Organic Nanostructures€¦ · 1 Artificial Photochemical Devices and Machines 1 Vincenzo Balzani, Alberto Credi, and Margherita Venturi 1.1 Introduction 1 1.2 Molecular and Supramolecular

Organic Nanostructures

Edited byJerry L. Atwood and Jonathan W. Steed

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Edited by

Jerry L. Atwood and Jonathan W. Steed

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Edited byJerry L. Atwood and Jonathan W. Steed

The Editors

Prof. Dr. Jerry L. AtwoodUniversity of Missouri–ColumbiaDepartment of Chemistry125 Chemistry BuildingColumbia, MO 65211USA

Prof. Jonathan W. SteedUniversity of DurhamDepartment of ChemistrySouth RoadDurham, DH1 3LEUnited Kingdom

Cover illustrationThe front cover shows a space-filling imageillustrating the packing of the ligands in the opticallypure cage complex [Zn4(L

o-Ph�)6(ClO4)](ClO4)7 and isadapted from Figure 9.5 with the permission ofMichael Ward. The structure is superimposed on anSEM image of the helical fibrous structure of a chiralsupramolecular xerogel.

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In memory of Professor Dimitry M. Rudkevich(1963–2007)

Contents

Preface XIIIList of Contributors XV

1 Artificial Photochemical Devices and Machines 1Vincenzo Balzani, Alberto Credi, and Margherita Venturi

1.1 Introduction 11.2 Molecular and Supramolecular Photochemistry 21.2.1 Molecular Photochemistry 21.2.2 Supramolecular Photochemistry 41.3 Wire-Type Systems 51.3.1 Molecular Wires for Photoinduced Electron Transfer 51.3.2 Molecular Wires for Photoinduced Energy Transfer 91.4 Switching Electron-Transfer Processes in Wire-Type Systems 111.5 A Plug–Socket Device Based on a Pseudorotaxane 131.6 Mimicking Electrical Extension Cables at the Molecular Level 141.7 Light-Harvesting Antennas 171.8 Artificial Molecular Machines 191.8.1 Introduction 191.8.2 Energy Supply 201.8.3 Light Energy 211.8.4 Threading–Dethreading of an Azobenzene-Based Pseudorotaxane 211.8.5 Photoinduced Shuttling in Multicomponent Rotaxanes: a Light-Powered

Nanomachine 231.9 Conclusion 27

References 28

2 Rotaxanes as Ligands for Molecular Machines and Metal–OrganicFrameworks 33Stephen J. Loeb

2.1 Interpenetrated and Interlocked Molecules 332.1.1 Introduction 33

VII

2.1.2 Templating of [2]Pseudorotaxanes 332.1.3 [2]Rotaxanes 362.1.4 Higher Order [n]Rotaxanes 372.1.5 [3]Catenanes 402.2 Molecular Machines 412.2.1 Introduction 412.2.2 Controlling Threading and Unthreading 412.2.3 Molecular Shuttles 422.2.4 Flip Switches 442.3 Interlocked Molecules and Ligands 462.3.1 [2]Pseudorotaxanes as Ligands 462.3.2 [2]Rotaxanes as Ligands 462.4 Materials from Interlocked Molecules 482.4.1 Metal–Organic Rotaxane Frameworks (MORFs) 482.4.2 One-dimensional MORFs 492.4.3 Two-dimensional MORFs 512.4.4 Three-dimensional MORFs 512.4.5 Controlling the Dimensionality of a MORF 542.4.6 Frameworks Using Hydrogen Bonding 572.5 Properties of MORFs: Potential as Functional Materials 572.5.1 Robust Frameworks 572.5.2 Porosity and Internal Properties 592.5.3 Dynamics and Controllable Motion in the Solid State 59

References 59

3 Strategic Anion Templation for the Assembly of InterlockedStructures 63Micha» J. Chmielewski and Paul D. Beer

3.1 Introduction 633.2 Precedents of Anion-directed Formation of Interwoven

Architectures 643.3 Design of a General Anion Templation Motif 703.4 Anion-templated Interpenetration 723.5 Probing the Scope of the New Methodology 743.6 Anion-templated Synthesis of Rotaxanes 793.7 Anion-templated Synthesis of Catenanes 823.8 Functional Properties of Anion-templated Interlocked Systems 883.9 Summary and Outlook 93

References 94

4 Synthetic Nanotubes from Calixarenes 97Dmitry M. Rudkevich and Voltaire G. Organo

4.1 Introduction 974.2 Early Calixarene Nanotubes 984.3 Metal Ion Complexes with Calixarene Nanotubes 99

VIII Contents

4.4 Nanotubes for NOx Gases 1014.5 Self-assembling Structures 1074.6 Conclusions and Outlook 108

References 109

5 Molecular Gels – Nanostructured Soft Materials 111David K. Smith

5.1 Introduction to Molecular Gels 1115.2 Preparation of Molecular Gels 1145.3 Analysis of Molecular Gels 1155.3.1 Macroscopic Behavior – ‘‘Table-Top’’ Rheology 1155.3.1.1 Tube Inversion Methodology 1165.3.1.2 Dropping Ball Method 1165.3.2 Macroscopic Behavior – Rheology 1175.3.3 Macroscopic Behavior – Differential Scanning Calorimetry 1175.3.4 Nanostructure – Electron Microscopy 1185.3.5 Nanostructure – X-Ray Methods 1205.3.6 Molecular Scale Assembly – NMR Methods 1205.3.7 Molecular Scale Assembly – Other Spectroscopic Methods 1225.3.8 Chirality in Gels – Circular Dichroism Spectroscopy 1235.4 Building Blocks for Molecular Gels 1245.4.1 Amides, Ureas, Carbamates (–XCONH– Groups, Hydrogen

Bonding) 1255.4.2 Carbohydrates (Multiple –OH Groups, Hydrogen Bonding) 1275.4.3 Steroids/Bile Salts (Hydrophobic Surfaces) 1295.4.4 Nucleobases (Hydrogen Bonding and �–� Stacking) 1305.4.5 Long-chain Alkanes (van der Waals Interactions) 1325.4.6 Dendritic Gels 1335.4.7 Two-component Gels 1375.5 Applications of Molecular Gels 1415.5.1 Greases and Lubricants 1425.5.2 Napalm 1425.5.3 Tissue Engineering – Nerve Regrowth Scaffolds 1425.5.4 Drug Delivery – Responsive Gels 1445.5.5 Capturing (Transcribing) Self-assembled Architectures 1455.5.6 Sensory Gels 1475.5.7 Conductive Gels 1475.6 Conclusions 148

References 148

6 Nanoporous Crystals, Co-crystals, Isomers and Polymorphsfrom Crystals 155Dario Braga, Marco Curzi, Stefano L. Giaffreda, Fabrizia Grepioni,Lucia Maini, Anna Pettersen, and Marco Polito

6.1 Introduction 155

Contents IX

6.2 Nanoporous Coordination Network Crystals for Uptake/Release of SmallMolecules 156

6.3 Hybrid Organic–organometallic and Inorganic-organometallicCo-crystals 161

6.4 Crystal Isomers and Crystal Polymorphs 1676.5 Dynamic Crystals – Motions in the Nano-world 1706.6 Conclusions 172

References 173

7 Supramolecular Architectures Based On Organometallic Half-sandwichComplexes 179Thomas B. Rauchfuss and Kay Severin

7.1 Introduction 1797.2 Macrocycles 1807.3 Coordination Cages 1877.3.1 Cyanometallate Cages 1877.3.1.1 Electroactive Boxes 1897.3.1.2 Defect Boxes {[(C5R5)M(CN)3]4[Cp

�M]3}z 190

7.3.2 Expanded Organometallic Cyano Cages 1917.3.3 Cages Based on N-Heterocyclic Ligands 1937.4 Expanded Helicates 1987.5 Clusters 2007.6 Conclusions 200

References 201

8 Endochemistry of Self-assembled Hollow Spherical Cages 205Takashi Murase and Makoto Fujita

8.1 Introduction 2058.2 Biomacromolecular Cages 2068.3 Polymer Micelles 2078.4 M12L24 Spheres 2078.4.1 Self-assembly of M12L24 Spheres 2078.4.2 Endohedral Functionalization of M12L24 Spheres 2098.4.3 Fluorous Nanodroplets 2108.4.4 Uptake of Metal Ions into a Cage 2128.4.5 Polymerization in a Nutshell 2138.4.6 Photoresponsive Molecular Nanoballs 2168.4.7 Peptide-confined Chiral Cages 2178.5 Conclusions and Outlook 219

References 220

9 Polynuclear Coordination Cages 223Michael D. Ward

9.1 Introduction 2239.2 Complexes Based on Poly(pyrazolyl)borate Ligands 225

X Contents

9.3 Complexes Based on Neutral Ligands with Aromatic Spacers 2279.3.1 Complexes Based on Lo-Ph and L12-naph 2279.3.2 Larger Tetrahedral Cages Based on Lbiph 2349.3.3 Higher Nuclearity Cages Based on Other Ligands 2359.4 Mixed-ligand Complexes: Opportunities for New Structural

Types 243References 248

10 Periodic Nanostructures Based on Metal–Organic Frameworks (MOFs):En Route to Zeolite-like Metal–Organic Frameworks (ZMOFs) 251Mohamed Eddaoudi and Jarrod F. Eubank

10.1 Introduction 25110.2 Historical Perspective 25210.2.1 Metal–Cyanide Compounds 25210.2.2 Werner Complexes 25410.2.3 Expanded Nitrogen-donor Ligands 25510.2.4 Carboxylate-based Ligands 25810.3 Single-metal Ion-based Molecular Building Blocks 26110.3.1 Discrete, 2D and 3D Metal–Organic Assemblies 26210.3.2 Zeolite-like Metal–Organic Frameworks (ZMOFs) 26410.3.2.1 sod-ZMOF 26510.3.2.2 rho-ZMOF 26610.4 Conclusion 270

References 271

11 Polyoxometalate Nanocapsules: from Structure to Function 275Charalampos Moiras and Leroy Cronin

11.1 Introduction 27511.2 Background and Classes of Polyoxometalates 27711.3 Wells–Dawson {M18O54} Capsules 27811.4 Isopolyoxometalate Nanoclusters 28011.5 Keplerate Clusters 28211.6 Surface-Encapsulated Clusters (SECs): Organic Nanostructures with

Inorganic Cores 28511.7 Perspectives 287

References 287

12 Nano-capsules Assembled by the Hydrophobic Effect 291Bruce C. Gibb

12.1 Introduction 29112.2 Synthesis of a Water-soluble, Deep-cavity Cavitand 29212.2.1 Structure of the Cavitand (What It Is and What It Is Not) 29212.2.2 Assembly Properties of the Cavitand 29412.2.3 Photophysics and Photochemistry Within Nano-capsules 29912.2.4 Hydrocarbon Gas Separation Using Nano-capsules 301

Contents XI

12.3 Conclusions 302References 303

13 Opportunities in Nanotechnology via Organic Solid-state Reactivity:Nanostructured Co-crystals and Molecular Capsules 305Dejan-Kre4simir Bu4car, Tamara D. Hamilton, and Leonard R. MacGillivray

13.1 Introduction 30513.2 Template-controlled [2 þ 2] Photodimerization in the Solid State 30513.3 Nanostructured Co-crystals 30713.3.1 Organic Nanocrystals and Single Crystal-to-single Crystal

Reactivity 30813.4 Self-assembled Capsules Based on Ligands from the Solid State 30913.5 Summary and Outlook 312

References 313

14 Organic Nanocapsules 317Scott J. Dalgarno, Nicholas P. Power, and Jerry L. Atwood

14.1 Introduction 31714.2 First Generation Nanocapsules 31714.3 Second Generation Nanocapsules 32014.4 Third Generation Nanocapsules 32314.5 Fourth Generation Nanocapsules 32914.6 Fifth Generation Nanocapsules 33114.7 Sixth Generation Nanocapsules 33914.8 From Spheres to Tubes 34214.9 Conclusions 344

References 345

Index 347

XII Contents

Preface

Current research in chemistry and materials science is now vigorously pushing theboundaries of the components studied firmly into themulti-nanometer length scale.In terms of traditional ‘‘molecules’’ a nanometer (10–9 m) is relatively large. As aresult, it is only relatively recent advances in analytical instrumentation capable ofdelivering amolecular-level understanding of structure and properties in this kind ofsize regime that have allowed access to and the study of such large molecules andassemblies. The key interest in multi-nanometer-scale structures (nanostructures) isthe fact that their size allows them to exhibit a significant degree of functionality andcomplexity – complexity that is mirrored in biological systems such as enzymes andpolynucleotides, Nature’s own nanostructures. However, this functionality is com-pressed into a space that is very small on the human scale, sparking interest in fieldssuch as molecular computing and molecular devices. Thus one of the great openingfrontiers in molecular sciences is the upward synthesis, understanding of structureand application of molecules and molecular concepts into the nanoscale.In compiling this book we have sought to bring together chapters from leading

experts working on the cutting edge of this revolution on the nanoscale. Each chapteris a self-contained illustration of the way in which the nanoscale view is influencingcurrent thinking and research across the molecular sciences. The focus is on the‘‘organic’’ (loosely applied) since it is generally carbon-based building blocks that arethe most versatile molecular components that can be induced to link into nanoscalestructures. As chapters by Mohammed Eddaoudi and Lee Cronin show, however,hybrid organic–inorganic materials and well-defined inorganic building blocks asjust as capable of assembling into well-defined and well-characterized discrete andpolymeric nanostructures.Crucial to the whole field of nanochemistry is the cross-fertilization between

researchers from different disciplines that are approaching related structuresfrom very different perspectives. It is with this aspect in mind that we havedeliberately mixed together contributions from the solid-state materials communityas in Dario Braga’s perspective on the crystal engineering or organic nanostructuresand from experts in discrete molecular assemblies such as Dimitry Rudkevich, KaySeverin, Thomas Rauchfuss and Bruce Gibb. Of course, nanostructures are not

XIII

always so well defined and so these aspects are balanced nicely by David Smith’schapter on gel-phase materials – in some respects a ‘‘halfway house’’ betweensolution-phase and solid-state assemblies. We also felt it of key importance toillustrate ways to use small-scale molecular concepts in order to ‘‘synthesize-up’’nanostructures. Chapters by Paul Beer, Steve Loeb and Len MacGillivray providevery different perspectives on templation and assembly in the field, while MakotoFujita and Mike Ward deal with larger-scale self-assembly. Finally, all-importantfunctional nanostructured devices are illustrated by Vincenzo Balzani’s chapter.Although a book of this size can only be illustrative of such a burgeoning field, it is

our sincere hope that the juxtaposition of these different perspectives and systems inone place will stimulate and contribute to the ongoing process of cross-fertilizationthat is driving this fascinating and emerging area of molecular science. It hascertainly been a fascinating and pleasurable experience to work on this projectand we thank all of the authors wholeheartedly for their enthusiastic contributionsto this project. We are grateful also to Manfred Köhl and Steffen Pauly at Wiley-VCHfor their belief in the book and for their help in making it a reality. As this book wentto press we learned of the sad and untimely death of Dimitry Rudkevich. We wouldlike to dedicate this book to his memory and legacy to science.

December 2007 Jonathan W. Steed, Durham, UKJerry L. Atwood, Columbia,MO,USA

XIV Preface

List of Contributors

XV

Jerry L. AtwoodUniversity of Missouri–ColumbiaDepartment of Chemistry125 Chemistry BuildingMO 65211 ColumbiaUSA

Vincenzo BalzaniUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

Paul D. BeerUniversity of OxfordDepartment of ChemistryInorganic Chemistry LaboratorySouth Parks RoadOxford OX1 3QRUK

Dario BragaUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

Dejan-Krešimir BučarUniversity of IowaDepartment of ChemistryIowa CityIA 52245USA

Michał J. ChmielewskiUniversity of OxfordDepartment of ChemistryInorganic Chemistry LaboratorySouth Parks RoadOxford OX1 3QRUK

Alberto CrediUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

Leroy CroninUniversity of GlasgowDepartment of ChemistryGlasgow G12 8QQUK

Marco CurziUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

Scott J. DalgarnoHeriot–Watt UniversitySchool of Engineering and PhysicalSciences – ChemistryEdinburgh EH14 4ASUK

Mohamed EddaoudiUniversity of South FloridaDepartment of Chemistry4202 East Fowler Avenue (CHE 205)TampaFL 33620USA

Jarrod F. EubankUniversity of South FloridaDepartment of Chemistry4202 East Fowler Avenue (CHE 205)TampaFL 33620USA

Makoto FujitaThe University of TokyoSchool of EngineeringDepartment of Applied Chemistry7-3-1 HongoBunkyo-kuTokyo 113-8656Japan

Stefano Luca GiaffredaUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

Bruce C. GibbUniversity of New OrleansDepartment of ChemistryNew OrleansLA 70148USA

Fabrizia GrepioniUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

Tamara D. HamiltonUniversity of IowaDepartment of ChemistryIowa CityIA 52245USA

Stephen J. LoebUniversity of WindsorDepartment of Chemistry andBiochemistryWindsorOntario N9B 3P4Canada

Leonard R. MacGillivrayUniversity of IowaDepartment of ChemistryIowa CityIA 52245USA

XVI List of Contributors

Lucia MainiUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

Charalampos MoirasUniversity of GlasgowDepartment of ChemistryGlasgow G12 8QQUK

Takashi MuraseThe University of TokyoSchool of EngineeringDepartment of Applied Chemistry7-3-1 HongoBunkyo-kuTokyo 113-8656Japan

Voltaire G. OrganoUniversity of Texas at ArlingtonDepartment of Chemistry andBiochemistryArlingtonTX 76019-0065USA

Anna PettersenUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

Marco PolitoUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

Nicholas P. PowerUniversity of Missouri–ColumbiaDepartment of Chemistry125 Chemistry BuildingMO 65211 ColumbiaUSA

Thomas B. RauchfussUniversity of IllinoisDepartment of ChemistryUrbanaIL 61801USA

Dmitry M. RudkevichUniversity of Texas at ArlingtonDepartment of Chemistry andBiochemistryArlingtonTX 76019-0065USA

Kay SeverinÉcole Polytechnique Fédérale deLausanneInstitut des Sciences et IngénierieChimiquesCH-1015 LausanneSwitzerland

David K. SmithUniversity of YorkDepartment of ChemistryHeslingtonYork YO10 5DDUK

Margherita VenturiUniversità di BolognaDipartimento di Chimica “G.Ciamician”Via Selmi 240126 BolognaItaly

List of Contributors XVII

Michael D. WardUniversity of SheffieldDepartment of ChemistryDainton BuildingSheffield S3 7HFUK

XVIII List of Contributors

1Artificial Photochemical Devices and MachinesVincenzo Balzani, Alberto Credi, and Margherita Venturi

1.1Introduction

The interaction between light and matter lies at the heart of the most importantprocesses of life [1]. Photons are exploited by natural systems as both quanta of energyand elements of information. Light constitutes an energy source and is consumed (or,more precisely, converted) in large amount in the natural photosynthetic process,whereas it plays the role of a signal in vision-related processes, where the energy usedto run the operation is biological in nature.A variety of functions can also be obtained from the interaction between light and

matter in artificial systems [2]. The type and utility of such functions depend on thedegree of complexity and organization of the chemical systems that receive andprocess the photons.About 20 years ago, in the frame of research on supramolecular chemistry, the idea

began to arise [3–5] that the concept of macroscopic device and machine can betransferred to themolecular level. In short, amolecular device can be defined [6] as anassembly of a discrete number of molecular components designed to perform afunction under appropriate external stimulation. A molecular machine [6–8] is aparticular type of device where the function is achieved through the mechanicalmovements of its molecular components.In analogy with their macroscopic counterparts, molecular devices and machines

need energy to operate and signal to communicate with the operator. Light providesan answer to this dual requirement. Indeed, a great number ofmolecular devices andmachines are powered by light-induced processes and light can also be useful toread the state of the system and thus to control and monitor its operation. Beforeillustrating examples of artificial photochemical molecular devices and machines, itis worthwhile recalling a few basic aspects of the interaction between molecular andsupramolecular systems and light. For a more detailed discussion, books [9–15] canbe consulted.

j1

1.2Molecular and Supramolecular Photochemistry

1.2.1Molecular Photochemistry

Figure 1.1 shows a schematic energy level diagram for a generic molecule that couldalso be a component of a supramolecular species. In most cases the ground state of amolecule is a singlet state (S0) and the excited states are either singlets (S1, S2, etc.) ortriplets (T1, T2, etc.). In principle, transitions between states having the same spinvalue are allowed, whereas those between states of different spin are forbidden.Therefore, the electronic absorption bands observed in the UV–visible spectrum ofmolecules usually correspond to S0! Sn transitions. The excited states so obtainedare unstable species that decay by rapid first-order kinetic processes, namelychemical reactions (e.g. dissociation, isomerization) and/or radiative and nonradia-tive deactivations. In the discussion that follows, excited-state reactions do not needto be explicitly considered and can formally be incorporated within the radiation-less decay processes. When a molecule is excited to upper singlet excited states(Figure 1.1), it usually undergoes a rapid and 100% efficient radiationless deactiva-tion [internal conversion (ic)] to the lowest excited singlet, S1. Such an excited stateundergoes deactivation via three competing processes: nonradiative decay to theground state (internal conversion, rate constant kic); radiative decay to the groundstate (fluorescence, kfl); conversion to the lowest triplet state T1 (intersystem crossing,kisc). In its turn, T1 can undergo deactivation via nonradiative (intersystem crossing,k0isc) or radiative (phosphorescence, kph) decay to the ground state S0. When themolecule contains heavy atoms, the formally forbidden intersystem crossing and

Figure 1.1 Schematic energy level diagram for a generic molecule. For more details, see text.

2j 1 Artificial Photochemical Devices and Machines

phosphorescence processes become faster. The lifetime (t) of an excited state, that is,the time needed to reduce the excited-state concentration by 2.718 (i.e. the basis fornatural logarithms, e), is given by the reciprocal of the summation of the deactivationrate constants:

tðS1Þ ¼ 1ðkicþkflþkiscÞ ð1Þ

tðT1Þ ¼ 1ðk0 iscþkphÞð2Þ

The orders of magnitude of t(S1) and t(T1) are approximately 10�9� 10�7 and

10�3� 100 s, respectively. The quantum yield of fluorescence (ratio between thenumber of photons emitted by S1 and the number of absorbed photons) andphosphorescence (ratio between the number of photons emitted by T1 and thenumber of absorbed photons) can range between 0 and 1 and are given by

Ffl ¼ kflðkicþkflþkiscÞ ð3Þ

Fph ¼kph � kisc

ðk0iscþkphÞ � ðkicþkflþkiscÞð4Þ

Excited-state lifetimes and fluorescence and phosphorescence quantum yields of agreat number of molecules are known [16].When the intramolecular deactivation processes are not too fast, that is, when the

lifetime of the excited state is sufficiently long, an excited molecule �A may have achance to encounter amolecule of another solute, B (Figure 1.2). In such a case, somespecific interaction can occur leading to the deactivation of the excited state bysecond-order kinetic processes. The two most important types of interactions in anencounter are those leading to electron or energy transfer. The occurrence of theseprocesses causes the quenching of the intrinsic properties of �A; energy transfer also

Figure 1.2 Schematic representation of bimolecular electron-and energy-transfer processes that may occur following anencounter between an excited state, �A, and another chemicalspecies, B.

1.2 Molecular and Supramolecular Photochemistry j3

leads to sensitization of the excited-state properties of the B species. Simple kineticarguments show that only the excited states that live longer than ca. 10�9 smay have achance to be involved in encounters with other solute molecules.An electronically excited state is a species with completely different properties to

thoseof theground-statemolecule. Inparticular, because of its higher energy content,an excited state is both a stronger reductant and a stronger oxidant than the corre-sponding ground state [17]. To afirst approximation, the redox potential of an excited-statecouplemaybecalculatedfromthepotentialof therelatedground-statecoupleandthe one-electron potential corresponding to the zero–zero excited-state energy, E0–0:

EðAþ=�AÞ � EðAþ=AÞ�E0--0 ð5Þ

Eð�A=A�Þ � EðA=A�ÞþE0--0 ð6ÞDetailed discussions of the kinetics aspects of electron- and energy-transfer process-es can be found in the literature [11,18–20].

1.2.2Supramolecular Photochemistry

A supramolecular system can be preorganized so as to favor the occurrence ofelectron- and energy-transfer processes [10]. The molecule that has to be excited, A,can indeed be placed in the supramolecular structure nearby a suitable molecule, B.For simplicity, we consider the case of an A–L–B supramolecular system, where A

is the light-absorbing molecular unit [Eq. (7)], B is the other molecular unit involvedwith A in the light-induced processes and L is a connecting unit (often called bridge).In such a system, after light excitation of A there is no need to wait for a diffusion-controlled encounter between �AandB as inmolecular photochemistry, since the tworeaction partners can already be at an interaction distance suitable for electron andenergy transfer:

A�L�Bþhn ! �A�L�B photoexcitation ð7Þ�A�L�B ! Aþ�L�B� oxidative electron transfer ð8Þ�A�L�B ! A��L�Bþ reductive electron transfer ð9Þ�A�L�B ! A�L��B electronic energy transfer ð10Þ

In the absence of chemical complications (e.g. fast decomposition of the oxidizedand/or reduced species), photoinduced electron-transfer processes [Eqs. (8) and (9)]are followed by spontaneous back-electron-transfer reactions that regenerate thestarting ground-state system [Eqs. 80 and 90] and photoinduced energy transfer [Eq.(10)] is followed by radiative and/or nonradiative deactivation of the excited acceptor[Eq. 100]:

Aþ�L�B� ! A�L�B back oxidative electron transfer ð80Þ


A��L�Bþ ! A�L�B back reductive electron transfer ð90Þ

A�L��B ! A�L�B excited state decay ð100ÞIn supramolecular systems, electron- and energy-transfer processes are no longer

limited by diffusion and occur by first-order kinetics. As a consequence, in suitablydesigned supramolecular systems these processes can involve even very short-livedexcited states.

1.3Wire-Type Systems

An important function at the molecular level is photoinduced energy and electrontransfer over long distances and/or along predetermined directions. This functioncan be performed by rod-like supramolecular systems obtained by linking donor andacceptor components with a bridging ligand or a spacer.

1.3.1Molecular Wires for Photoinduced Electron Transfer

Photoinduced electron transfer in wire-type supramolecular species has beenextensively investigated [6,10]. The minimum model is a dyad, consisting of anelectron donor (or acceptor) chromophore, an additional electron acceptor (or donor)moiety and an organizational principle that controls their distance and electronicinteractions (and therefore the rates and yields of electron transfer). A great numberof such dyads have been constructed and investigated [6,10].The energy-level diagram for a dyad is schematized in Figure 1.3. All the dyad-

type systems suffer to a greater or lesser extent from rapid charge recombination

Figure 1.3 Schematic energy-level diagram for a dyad.

1.3 Wire-Type Systems j5

[process (4)]. An example of a systematic study on dyads is that performed oncompounds 15þ–55þ (Figure 1.4) [21,22]. When excitation is selectively performedin the Ru(II) chromophoric unit, prompt intersystem crossing from the originallypopulated singlet metal-to-ligand charge-transfer ð1MLCTÞ excited state leads tothe long-lived 3MLCT excited state which lies �2.1 eV above the ground state, canbe oxidized approximately at �0.9 V (vs. SCE) and has a lifetime of �1 ms indeaerated solutions [23]. Before undergoing deactivation, such an excited statetransfers an electron to the Rh(III) unit, a process that is then followed by a backelectron-transfer reaction.Comparison of compounds 15þ and 25þ shows that, despite the longer metal–

metal distance, the forward electron transfer is faster across the phenylene spacer(k¼ 3.0· 109 s�1) than across the two methylene groups (k¼ 1.7· 108 s�1). Thisresult can be related to the lower energy of the LUMO of the phenylene group, whichfacilitates electronic coupling. In the homogeneous family of compounds 25þ–45þ,the rate constant decreases exponentially with increasing metal–metal distance.For compound 55þ, which is identical with 45þ except for the presence of two

solubilizing hexyl groups on the central phenylene ring, the photoinduced electron-transfer process is 10 times slower, presumably because the substituents increase thetwist angle between the phenylene units, thereby reducing electronic coupling.Photoinduced electron transfer in three–component systems (triads) is illustrated

in Figure 1.5 [24]. The functioning principles are shown in the orbital-type energydiagrams of the lower part the figure. In both cases, excitation of a chromophoriccomponent (step 1) is followed by a primary photoinduced electron transfer to aprimary acceptor (step 2). This process is followed by a secondary thermal electron-transfer process (step 3): electron transfer from a donor component to the oxidizedchromophoric component (case a) or electron transfer from the primary acceptor to asecondary acceptor component (case b). The primary process competes with excited-state deactivation (step 4), whereas the secondary process competes with primarycharge recombination (step 5). Finally, charge recombination between remotemolecular components (step 6) leads the triad back to its initial state.For case a, the sequence of processes indicated above (1–2–3) is not unique.

Actually, the alternative sequence 1–3–2would also lead to the same charge-separatedstate. In general, these two pathwayswill have different driving forces for the primaryand secondary steps and thus one may be kinetically favored over the other.Occasionally one of the two pathways is thermodynamically allowed and theother is not, although in a simple one-electron energy diagram like that shown inFigure 1.5a this aspect is not apparent.The performance of a triad for wire-type applications is related to the rate and

quantum yield of formation of the charge separated state (depending on thecompetition between forward and back processes, F¼ [k2/(k2þ k4)][k3/(k3þ k5)]).For energy conversion purposes, important parameters are also the lifetime ofcharge separation (depending on the rate of the final charge-recombination process,t¼ 1/k6) and the efficiency of energy conversion (Zen.conv.¼F ·F, where F is thefraction of the excited-state energy conserved in the final charge-separated state). Toput things in a real perspective, it should be recalled that the triad portion of the


Figure 1.4 Binuclear metal complexes 15þ–55þ used forphotoinduced electron-transfer experiments [21,22].


reaction center of bacterial photosynthesis converts light energy with t� 10ms,F¼ 1 and Zen.conv.� 0.6.The introduction of further molecular components (tetrads and pentads) leads to

the occurrence of further electron-transfer steps, which, in suitably designedsystems, produce charge separation over larger and larger distances [6,10]. As thenumber of molecular components increases, also the mechanistic complexityincreases and charge separation may involve energy-transfer steps.Several triads have been designed and investigated. Avery interesting system is the

4-nm long triad 63þ shown in Figure 1.6, which consists of an Ir(III) bis-terpyridine

Figure 1.5 Schematic representation of the two possiblearrangements for charge-separating triads.

Figure 1.6 Electron-transfer processes in triad 63þ [25].


complex connected to a triphenylamine electron donor (D) and a naphthalenebisimide electron acceptor (A) [25]. Upon excitation of the electron donor D (oreven the Ir-based moiety), a charge separated state Dþ–Ir�–A is formed with 100%yield in less than 20 ps that successively leads to Dþ–Ir–A� with 10% efficiency in400 ps. Remarkably, the fully charge-separated state Dþ–Ir–A� has a lifetime of120 ms at room temperature in deaerated acetonitrile solution.

1.3.2Molecular Wires for Photoinduced Energy Transfer

Many investigations on electronic energy transfer in supramolecular species havebeen performed in the past few years [6,10], a relevant fraction of which have beenobtained for systems containing polypyridinemetal complexes as donor and acceptorunits. Usually, the photoexcited chromophoric group is [Ru(bpy)3]

2þ (bpy¼ 2,20-bipyridine) and the energy acceptor is an [Os(bpy)3]

2þ unit. The excited state of [Ru(bpy)3]

2þ playing the role of energy donor is the lowest, formally triplet, metal-to-ligand charge-transfer excited state, 3MLCT, which, as we have seen above, can beobtained by visible light excitation (lmax� 450 nm), lies �2.1 eV above the groundstate and has a lifetime of �1 ms in deaerated solutions [23]. This relatively longlifetime is very useful because it permits the study of energy transfer over longdistances. The occurrence of the energy-transfer process promotes the ground-state[Os(bpy)3]

2þ acceptor unit to its lowest energy excited state 3MLCT, which liesapproximately 0.35 eV below the donor excited state. Both the donor and the acceptorexcited states are luminescent, so that the occurrence of energy transfer can bemonitored by quenching and/or sensitization experimentswith both continuous andpulsed excitation techniques.Ru(II) and Os(II) polypyridine units have been connected by a variety of bridging

ligands and spacers. When the metal-to-metal distance is very short, fast energytransfer occurs by a F€orster-type resonancemechanism [26]. In other systems the twophotoactive units are separated by a more or less long spacer. When the spacer isflexible [e.g.�(CH2)n� chains], the geometry of the system is not well defined and itis difficult to rationalize the results obtained.These problems are overcome by using rigid and modular spacers to connect the

two chromophoric units; the systems so obtained have a well-characterized geometryand the energy transfer can occur over long distances. Interesting examples of thistype of systems are the [Ru(bpy)3]

2þ–(ph)n–[Os(bpy)3]2þ (ph¼ 1,4-phenylene; n¼ 2,

3, 4, 5) species [27] shown in Figure 1.7. In such compounds, excitation of the [Ru(bpy)3]

2þ moiety is followed by energy transfer to the [Os(bpy)3]2þ unit, as shown by

the sensitized emission of the latter (CH3CN, 293K). The energy-level diagram isshown schematically in Figure 1.7. The lowest energy level of the bridge decreasesslightly as the number of phenylene units is increased, but always lies above thedonor and acceptor levels involved in energy transfer. A further decrease in the energyof the triplet excited state of the spacer would be expected to switch the energy-transfer mechanism from superexchange-mediated to hopping, similar to whathappens for photoinduced electron transfer. In the series of compounds shown in


Figure 1.7, the energy-transfer rate decreases with increasing length of the oligo-phenylene spacer. Such rate constants are much higher than those expected for aF€orster-type mechanism, whereas they can be accounted for by a superexchangeDexter mechanism [28]. The values obtained for energy transfer in the analogousseries of compounds [Ru(bpy)3]

2þ–(ph)nR2–[Os(bpy)3]2þ [29], in which the central

phenylene unit carries two hexyl chains, are much lower than those found for theunsubstituted compounds, most likely because the bulky substituents R increase thetilt angle between the phenyl units. A strong decrease in the rate constant is observedwhen the Ru-donor and Os-acceptor units are linked via an oligophenylene bridgeconnected in the meta position [30].In an another family of similar compounds, [Ir(ppyF2)2(bpy)]

þ–(ph)n–[Ru(bpy)3]2þ

(ph¼ 1,4-phenylene; n¼ 2, 3, 4, 5) [31], the energy-transfer rate constant is muchhigher and substantially independent of the length of the spacer. The energy-level

Figure 1.7 Structureofcompounds [Ru(bpy)3]2þ–(ph)n–[Os(bpy)3]

2þ

and energy-level diagram for the energy-transfer process [27].


Documents

Organic Nanostructures€¦ · 1 Artificial Photochemical Devices and Machines 1 Vincenzo Balzani, Alberto Credi, and Margherita Venturi 1.1 Introduction 1 1.2 Molecular and Supramolecular