Iridium(III) in Optoelectronic andPhotonics Applications

Iridium(III) in Optoelectronic and PhotonicsApplications

Edited by

Eli Zysman-ColmanOrganic Semiconductor CentreEaStCHEM School of ChemistryUniversity of St AndrewsUK

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Names: Zysman-Colman, Eli, 1975– editor.Title: Iridium(III) in Optoelectronic and Photonics Applications /edited by Eli Zysman-Colman.

Description: Chichester, West Sussex : John Wiley & Sons, Inc., 2017. |Includes index.

Identifiers: LCCN 2016055403 (print) | LCCN 2016057073 (ebook) | ISBN9781119007135 (cloth) | ISBN 9781119007142 (Adobe PDF) | ISBN9781119007159 (ePub)

Subjects: LCSH: Iridium–Industrial applications. | Iridium alloys. |Optoelectronic devices. | Photonics.

Classification: LCC TP245.I7 I74 2017 (print) | LCC TP245.I7 (ebook) | DDC621.36/5–dc23

LC record available at https://lccn.loc.gov/2016055403

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Dedicated to my parents Margo and Neil whose supporthas been continuous and unwavering

Contents

List of Contributors xvForeword xviiPreface xix

Volume 1

1 Archetypal Iridium(III) Compounds for Optoelectronic and PhotonicApplications: Photophysical Properties and Synthetic Methods 1Joseph C. Deaton and Felix N. Castellano

1.1 Introduction 11.2 Iridium Complex Ion Dopants in Silver Halide Photographic Materials 11.3 Overview of the Photophysical Properties of C^N and C^C: Cyclometalated

Ir(III) Complexes 21.4 Importance of Ir─C Bonds in the Archetypal Ir(III) Complexes for

Optoelectronic and Photonic Applications 91.5 Tuning Emission Color 141.6 Absorbance and Photoluminescence of C^N Cyclometalated Ir(III)

Complexes 171.7 SOC Mechanism: Radiative Decay Rates and ZFS 231.8 Non-Radiative Decay Rates 391.9 Synthetic Methods Targeting C^N Cyclometalated Ir(III) Compounds 421.10 Synthetic Methods for Cyclometalated Ir(III) Compounds Containing

Carbenes 471.11 Conclusions 48

Acknowledgements 49Abbreviations for Ligands in Ir(III) Complexes 49References 50

2 Multinuclear Iridium Complexes 71J. A. Gareth Williams

2.1 Introduction 712.2 Compounds Incorporating ‘Single Atom Bridges’: μ-Chloro, μ-Oxo

and μ-Aza 722.2.1 μ-Chloro-Bridged Complexes 722.2.2 μ-Aza-Bridged Complexes 742.2.3 μ-Hydroxo-Bridged Complexes 76

vii

2.3 Polyatomic Acyclic Bridges: Acetylides, Cyanides and Hydrazides 782.4 Compounds with Heterocyclic Bridges 822.4.1 Bis-(N^N)-Coordinating Ligands and Related Systems Incorporating At Least

One N^N Unit 832.4.2 Bis-(N^C)-Coordinating Ligands 892.5 Multinuclear Complexes Featuring Conjugated Bridges between Iridium-

Bound Polypyridyl or Arylpyridyl Ligands 932.5.1 Systems Incorporating C≡C or N=N Bridges with One or More

[Ir(N^C)2(N^N)]+ Units 95

2.5.2 Multinuclear Complexes Incorporating Phenyl and Polyphenylene Bridgesbetween the Ligands: ‘Supramolecular Assemblies’ 96

2.6 Concluding Remarks 104Acknowledgements 104References 104

3 Soft Materials and Soft Salts Based on Iridium Complexes 111Etienne Baranoff and Yafei Wang

3.1 Introduction 1113.2 Liquid Crystals 1123.3 Gels 1153.4 Micelles 1163.5 Langmuir–Blodgett Films 1183.6 Soft Salts 1183.7 Conclusion 123

Acknowledgements 123References 123

4 Porous Materials Based on Precious Metal Building Blocks for Solar EnergyApplications 127Daniel Micheroni and Wenbin Lin

4.1 Introduction 1274.2 The Luminescent Nature of MOFs and Their Use in Chemical

Applications 1294.3 Energy Transfer in Porous Materials 1344.4 Porous Materials for Water Oxidation 1364.5 Porous Materials for Proton Reduction 1384.6 Porous Materials for CO2 Reduction 1404.7 Conclusions and Outlook 141

References 141

5 Polymeric Architectures Containing Phosphorescent Iridium(III)Complexes 145Andreas Winter and Ulrich S. Schubert

5.1 Introduction 1455.2 Ir(III)-Containing Polymers: Classification, Design Principles, and

Syntheses 1465.2.1 Classification of Ir(III)-Containing Polymers 1465.2.2 Design Principles for Metal-Containing Polymers 147

Contentsviii

5.2.2.1 Decoration of Preformed Polymers with Ir(III) Complexes 1495.2.2.2 Coordination of Ir(III) Precursor Complexes

to Preformed Polymers 1515.2.2.3 (Co)Polymerization of Ir(III)-Containing Monomers 1575.2.2.4 Electropolymerization of Ir(III)-Containing Complexes 1825.2.2.5 Synthetic Approaches Toward Ir(III)-Containing Polymers: The Roads Not

Taken 1865.3 Hyperbranched and Dendritic Architectures 1875.3.1 Ir(III)-Containing Hyperbranched Polymers 1875.3.2 Ir(III)-Containing Dendritic Systems 1885.4 Concluding Remarks 191

References 192

6 Iridium(III) Complexes for OLED Application 205Elena Longhi and Luisa De Cola

6.1 Introduction 2056.2 Iridium Complexes 2066.2.1 General Synthesis of Ir(III) Complexes 2076.2.2 Luminescence of Iridium(III) Complexes 2086.2.3 Emission Color Tuning in Iridium(III) Complexes 2096.2.3.1 Influence of the (C^N) Ligand 2106.2.3.2 Influence of the Ancillary Ligand 2126.3 Organic Light-Emitting Diodes 2166.3.1 Device Architecture and Fabrication 2176.3.2 Device Lifetime 2186.3.3 Device Efficiency 2206.3.4 Phosphorescent Materials 2216.3.5 Host Materials 2226.4 Iridium(III) Complexes for PHOLED Application 2276.4.1 Green Emitters 2276.4.1.1 Role of the Ancillary Ligand 2286.4.1.2 Modification of the Phenylpyridine Ring 2296.4.1.3 Use of Different Tris-cyclometalated Motifs 2306.4.2 Red Emitters 2326.4.3 Blue Emitters 2386.5 Conclusions and Perspectives 262

References 262

7 A Comprehensive Review of Luminescent Iridium Complexes Used inLight-Emitting Electrochemical Cells (LEECs) 275Adam F. Henwood and Eli Zysman-Colman

7.1 Introduction 2757.2 Device Fundamentals 2787.3 Green Emitters 2807.3.1 Archetypal Emitters 2827.3.2 Pyrazoles 2897.3.3 Imidazoles 2927.3.4 Triazoles and Tetrazoles 293

Contents ix

7.3.5 Oxadiazoles 2947.3.6 Thiophenes 2967.3.7 Intramolecular π-Stacked Emitters 2967.3.8 Supramolecular Emitters 3007.4 Blue Emitters 3017.4.1 [Ir(ppy)2(bpy)]

+-Type Emitters 3027.4.2 Pyrazoles 3077.4.3 Imidazoles 3127.4.4 Triazoles 3137.4.5 Oxadiazoles 3167.4.6 N-Heterocyclic Carbenes 3207.4.7 Phosphines 3227.5 Yellow Emitters 3237.5.1 [Ir(ppy)2(bpy)]

+-Type Emitters 3247.5.2 Imidazole Emitters 3277.5.3 Anionic Emitters 3287.5.4 Intramolecularly π-Stacked Emitters 3287.5.5 Multifunctional or Supramolecular Emitters 3327.6 Orange-Red Emitters 3347.6.1 [Ir(ppy)2(bpy)]

+-Type Emitters 3357.6.2 Emitters Bearing Five-Membered Heterocyclic Rings 3407.6.3 Intramolecular π-Stacked Emitters 3417.6.4 Multifunctional Emitters 3457.7 Conclusions and Outlook 348

Acknowledgements 349References 349

Volume 2

8 Electrochemiluminescence of Iridium Complexes 359Sarah E. Laird and Conor F. Hogan

8.1 Background and Overview of Electrochemiluminescence 3598.1.1 ECL from Metal Complexes 3628.2 Iridium ECL 3638.2.1 First Examples 3638.2.2 Renewed Interest in Iridium ECL Stimulated by Progress in the Field of

Light-Emitting Devices 3648.2.3 Early Advances in Theoretical Understanding and Electrochemiluminophore

Design 3668.2.4 Modified Electrode Systems 3708.2.5 ECL-Based Sensing Strategies 3728.2.6 Issues Related to ECL of IridiumComplexes in AqueousMedia andQuenching

by Oxygen 3848.2.7 Tuning ECL Emission Colour and Redox Properties 3868.2.8 Potential-Resolved Multicolour ECL 3998.2.8.1 Miscellaneous ECL Systems Involving Iridium Complexes 4058.2.9 Conclusion and Future Prospects 406

List of Ligand Abbreviations Used in Text 406References 407

Contentsx

9 Strategic Applications of Luminescent Iridium(III) Complexes asBiomolecular Probes, Cellular Imaging Reagents, and PhotodynamicTherapeutics 415Karson Ka-Shun Tso and Kenneth Kam-Wing Lo

9.1 Introduction 4159.2 General Cellular Staining Reagents 4169.3 Hypoxia Sensing Probes 4239.4 Molecular and Ion Intracellular Probes 4279.4.1 Intracellular Probes for Sulfur-Containing Species 4279.4.2 Intracellular Probes for Metal Ions 4339.4.3 Intracellular Probes for Hypochlorous Acid and Hypochlorite 4379.4.4 Intracellular Probes for Nitric Oxide 4399.5 Organelle-Targeting Bioimaging Reagents 4419.5.1 Nucleus 4419.5.2 Nucleoli 4439.5.3 Golgi Apparatus 4459.5.4 Mitochondria 4479.6 Functionalized Polypeptides for Bioimaging 4509.7 Polymers and Nanoparticles for Bioimaging 4549.8 Photocytotoxic Reagents and Photodynamic Therapeutics 4589.9 Conclusion 466

Acknowledgements 466Abbreviations 466References 469

10 Iridium Complexes in the Development of Optical Sensors 479Teresa Ramón-Márquez, Marta Marín-Suárez, Alberto Fernández-Gutiérrezand J. F. Fernández-Sánchez

10.1 Generalities of Optical Sensors 47910.2 Ir(III) Used as Optical Probes 48110.2.1 Optical Probes for the Detection of Gaseous Species 48110.2.1.1 Oxygen 48210.2.1.2 Other Gaseous Species 48310.2.2 Optical Probes for the Detection of Ionic Species 48510.2.2.1 Cations 48510.2.2.2 pH 49110.2.2.3 Anions 49310.2.3 Optical Probes for the Detection of Biomolecules 49810.2.3.1 Amino Acids and Proteins 49810.2.3.2 Nucleotides and Nucleic Acids 50610.2.4 Optical Probes for the Detection of Other Small Molecules 50610.2.4.1 Explosives 50610.2.4.2 Free Radicals 50710.2.4.3 H2O2 50810.2.4.4 Amines 50810.2.4.5 Silver Salts 50810.2.4.6 Hypochlorous Acid (HOCl) 50810.3 Ir(III) Used in the Development of Sensing Phases 50910.3.1 Sensing Phases for the Detection of Gases 509

Contents xi

10.3.1.1 Oxygen 50910.3.1.2 Others Gases 51610.3.2 Sensing Phases for the Detection of Ions 51610.3.3 Sensing Phases for the Detection of Biomolecules 51710.3.3.1 Glucose 51810.3.3.2 BSA 52010.3.3.3 Cysteine and Homocysteine 52010.3.3.4 Heparin 52010.3.3.5 Histone 52110.3.4 Sensing Phases for Multiparametric Sensing 52110.4 Conclusion and Future Challenges 522

Acronyms Used in the Names of the Complexes 525References 528

11 Photoredox Catalysis of Iridium(III)-Based Photosensitizers 541Timothy M. Monos and Corey R. J. Stephenson

11.1 Introduction 54111.1.1 Photoredox Catalysis 54111.1.2 Principles of Photoredox Catalysis 54211.1.3 Iridium(III) Photocatalyst Design 54211.1.4 Ir(III) Photocatalyst synthesis 54511.2 Iridium-Based Photoredox Catalysis in Organic Synthesis 54711.2.1 Net Oxidative Reactions 54711.2.1.1 Amine Oxidation and Functionalization 54711.2.1.2 Arene Oxidation 55111.2.2 Net Reductive Reactions 55111.2.2.1 Dehalogenation Reactions 55111.2.2.2 Ketyl Radical Chemistry 55311.2.3 Redox-Neutral Reactions 55411.2.3.1 Atom Transfer Radical Addition 55511.2.3.2 Radical-Based Arene Addition Reactions 56111.2.3.3 Tandem Catalysis Methods 56511.2.4 Amine Fragmentation 57111.3 Conclusion 574

References 574

12 Solar Fuel Generation: Structural and Functional Evolution of IridiumPhotosensitizers 583Husain N. Kagalwala, Danielle N. Chirdon and Stefan Bernhard

12.1 Introduction 58312.2 Fundamentals of [Ir(C^N)2(N^N)]+ Photosensitizers 58512.2.1 Synthesis and Structure 58512.2.2 Electronics: Photophysics and Electrochemistry 58512.2.3 Complexes Made to Order 58812.3 Application of [Ir(C^N)2(N^N)]+ in Photocatalytic Water Reduction 58912.3.1 Initial Exploration 58912.3.2 Systems with Non-precious Components 59112.3.3 Strategies for Improved Efficiency 594

Contentsxii

12.3.3.1 New C^N Ligands 59412.3.3.2 New N^N Ligands 59712.3.3.3 Orchestration 59912.4 Alternative Iridium Structures 60312.4.1 Tridentate Coordination 60312.4.2 Tris-Cyclometalated Complexes 60512.4.3 Dinuclear Iridium Complexes 60612.5 Outlook 607

Acknowledgements 609References 610

13 Iridium Complexes in Water Oxidation Catalysis 617Ilaria Corbucci, Alceo Macchioni and Martin Albrecht

13.1 Introduction 61713.2 Sacrificial Oxidants 61913.2.1 Cerium(IV) Ammonium Nitrate 62013.2.2 Sodium Periodate 62013.3 Molecular Iridium Catalyst for Water Oxidation 62113.3.1 Ir WOCs without Cp∗ 62113.3.2 Ir WOCs with Cp∗ 62413.3.3 Cp∗Ir WOCs Based on Carbene-Type Ligands 63213.3.3.1 Cp∗Ir WOCs Bearing Normal Carbene-Type Ligands 63313.3.3.2 Cp∗Ir WOCs Bearing Abnormal Carbene-Type Ligands 63613.3.3.3 Comparison of Catalytic Activity of Cp∗Ir Bearing Mesoionic Imidazolylidene

Ligand or the Mesoionic Triazolylidene Analogue 63813.3.4 Heterogenized Molecular Iridium Catalyst for Water Oxidation 64013.3.5 Iridium WOC as Photocatalyst for Water Oxidation under Visible Light

Irradiation 64513.4 Conclusions 647

Acknowledgements 648Glossary of Terms and Abbreviations 648References 649

14 Iridium Complexes as Photoactive Center for Light Harvesting and Solar CellApplications 655Etienne Baranoff and Prashant Kumar

14.1 Introduction 65514.2 Photoinduced Electron Transfer in Multicomponent Arrays 65614.2.1 Ir(tpy)2 Fragment (tpy = 2,2 :6 -2 -terpyridine) 65614.2.2 Cyclometalated Iridium(III) 66014.3 Iridium Complexes as Photoactive Center for Solar Cell Applications 66514.3.1 Sensitizer for Dye-Sensitized Solar Cells 66514.3.2 Iridium Complexes for Organic Photovoltaic Devices 67314.4 Conclusions 676

References 677

Index 683

Contents xiii

List of Contributors

Martin AlbrechtDepartment of Chemistry andBiochemistryUniversity of BernSwitzerland

Etienne BaranoffSchool of ChemistryThe University of BirminghamUK

Stefan BernhardDepartment of ChemistryCarnegie Mellon UniversityPittsburgh, PA, USA

Felix N. CastellanoDepartment of ChemistryNorth Carolina State UniversityRaleigh, NC, USA

Danielle N. ChirdonDepartment of ChemistryCarnegie Mellon UniversityPittsburgh, PA, USA

Ilaria CorbucciDepartment of ChemistryBiology and Biotechnology and CIRCCUniversity of PerugiaItaly

Luisa De ColaInstitut de Science et d’IngénierieSupramoléculaires (ISIS)Université de StrasbourgFrance

Joseph C. DeatonDepartment of ChemistryNorth Carolina State UniversityRaleigh, NC, USA

Alberto Fernández-GutiérrezDepartment of Analytical ChemistryFaculty of ScienceUniversity of Granada, Spain

J. F. Fernández-SánchezDepartment of Analytical ChemistryFaculty of ScienceUniversity of Granada, Spain

Adam F. HenwoodOrganic Semiconductor CentreEaStCHEM School of ChemistryUniversity of St AndrewsUK

Conor F. HoganDepartment of Chemistry and PhysicsLa Trobe Institute for Molecular ScienceLa Trobe UniversityMelbourne, Victoria, Australia

Husain N. KagalwalaDepartment of ChemistryCarnegie Mellon UniversityPittsburgh, PA, USA

Kenneth Kam-Wing LoDepartment of Biology and ChemistryState Key Laboratory of Millimeter WavesCity University of Hong KongP.R. China

xv

Karson Ka-Shun TsoDepartment of Biology and ChemistryState Key Laboratory of Millimeter WavesCity University of Hong KongP.R. China

Prashant KumarSchool of ChemistryThe University of BirminghamUK

Sarah E. LairdDepartment of Chemistry and PhysicsLa Trobe Institute forMolecular ScienceLa Trobe UniversityMelbourne, Victoria, Australia

Wenbin LinDepartment of ChemistryThe University of ChicagoIL, USA

Elena LonghiInstitut de Science et d’IngénierieSupramoléculaires (ISIS)Université de StrasbourgFrance

Alceo MacchioniDepartment of ChemistryBiology and Biotechnology and CIRCCUniversity of PerugiaItaly

Marta Marín-SuárezDepartment of Analytical ChemistryFaculty of ScienceUniversity of Granada, Spain

Daniel MicheroniDepartment of ChemistryThe University of ChicagoIL, USA

Timothy M. MonosDepartment of ChemistryUniversity of MichiganAnn Arbor, MI, USA

Teresa Ramón-MárquezDepartment of Analytical ChemistryFaculty of ScienceUniversity of GranadaSpain

Ulrich S. SchubertInstitute for Organic and MacromolecularChemistry (IOMC)Jena Center for Soft Matter (JCSM)Center for Energy and EnvironmentalChemistry Jena (CEEC Jena)Friedrich Schiller University JenaGermany

Corey R. J. StephensonDepartment of ChemistryUniversity of MichiganAnn Arbor, MI, USA

Yafei WangSchool of ChemistryThe University of BirminghamUKSchool of Materials Science andEngineeringChangzhou UniversityChina

J. A. Gareth WilliamsDepartment of ChemistryDurham UniversityUK

Andreas WinterInstitute for Organic and MacromolecularChemistry (IOMC)Jena Center for Soft Matter (JCSM)Center for Energy and EnvironmentalChemistry Jena (CEEC Jena)Friedrich Schiller University JenaGermany

Eli Zysman-ColmanOrganic Semiconductor CentreEaStCHEM School of ChemistryUniversity of St AndrewsUK

List of Contributorsxvi

Foreword

When I was an undergraduate student in the late 1980s, the quintessential photoactivecoordination compound was [Ru(bpy)3]

2+. I still remember the astonishment in learningthe apparently unique combination of ground and excited state electronic properties ofthis complex and its derivatives. It looked sort of impossible to discover counterpartscomparably good by browsing the periodic table and designing new chelating ligands.Of course, this proved incorrect. Indeed, the number of potential competitors ofRu(II) complexes in the area of photochemistry and its applications has increased,Ir(III) systems being a notable case.In the late 1990s, iridium compounds were still a minor area of research. A quick bib-

liometric analysis shows that the number of papers published in the field at that time isbasically background noise when compared to the skyrocketing trends of the last 15 years.In retrospect, this is not surprising; some physical properties of iridium and its scarcityon the Earth’s crust did not make it a particularly sexy candidate to open new avenuesin synthetic chemistry and advanced technological and biomedical applications.The “iridium fever” is one of the most notable recent happenings in the domain of

inorganic and organometallic chemistry. It has been driven by a relentless demand forefficient and sustainable solutions in the areas of photonics, analytical chemistry andcatalysis. But it is not limited to applicative fields, because iridium compounds have alsoproven a remarkable playground to test and advance theoretical methods in complexsystems. Despite this amazing success, books devoted to the chemistry of thiselement are rather limited or outdated. Surprisingly, a book covering optoelectronicsand photonics of iridium materials was still missing.This volume fills the gap. It encompasses discussions on consolidated aspects of

iridium (photo)chemistry such as (i) design and synthesis (mono- and polynuclearcomplexes; soft, porous and polymeric materials); (ii) use in electroluminescent devices(OLEDs, LEECs); (iii) electrochemiluminescence, sensing, theranostics. Remarkably, italso presents developments in the area of photoredox catalysis and solar energy conver-sion, where iridium compounds can serve both as sensitizers and catalysts. These latterfields are still in their infancy.Twenty years ago nobody anticipated the surge of iridium-based photoactive materials.

Now we may foresee that the follow-up of this book will include applications that we donot envisage yet. Most probably, this will happen also because students and scholars willfind these pages stimulating and inspiring.

Nicola Armaroli, ISOF-CNRBologna, January 2017

xvii

Preface

In the digital age, humanity has become reliant on myriad technologies, many of whichdepend on scarce elements. Of these, iridium is one of the most rare, about 40 times lessabundant than gold. Most iridium is found in a thin layer of sediment, which coincideswith the so-called K–Pg boundary that marks the Cretaceous–Paleogene extinctionevent where approximately 70% of plant and animal species, including the dinosaurs,were lost. The demise of the dinosaurs 65 million years ago to an iridium-rich asteroidand the discovery of this element in 1803 by British chemist Smithson Tennant have alsonow permitted some of the most promising optoelectronic materials to be developed.Iridium and its alloys and organometallic complexes have become very important acrossa wide range of applications – from ballpoint pen tips, to contacts in spark plugs, to catal-ysis. However, it is the desirable photophysical properties of iridium complexes that havemade these compounds so attractive as components in optoelectronic applications.This goal of this book is to provide the most comprehensive account of photoactive

iridium complexes and their use across a wide variety of applications. The book will startwith an overview of the synthesis of these complexes and discuss their photophysicalproperties. It will highlight not only mononuclear complexes but also the propertiesof multinuclear and polymeric iridium-basedmaterials and the assembly of iridium com-plexes into larger supramolecular architectures such as MOFs and soft materials. Chap-ters devoted to the use of these iridium-based materials in diverse optoelectronicapplications follow, including electroluminescent devices such as organic light-emittingdiodes (OLEDs) and light-emitting electrochemical cells (LEECs), electrochemilumines-cence (ECL), bioimaging, sensing, light harvesting in the context of solar cell applica-tions, in photoredox catalysis and as components for solar fuels.The different contributions are written by leading experts in the field and are in a style

that clearly explains the link between the structure of the iridium complex and its prop-erties and use in optoelectronic applications. I am convinced that this book will becomean important resource to both those in academia, including postgraduate students, andthose in industry. I hope that these pages will stimulate further research to develop evenmore potent and attractive iridium-based materials.I would like to take the opportunity to thank all of the contributors to this book for

their efforts and for sharing my vision to develop a definitive resource for the synthesis,properties and applications of photoactive iridium complexes.

Eli Zysman-ColmanSt Andrews, Fife, UK2016

xix

1

Archetypal Iridium(III) Compounds for Optoelectronicand Photonic Applications: Photophysical Properties andSynthetic MethodsJoseph C. Deaton and Felix N. Castellano

Department of Chemistry, North Carolina State University, Raleigh, NC, USA

1.1 Introduction

In the early years following the discovery and isolation of the element iridium, it wasregarded as “not useful for anything” because of its apparent chemical inertness as anoble metal and the high temperatures required for forging iridium-based metal objects[1]. But steady advances in the fields of metallurgy, chemistry, physics, and materialsscience have culminated in numerous applications for the element. In particular,the succeeding chapters of this volume will each describe in detail an applicationutilizing the extraordinary photophysical properties and reversible electrochemistryof organometallic complexes of iridium. In this opening chapter, the photophysicalproperties and synthesis of the archetypal complexes suitable for these applications willbe presented.

1.2 Iridium Complex Ion Dopants in Silver HalidePhotographic Materials

Before proceeding with the review of their modern optoelectronic applications, it isworth noting that iridium complexes have already been in use for decades in a specialtype of optoelectronic product: silver halide photographic films and papers. The silverhalide process is a unique optical process in a semiconductor because of a remarkablecombination of solid state properties of the material [2, 3]. Well into the twentiethcentury, development of this technology proceeded in an empirical manner, mainly inindustrial laboratories where trade secrets were more highly valued than scientific pub-lications and sometimes even patents. Therefore the origin of the use of iridium com-plexes and their effects can be difficult to discern from the early literature andpatents [4, 5 and references therein]. But in more recent literature, it has been shown

1

Iridium(III) in Optoelectronic and Photonics Applications, First Edition.Edited by Eli Zysman-Colman.© 2017 John Wiley & Sons Ltd. Published 2017 by John Wiley & Sons Ltd.

that iridium complexes such as [IrCl6]3−, [IrBr6]

3−, and even molecules containing smallorganic ligands may be incorporated as impurity ion dopants into silver halide crystals ormicrocrystals where they function as traps for photoelectrons generated during lightexposure, thus modulating the life cycle of the photoelectrons in the latent image form-ing process [6, 7]. In more recent patent disclosures, it was shown that an importanteffect of these dopants is to control what is known as reciprocity law failure for photo-graphic exposures [8, 9]. Ideally, according to the reciprocity law, the image optical den-sity formed after development of an exposed photographic film should be the same forthe same value of total exposure, E, regardless of the combination of intensity, i, andexposure time, t, used to produce that exposure according to Equation 1.1:

E = it 1 1

In practice, photographic films and papers exhibit lower developed optical density forexposures made with relatively low intensity over longer time (low-intensity reciprocityfailure) or for exposures made with high intensity over shorter time (high-intensityreciprocity failure) or both. Doping the silver halide microcrystals during precipitationwith small concentrations of iridium complexes such as [IrCl6]

3− has been shown toreduce reciprocity law failure, although it cannot be totally eliminated for exposureextremes [8, 9]. The effect of the iridium dopant is not based on a photophysical propertyin the same sense as for the iridium complexes in applications described in succeedingchapters. Rather, the presence of the iridium dopant and its naturally accompanyingcharge compensation defect introduces a trap for electrons below the conduction bandof the silver halide and thereby modulates the lifecycle of photoelectrons in the micro-crystal. Dopants, such as the iridium complexes and a special class known as chemicalsensitizers [2], do not affect how much light is absorbed by the silver halide, at leastto a first approximation, but rather control how efficiently the light is used to formthe latent image. Chemical sensitizers and dopants are therefore distinguished fromorganic sensitizing dyes, known as spectral sensitizers, which function through increas-ing light absorption and injecting the resultant photoelectrons into the silver halideconduction band [2], much like the Ru(II) dyes found in Grätzel-type dye sensitizedsolar cells.

1.3 Overview of the Photophysical Properties of C^Nand C^C: Cyclometalated Ir(III) Complexes

Ir(III) complexes bearing C^N and C^C: cyclometalated ligands possess impressivephotophysical properties that make these compounds highly desirable for the optoelec-tronic and photonic applications covered in this volume. Stereochemical illustrations ofrepresentative archetypal Ir(III) cyclometalates (Section 1.4) are presented in Scheme 1.1,and structural formulae for additional examples are shown in Scheme 1.2 in 2D. In thesestructures, the C^N and C^C: bidentate ligands are monoanionic, and the negativecharge is donated by a C atom occupying one coordination site. In the case of theC^C: cyclometalates, the second coordination site is occupied by the neutral C-donor(designated C:) of the carbene moiety (Scheme 1.3). The :C^N and :C^C: types of carbeneligands are charge neutral and have been incorporated in heteroleptic complexes with

Iridium(III) in Optoelectronic and Photonics Applications2

monoanionic C^N or C^C: ligands. Examples of Ir(III) cyclometalates with tridentateligands are shown in Scheme 1.4. A list of abbreviations for ligands illustrated in theschemes and used in the text may be found at the end of the chapter. The Ir(III) com-plexes are generally obtained as racemic mixtures, and the structural diagrams inSchemes 1.1, 1.2, 1.3, and 1.4 are not meant to limit the representations to specificenantiomers. Because the properties of enantiomers differ only in optical activity,their properties will not be covered in succeeding sections, but the preparation and iso-lation of enantiomers and diastereomers will be covered in Section 1.9.The proper nomenclature, for example, of the prototype compound is fac-tris(2-phe-

nylpyridinato-N^C2 )iridium(III) (fac-Ir(ppy)3, Scheme 1.1). But often in the literaturethe prefix is omitted, and it is assumed that the facial isomer is being discussed becausethese are generally much more emissive than the meridional isomers (mer-Ir(ppy)3,Scheme 1.1), at least in the case of the more common C^N cyclometalates(Section 1.4). The nomenclature is often further simplified to Ir(phenylpyridine)3, forexample, with the assumption that it is understood that the ortho-deprotonated formof the ligand (2 carbon) is intended, not the neutral form. The C on the phenyl ring thatis bonded to the pyridine ring is designated the 1 position, and in numbering substitu-ents on the phenyl ring, the point of metalation takes precedence as the 2 position. Num-bering positions of further substituents on the phenyl ring can therefore be differentin the complex than in the free ligand and sometimes may be a point of confusion.

fac-Ir(ppy)3

NN N

N

N

N

N

+

N

N

N

NN

N

Ir Ir

Ir

NO

O

N

Ir

Ir

Ir(ppy)2(acac) [Ir(ppy)2(bpy)]+

mer-Ir(ppy)3 mer-Ir(ppy)3(piq)

Scheme 1.1 Sterochemical diagrams of representative archetypal Ir(III) cyclometalates.

Archetypal Iridium(III) Compounds for Optoelectronic and Photonic Applications 3

Still other researchers designate the site of metalation as the 1 C atom and number sub-stituents accordingly.The cyclometalated Ir(III) compounds are highly emissive because the lowest energy

excited states are a mixture of metal-to-ligand charge transfer (MLCT) and ligand-centered (LC) π–π∗ states, not the non-radiative d–d states (Section 1.4). Schematicenergy level diagrams of the frontier one-electron orbitals and the resultant zero-order

FIrpic

IrN

IrN

N

N

O

F

F

2

3

IrN

3

Ir

N

Ir

+

N N

NS

S

O

O

22

IrN

3

IrN

3

IrO

O

N

2

Ir Ir Ir

N N NN

NC6H11NN

3 3 3

Ir BN N N

N N

N N

N N

F

F Ir IrNN N

N

NNN

CF3 CF3

N N

F

F F

F2 2 2

O

Ir(ppz)3

Ir(piq)3

Ir(btp)2(acac)

Ir(iprpim)3

Ir(3-piq)3

[Ir(thpy)2(bpy)]+

Ir(dfppy)2(pz2Bpz2) Ir(dfppy)2(fppz)

Ir(imphn)3

Ir(phq)3

Ir(dfpypy)2(fppz)

Ir(cytrz)3

Ir(dmphq)2(acac)

Scheme 1.2 Structural formulae of additional examples of Ir(III) cyclometalates.

Iridium(III) in Optoelectronic and Photonics Applications4

Ir(pmi)3

IrIr

N

N

N

N

Ir

N

N Ir

IrIrIr

N

N

NNNN

N

N

N

N

NN

NN

FF

F F

CN3 3 3 3

22

2

Ir(ppy)2(pmi) [Ir(dfppy)2(pymi)]+ [Ir(dfppy)2(dmi)]+

Ir(pti)3 Ir(pmb)3 Ir(cnpmb)3

+ +

Scheme 1.3 Structural formulae of representative Ir(III) cyclometalates comprising carbene ligands.

[Ir(phbpy)2]+

IrN

Ir IrN

N

NN

NN

N

N

NIr

nBu

nBu

nBu

nBu

[Ir(bbi)2]+

N

NN

NN

N

N

N

N

+ +

+

Ir(dpyx)(dphpy) [Ir(dpyx)(phbpy)]+

Scheme 1.4 Sterochemical diagrams of representative Ir(III) cyclometalates comprising tridentateligands.

Archetypal Iridium(III) Compounds for Optoelectronic and Photonic Applications 5

many-electron states are shown in Figures 1.1 and 1.2, respectively. Note that the zero-order (i.e., prior to anymixing interactions)MLCT state is the lowest energy excited statewithin the singlet manifold, but as shown in Figure 1.2 often the LC is lowest within thetriplet manifold because of greater electron-exchange interaction (Section 1.6).In heteroleptic complexes wherein the HOMO comprises a mixture of Ir dπ orbitals

and π orbitals of one ligand while the LUMO comprises a π∗ orbital on another ligandsuch that the latter becomes the chromophoric ligand, the lowest triplet excited statemay be described as a mixture of MLCT and ligand-to-ligand charge transfer (LLCT),sometimes referred to as metal–ligand-to-ligand charge transfer (MLLCT). This is com-monly the case in cationic complexes of the type [Ir(C^N)2(N^N)]+ where N^N is a dii-mine (Section 1.4) and is the chromophoric ligand. A less commonly encounteredsituation occurs when strong electron donor groups are substituents on the chromo-phoric ligand and an intra-ligand charge transfer (ILCT) occurs [10].The photophysical properties of greatest practical value include rapid singlet-to-triplet

intersystem crossing (ISC), high photoluminescence (PL) quantum yields, spectral tun-ability over the entire visible range through chemical structure variation, and fast phos-phorescence decay rates generating bright triplet state PL. The zero-field splitting (ZFS)within the excited triplet state is an important property in characterizing the nature ofthis emissive state. In addition to meritorious photophysical properties, the reversibleelectrochemical properties of many of these compounds (Section 1.4) are necessaryfor robust stability in many applications, especially in optoelectronics.The ISC rate, the radiative decay rate, the ZFS, and other spectroscopic features all

depend upon the spin–orbit coupling (SOC) interaction between the lowest tripletand one or more singlet excited states, resulting in an admixture of some singletcharacter into the lowest triplet state (Section 1.7). The ZFS in the lowest triplet excitedstate may additionally depend upon SOC with a higher triplet state. Large SOC interac-tions are found in molecules comprising heavy atoms because the SOC constant gener-ally increases with atomic number and is therefore quite large for Ir (3909 cm−1) [11].Note that this value is for zero-valent iridium, and in Ir(III) complexes the SOC constantwould necessarily increase [12] and has been calculated to be 4430 cm−1 [13]. Eventhough the lowest triplet excited state may be dominantly LC in nature, it is evident thatiridium orbitals must be involved through some admixture of MLCT character in orderto produce the extraordinary photophysical properties in comparison with those of thefree ligand (Section 1.7).When some singlet character ismixed into triplet states, singlet excited states can rapidly

intersystem cross to the triplet manifold with yields approaching unity and thermalize tothe lowest triplet excited state. Thus, spin-allowed photon absorption that typically resultsin initial population of a Franck–Condon singlet excited state results in emission from thelowest triplet state when ISC is fast relative to the fluorescence emission rate from the sin-glet state. For this reason, such phosphorescent emitters can harvest both singlets and tri-plets formed in an “organic” light-emitting diode (OLED). According to the spindegeneracies, triplets comprise 3/4 and singlets comprise ¼ of the excitons formed uponrecombination of the electrochemically generated electrons and holes injected into anOLED. Consequently, the organometallic iridium cyclometalates are muchmore desirableas emitters for OLED applications (Chapter 6) than fluorescent emitters having equallyhigh PL quantum yields because fluorescent dopants are limited to harvesting only singletexcitons. Thus, an OLED with a phosphorescent emitter can have an electroluminescence

Iridium(III) in Optoelectronic and Photonics Applications6

E

t2

Δo

e*

π*

π

Figure 1.1 Energy level diagram for one-electron orbitals in d6 MLCT–LC complexes.

1LC

1MLCTISC

ΔE

E

kr knr

knrhv

S0

3MLCT

3LC

3MC

Figure 1.2 Jablonski diagram for the many-electron states in d6 MLCT–LC complexes.

Archetypal Iridium(III) Compounds for Optoelectronic and Photonic Applications 7

quantum yield (excluding optical out-coupling losses) approaching 100% of the phosphor’sPL quantum yield, while the upper limit for an OLED comprising a fluorescent emitter is25% of the fluorophore’s PL quantum yield (ignoring additional singlets that may beformed through triplet–triplet annihilation [14]).Determination of ISC rates requires ultrafast absorption or fluorescence techniques,

often requiring deconvolution of the transient signal(s) from the instrument responsefunction and has been reported for relatively few representative Ir(III) compounds todate [15–17]. The ISC rate for Ir(ppy)3 was reported to be within 100 fs [16], and theISC rate of Ir(piq)3 wasmore distinctly determined to be 70 fs [17]. In view of calculationsand experimental evidence (Section 1.7) that illustrate the excited states are highly mixedin terms of singlet and triplet character, some authors have even suggested that ISC isnot a meaningful descriptor for the process of converting the initial Franck–Condonstate to the lowest energy excited state and instead propose that this be viewed as a formof internal conversion (IC).The PL quantum yield of the prototype fac-tris(2-phenylpyrdinato-N^C2 -)

iridium(III) (Ir(ppy)3) in toluene (λmax = 516 nm) was reported [18] to be 40% whenit was first isolated. Subsequent determinations [19–21] revealed it to be muchhigher, up to 97% in 2-MeTHF [21] although it varies with solvent [22]. Quantumyields over 90% have also been reported for Ir(ppy)3 diluted into polymers andorganic semiconductor thin film host materials [19, 20, 23, 24]. This is an extraor-dinarily high quantum yield for a phosphorescent emitter, that is, one that emits via aformally spin-forbidden transition that gains intensity through SOC. The efficienciesachieved with Ir(III) cyclometalates in optimized OLEDs are indeed consistent withvery high phosphorescence quantum yields and harvesting both singlet and tripletexcitons [25].In many reports of Ir(III) complexes, the phosphorescence quantum yields and

observed decay times, τobs, in solution at room temperature are reported. The observedintensity decay rate, kobs, is simply the reciprocal of the observed decay time constant(Equation 1.2). As implied by a single value of τobs or kobs, the emission decay is nor-mally a single exponential that obeys Equation 1.3 in solution samples where the envi-ronment is rapidly averaged. kobs is obtained by fitting Equation 1.3 to the experimentalPL intensity decay data. kobs is simply the sum of radiative (kr) and non-radiative (knr)decay rates (Equation 1.4). The PL quantum yield (Φ) is related to kr and knr byEquation 1.5:

kobs =1τobs

1 2

I t = I0 exp −kobst 1 3

kobs = kr + knr 1 4

Φ =krkobs

1 5

Clearly, when the quantum yield is very high, kobs is close to kr. Reporting kobs is mostuseful when quantum yields are also reported because then the values of kr and knr can beseparately obtained. The radiative rate constant kr may be related to the strength ofthe SOC (Section 1.7), while knr depends additionally on other factors (Section 1.8).

Iridium(III) in Optoelectronic and Photonics Applications8