent interest both for their bearing on redox processes in biological systems, where redox centers may be buried in a hydrophobic protein core, as well as for their relation to the development of molecular information and storage devices. Other examples of systems in which the electrochemistry of redox active moieties is affected by fixed association have been noted in the literature. One example, presented in Chapter 12, involves a well known bis-paraquat cyclophane host, bearing a 4+ net charge, acting as a bead in a rotaxane. The highly charged cyclophane exerts a dramatic effect on the redox behavior of an aromatic unit in the thread of the rotaxane. This effect, both thermodynamic and kinetic, reflects the substantial electrostatic repulsion created by oxidation of the thread moiety, which generates a more highly charged (from overall charge of +4 to the oxidized states of +5 and +6) system. Due to the dramatic shift in the oxidation potential of the thread moiety and its sluggish kinetics, the usefulness of this particular system as a redox-switch is lost. Thus, the potential for loss of facile electrochemical reversibility in such systems is an important consideration for the supramolecular chemist, because of the implications for switching control. Ideally, electrochemical switching control in supramolecular systems should be via processes that are fast and reversible, in order to assure complete conversion (switching)within reasonable potential limits.

8.3 Systems Involving Multiple Identical or Non-Identical Redox-Active Moieties Many redox-switchable supramolecular systems are designed with the capacity to undergo electron transfer at multiple sites. The voltammetric behavior of such systems can be strongly influenced by the extent of electronic coupling between these sites. In this section we consider the voltammetric behavior presented in three different scenarios- one involving non-identical redox sites and two cases, in which identical redox moieties may be uncoupled or strongly coupled.

8.3

Systems Involving Multiple Identical or Non-Identical Redox-Active Moieties

95

In systems with several non-identical redox active moieties, the electronic effect of one redox-active substituent on another may exert an d u e n c e on both the thermodynamics, e.g. El/& and the kinetics, e.g. the magnitude of AEp, of electron transfer of either or both moieties. Interpretation of the observed electrochemical behavior should consider whether the sites are covalently or mechanically linked. In contrast to the mechanically linked structures mentioned above, the comparison of monomeric to dimeric (or higher order) structures may be less straightforward because of changes in the electrondonating or -accepting character of the extended structural framework. Here we illustrate one such example by considering a simple system composed of two metal cluster sites. Ligand 1, prepared by Haga and coworkers, provides two benzimidazole sites that can coordinate to Ru(I1) and Os(I1) ionsP" The authors also examined the analogous complexes formed with monomeric benzimidazole 2. The metal complexes employ two bipyridine (bipy) ligands to complete the coordination sphere of each metal ion.

2

First we shall consider the redox behavior of the monomeric structures. Os(II)2(bipy)ndemonstrates reversible oxidation of Os(I1) at 0.59 V vs. Ag/AgCl, while the Ru(II)2(bipy)zcomplex exhibits its (reversible) oxidation at 0.96 V vs. Ag/ AgC1. The mixed ligand (bipy)zOs(II)lRu(II)(bipy)z exhibits small shifts in the oxidation potentials of the metal ions. The oxidation of Os(I1) occurs at 0.56 V vs. Ag/AgCl, while that of Ru(I1) occurs at 0.99 V. Contrary to what we might expect, that the oxidation potential of the Os(I1) site to its higher redox state might be negatively mfluenced electrostatically by the presence of the Ru(I1) site, the Os(I1) site in the dimer is actually somewhat more easily oxidized. Oxidation of the Ru(I1) site is rendered slightly more difficult, however. Several factors may determine the extent of interdependence on the electrochemical behavior of

96

8

Electrochemical Considerations for Supramolecular Systems

the neighboring non-identical sites. For covalently linked redox sites both the separation distance between the sites, and the extent to which the molecular framework permits electronic communication between sites, are obviously sigmficant determinants of the electronic coupling. The differences between the oxidation potential for the respective metal centers in the monomeric complexes and the asymmetric dimeric complex are relatively minor. In fact, similar shifts in the oxidation potentials are also exhibited with the symmetric bis-Os(I1) and bis-Ru(I1) complexes of 1. The bis-Os(I1) complex shows its oxidations at 0.53 and 0.58 V vs. Ag/AgCl, while the bis-Ru(I1) complex shows its oxidations at 0.94 and 0.98 V vs. Ag/AgCl. Thus, the more facile oxidation of the first site in complexes of 1 appears to reflect the greater electron-donating capacity of the dimer's extended ligand. The extended x-ligand can offer greater stabilization of the higher charge state on the first oxidized metal center. The small positive shift in the oxidation of the second metal center probably reflects electrostatic repulsion, although the reduced electron-donating capacity of the ligand (which is stabilizing the first oxidized site) may also play a role. In the case of mechanically linked redox sites no such molecular framework communication is present. Clearly the extent of electrostatic attraction or repulsion is affected by the proximity of the redox sites. Controlling the proximity of redox-active sites in mechanically linked structures has been successfully employed in molecular switching. For instance, redox-switched changes in the proximity of electroactive components of rotaxanes form the basis of redox-controlled molecular shuttles (see Chapter 12). When the proximity between the sites is not altered by redox-switching, the resulting electrostatic repulsion or attraction may yield sigmficant changes in the observed kinetics and half-wave potentials of one or both sites, as was mentioned in the previous section. The symmetric metal complexes with ligand 1 allow us to pose the question of what behavior we should expect for structures bearing multiple identical redox sites. Should we expect that both Os(I1) sites will be oxidized at the same potential in the symmetric dimer? How can we interpret the 50 mV separation between the first and second half-wave potentials for the Os(II/III) redox couples? The redox behavior of molecules bearing multiple identical redox sites has been examined in detail by Shain,lW and Bard[l'bI and has continued to be a topic of interest for a number of electrochemists. Sav6ant was the first to point out that a molecule bearing two identical noninteracting redox sites would yield a voltammetric wave with the shape of a single electron transfer reaction, although more than a single electron is transferredP1 The characteristic separation between the half-wave potentials of the two electron transfer processes, in this case equal to a value in volts of (RT/F) In 4, is determined by simple statistics. Bard and Anson further studied the voltammetric response of poly(vinylferrocene) of various weight distributions and extended the statistical treatment.[W Based on the work of these authors, consideration of the expected behavior for systems bearing multiple identical noninteracting electroactive centers is quite straightforward.

8.3

Systems lnvolving Multiple ldentical or Non-ldentical Redox-Active Moieties

97

For a polymeric molecule bearing n independent identical electroactive sites each site should have the same standard potential, E,o and a corresponding half reaction can be defined for the reduction of any one of these sites:

Emo

XXXOXXXX +

e-

+-

e-

XXXXXXXO

Emo

XXXRXXXX

XXXXXXXR

0 and R represent the oxidized and reduced states of the center while X represents a site in either state. At equilibrium the probability that any site i is in the reduced state is given by:

where 0 is given by: 8 = exp[ &(E

-

EO,

i]

and E is the equilibrium potential. The net oxidation state of the molecule is given by the sum of the difference between the total number of sites n and the number of sites in the reduced state, j , that is ( n - j ). Simple binomial distribution leads to the fraction of polymer molecules present with exactly j number of redwed centers: f. = I

[;)(

0 )(n-j)( 1 )i

1+0

(3)

1+0

In a molecule bearing noninteracting centers it is possible to calculate formal potentials for the successive oxidation states. By use of eq. 2 and 3 it can

98

8 Electrochemical Considerations for Supramolecular Systems

be shown that the formal potential for the molecule in oxidation state given byh is:

Based on eq. 5 we can see that for n = 2 the first formal potential (for XR + XO or RX + OX) occurs at -17.8 mV from the observable oxidation wave ( E I ~ ) , while the second formal potential (RO + RR or OR + RR) is symmetrically distributed about this point at +17.8 mV from the observable E 1 p . These small shifts are not discernable by most voltammetric methods. Thus, the voltammogram has the appearance of a normal, single electron process, albeit with two superimposed peaks with differences in their half-wave potentials of 35.6 mV, and a higher current intensity, reflecting the fact that a two electron process is occurring. The peak current intensity may be close to twice that expected for a single electron process. We should note that current intensity is nof an accurate method for assessing the number of electrons transferred. Bulk electrolysis is the appropriate method for this determination. In the case of a dimeric species, the diffusion coefficient is likely to be lower than that of the monomer and thus we would anticipate that the current intensity will be proportionately lower. (Recall from Chaps. 3 and 4 that the current intensity in potential sweep methods is proportional to the square root of the diffusion coefficient.) For molecules with three identical sites (n = 3) we can see that the three formal potentials are distributed about the observed E1/2 at-28.5, 0.0 and +28.5 mV. In a molecule with four identical sites (n = 4) the spacing of the formal potentials occurs at -35.6, -10.4, +10.4 and +35.6 mV. For n = 5 the spacing between the successive formal potentials is distributed about the half-wave potential in according to -41.4, -17.8, 0.0, +17.8, and 41.4 mV. As the number of identical sites increases, the large overlap of concentrations of the partially reduced species begins to affect the appearance of the voltammogram. The observed voltammetric wave begins to broaden. Fig. 8.1 shows the positions of the successive formal potentials for n = 2-6. The formal potentials for the first and last pair of oxidation states in the molecule are given by:

Thus for a molecule of ten identical noninteracting sites the first and last formal potential are separated by 118.3 mV. Again, the voltammetry of such a molecule would reflect both the slower diffusion of the large structure to the electrode surface and a broadened appearance, attributable tp the multiple species undergoing oxidation as the region of the half-wave potential is scanned.

8.3 Systems Involving Multiple Identical or Non-ldentical Redox-Active Moieties

ElF- EnF

E1,20bS

I

n=2

I I

I

n=4

c

n=5

n=6

35.6

I

n=3

I

57

I

71.2

I

I

I

99

I

82.8

90.9

Figure 8.1: Separations for the successive formal potentials of molecules with n identical, noninteracting redox sites. Note that for n 2 4 the separations become nonuniform.[W E i F - EJ is the difference between the formal potentials for the first and last oxidation states.

Based on the foregoing discussion we can see that the symmetric bisOs(I1) complex of 1 shows very slight evidence of interaction between the two redox sites. Rather than displaying the anticipated single wave with the expected but not observable peak to peak separation of 35.6 mV, the separation between the oxidation potentials, El - Ez, was estimated to be around 50 mV. Such small separations in half-wave potentials are often difficult to discern accurately and are best examined by differential pulse voltammetry (see Chap. 4). This value presents a deviation from the behavior expected for a system with identical noninteracting sites. The magnitude of the separation between halfwave potentials of each site is a measure of the extent of electronic coupling interaction between the redox sites. Isomeric covalently linked systems with multiple identical sites provide examples of the variation in the extent of intramolecular interaction according to changes in the molecular framework. Metal-metal interactions can provide particularly dramatic examples of electronic cn*ipling. Below we briefly mention two cases of electronically

100

8

Electrochemical Considerations f o r Supramolecular Systems

coupled redox sites, one on isomeric calixarene-based structures and one based on metal helicates. Diquinonecalix[4]arenes3 and 4 differ in their placement of the quinone rings. Not surprisingly, this leads to differing degrees of interaction between the two quinone sites. Diquinone 3, in which the quinones are proximal, shows two well-separated redox waves corresponding to the first reduction of each of the two quinone moieties, as shown in Fig. 8.2a. The difference between the two half-wave potentials (AE1/2), is 297 mV. In contrast, the voltammetric response for 4 reveals that when the quinones oppose one another, the extent of electronic communication is lessened: AE1/2 decreases to 141 mV.I*41

Figure 8.2: Cyclic voltammetric response on a GC electrode obtained for 1mM solutions of 3 (a) and 4 (b) in CH3CN/O.l M TBAPF6. Scan rate was 100 mV/s, potentials referenced to Ag/AgCl.[**l Used with permission of the author.

8.3

Systems Involving Multiple Identical or Non-Identical Redox-Active Moieties

101

Helicates are a special class of metal complexes covered in detail in Chap. 13. Ligand 5 (a terpyridine derivative) forms Cu(I/II) helicates composed of two metal ions and two ligands. The solution structure of the Cu(1) complexes of 5 has been reported to have an unusual diamond-like tetrahedral ge~rnetry"~] (see inset in Fig. 8.3). This compIex, which is discussed in greater detail in Chap. 13, is novel in that it shows an extremely dramatic separation of 860 mV between the first and second Cu(I/II) redox couples, as shown in Fig. 8.3. Both processes were quasireversible in CH3CN. n

= Pyridine

I

1

1

1

+1.W

I

I

I

I

I

0= Cu(1)

0 = Phenyl

I

M.50

I

I

1

1

I

I

I

0.0

E vs SSCE Figure 8.3: Cyclic voltammograms of a 1.1mM solution of [Cu(II)52] in CH3CN/TBAPF6. The scan rate was 100 mV/s and the electrode surface was platinum. The inset shows the space-filling representation of [Cu(I)5,] based on the x-ray structure of right-handed helicate.Ps1 Reprinted w i t h permission of the American Chemical Society.

102

8 Electrochemical Considerations for Supramolecular Systems

8.4 References 1. C. Reichardt, Solvent Efects in Organic Chemisfy, 2nd Ed., VCH, New York, 1988, p. 408 - 410. 2. Clearly here we take into consideration only the decrease in solution resistance provided by the higher dielectric solvent. Other considerations such as solvent volatility may make CH2C12 more ideal from the standpoint of sample recovery. 3. Under these conditions the reader is referred to Chapter 4 for the discussion of normal pulse voltammetry, which may aid in the study of analytes which tend to precipitate on the electrode surface in their switched oxidation states. 4. [a] D. B. Smithrud, T. B. Wyman, F. Diederich, J. Am. Chem. SOC., 1991, 113, 5420 - 5426; [b] D. B. Smithrud, F. Diederich, J. Am. Chem. SOC., 1990,112,339 - 343. 5. D. A. Dougherty, Science, 1996,271,163 - 168. 6. M. A. Petti, T. J. Shepodd, R. E. Barrans, Jr., D. A. Dougherty, J. Am. Chem. SOC., 1988,110,6825 - 6840. 7. [a] M. T. Rojas, R. Koniger, J. F. Stoddart, A. E. Kaifer, J. Am. Chem. SOC., 1995, 11 7,336 -343; [b]M. T. Rojas, A. E. Kaifer, 1. Am. Chem. SOC., 1995,117,5883 5884; [c] 0.Chailapakul, D. Crooks, Langrnuir, 1995,ZZ,1329 - 1340. 8. T. Matsue, D. H. Evans, T.Osa, N. Kobayashi, J. Am. Chem. SOC.,1985,107,3411 - 3417. 9. S. Mendoza, P. D. Davidov, A. E. Kaifer, Chem. Eur. J., 1998,864 - 870. 10. M.-a. Haga, T.-a. Ano, T. Ishizaki, K. Kano, K. Nozaki, I. Chern. SOC., Dalfon, 1994,263 - 272. 11. [a] R. L. Myers, 1. Shain, Anal. Chem. 1969, 41 980-990. [b] A. J. Bard, Pure App. Chem., 1971,25,379-393. 12. F. Ammar, J.-M. Saveant, Electroanal. Chem., 1973,47,215-221. 13. J. B. Flanagan, S. Margel, A. J. Bard, F. Anson, J. Am. Chem. SOC., 1978, ZOO, 4248 - 4253. 14. M. Gomez-Kaifer, Ph.D. Dissertation, University of Miami, 1997, 145-147. Used with with permission of the author. 15. K. T. Potts, M. Keshavarz-K, F. S. Tham, H. D. Abruiia, C. Arana, Inorg. Chem., 1993,32,4450-4456.

Supramolecular Electrochemistry

Angel Kaifer, Marielle G6mez-Kaifer 0 WILEY-VCH Vcrlag GmbH. 1999

9 Electrochemical Switching

Electrochemical switching is perhaps the most important application of electrochemistry in the field of supramolecular chemistry. This concept, whose use has been widespread in the past two decades, has been exploited to such an extent that a thorough examination of its use is well beyond the scope of this book.111 The early focus of this concept in supramolecular research was primarily cation recognition. During the 1980s much interest centered on redox-switchable cation binding systems as potential mimics of biological cation transport or the development of analytical methods for cation sensing. More recently this emphasis has broadened to include anion and molecular recognition. Without a doubt however, the predominant interest in this field has shifted toward the development of switchable molecular devices. Electrochemical switching provides an easy means for con trolling the molecular architecture of redox-active supramolecular systems. In the four chapters that follow, we will see the prominence of this concept in the field. In this chapter we will examine the basic concept of electrochemical switching, and by way of example, contemplate its application in two areas: cation binding, and its power to exert control over molecular architecture.

9.1 The Concept of Electrochemical Switching The concept of an electrochemically switchable molecule is a simple one. Such a molecule displays differing affinity with a second species based on its redox state. The oxidation state of the redox-switched component of the pair determines the thermodynamic stability of the complex formed between the two species. The basis of this differential affinity is purely electrostatic. Perturbation of the charge in a redox-active host or guest can result in increased or decreased binding affinity. When the magnitude of this change in interaction energy is strong, the electrochemistry may clearly reflect two different redox states, i.e. the interacting and noninteracting species may give rise to different half-wave potentials. In electrochemically-switchable systems either host, guest, or both, may be redox active. The requirements for such a host or guest are essentially the same. First, in order to provide meaningful switching control, the redox-active moiety to be switched should exhibit reversible heterogeneous electron transfer kinetics. Without reversible kinetics, the switch itself is in essence rendered too slow to be useful. A second requirement is that at least one of the redox states

104

9

Electroclleniical Switching

must interact strongly with its targeted species. These two prerequisites apply for any switchable system- from ion binding agents to molecular shuttles. Analytical work presents additional requirements. In general, for work involving sensors, the half-wave potentials of the free and bound states should ideally be well separated from one another in order to see true two wave behavior. This point is important because the intensity of a voltammetric wave is proportional to the concentration of the corresponding complexed or free molecule. Thus, for most quantitative analytical work there should be two clearly defined states that are readily discerned via voltammetric techniques. These states are defined as "on-off" or more accurately, "high-low" states of interaction. For the easiest analysis of a switchable binding scenario, both forms of the redox-active host or guest should have sufficiently high binding affinity if separate redox waves for the two states are to be observed by voltamrnetry.[21 This is an important consideration when trying to extract accurate binding constant information from voltammetric data. In most instances however, digital simulations provide the best analysis. A simple thermodynamic square scheme can be employed to elucidate the equilibria in a redox-switchable system. Such a scheme is shown in Fig. 9.1. In this system, the redox equilibrium is coupled with a reversible binding reaction. For this example, we assume that the host H is electroactive and is switched from its low to high binding state by reduction. In this square scheme, host H forms more stable complexes with guest G when it is reduced (H-)than when it is oxidized (H). Therefore, the association constant KH is much larger than KL. In this scenario, when the initial state shows a lower association constant than the electrochemically switched state the magnitude of the ratio of association constants KH/KL is defined as the binding enhancement. The value of this term may be estimated (vide inpa) by the difference in formal potentials for the free and complexed host as shown in Eq. (1):

KH/KL

= exp[-F(E$

-Eg)/RT]

In this expression EF is the formal potential of the free ligand, Ec the formal potential of the complexed ligand, (both values are usually approximated by the

Figure 9.1:A square scheme for binding equilibria with a redox switchable host.

9.2

Switcluzble Binding in a Redox-Active Cation Host

105

half-wave potentials), F is the Faraday constant and R the molar gas constant in J/mol.K. The larger the difference in half-wave potentials, the greater the magnitude of the binding enhancement. The value of KL determines whether H or H-G is reduced at the electrode surface. When KL is large, the complex H-G, already formed, is electrochemically reduced to a higher affinity state, H--G. In this situation, the diffusion of the guest species is not a relevant factor. In contrast, if KL is not large the species undergoing reduction will be H, which will subsequently bind the guest to yield H--G. In this instance, after reduction of the free host to the "high" binding state (H-), the complexation process is essentially diffusion controlled, i.e. determined by the diffusion of available guest to the reduced receptor. Generally, two separate voltammetric waves corresponding to the redox processes of the free (H) and complexed (H-G) species will not be observed for systems with a low KL. In this binding regime the rate constants for complexation and decomplexation may be an important consideration (vide infiu). The typical electrochemical response observed for a system with a very low KL will be a shift of the half-wave potential for the free host species as guest is added to the solution, i.e., separate waves will likely not be observed for the free and bound species.PI We shall consider these various aspects in greater depth in the foIIowing section.

9.2 Switchable Binding in a Redox-Active Cation Host Crown ether and cryptand structures are well known hosts for a variety of metaI ions. When redox-active substituents are added to a crown ether or cryptand structure, the binding of a metal ion may affect the redox behavior of the host. Ferrocenyldimethyl-[2.2]-cryptand1 is a sensitive redox probe for the presence of a variety of ions, including Na+, K+, Ca*+,and Ag+.[4]1exhibits the expected voltammetric behavior of a ferrocene derivative, with a reversible monoelectronic oxidation at +0.216V vs. SSCE. Based on the nature of ferrocene electrochemistry, we would expect that binding of a cation in the cryptand region of this host would shift the oxidation of ferrocene to more positive potentials, due largely to electrostatic repulsion. This expectation is confirmed when we examine the cyclic voltammetric behavior of 1in the presence of an

1 ' Oq\

I I

106

9

I

-0.2

Electrochemical Switching

I

0.7 -0:2 €/V vs. SSCE

0.7

Figure 9.2: Cyclic voltammetry of 1 in CHKN/O.lO M TBAPF6, scan rate of 50 mV/s. (a) No Na'; (b) 0.25 equivalents of Na'; (c) 0.50 equivalents; (d) 0.75 equivalents; (e) 1.0 equivalents, ( f ) 3.0 Reprinted with permission of the Royal Society of Chemistry.

increasing concentration of Na', as shown in Figure 9.2. In the presence of 0.25 equivalents of Na+, a second redox wave emerges at 0.402 V vs. SSCE (Fig. 9.2b) As the concentration of the salt is increased this second wave grows in intensity at the expense of the current observed for the original. The fact that 1-Na' is oxidized at more positive potentials confirms that the binding of Na' to the cryptand destabilizes the ferrocenium form of the complex due to the increased positive charge near the redox active site. Examining Fig. 9.2, we can easily see that both the free and complexed 1 yield reversible voltammetric behavior. What can we assume about the nature of 1-Na' complex from the fact that it too, yields a reversible redox couple? Here we can pause to analyze each of the components in the switching scheme. Fig 9.3 shows the square scheme for 1, along with the association constants for 1 and 1+ with Na+, as determined for 1 by potentiometric experiments and for 1' as calculated by the shift in the half-wave potentials (more on this point later).

9.2

Switchable Binding in a Redox-Active Cation Host

107

While it is evident that 1 has a high binding association constant for Na+ it would seem that 1' also has a moderate binding affinity for Na+. Based on the

Kal= 2 x lo6

1

+ Na'

1' + Na'

-

-

L

1-Na'

-

1'-Na'

Figure 9.3: A square scheme for the equilibria of 1and it's Na+complex.

voltammetric results, this is a reasonable assumption since the new redox wave that appears for the 1-Na+ complex is reversible. What behavior would we expect if the binding constant for 1' were substantially lower, for instance on the order of 1 M-I? We might expect the appearance of the voltammogram to be different, namely in that after oxidation of the complex 1-Na', the significant loss of binding avidity might result in decomplexation of Na' and a subsequent decrease in the currents attributable to the reduction of 1'-Na'. This argument makes sense, assuming that the rate of decomplexation of the Na' ion is very fast. If we begin to examine the kinetics regime for ion complexation and decomplexation however, we know that the rate of decomplexation, from cryptands in particular, can actually be quite slow. What effect would the kinetics of decomplexation have on the voltammetric appearance of 1-Na'? If the value of the association constant were very low and the rate of decomplexation very slow, the voltammetric behavior might approach reversibility. If Na' cannot decomplex rapidly, even with low affinity for its host's new redox state, on the voltammetric timescale we might still see the reduction of 1-Na+with currents approaching those observed for the oxidation process. In this instance scan rate studies might be useful, although there are lower limits on how slowly we can sweep the potential and still avoid convection. In such situations, the only way to truly comprehend the electrochemical binding scenario is via digital simulations. Digital simulations of 1reveal that the ratio of binding constants for 1is actually much higher than that estimated by the difference in half-wave potentials. Although K1/KJ51 was initially estimated at 1500, digital simulations of this system later showed that this value must be closer to 3 x lo4 in order to reproduce the voltammetric behavior.@a] This ratio of constants drops the association constant for 1+-Na+to 64 M-I! Yet, even this low value does not necessarily imply that the kinetics of decomplexation is the determining factor in the voltammetric appearance of the reversible redox wave for 1-Na+. In the

108

9

- 0.2

~

Electrochemical Switching

~1

+ 0.8

Potential, V us SSCE a solution containing 1.0 mM 1 and 0.50 mM NaC104. (a) Experimental voltammogram at 100 mV/s; (b) simulated voltammogram using KI= 2 x 106 M-1 and K2=50.[4al Reprinted with permission of the American Chemical Society.

Figure 9.4:Voltammetric response of

simulation described, no assumptions were made about the kinetics of decomplexation, i.e., it is implicit that the rate of decomplexation is fast on the timescale of the vokammetric experiment, in order to allow all species to be at equilibrium. In spite of this lack of kinetic considerations, the excellent fit of the simulations to the experimental data is evident in Fig. 9.4. The authors have theorized that while the rate of decomplexation of Na' may indeed be slow, the complexation equilibria appear to be frozen on the voltammetric timescale.[4] Even very slow scan rate studies were unable to differentiate a decrease in the reduction wave for 1-Na' complex that would suggest decomplexation. Evidence of decomplexation effects has been noted in related systems, however. Ferrocenyl macrocycle 2 is the synthetic precursor of 1.This cryptand also exhibits significant binding and distinct oxidation waves for both the free and bound forms of 2 in the presence of Be2'.[61 In this instance however,, differences in the cathodic and anodic current intensities suggested dissociation of the 2+-Be2' complex on the cyclic voltammetric timescale. These effects were evident even with moderately fast scan rates (400 mV/s). In the presencqof Mg2', Ca2+,Sr2+or Ba2+,2 exhibited voltammetric behavior typical of lower KL

9.3

Electroclleniical Switching

a5

a Means of Controlling Molecular Devices

109

systems: a single oxidation wave was observed for the free and bound ligand, and the E1/2 shifted from that of the free ligand.161 What factors can aid in the analysis of this class of redox-switchable binding systems? Digital simulation is clearly one of the most important tools. In titration experiments a wide concentration range of the bound species should be examined, i.e., ranging from substoichiometric amounts to as large an excess number of equivalents as solubility permits. Examining the redox behavior in the presence of excess guest permits the determination of more accurate halfwave potentials for bound states of the redox-active host. When KLis quite low, recaIl that shifts in the half-wave potential may occur, thus the true position of voltammetric peaks may not be evident without such a thorough titration study. NMR experiments may yield information about the rate of guest complexation or decomplexation, while both NMR and potentiometric studies (when possible) may provide a good experimental value for the binding constant of the initial state of the host. The foregoing analysis points to the complex factors that are at work in a redox-switchable host-guest system. These points are important when one is concerned with switchabIe systems for transport or sensing. However, in many instances the more recent applications of switching have a completely different emphasis. In contrast to sensing or transport systems, which may operate on a "high/low" principle, redox-switching in many of the other avenues of supramolecular research may truly operate on an "on/off" principle. In these cases, the idea of electrochemical switching is applied with a different focus.

9.3 Electrochemical Switching as a Means of Controlling Molecular Devices and Other Structures The goal of one avenue of supramolecular research is the development of switchable molecular devices. These electrochemically or photochemically active molecules can undergo a change in molecular structure that permits assignment of clearly defined on/off states. To be useful, a switch must show reversibility, and as we have mentioned in Section 9.1, a redox-switchable structure must have fast heterogeneous kinetics. This implies that the rate of switching itself will not be limited by the rate of electrochemical processes.

110

9

Electrocllernical Switching

What types of molecules display this device-like switching? And what types of changes are induced by chemical or electrochemical redox-switching? If we consider that a switch requires clearly defined on/off states we can relax the requirement that both states of the redox-switchable species interact with a second species. For instance, in a molecular shuttle the bead-like host is switched in between two topologcally linked guest sites on the rotaxane thread.l71 Electrochemical-switching of one of the guest sites results in translocation of the bead along the thread to the other site. In this instance, while the bead still interacts with the thread, in reality, the host is now interacting with a different moiety on the thread. The same type of reactions could be observeU if all three individual components (the host and two nonlinked guests) were in solution. However, the great limiting factor of such a switching system would be the slow rates of diffusion, as the host searches for a new guest that exists in a more compatible redox state. A design in which the host and two guests are linked in a rotaxane structure, greatly enhances the functional rate of the switch. Nevertheless, occasionally effective switchable systems can involve more than two solution species. Rotello and coworkers have developed just such a three component system, in which a change in the oxidation state of one molecule switches its affinity for a second species to a thirdP1 Broadening the definition of switching even further, we can eliminate the requirements for a second species and consider the effects of redox-switching on the molecule’s self-interaction. A process as familiar as electrochemicallyinduced dimerization could be considered an example of switching: a different species, with potentially very different structural and spectral properties, is generated. A prominent example of this concept is the helicates, a class of molecules in which redox-switching can induce the reversible self-assembly of new species.I91 These various systems are shown schematically in Fig. 9.5.

Figure 9.5: Concepts of several electrochemically switchable systems. From top: a three component switch, a molecular shuttle and a helicate.

9.3 Electroclzemical Switching as a Means of Controlling Molecular Devices

111

Drawing upon the above-mentioned concept, many of the most promising switchable systems are those in which all the switching components are topologically linked. In this sense, while the components of the system may not be covalently bound to one another, their proximity to one another is assured. In this class of molecules there are many examples of redox-switchable intertwined or helical structures. Although many of these systems have been explored eIectrochemically, there are s t i l l a sigruficant number whose redoxswitching has been examined by chemical means. One such intriguing molecule is 3, a helical iron binding ligand that can translocate iron ions to two distinct coordination sites on the ligand as their redox state is changed from Fe2+and Fe3+ states.[I01 Readily discerned spectroscopic changes in the visible range sigrufy the change in the redox state and coordination sphere. 3 is a three armed ligand with an internal tris(hydroxymate) binding Fe3+ site and an external tris(2,2'-bipyridine) Fe*+ coordination site. The Fe3+ coordinated system is readily reduced by ascorbic acid and rapidly generates the tris(bpy) complex of Fe2+.The reverse process, oxidation with ammonium persulfate, regenerates the hydroxymate Fe3+complex on a slightly slower timescale, with the half-life of the reduction process on the order of 15 s. Curiously, a chiral ligand structurally related to 3 (bearing an alanyl residue) displays an even longer half-life for the reduction process, -45 s. In an important experiment pointing to the efficacy of modular design, the authors were unable to accomplish the analogous complexation intermolecular processes by employing two tridentate ligands bearing the hydroxymate and bipyridine residues, respectively. These

s I

,1

0

Fez+ -binding site

P O
475 nm

electron transfer

Figure 15.9 Willner's photoswitchable SAM. Photochemical control of communication between the electrode and cytochrome c.

the

Very recently the Willner group has also reported a method to photochemically imprint molecular recognition sites in SAMs.[l71Their approach relies on the preparation of mixed SAMs from tetradecanethiol and a thiolfunctionalized naphthacene quinone. Irradiation of this monolayer (320 < h < 380 nm) photoisomerizes the quinone units to their "anal'-quinone forms (Fig. 15.10), which in turn can be removed from the monolayer by reaction with free amines, leaving behind well-defined empty sites which can recognize diffusing

15.4

Pkotosruitcliable SAMs

205

naphthacene quinone molecules. These sites reveal high selectivity in their recognition ability as they do not show any significant binding affinity for structurally related molecules.

Figure 15.10: Willner's method for the photochemical imprinting of recognition sites in SAMs.

These examples illustrate some of the possibilities that are being explored with photoswitchable SAMs. Other groups have used light to fabricate two-dimensional patterns on monolayer assemblies. While basic research in SAMs appears to have reached maturity, this area is still very active and new developments continue to appear in the literature.

206

15

Self-Assembled Monolayers

15.5 References 1. (a) L. H. Dubois and R. G. Nuzzo Ann. Rev.Phys. Chem. 1992,43,437-63. (b) G. M. Whitesides and P. E. Laibinis Langmuir 1990, 6,87-96. 2. (a) H. 0. Finklea in Electroanalyfical Chemistry, A. J. Bard and I. Rubinstein, Eds.; Dekker: New York, 1996; Vol. 19, pp109-335. (b) D. Mandler and I. Turyan Electroanalysis, 1996, 8, 207-213. 3. C. Miller, P. Cuendet and M. Gratzel, J. Phys. Chem. 1991,95,877-886. 4. Y. Wang and A. E. Kaifer 1. Phys. Chem. B 1998,102,9922-9927. 5. R. Bilewicz, T. Sawaguchi, R. V. Chamberlain I1 and M. Majda Langmuir 1995, 11,2256-2266. 6 . 0 . Chailapakul and R. M. Crooks Langmuir 1993,9,884-888. 7. C. E. D. Chidsey Science 1991,251,919-922. 8. G. K. Rowe and S. E. Creager Langmuir 1991, 7,2307-2312. 9. I. Rubinstein, S. Steinberg, Y. Tor, A. Shanzer and J. Sagiv Nature 1988, 332,426-429. 10. S. Steinberg, Y. Tor, E. Sabatani and I. Rubinstein J. Am. Chem. SOC.1991, 113, 5176-5182. 11. S. Flink, B. A. Boukamp, A. van den Berg, F. C. J. M. van Veggel and D. N. Reinhoudt J. Am. Chem. SOC.1998,120,4652-4657. 12. See, for instance: E. U. T. van Velzen, J. F. J. Engbersen, P. J. de Lange, J. W. G. Mahy and D. N. Relnhoudt 1. Am. Chem. SOC.1995,117,6853-6862. 13. M. T. Rojas, R. Koniger, J. F. Stoddart and A. E. Kaifer J. Am. Chem. SOC.1995, 117,336-343. 14. For reviews, see: (a) R. M. Crooks and A. J. Ricco Acc. Chem. Res. 1998, 31, 219-227. (b) A. J. Ricco, R. M. Crooks and G. C. Osbourn Acc. Chem. Res. 1998, 31,289-296. 15. I. Willner Acc. Chem. Res. 1997, 30,347-356. 16. M. Lion-Dagan, E. Katz and I. Willner J. Chem. Soc., Chem. Commun. 1994, 2741-2742. 17. M. Lahav, E. Katz, A. Doron, F. Patolsky and I. Willner J.Am. Chem. SOC.1999, 121,862-863.

Supramolecular Electrochemistry

Angel Kaifer, Marielle G6mez-Kaifer 0 WILEY-VCH Vcrlag GmbH. 1999

16 Electroactive Dendrimers

During the past few years one of the most active areas of research in chemistry has focused on dendrimers. This term describes a special type of macromolecules that are built from a central core and expand in three dimensions forming globular structures with well-defined surfaces.il1 Quite often, dendrimers are prepared layer by layer using a repetitive synthetic sequence. Therefore, the term generation was coined to describe the extent of growth of these macromolecules as measured by the number of synthetic iterations needed for their preparation. Typically, a first generation dendrimer is a rather simple molecule with 3 or 4 identical functional groups on its periphery, while a fourth or fifth generation dendrimer may contain nearly 100 tightly packed functional groups and have a molecular mass in the range of 10-100K daltons. A distinguishing feature of dendrimers as compared to linear chain polymers is that they typically exhibit a much higher degree of monodispersity. As supramolecular chemists strive to prepare nanometer scale structures, dendrimers have become a natural framework for this type of work. In this chapter, we will limit ourselves to review some of the recent developments involving electroactive dendrimers, as well as dendrimer use in electrochemistry. We define electroactive dendrimers as those that contain functional groups capable of fast electron transfer reactions. We can class+ all electroactive dendrimers into two general classes: (i) Dendrimers with peripheral electroactive groups, and (ii) dendrimers with internal electroactive groups. In the second class, the most common situation arises when the central core of the dendrimer is the single electroactive group. However, the literature contains several examples of dendrimers with multiple internal electroactive groups.

16.1 Dendrimers with Peripheral Electroactive Groups These dendrimers are prepared by attaching a number of identical electroactive functional groups to the periphery of a dendritic macromolecular framework. Each dendrimer macromolecule is thus capable of exchanging mxn electrons, where m is the number of electroactive residues and n is the number of electrons exchanged per residue in the corresponding electrochemical reaction. The multielectron character of the electrochemical reactivity of these dendrimers might find use in catalysis or in other applications involving

208

I6

Electronctive Dendrimers

me

Polar, cationic dendrimer

Nonpolar, neutral dendrimer

Figure 16.1:Polarity changes experienced upon electron transfer reactions by dendrimers containing peripheral electroactivegroups.

chemical amplification, although these possibilities have not been explored yet. One of the key problems in this type of work is that the polarity and solubility properties of these dendrimers change drastically as a result of the multiple electron transfer reactions that they can experience, hindering their use in catalytic cycles. Bryce and coworkers reported one of the first electroactive dendrimers of this class in 1994.[21 Using a convergent synthetic approach relying on a repetitive coupling/ deprotection sequence, they prepared the symmetric dendrimer 1, which has 12 tetrathiafulvalene (TTF) units in the periphery, as well as several other related TTF-containing macromolecules. Dendrimer 1 exhibits the electrochemical behavior typical of TTF, with two quasi-reversible oxidations in acetonitrile solution at half-wave potentials of 0.43 and 0.81 V vs Ag/AgCl. The authors concluded that there is no significant interaction between the TTF units in any of their oxidation states. In a later paper,c31 they report thin layer cyclic voltammetric data confirming that all TTF subunits are oxidized. One of the most interesting properties of TTF is its ability to form charge transfer complexes with electron acceptors. The TTF cation radical (TTF") dimerizes, giving rise to a characteristic band at 830 nm. The association of TTF" to form dimers and ordered stacks is of considerable interest as they give rise to organic solids with electronic conductivity. These authors explored the behavior of oxidized 1 and similar TTF-containing dendrimers and found spectral evidence for inter-dendrimer interactions relying on TTF+'- TTF" contacts. The group of Cuadrado and Moran have also prepared several classes of dendrimers with organometallic subunits on their periphery. In 1995 these authors reported on two novel dendrimers with a silicon backbone and 4 or 8

16.1

Dendrimers with Peripheral Elecfroactive Groups

209

O Y 0

g '0 0

3 0

I 7TF

I TTF ~ T F

STF

equivalent ferrocene subunits on the periphery.[41 Voltammetric results in CHzC12 solution indicate that the oxidized forms of these dendrimers deposit on the electrode surface forming stable electroactive films. Platinum electrodes modified in this fashion could be transferred to fresh solutions containing no dendrimers, yielding persistent voltammetric responses. A similar class of dendrimers with peripheral ferrocene subunits has been utilized by Astruc and coworkers to demonstrate the so-called dendritic effect in molecular recognition interactions.151 Oxidation of the nine ferrocene groups of dendrimer 2 yields a species with a 9+ charge, which is capable of anion recognition. Cyclic voltammetry of 2 in CH2C12 shows a single anodic reversible wave at E1p = 0.69 V vs SCE. Titration with n-Bu4N+H~P04-,for instance, leads to the gradual development of a new wave at less positive potentials at the expense of the original wave. The replacement of the initial wave by the anion-induced one is completed after the addition of 1 equiv of nBu4N+H2P04-per ferrocene group. This reveals that the oxidized dendrimer binds the anions much more strongly than the reduced dendrimer. A more quantitative analysis of the data shows that the spatial accumulation of positive charges on the dendrimer surface (dendritic effect) leads to substantial increases of the apparent binding constants compared to those that would be measured with isolated ferrocene receptors. Even larger dendritic effects are observed with a higher generation dendrimer analog possessing 18 ferrocene groups in its surface. Since anion recognition results from the combination of hydrogen

210

16

Electroactiue Dendrimers

0

2

bonding to the amidic N-H and coulombic attraction between the anion and the positive charge of the oxidized ferrocenium residue, the binding ability and the dendritic effect are larger for H2P04- and HS04- than for C1- and NOs-.[5l These results open interesting research avenues for the use of dendrimers as active components in electrochemical sensors. The group of Cuadrado and Mordn has also reported on another series of dendrimers possessing 4,8,16,32 and 64 peripheral ferrocene groups.[61 This series of macromolecules was built around poly(propy1eneimine) frameworks. The electrochemistry of these dendrimers reveals the non-interacting character of all the ferrocene subunits. In nonpolar solvents such as CH2C12, the oxidized form of the dendrimers was found to deposit as robust films on the working electrode surface. These dendrimers were also shown to act as multisite guests for complexation by P-cyclodextrin hosts,[? yielding large supramolecular assemblies in aqueous solution that could be disrupted by electrochemical or chemical oxidation of the ferrocene residues. The same group has also reported a unique example of silicon-based dendrimers that exhibit a substantial extent of interaction among their peripheral ferrocene For instance, the cyclic voltammetry of 3 (see structure in the next page) in CH2Clz:CHKN (5:l v/v) reveals two distinct waves (separated by 190 mV) for the oxidation of the peripheral ferrocene

16.1

Dendrimers with Peripheral Electroactive Groups

4.0

f 1Vvr.SCE

211

Q.Q

Figure 16.2: Cyclic voltammogram of 3 in CH$&/CfiCN (5:l) v/v with 0.10 M TBAPF,j.[al Reprinted w i t h permission of the American Chemical Society.

212

16 Electroactive Dendrimers

groups. This finding indicates that the two ferrocene centers attached to the terminal silicon atom in each of the dendritic wedges undergo oxidation at different potentials, due to their strong mutual interactions. Dendrimer 5 and a higher generation analog (16 ferrocenes) deposit on the working electrode surface upon oxidation in CHzClz, yielding electroactive films which maintain the same kind of two-wave voltammetric response. A recent report from the same authors (working in collaboration with the authors’ group) has focused on the synthesis and electrochemistry of dendrimers based on the poly(propy1eneimine) backbone and containing 4, 8, 16, and 32 cobaltocenium subunits on their (the structure of the tetrameric dendrimer is shown in Fig. 16.3.). These cationic dendrimers were isolated as their hexafluorophosphate salts. The voltammetric behavior of these dendrimers in aqueous media resembles that of simple cobaltocenium, which undergoes a fast one-electron reduction to yield cobaltocene. This neutral and hydrophobic compound is not soluble in water and, therefore, precipitates on the working electrode surface. The shape of the anodic peak in the reverse voltammetric scan, a very sharp spike, reflects the stripping and re-dissolution of the deposited cobaltocene upon oxidation. In spite of this complication, the reduction of all the cobaltocenium subunits in these dendrimers takes place in a single, if distorted, wave. In other words, all the cobaltocenium subunits are independent or non-interacting. We have shown previously (Chapter 7) that

Figure 16.3: Electrochemically-driven binding of cobaltocene dendrimers by p-CD hosts.

7 6.2

Dendrimers with Internal Electroactive Groups

213

cobaltocene forms a stable inclusion complex with p-CD, while cobaltocenium is not bound by this host. Are these binding interactions maintained between the peripheral subunits of these dendrimers and the p-CD hosts in the solution? Voltammetric data obtained with the cobaltocenium-containing dendrimers (4, 8, and 16 residues) in the presence of p-CD strongly suggest that, while the oxidized dendrimers do not interact with the CD hosts, multiple inclusion complexes are formed upon dendrimer reduction to the cobaltocene form (see Fig. 16.3). This constitutes an excellent example of large molecular weight supramolecular assemblies that are formed only after appropriate electrochemical stimuli are applied to the system.

16.2 Dendrimers with Internal Electroactive Groups The previous section has shown that the surface functionalization of dendrimers with redox active residues yields macromolecules capable of multielectron transfer reactions. A general observation common to the majority of these systems is that the electrochemical properties of the dendrimers resemble those of the monomeric redox active species. Furthermore, the electroactive centers usually behave as non-interacting subunits. In contrast to these findings, one would anticipate that internal electroactive residues should exhibit electrochemical properties reflecting their environment and, thus, be more sensitive to the surrounding dendritic backbone. In fact, this is often the case. In some instances (vide infra), analogies can be established between dendrimers having internal electroactive groups and redox proteins. Furthermore, internal redox active groups provide interesting possibilities regarding electric charge and information storage in these macromolecules. Diederich and coworkers pioneered the exploration of these issues by preparing dendritic systems around a metal porphyrin core.[101 The two dendrimers shown below are representative structures of the two series of macromolecules synthesized by this group.1111 Dendrimer 4 has a zinc porphyrin core and a lipophilic surface, which renders it soluble in nonpolar solvents, such as THF and CH2C12. On the other hand, 5 has a Fe core (with an additional chloride in the fifth coordination site) and more hydrophilic peripheral groups that make it water soluble. The third generation analog of 4 has 108 methyl carboxylates on its surface and a molecular weight of 19,054 daltons. Computer modeling suggests that it has a dense structure and globular shape, with a diameter of ca. 4 nm, a size similar to that of the redox protein cytochrome c. These dendrimers exhibit half-wave potentials for the first porphyrin-centered oxidation and reduction processes that shift to more negative values with increasing dendrimer generation.[lOJlI This finding is counterintuitive, because it means that the generation of positive charge on the porphyrin residue is thermodynamically favored by the growth of the surrounding dendritic mass. The voltammetric behavior of these dendrimers also reveals that the kinetics of the electron transfer reactions slow down as the dendrimer generation increases.

214

16

Electroactive Dendrimers

Y

O M 0

e

The series of iron dendrimers was also studied in aqueous solution.[111 In going from dendrimer 5 to its next generation analog the potential for the Fe(III)/Fe(II)redox couple becomes more positive by 420 mV. This finding was rationalized by the increased hindrance experienced by water molecules as they solvate the more highly charged Fe(II1) oxidation state through the increased dendrimer mass. These same Fe dendrimers showed only small potential differences when their electrochemical behavior was recorded in CH2Ch. In collaboration with Collman's group, these authors have also reported on the 0 2 and CO binding affinity of the Fe(I1) forms of these dendrimersP21 (with imidazole replacing chloride at the fifth coordination spot). They found that the oxygen affinity is 1,500 times higher than that of hemoglobin, while the CO affinity is similar. Both diatomic gases were bound reversibly by the Fe(I1) porphyrin dendrimers. Aida and coworkers have also reported on the electron transfer reactions,[131 and 0 2 and CO binding of Frechet-type, porphyrincontaining dendrimers"41. Recently, Gorman and coworkers have described the synthesis and properties of a novel series of dendrimers"51 built around a redox active ironsulfur core of the form [Fe4S4(SR)4I2-.This core exhibits a quasi-reversible oneelectron reduction. The voltammetric behavior of these dendrimers demonstrates that the reduction processes become kinetically and thermodynamically hindered with increasing dendrimer generation (zeroth to fourth). This finding was interpreted to result from the increased steric bulk imposed by the dendritic ligands R.

16.2 Dendrimers with Internal Electroactive Groups

215

Figure 16.4 The basic concept of Newkome's dendrimer systems, which contain a single bis(terpyridyl)Ru(II) center.

Newkome and coworkers have developed an interesting approach to the assembly of dendrimers that relies on the coordination of metal ions.[161 The main idea is represented in Fig. 16.4. Two terpyridine-derivatized dendritic wedges can be assembled into a larger structure by using an appropriate metal ion to which the terpyridines will act as ligands. These authors have used Ru(I1) for the preparation of a series of dendrimers containing either a single[161 or several[171bis(terpyridyl)Ru(II) centers. For the systems with a single metallic center, the kinetics of the electron transfer reactions was found to slow down as the dendritic wedges became bulkier and surround the metal complex. The group of Balzani, at the University of Bologna, has also made extensive use of transition metal coordination in the design and preparation of many novel dendrimers.['s] Their systems are completely synthesized relying on metal coordination, using the directionality of ligand-to-metal bonds for the structural design of the dendrimers. The main building blocks used in this approach, along with Ru(I1) and Os(I1) nucleating metal ion centers, are illustrated in Fig. 16.5. The resulting dendrimers can be considered as organized assemblies of coordinated metal centers. In addition to very interesting spectroscopic properties, these dendrimers exhibit rich electrochemical behavior related to metal-centered one-electron oxidations or ligand-centered reductions. The magnitude of the interactions among the metal centers varies from small to considerable depending on their relative proximity. Therefore, the electrochemical behavior can be predicted if one considers these effects as well as others resulting from the macromolecular nature of these systems. As an illustrative example, let us look at the behavior of the decanuclear Ru(I1) dendrimer 6. This dendrimer contains three kinds of Ru(I1) centers: One in the exact center of the molecule, three internal ones, and six peripheral ones. Since

216

16

Electroactive Dendrimers

\

/

, /

Terminal ligands

Bridging ligands

bPY

Q-Me

Q / -\ ( = = \

2,3-Medpp+

Figure 16.5: Ligands used by Balzani and coworkers for dendrimer assembly.

120+

bpy is known to be a better electron donor than 2,3-dpp, the six peripheral. centers are expected to undergo oxidation at less positive potentials than the four more deeply buried redox centers. The differential pulse voltammogram shows only one anodic peak at 1.53 V vs SCE that involves six electrons. The

16.2

Dt-ndrirners with Internal Electroactive Groups

217

oxidation of the internal Ru(I1) centers was not observed probably due to the building u p of positive charge in the periphery of 6 which hinders the extraction of more electrons from the macromolecule, shifting the oxidation potentials beyond the accessible potential window. Synthetic replacement of the central Ru(I1) center by a more easily oxidizable Os(I1) results in a differential pulse voltammogram exhibiting the same six-electron oxidation of the peripheral Ru(I1) units at 1.53 V plus a poorly resolved shoulder at 1.35 V corresponding to the oxidation of the central Os(1I) metal center. The utilization of different ligands and metal ions makes it indeed possible to design and establish electrochemical potential gradients in these dendrimers. The generation of radial electrochemical potential gradients is crucial to afford directional electron transfer properties in dendritic macromolecules, an important feature for the development of materials with useful charge and/or mformation storage properties. Selby and Blackstock have recently reported on a new redox active polyarylamine dendrimer (7,see structure in the next page) that exhibits a radial redox gradientJ91 The cyclic voltammogram of 7, as shown in Fig. 16.6, exhibits three well-defined anodic waves. The first two (at halfwave potentials of 0.48 and 0.63 V vs SCE) involve one and two electrons and correspond to the oxidation of the three internal p-phenylenediamine residues. The last wave (at 0.88 V vs SCE) involves three electrons and is assigned to the oxidation of peripheral arylamino groups. Dendrimer 7 exhibits other irreversible oxidation waves at more positive potentials. The gradient of

Meo\O_NMoMe

OMe

I

I 7

OMe

OMe

218

16

Electroactive Dendrirners

-a, a0 -16.

an

- 1 p. OD

-B. aU

-4.130 0.00

4, DO

Figure 16.6: Cyclic voltammogram of 7 in CH2C12/0.10 M TBABF4 at 25 C; the scan rate l with permission of the American Chemical Society. was 200 m V / ~ . " ~Reprinted

electrochemical potentials present in this molecule leads the authors to hypothesize that oxidation of the core to yield ' 1 should be a facile process, while reduction of 1+ must be hindered. Measurements of the self-exchange electron transfer rate constant for the 1+/1 couple yield a value of 1.8 x lo5 M-1 s-1. Self-exchange electron transfer rate constants for simpler, unprotected p-phenylenediamine derivatives were found to show values consistently above 1x l o 9 M-1 s-1. This difference was attributed to the countergradient charge transfer barrier imposed by the peripheral arylamino groups in the dendrimer. One feature that is commonly observed in redox proteins is the unsymmetric positioning of the prosthetic group. Often, the redox active center is partially buried in the polypeptide backbone and unsymmetrically located "off center" in the protein structure. This is one of Nature's methods to control electron transfer reactivity in biological systems, favoring reactions with desirable partners and preventing reactions with unwanted partners. For instance, in cyctochrome c, the redox active iron-heme group is buried in the backbone, but remains close to several surface lysine residues that are positively charged at physiological pH values. This protein undergoes rather facile electron transfer with appropriate partners that interact with the lysinedz01 However, reactions with other partners, those that approach the protein from other directions, are more sluggish. Recently, the authors' group begun to use dendritic frameworks to express this type of directional redox reactivity. Using ferrocenecarboxylic acid as the redox active nucleus and Newkome's synthetic

16.2

Dendrimers with Internal Electroactive Groups

219

methodology, we synthesized a series of three novel dendrimers containing a single, unsymmetrically located ferrocene center.[211 Compound 8 is the third generation and largest member of the series. The electrochemical behavior of these molecules was investigated in CHzC12 solution. As expected the rate constant for heterogenous electron transfer decreased as the dendrimer generation increased. The half-wave potential for the one-electron oxidation of the ferrocene subunit was found to shift to less positive potentials in going from the first to the third generation compound. Again, this is not an intuitive finding, but it is in excellent agreement with the trend observed by Diederich's group on the oxidation of their Zn-porphyrin dendrimers in nonpolar solvents[101 (CH2C12 and THF). Hydrolysis of the peripheral esters in 8 and lower generation analogs produces a new series of three carboxylate-terminated, water-soluble dendrimers. In pH 7 buffered solution, these dendrimers are almost fully ionized and exhibit large negative charges. Their electrochemical behavior is still under investigation as this is written, but their anionic nature at neutral pH offers an obvious "handle" to restrict their free rotation at the electrode-solution interface and force them to adopt specific orientations that may affect their rates of electron transfer with the electrode surface.

220

16 Electroactive Dendrimers

There is no question that the chemistry of dendrimers offers extraordinary promise. The work summarized in this chapter is concerned only with dendrimers functionalized with electroactive groups. We have not addressed other interesting topics such as the modification of electrode surfaces with dendrimerdzl or their use as templates for the preparation of catalytically active metal colloidal particles.[~l The work described here suggests that dendrimers may find interesting applications as materials for molecule-based storage of charge and/or information. In addition to this, the analogies between some electroactive dendrimers and redox proteins are very interesting and open research avenues geared to mimicking these important biological systems and their functions.

16.3 References 1.See, for instance: (a) G. R. Newkome, C. N. Moorefield and F. Vogtle, Deizdritic Molecules: Concepts, Syntheses, Perspectives, VCH, Weinheim, 1996. (b) F. Zeng and S. C. Zimmerman, Chem. Rev. 1997,97,1681-1712. 2. M. R. Bryce, W. Devonport and A. J. Moore, Angew. Chem. Int. Ed. Engl. 1994, 33,1761-1763. 3. W. Devonport, M. R. Bryce, G. J. Marshallsay, A. J. Moore and L. M. Goldenberg, J. Muter. Chem. 1998,8,1361-1372. 4. B. Alonso, M. Moran, C. M. Casado, F. Lobete, J. Losada and I. Cuadrado, Chem. Muter. 1995, 7,1440-1442. 5. C. Valerio, J.-L. Fillaut, J. Ruiz, J. Guittard, J.-C. Blais and D. Astruc, J. Am. Chem. SOC.1997,119,2588-2589. 6. I. Cuadrado, M. Moran, C. M. Casado, B. Alonso, F. Lobete, B. Garcia, M. Ibisate, J. Losada, Orgunometullics 1996, 15,5278-5280. 7. R. Castro, I. Cuadrado, B. Alonso, C. M. Casado, M. Moran and A. E. Kaifer, J. Am. Chem. SOC.1997,119,5760-5761. 8. I. Cuadrado, C. M. Casado, B. Alonso, M. Moran, J. Losada and V. Belsky, ,i. Am. Chem. SOC.1997,119,7613-7614. 9. B. Gonzalez, C. M. Casado, B. Alonso, I. Cuadrado, M. Moran, Y. Wang and A. E. Kaifer, Chem. Commun. 1998,2569-2570. 10. P. J. Dandliker, F. Diederich, M. Gross, C. B. Knobler, A. Louati and E. M. Sanford, Angew. Chem. lnt. Ed. Engl. 1994,33,1739-1742. 11.P. J. Dandliker, F. Diederich, A. Zingg, J.-P. Gisselbrecht, M. Gross, A. Louati and E. Sanford, Helv. Chim. Actu 1997,80,1773-1801. 12. J. P. Collman, L. Fu, A. Zingg, F. Diederich, Chem. Commun. 1997,193-194. 13. R. Sadamoto, N. Tomioka and T. Aida, J. Am. Chem. SOC.1996,118,3978-3979. 14. D. L. Jiang and T. Aida, Chem. Commun. 1996,1523-1524. 15. C. B. Gorman, B. L. Parkhurst, W. Y. Su and K.-Y. Chen, J. Am. Chem. SOC. 1997,119,1141-1142. 16. G. R. Newkome, R. Guther, C. N. Moorefield, F. Cardullo, L. Echegoyen, E. Perez-Corder0 and H. Luftmann, . Angew. Chem. Int. Ed. Engl. 1995, 34, 20232026.

16.3

References

221

17. G. R. Newkome, F. Cardullo, E. C. Constable, C. N. Moorefield, A. M. W. C. Thompson, J. Chem. SOC., Chem. Commun. 1993,925-926. 18. V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni and M. Venturi, Acc. Chem. Res. 1998,31,26-34. 19. T. D. Selby and S. C. Blackstock, I. Am. Chem. SOC. 1998,120,12155-12156. 20. E. M. Bowden, Interface 1997, 6(4), 40-44. 21. C. M. Cardona and A. E. Kaifer, 1. Am. Chem. SOC.1998,120,4023-4024. 22. M. Wells and R. M. Crooks, 1.Am. Chem. SOC.1996,118,3988-3989. 23. M. Zhao and R. M. Crooks, Angew. Chem. Int. Ed. Engl. 1999,38,364-366.

Supramolecular Electrochemistry

Angel Kaifer, Marielle G6mez-Kaifer 0 WILEY-VCH Vcrlag GmbH. 1999

17 Molecular Wires

In this chapter we shall briefly survey an area of supramolecular chemistry with great potential for development. It has been more than twenty five years since the theory of a "molecular wire" was first described."] The concept of molecular electronics has progressed to include molecular switches which will be interconnected via molecular wires to form arrays of sites that can perform a variety of functions. Unlike the plethora of successful molecular switches that have been reported in the literature, the preparation of successful molecular wires has proven a challenging undertaking. In this chapter we shall examine the design goals of molecular wires and review a few of the wires that have been the subject of published electrochemical studies. The majority of work in this area has involved photochemical rather than electrochemical methods. It is our hope that this brief chapter will incite further interest in the area. We should note at the outset that as conduction in the wires becomes more efficient, electron transfer rates exceed a level that can be probed by common electrochemical methods. Alternative voltammetric techniques such as AC voltammetry have been employed by some

17.1 The Concept of a Molecular Wire and its Electron Transfer Kinetics Perhaps the simplest description of a molecular wire is that of a conductive path of atomic orbitals permitting long-range electron or hole transfer.f31 Most proposed structures have involved bridged donor-acceptor pairs. Considerable insight into the behavior of these systems has been reaped from extensive research in donor-acceptor systems over the past forty yearsJ41 The first reported molecules purporting to exhibit wire-like behavior were naturally occurring chlorophyll and acceptor assemblies bearing polyisoprene chains.[51 While the possibility of efficient conduction of electrons along the molecular framework has, as mentioned above, been around for twenty five years, bringing this idea to reality has proven a challenge. In most systems prepared thus far, the rate of electron transfer has remained linked to the donoracceptor distance, with increasing separation between the sites adversely effecting the rate of electron transfer. This distance dependence of the observed electron transfer rate has been described by the expression:

k,, = k, expI-PR,,l

(17.1)

17.2

Electrocllenzical Studies of Molecular Wires

223

where ko is a kinetic prefactor, RDA the center to center distance between the donor and acceptor and p a scaling factor of units A-1, characteristic of the nature of the bridge moiety. Typical reported values of J3are 1.0-1.4 A-1 for proteins, 20.2- 1.4 A-1 for DNA, 0.8 -1.0 A-1 for saturated hydrocarbons, and 0.2 - 0.6 A-1 for unsaturated phenylene, polyene, polyyne bridges.[6] Recently however, Wasielewski and coworkers have reported truly impressive p values including an astonishing p of 0.04 A-1 for wire 1.161

1 Clearly from the standpoint of electrochemical conversion, as values of p approach the theoretical minimum, the electron transfer rate increases toward the limit of the heterogeneous electron transfer rate for the redox moiety in question. As mentioned above, when the rate (ko) becomes large it can no longer be measured using the typical electrochemical methods. Creager and coworkers have employed AC voltammetry,[2] while Chidsey has used a coulostatic indirect laser induced temperature jump method for assessing electron transfer in ferrocenyl wires, vide infua."I Assessment of the electron transfer kinetics in slower systems is still quite complicated. Both hole tunneling and electron tunneling contribute to the overall observed transfer rates. The contribution of each component may be resolved in order to give insight into the nature of conduction in the bridge. The results obtained must be examined carefully however, with interpretation of true conduction tempered by consideration of interchain percolation effects along with other factors. Allara, Tour, Weiss and coworkers have advocated use of STM to prove both conductivity and to image the molecule in question to verlfy that it is, in actuality, the conductive component in an assembly.[*] Bearing in mind these caveats, electrochemical study of wires yields important insights and is a direct step toward the development of electrically switched sensors.

17.2 Electrochemical Studies of Molecular Wires In this section we shall address four recent reports on molecular wires. This area of research is enjoying rapid growth with a number of recent successes in increased efficiency of electron transfer. Thus, this section can provide only a

17 Molecular Wires

224

cursory overview of the field. We have selected several studies that have included electrochemical work and that are illustrative of the salient factors for research in the area. Harriman and Ziessel have examined electron localization, exchange and transfer in alkyne bridged metal terpyridyl complexes in some detail.191 While the majority of their studies revolve around photophysical measurements, the authors report half-wave potentials for their trinuclear complexes 2 where M is variously Fe, Zn or Co. Based on a variety of laser excited lifetime measurements the p value in 2-Co(II) system was estimated to be in the range of 0.17 A-1 with the contribution of hole tunneling to p estimated to be more substantial than that of electron tunneling.

2 Tolbert and coworkers reported a prototype for a molecular wire based on bis(ferrocenyl)polymethines.l~~ Their system employs an odd-alternant polyenyl (polymethine ion) bridge, termed a soliton. In this system the soliton, which exhibits its own conduction of negative or positive charge, enhances communication between the terminal acceptor groups, in this instance the ferrocene moieties. Wires of general structure 3' were probed by absorption spectroscopy and voltammetry, with the latter technique used to assess the extent of electronic communication between the terminal ferrocene groups. Amazingly, separate voltammetric waves are observed for the oxidation of the ferrocenyl groups even when the soliton chain length is increased to 13 carbons between the metal centers. The separation between successive oxidations ranges from 0.33 V for n=l to 0.04 V for the n=13 case.131 (This finding is highly unusual in dimeric ferrocenes with bridging of more than two carbons.) The timescale of the electrochemical measurements did not permit resolution of the dynamic events leading to the peak separation, i.e. the effects of the soliton migration or mixed valence states could not be distinguished.

7 7.2 Electroclteniicul Studies ofMoleculur Wires

3

3+

Ferrocenyl wires have also been examined by Chidsey and coworkersJV Mixed SAMs composed of ferrocene terminated polyakynylthiols, general structure 4, and various alkyltl-dols on gold were employed for studies of electron transfer rates by a coulostatic indirect laser induced temperature jump method (ILIT). Surface coverages on the gold electrodes were measured by cyclic voltammetry. The estimated p value for these systems was 0.57 A-l, lower than that for the analogous systems with saturated bridges,[*O]although still high in comparison to those observed for polyene spacers.

0 4

Creager and coworkers have recently reported the electron transfer rates of conjugated ferrocenyl wires based on the general structure 5 where n=O - 2 and X is an alkyl extension bearing variously a pyridyl or silyl group.[lll Mixed monolayers of the ferrocenyl wires and 16-mercaptohexadecanol were prepared on gold bead electrodes and studied by AC voltammetry, a method that along with other impedance methods has emerged as a valuable tool in studying redox kinetics of mixed monolayers with dilute surface coverages of electroactive material.[2J21 By varying the frequency of application of the AC potential insight into the electron transfer rate can be deduced. Fig. 17.1 shows

5

17 Molecular Wires

226

the AC voltammogram for a mixed monolayer of 5 in which n=l. At a low frequency of 100 Hz for the applied AC potential a peak is observed at +0.274 V vs. Ag/AgCl, while no such peak is clearly distinguishable at a frequency of 104 Hz. This indicates that the alternation of potential exceeds the rate of the ferrocene redox reaction. The diminution of the ac peak current relative to the baseline in a series of AC voltammograms acquired at different frequencies can yield a value for the standard electron transfer rate constant ko of an immobilized species.[21 Although the structures employed by Creager and coworkers are similar to those of Chidsey, Creager’s group found a p value of 0.36 A-1 for their systems. While Creager’s monolayers are more dilute (by a 4.Oe-7 D

3.5e-7

{

3.0e-7

6 0

E

2.5t-7

2.08-7

+. E3

u

2

1%-7 1.0s-7

5.00-6 O.Oe+O

J 0.1

0.2

0.3

0.4

0.5

0.6

0.5

0.6

Potential (Volts vs. ref) 4.0e-5

g

-, 10,OOO Hr

3.s-5

E q 3.0e-5

E

1Se-5

P(

1.00-5

2

5.0e-6 O.Oe+O

J

0. I

0.2

0.3

0.4

Potential (Volts vs. ref)

Figure 17.1: AC voltammograms for mixed monolayers of 5 (n=l) and 16mercaptohexadecanol on a gold bead electrode in 1M NaC104.[111 Potentials are reported vs. a Ag/AgCI reference. Peak amplitude of applied voltage was 25 mV 950mV peak to peak), sampling period was Is and sampling interval 10 s. The top voltammogram was acquired at 100 Hz, the bottom at 10,000 Hz. Reprinted with permission of the American Chemical Society.

17.3

References

227

factor of ten in some instances) the reasons for the differing values observed are not clear. They may reflect an effect of differing diluent alkylthiols or perhaps variations of the two methods employed to estimate ko or, a combination of these two factors. One important difference to be noted between the two methods is that ILIT is suited for the measurement of very rapid rates of electron transfer (up to 106 s-1 reportedl71) , while the AC voltammetric method has been recommended by its author for probing slightly slower electron transfer kinetics regimes in the range of 105 s-1.[21

17.3 References 1.A. Aviram, M. A. Ratner, Chem. Physics Lett., 1974,29,277- & X J . 2. S. E. Creager, T. T. Wooster, Anal. Chem., 1998, 70,4257-4263. 3. L. Tolbert, X. Zhao, Y. Ding, L. A. Bottomley, 7. Am. Chem. Soc., 1995, 117, 12891-12892. 4. P. F: Barbara, T. J. Meyer, M. A. Ratner, I. Phys. Chem., 1996,100,13148. 5. A. F. Janzen, J. R.Bolton, (Eds. K. W. Boeer, B. H. Glenn,) Sun 2, Proc. lnt. Sol. Energy SOC. Silver Jubilee Congr., (Pergamon, Elmsford, N. Y.) 1979,1,117-120. 6. W. B. Davis, W. A. Svec, M. A. Retner, M. R. Wasielewski, Nahtre, 1998,396, 80-83. 7. S. B. Sachs, S. P. Dudek, R. P. Hsung, L. R. Sita, J. F. Smalley, M. D. Newton, S. W. Feldberg, C. E. D. Chidsey, J. Am. Chem. Soc., 1997,119,10563-10564. 8. L. A. Bumm, J. J. Arnold, M. T. Cygan, T. D. Dunbar, T. P. Burgin, L. Jones 11, D. L. Allara, J. M. Tour, P. S. Weiss, Science, 1996,271,1705-1707. 9. V. Grosshenny, A. Harriman, R. Ziessel, Angew. Chem. lntl. Ed. Engl., 1995,35, 2705-2708. 10. J. F. Smalley, S. W. Feldberg, C. E. D. Chidsey, M. R. Linford, M. D. Newton, Y.-P. Liu, J. Phys. Chem., 1995,99,13141-13149. 11. S. E. Creager, C. J. Yu, C. Bamdad, S. OConnor, T. MacLean, E. Lam, Y. Chong, G. T. Olsen, J. Luo, M. Gozin, J. Faiz Kayyem, 7. Am. Chem. Soc., 1999, 121, 1059-1064. This paper is strongly recommended as an introduction to methods for determining ko, and electronic coupling in wire systems.

SupramolecularElectrochemistry Angel Kaifer, Marielle G6mez-Kaifer 0 WILEY-VCH Vcrlag GmbH. 1999

18 Conclusions and Outlook

We hope that the readers will agree with us that the last two decades have witnessed fast progress in supramolecular chemistry and that electrochemical concepts and techniques have played a significant role in many of the advances in this active research field. In this book we have shown how changes in the oxidation states of appropriate subunits can be used to exert control on supramolecular structures and assemblies. We have also discussed many examples that illustrate how supramolecular structure may affect the kinetics and thermodynamics of electron transfer reactions. These ideas constitute the central core of supramolecular electrochemistry and have been established in a wide variety of systems. The key question that this chapter attempts to address is: Where can we go from here? This type of question is indeed extremely difficult to answer. Innovation has always been one of the driving engines of science and predicting the scientific directions and accomplishments of the future is obviously risky. There are some trends, however, that are clearly visible today. We can extrapolate them and make reasonable guesses on what the near future will bring us in this field of science. During the last two decades electrochemists have learned to manufacture and use electrodes of micrometer and even nanometer dimensions. Electrode miniaturization has brought about the miniaturization of the systems that can be addressed with electrochemical techniques. In chapter 6 we described some of the recent results on single molecule electrochemistry. As this is written, a report by Crooks and coworkers[11demonstrates the feasibility of using carbon nanotubes as ultramicroelectrodes. While these developments have changed the field of electrochemistry, supramolecular chemists have been at work preparing, characterizing and functionalizing larger and larger molecules and supramolecular assemblies. For instance, we can now easily prepare dendrimers having dimensions in the nanometer range. We must realize, thus, that electrodes and molecules (or assemblies) are reaching similar sizes simultaneously, opening new and fascinating possibilities for singlemolecule manipulation and study. Although considerable technical difficulties need to be overcome, these ideas should no doubt develop into a highly exciting area of research during the next few years. Electrochemically or, more generally, redox switchable molecules are one of the most promising types of multi-stable molecular systems. Electrochemicalreactions can be effectively used to reversibly control the state of these interesting molecules. However, in order to move this research work towards more practical applications it is necessary to find ways to address a single molecule or a small group of molecules at a time. Switching large

18

Conclusions and Outlook

229

numbers of molecules in the solution phase is an elegant and worthwhile research accomplishment, but unrealistic and unpractical if we want to build information storage or processing systems, which truly resemble our current electronic circuits. The key reason to replace silicon-based circuit elements by molecdes is miniaturization. Therefore, switching or addressing trillions of molecules at the same time serves no useful purpose. To accomplish the level of required addressability researchers must move away from the solution phase to work either in the solid state or at surfaces, media in which molecules can be anchored at specific locations and addressed individually or as small groups. In this regard, the equalization of sizes between electrode surfaces and molecular systems (vide supra) opens the possibility of using each molecule as an individual circuit component addressable through its corresponding electrode. Scanning probe microscopic techniques may also play an important role in this area. Molecular recognition in interfacial environments has extraordinary relevance in biochemistry as many important recognition and binding phenomena take place on the surface of cells and membranes. It is thus hardly surprising that investigations on interfacial molecular recognition have become more commonplace in the last decade. In this regard, electrochemical techniques occupy a privileged position because the electrode-solution interface offers an excellent framework for these studies. In the next few years, we should see increasing research activity on host-guest phenomena at electrochemical interfaces. We should note here that the field of supramolecular chemistry keeps expanding and that researchers in this field are continuously applying their basic concepts and ideas to new systems. As an illustrative and pertinent example, we can mention the metal-solution interface of colloidal particles as a novel platform for molecular recognition studies that has attracted some recent attention. Work by several groups[241is breaking ground in this novel area and expanding the horizons of supramolecular electrochemistry. In the past, electrochemical experiments were customarily carried out with a millimolar solution of the electroactive species in an appropriate supporting electrolyte/solvent medium. Nowadays, the sensitivity of electrochemical techniques is greatly improved. Pulse voltammetric techniques are gaining wider popularity, among other reasons, because they afford excellent sensitivity. Square wave voltammetry, for instance, can be used in solutions with concentrations of electroactive species as low as 10 pM. In addition to this, ultramicroelectrodes can be employed in fairly resistive solutions, eliminating the requirement for large concentrations of supporting electrolyte that may be incompatible with certain supramolecular systems. Therefore, the possibilities for applications of electrochemical techniques from an experimental standpoint are only increasing. A key problem in supramolecular chemistry is the characterization of high molecular weight assemblies. As the complexity and molecular weight of the supramolecular assemblies prepared increase so do the difficulties associated with their characterization. Chemists have at their disposal a rather limited number of techniques useful for these purposes and the use of electrochemical techniques may prove increasingly fruitful as long as electroactive subunits are

230

18

Conclusions and Outlook

suitably inserted as markers in the supramolecular structures to be characterized. Diffusion coefficients, electrochemical potentials, and electron transfer rate constants may provide different types of information that can be of assistance in the characterization of high molecular weight supramolecular structures. Selectivity is a perennial problem in modern chemical research. We measure enviously the high binding selectivities exhibited by biological hosts, such as antibodies or enzymes. As chemists, we strive to reach similar selectivity values when we design a molecular host for a particular guest. Unfortunately, the selectivity presented by synthetic receptors, with very few exceptions, is orders of magnitude below that exhibited by natural systems. Of course, biological systems are the result of extremely lengthy processes of molecular evolution, and may contain large fractions of material without any useful purpose, accumulated over millions of years. Chemists must design within a much shorter time scale and with material economy in mind. We must admit, however, that we cannot compete with Nature, at least, not yet. From the standpoint of the development of sensors, catalytic systems, and other practical systems we must be concerned with selectivity since it often determines the difference between a practical development and a system with mere academic interest. Supramolecular chemists must be at the forefront in searching for solutions to this problem. In particular, electrochemists can perhaps make use of the peculiar character of interfacial environments to enhance selectivity properties. Alternatively, sensor arrays, aided by pattern recognition software, may offer practical ways to circumvent problems associated with lack of selectivity. Combinatorial chemistry has changed the approach to chemical problem solving in many branches of chemistry. Not surprisingly, combinatorial approaches are also starting to emerge and gain importance in supramolecular chemistry. Electrochemical techniques and concepts may play a significant role in the future development of this field providing methods to analyze and sort the most effective compounds within a library. Also, molecular recognition and selectivity at electrochemical interfaces may also benefit from combinatorial approaches. In general, the future of supramolecular electrochemistry appears bright and exciting. We have only started to develop the possibilities of this field. Research in supramolecular electrochemistry ideally requires collaborative efforts from several groups because of its inherent multidisciplinary character. As more researchers travel across and along the rather arbitrary frontiers between the several branches of chemistry and other sciences, the frontiers themselves will tend to disappear. Multidisciplkary research areas, such as supramolecular electrochemistry, will thus enjoy their best days.

18.1

References

231

18.1 References 1.J. K. Campbell, L. Sun, R. M. Crooks, J. Am. Chem. SOC.1999,121,3779-3780. 2. J. J. Storhoff, R. Elghanian, R. C. Mucic, C. A. Mirkb, R. L. Letsinger, J. Am. Chem. SOC.1998,120,1959-1964. 3. J. Liu, S. Mendoza, E. Roman, M. J. Lynn, R. Xu, A. E. Kaifer, J. Am. Chem. SOC. 1999,121,4304-4305. 4. A. K. Boal, V. M. Rotello, J. Am. Chem. SOC.1999,121,49154915.

Supramolecular Electrochemistry Angel Kaifer, Marielle G6mez-Kaifer 0 WILEY-VCH Vcrlag GmbH. 1999

Index

AN conditions, bulk electrolysis 29 acceptors - catenanes 150 - supramolecular systems 91 acetonitrile 65 f acidic solutions, cleaning 59 Ag/AgCl electrodes 59 alkali chlorides, supporting electrolytes 67 alkenethiols, SAMs 191 alkylammonium, supramolecular systems 92 alkyne, molecular wires 224 alumina, polishing compounds 57 amalgam electrodes 55 amphiphiles, LB films 180 amphiphilic aggregation 93 anion binding 114 ff, 122 anodes 2 f anthracene hosts 137 anthraquinone moiety 117 approxima tion methods, numerical 77 f aproticDMF 91 arene bindings 91 argon atmosphere 65 aromatic amino acids 132 f aromatic systems 91 array ultramicroelectrodes 46 associa tion - fixed 93 f - flavinophane 131 azobenzene derivatives, LB films 186 backward processes, electron transfer 8 band ultramicroelectrodes 46

bead electrodes 55,58 benzene - solvents 66 - supramolecular systems 91 benzidine rotaxanes 157 benzimidazole 95 benzonitrile - solvents 66 - supramolecular systems 91 binary systems, solvents 91 binding - calixarenes 119 - switching 103 f, 114 ff bipyridine ligands 95 bipyridinum 159 bipyridinum derivatives 19 bis(chlorophyl1) cyclophane system 127 f bis(cyclopentadienyl)iron(II) see ferrocene boiling points, solvents 64 bolaamphiphiles 112 Boltzmann constant 6 bromides, supporting electrolytes 68 BTS catalysts, drying agents 67 bulk electrolysis 29 - celldesign 70 bulk solutions, digital simulations 77 bulk techniques 13 Butler-Volmer equation 9

calcium hydridekhloride 66 calixarene based structures 100 calixarenes - cyclophanes 135 - switching 114, 119

234

calomel electrodes 59 ff capacitance, ultramicroelectrodes 45 carbon paste electrodes 55 carbonyl bonds 56 carboxylcoba1tocenium reduction 86 catechol solutions, cyclophanes 133 catenand effect 146 ca tenanes - cyclophanes 132 - intertwined structures 142 - n-donor/acceptor interactions 150 ff - redox-switchable 127 - self assembly 93 cathodes 2 ff cation binding, switching 114 ff cation host, redox switchable 105 cation-K interactions, supramolecular systems 89 f cation recognition, switching 103 cationic dendrimers 208 cavity binding, cyclophanes 132 cell design 68 f cell potentials 1 ff cell types 14 CH2C13 supramolecular systems 90 charging currents 12,45 chlorides supporting electrolytes 67 chlorophyll, molecular wires 222 chronoamperometry 13,% f chronocoulometry 28 cleaning - counter electrodes 59 - vacuum methods 73 cobaltocene - dendrimers 212 - redox couples 85 cobaltocenium - cyclophanes 135 - redox couples 18,85 concentration profiles, redox groups 80 constrictive binding 84 contamination, counter electrodes 70 controlling, molecular devices 109

Index

convection 5 coordination arrays 164,175 f copper helicates 164 ff Cotrell conditions - ultramicroelectrodes 49 - potential step methods 22 f coulombic interactions - dendrimers 210 - supramolecular systems 89 coulometric titrations 13 counter electrodes 14,59,70 coupling, redox sites 94 f covalently interlocking, self assembly 93 crown ethers, switching 105,114,153 cryptands, switching 105,114 ff crystal formation, reference electrodes 61 current genera tion modes, SECM 50 current potential curve, electrode reactions 7 currents - faradaic 11 - ultramicroelectrodes 45 cyclic voltammetry (CV) 16 - dendrimers 211 - digital simulations 79 - ECmechanism 85 - helicates 166 - potential sweep methods 34 f - ultramicroelectrodes 48 cyclodextrin - inclusion complexes 124 - redox couples 85 - self assembly 94 cyclodextrin based rotaxanes, intertwined structures 143 cyclophanes - intertwined structures 157 - redox-active 130 - redox-switchable 127 ff - supramolecular systems 91 - viologens 150

235

Index

decomposition, supporting electrolytes 55 dendrimers, electroactive 207 ff dendritic effect, molecular recognition 209 diamine ketone, switching 118 diamond paste, polishing compounds 57 diazacrown ethers, switching 117 dichloromethane 65 f dielectric constants 89 f diferrocinyl bolaampiphiles 112 differential pulse voltammetry (DPV) 16,39 diffusion - digital simulations 78 - mass transport 5 - potential step methods 23 f - chronocoulometry 29 - redox groups 81 diffusion layer thickness 46 DigiSim 79,82 digital simulations 77 ff dihydrogenphosphate binding 124 dimerization, helicates 165 dimethylformamide (DMF) 65 f dimethylsulfoxide (DMSO) - solvents 65 f - supramolecular systems 90 dioxynaphthalene moiety, cyclophanes 134 dipole-dipole interactions 89 diquinone structures 120 diquinonecalixarenes 100 discretization, electrolpc solutions 77 disk electrodes 55 f disk ultramicroelectrodes 46 dispersion 91 dissolution 1 disulfide attachement, self assembly 93 DNA, molecular wires 223 donor acceptor systems - intertwined structures 143

molecular wires 222 donors - catenanes 150 - supramolecular systems 91 dopamine solutions, cyclophanes 133 double bridges, reference electrodes 62 double layer, electrical 11 drierite, drying agents 67 drying, solvents 64 ff dynamic methods 13

-

EC/ECE/EE mechanisms 84 electroactive dendrimers 207 ff electroactive guests, cyclodextrin 87 electroactive intertwined structures 142 ff electroactive Langmuir-Blodgett films 180 ff electroactive SAMs 195 electrochemical conditions, intermolecular forces 89 f electrochemical reactions 1 ff electrochemical simulation package (ESP) 79 electrochemical switching 103 ff electrochemical techniques 11 ff electrochemistry, single molecules 51 f electrocrystallization 31,72 electrode blocking, SAMs 193 electrode size, potential step methods 24 electrode surfaces 45 ff, 55 ff - precipitation 91 electrode types 14,32,45,55 f electrodes 2 ff electrogravimetry 13 electroinactive cyclophane hosts 135 electrolytic solutions, discretization 77 electron movements 4 electron transfer lff, 8 - homogeneous processes 84 f

Index

236

LBfilms 183 molecular wires 222 SAMs 193 epinephrine solutions 133 equilibrum methods 13 error function, Cottrell conditions 25 ester hydrolysis, dendrimers 219 ethanol 66 ethers 114 excitation functions 15 - chronoamperometry 26 - cv 34 - DPV 40 - LSV 33 - NPV 37 - potential step methods 22 - S W V 42 experimental methods 55 ff explicite finite difference (EFD) 78 explosion hazard, NaK amalgam 66

-

faradaic currents 11 - digital simulations 78 - ultramicroelectrodes 45 Faraday law 3 ff, 30 feedback, ultramicroelectrodes 49 Fermilevel 3 ferrocene - cyclophanes 135 - dendrimers 209 - hemicarcerand encapsulation 84 - pseudoreference 62 - reversible redox couples 18 - SAMs 197 - self assembly 93 - switching 105 - ultramicroelectrodes 53 - vacuum methods 75 ferrocenyl - LBfilms 183 - molecular wires 225 - switching 114,118 ferrocyanine, switching 122 Fick law, digital simulations 6,78 flame polishing 59

flavinophane isoalloxazine 131 forward reactions, electron transfer 8 forward scan, CV 34 fullerenes - cyclophanes 137 - supramolecular systems 91 future trends 228 ff galvanic cells 2 ff galvanostatic methods 13 glass shrouds 56 glassblown cells, vacuum methods 74 glassy carbon surfaces 55 gold disk electrodes 55 f grid-like structures, self assembly 93 grids 164,175f guests - intertwined structures 143 - self assembly 93 half wave potentials cv 37 - helicates 167 - switching 103 ff halide adsorption 57 helicates 164 ff - redox-active moieties 101 - self assembly 93,110 hemicarcerand - ferrocene 84 - self assembly 94 heterogeneous electron transfers 11 hexaamineruthenium(I1) 20 hexacyanoferra to(I1) 20 hexafluorophosphate 68 hole transfer, molecular wires 222 homogeneous processes, electron transfer 84 f horizontal touching technique, LB films 182 hosts - electroinactive cyclophanes 135 - ferrocene 84 - intertwined structures 143, 150 -

Index

redox switchable 104 self assembly 93 - sterically hindered 122 - supramolecular systems 89 HPLC grade, solvents 65 hydrogen bonding - dendrimers 209 - electrode surfaces 56 - molecuar receptors 137 - supramolecular systems 89 f hydrophobic interactions 89 f hydroxymate, iron binding 111

-

-

implicite finite difference (IFD) 79 inclusion complexes 87,124 indicator electrodes 14 indole solutions, cyclophanes 133 interdigitated array ultramicroelectrodes 46 interfacial techniques 11 f intermolecular interactions 89 internal electroactive groups, dendrimers 213 internal reference 63 intertwined structures, electroactive 142 ff ion binding, switching 114 ion-dipole interactions 89 ion-ion interactions 89 ion movements 4 ion-quadrupole effects 92 iron binding ligand, helical 111 iron complexes, metallocyclophanes 130 isomeric covalently linked systems 99 isomers - catenanes 153 - cyclophanes 129 iterations, digital simulations 78 Kel-F 56 kine tics - digital simulations 77 - electrode reactions 6 ff

231

Langmuir-Blodgett (LB) films 180 ff lariat ethers 114 leakage 56 ligands - dendrimers 216 - helicates 164 ff - redox-active moieties 95 - switching 114 limiting currents, ultramicroelectrodes 47 linear sweep voltammetry (LSV) 16,32 linking, self assembly 93 lithium fluorides 67 macrocycles ferrocenyl 108 intertwined structures 146 - switching 114 macroelectrodes 45 mass transport 4 ff - potential step methods 23 - ultramicroelectrodes 45 mechanical linking 93 mercapto pyridine 203 mercury, toxicity 15 mercury electrodes 55,61 metal complexes, reversible redox couples 20 metallic circuits 1 metalloca tenanes, intertwined structures 143,145 f metallocyclophanes 127 ff metallorotaxanes, templated 145 f methanol 66 methyl carboxylates 213 micelles, switching 112 microelectrodes 45 moieties, redox-active 94 molecular devices 109,160 molecular receptors, cyclophanes 130,137 molecular recognition - dendrimers 209 - SAMs 198

-

238

Index

molecular wires 222 ff monodispersivity, dendrimers 207 monolayers - LBfilms 181 - self-assembled 191 - supramolecular systems 89 monomers, cyclophanes 129 monoquinone, SQV 119 multilayers, LB films 184 Nafion films, cyclophanes 133 naphthalene groups, cyclophanes 130 f naphthalene quinone 205 naphthalimides 137 negative feedback, ultramicroelectrodes 49 Nernst equation 4 ff - digital simulations 78 - potential controlled systems 17 - ultramicroelectrodes 47 neurotransmitters, catechol-type 133 nitrates 67 nitroaromatic groups, switching 114,119 nitrobenzene, reversible redox couples 20 nitrobenzene lariat ethers 115 nitrogen atmosphere 65 nitromerocyanine 203 nitromethane 66 non-faradaic currents 11 nonpolar solvents 91 nonsolid electrodes 55 norepinephrine solutions 133 normal hydrogen electrode (NHE) 3,62 normal pulse voltammetry (NPV) 16,37 numerical approxima tion methods 77 f ohmic drops 45 oligopyridine ligands

164 f

optically transparent electrodes 55,58 organic solvents - LBfilms 180 - supramolecular systems 90 f overpotential 8 oxydation reactions 1 ff, 6 f oxygen scavenger 67

PEEK 56 pentamethylphenyl moiety, molecular receptors 138 perchlorates 67 perfluorinated vacuum 74 peripheral electroactive groups, dendrimers 207 f pertubations, digital simulations 78 phenanthroline derivatives 112 phenylenediamine derivatives, dendrimers 218 phophates 67 phospholipid monolayers 186 photoswitchable SAMs 203 physical constants, solvents 66 n-acceptor hosts, cyclophanes 132 f n-donor/acceptor interactions - catenanes 150 ff - rotaxanes 155 f n-donors, intertwined structures 143 n-n interactions 89 f pinholes, SAMs 195 planar diffusion, potential step methods 24 plastic shrouds 56 platinum disk electrodes 57 platinum electrodes 209 platinum surfaces 55 poisoning, electrode surfaces 56 polar dendrimers 208 polar solvents 65 polishing, electrode surfaces 57 f polyaminemacrocycles 122 polyisoprene, molecular wires 222 poly(siloxyphtha1ocyanines) 188 poly(viny1ferrocene) 96

I?dex

porph yrin cyclophanes 128 - dendrimers 213 - intertwined structures 143 positive feedback, ultramicroelectrodes 49 potential controlled systems, Nernst equation 17 potential excitation functions see excitation functions potential step methods 22 ff potential sweep methods 32 ff potential window, cathodic 57 potentiostatic methods 13 precipitation, electrode surfaces 91 propylene carbonate 66 pseudoreference, silver wires 62 pseudorotaxanes 142 pulsed voltammetric techniques 12 f, 37 ff purification, solvents 64 purity, supporting electrolytes 68 purple red, helicates 111 pyridyl groups, molecular wires 225 pyridyne ligands, helicates 164 f

-

quinone based ligands, switching 114 ff quinones - electrode surfaces 56 - redox-active moieties 100 - reversible redox couples 20 - switching 119 racks 164,175 f Randles Sevcik equation 32 f receptors - molecular 127 ff - switching 114 redox active guests, anion binding 122 redox active moieties 94 - intertwined structures 142

239

redox active supramolecular systems 103 f redox groups - CV behavior 79f - surface confined 196 redox couples - pseudoreference 62 - reversible 18,35 - ultramicroelectrodes 47 redox switchable cation hosts 105 reducing agents, solvents 66 reduction 1 reference electrodes 14,32,59 resistances 45, 56 reverse scan, CV 34 Ridox, drying agents 67 ring ultramicroelectrodes 46 rotating electrode voltammetry 69 rotaxanes - cyclodextrin-based 143 - intertwined structures 142 - x-donor/acceptor chemistry 155 f - redox-switchable 127 - self assembly 93 ruthenium complexes 130 salt bridges 2,60 saturated calomel electrode (SCE) 59 f scanning electrochemically microscopy (SECM) 45,49 self assembled monolayers (SAM) 191 ff self assembly - helicates 164,175 - supramolecular systems 89, 93 semiconductor circiuts 1 semiconductor electrodes 58 serotonin solutions 133 shapes, ultramicroelectrodes 46 shroud types 56 shuttles - intertwined structures 142 - n-donor/acceptor chemistry 155 f - redox-switchable 127 - self assembly 93

240

silane attachement, self assembly 93 silyl groups, molecular wires 225 single molecule electrochemistry (SME) 51 f silver wires, pseudoreference 62 smooth bead electrodes 57 sodium chloride saturated calomel electrode (SSCE) 59 f sodium fluorides 67 sodium metal, explosion hazard 66 sodium potassium amalgam 65 solid electrodes 55 solitons, molecular wires 224 solubility, solvents 64 solution flow, mass transport 5 solution layer thickness 26 solvents 64 ff - supramolecular systems 91 solvophobic interactions 89 f spacers, switching 117 sphere ultramicroelectrodes 46 spontaneous formation, SAMs 191 square wave voltammetry (SWV) 16,42 - digital simulations 79 - redox groups 83 stacks, self assembly 93 staircase excitation functions 15 stirring, mass transport 5 Stokes-Einstein equation 5 f structural changes, helicates 165 f sulfates 67 supporting electrodes 5 supporting electrolytes 64 ff, 89 f - decomposition 55 - pseudoreference 63 supramolecular systems 13 ff, 89 ff - reversible redox couples 18 surface confined redox centers, SAMs 196 surfaces - electrodes 55 f - ultramicroelectrodes 45 ff swinging catenanes 147

Index

switching, electrochemical 103 ff switching potential, CV 34 Tafel equation 8 technique classifications 13 f teflon components, vacuum methods 73 teflon tubing, cell design 68 templa ted metallocatenanes/ rotaxanes 145 f terpyridyl, alkyne bridged 224 Tesla probe, vacuum methods 74 tetra butylammonium hexafluorophosphate (TBAPF6), supporting electrolytes 68 tetrachloroethane (TCE) 65 f tetrahydrofuran 65 f tetramethylammonium 92 tetrathiafulvalene (TTF) - dendrimers 208 - reversible redox couples 19 - supramolecular systems 90 thermodynamics, digital simulations 77 thin layer cells 69 thiobisethyl acetate (TBEA) 199 thioVdisulfide attachment, self assembly 93 thiolate gold SAMs 191 three electrode cells 14 time constant, electrochemical cell 12 time evolution, digital simulations 78 timescales, vacuum methods 75 tin oxide films, semiconductor electrodes 58 toluene - solvents 66 - supramolecular systems 91 toxicity - mercury 15 - solvents 65 trapping, electroactive molecules 52 triflates 68

24 1

Idex

triggers, intertwined structures 143 tunneling, molecular wires 223 two electrode cells 14 ultramicroelectrodes

45 ff

vacuum conditions 56 vacuum methods 72 ff van der Waals interactions, supramolecular systems 89 f Venus flytrap ligands 138 vertical dipping, LB films 181 vesicles, switching 112 vinylpyridinium derivatives, LB films 187 viologens - cyclophanes 132 f, 150

pseudorotaxanes 144 reversible redox couples voltammetry 13 ff, 32 f voltammograms see cyclic voltammograms 15 volume elements, digital simulations 77 vycor bridges 60

-

-

19

water 66,90 wave form potential parameters, swv 43 wire ultramicroelectrodes 46 wires, molecular 222 ff working electrodes 14,32,55 f xylylene bridges, cyclophanes

132