Vincenzo Balzani, Paola Ceroni, and Alberto Juris

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2012 ISBN: 978-3-527-33210-6 Vincenzo Balzani, Paola Ceroni, and Alberto Juris

Photochemistry and Photophysics

Concepts, Research, Applications The Authors All books published by Wiley-VCH are carefully produced. Nevertheless, authors, Prof. Vincenzo Balzani editors, and publisher do not warrant the information contained in these books, University of Bologna including this book, to be free of errors. ‘‘G. Ciamician’’ Department of Chemistry Readers are advised to keep in mind that Via Selmi 2 statements, data, illustrations, procedural 40126 Bologna details or other items may inadvertently be Italy inaccurate. Prof. Paola Ceroni University of Bologna Library of Congress Card No.: applied for ‘‘G. Ciamician’’ Department of Chemistry Via Selmi 2 British Library Cataloguing-in-Publication 40126 Bologna Data Italy A catalogue record for this book is available from the British Library. Prof. Alberto Juris University of Bologna Bibliographic information published by the ‘‘G. Ciamician’’ Department of Chemistry Deutsche Nationalbibliothek Via Selmi 2 The Deutsche Nationalbibliothek 40126 Bologna lists this publication in the Deutsche Italy Nationalbibliografie; detailed bibliographic data are available on the Internet at .

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VII

Contents

List of Boxes XVII Preface XIX Acknowledgments XXV List of Abbreviations XXVII

1 Introduction 1 1.1 Photochemistry and Photophysics in Science and Technology 1 1.2 Historical Notes 2 1.3 A New Dimension of Chemistry and Physics 3 1.4 The Nature of Light 5 1.5 Absorption of Light 7 1.6 Quantum Yield, Efficiencies, and Excited-State Reactivity 8 References 10

2 Elementary Molecular Orbital Theory 11 2.1 Introduction 11 2.2 The Hydrogen Atom 11 2.3 Polyelectronic Atoms 13 2.4 From Atoms to Molecules 17 2.5 Electronic Structure of Homonuclear Diatomic Molecules 21 2.6 Electronic Structure of Heteronuclear Diatomic Molecules 25 2.7 Simple Polyatomic Molecules and Elements of Group Theory 26 2.7.1 Elements of Group Theory 26 2.7.2 Water 29 2.7.3 Ammonia 31 2.8 Typical Organic Molecules 33 2.8.1 Methane 33 2.8.2 Ethene 35 2.8.3 Benzene 37 2.8.4 Formaldehyde 39 2.9 Transition Metal Complexes 41 2.9.1 General Concepts 41 VIII Contents

2.9.2 Typical Metal Complexes 48 References 52

3 Light Absorption and Excited-State Deactivation 55 3.1 Light Absorption 55 3.1.1 Selection Rules 57 3.1.2 Symmetry Selection Rules 58 3.1.3 Spin Selection Rules 59 3.1.4 The Franck–Condon Principle 60 3.1.5 Visualization of Photochemical Reactions on Potential Energy Surfaces 62 3.2 Jablonski Diagram 64 3.3 Excited-State Deactivation 68 3.3.1 Vibrational Relaxation 68 3.3.2 Radiationless Deactivation 68 3.3.3 Radiative Deactivation 71 3.3.4 Radiative Lifetime 72 3.4 Chemical Reactions 73 3.5 Kinetic Aspects 74 3.6 Solvent and Temperature Effects 75 3.6.1 Solvatochromic Shift 75 3.6.2 Crossing of States 77 3.6.3 Temperature Effects on Excited-State Lifetime 79 3.6.4 Thermally Activated Delayed Fluorescence 80 3.7 Selected Molecules 81 3.7.1 Oxygen 81 3.7.2 Naphthalene 83 3.7.3 Benzophenone 85 3.7.4 Zinc(II) Tetraphenyl Porphyrin 87 3+ 3.7.5 [Cr(en)3] 90 3+ 3.7.6 [Co(NH3)6] 92 2+ 3.7.7 [Ru(bpy)3] 94 3.8 Semiconductors 96 References 100

4 Excited States: Physical and Chemical Properties 103 4.1 Excited State as a New Molecule 103 4.2 Lifetime 103 4.3 Energy 104 4.4 Geometry 105 4.4.1 Small Molecules 106 4.4.2 Ethene 107 4.4.3 Ethyne 108 4.4.4 Benzene 109 4.4.5 Formaldehyde 109 Contents IX

4.4.6 Square Planar Metal Complexes 111 4.5 Dipole Moments 112 4.6 Electron Transfer 114 4.7 Proton Transfer 117 4.8 Excimers and Exciplexes 120 References 122

5 From Molecules to Supramolecular Systems 125 5.1 Supramolecular (Multicomponent) Systems and Large Molecules 125 5.2 Electronic Interaction in Mixed-Valence Compounds 127 5.3 Electronic Interaction in Donor–Acceptor Complexes 129 5.4 Electronic Stimulation and Electronic Interaction in the Excited State 131 5.5 Formation of Excimers and Exciplexes in Supramolecular Systems 134 References 136

6 Quenching and Sensitization Processes in Molecular and Supramolecular Species 139 6.1 Introduction 139 6.2 Bimolecular Quenching 140 6.2.1 Stern–Volmer Equation 140 6.2.2 Kinetic Details 143 6.2.3 Static versus Dynamic Quenching 144 6.2.4 Sensitized Processes 145 6.2.5 Spin Considerations 146 6.3 Quenching and Sensitization Processes in Supramolecular Systems 146 6.4 Electron-Transfer Kinetics 150 6.4.1 Marcus Theory 150 6.4.2 Quantum Mechanical Theory 153 6.4.2.1 The Electronic Factor 154 6.4.2.2 The Nuclear Factor 156 6.4.2.3 Optical Electron Transfer 156 6.5 Energy Transfer 157 6.5.1 Coulombic Mechanism 159 6.5.2 Exchange Mechanism 161 6.6 Role of the Bridge 163 6.7 Catalyzed Deactivation 164 References 166

7 Molecular Organic Photochemistry 169 7.1 Introduction 169 7.2 Alkenes and Related Compounds 169 X Contents

7.2.1 Basic Concepts 169 7.2.2 Photoisomerization of Double Bonds 170 7.2.3 Electrocyclic Processes 172 7.2.4 Sigmatropic Rearrangements 173 7.2.5 Di-π-Methane Reaction 174 7.2.6 Photocycloaddition Reactions 174 7.2.7 Photoinduced Nucleophile, Proton, and Electron Addition 175 7.3 Aromatic Compounds 176 7.3.1 Introduction 176 7.3.2 Photosubstitution 179 7.3.3 Photorearrangement 180 7.3.4 Phototransposition 181 7.3.5 Photocycloadditions 181 7.4 Carbonyl Compounds 182 7.4.1 Introduction 182 7.4.2 Photochemical Primary Processes 183 7.5 Photochemistry of Other Organic Compounds 185 7.5.1 Nitrogen Compounds 185 7.5.1.1 Overview 185 7.5.1.2 Photoisomerization of Azocompounds 186 7.5.2 Saturated Oxygen and Sulfur Compounds 186 7.5.3 Halogen Compounds 187 References 189

8 Photochemistry and Photophysics of Metal Complexes 191 8.1 Metal Complexes 191 8.2 Photophysical Properties 191 8.3 Photochemical Reactivity 192 8.4 Relationships between Electrochemistry and Photochemistry 194 8.4.1 Cobalt (III) Complexes 195 8.4.2 Copper (I) Complexes 196 8.4.3 Ru(II) Polypyridine Complexes 196 8.4.4 Excited-State Redox Potentials 199 8.5 Luminescent Metal Complexes 201 8.5.1 Polypyridine Metal Complexes 201 8.5.2 Cyclometallated Complexes 203 8.5.2.1 Ruthenium Complexes 204 8.5.2.2 Rhodium Complexes 204 8.5.2.3 Iridium Complexes 205 8.5.2.4 Platinum Complexes 207 8.5.2.5 Orbital Nature of the Emitting Excited State 212 8.5.3 Porphyrin Complexes 213 8.5.4 Chromium (III) Complexes 216 8.5.5 Lanthanoid Complexes 219 8.6 Photochemical Processes 223 Contents XI

8.6.1 Types of Photoreactions 223 8.6.1.1 Photodissociation and Related Reactions 223 8.6.1.2 Photooxidation–Reduction Reactions 224 8.6.1.3 Intramolecular Rearrangements 225 References 226

9 Interconversion of Light and Chemical Energy by Bimolecular Redox Processes 231 9.1 Light as a Reactant 231 9.2 LightasaProduct 232 9.3 Conversion of Light into Chemical Energy 233 9.4 Chemiluminescence 235 9.5 Electrochemiluminescence 235 9.6 Light Absorption Sensitizers 237 9.7 Light Emission Sensitizers 240 References 242

10 Light-Powered Molecular Devices and Machines 245 10.1 Molecules, Self-Organization, and Covalent Synthetic Design 245 10.2 Light Inputs and Outputs: Reading, Writing, and Erasing 246 10.3 Molecular Devices for Information Processing 247 10.3.1 Photochromic Systems as Molecular Memories 247 10.3.2 Molecular Logics 249 10.3.2.1 Luminescent Sensors as Simple Logic Gates 250 10.3.2.2 AND Logic Gate 251 10.3.2.3 XOR Logic Gate with an Intrinsic Threshold Mechanism 251 10.3.2.4 Encoding and Decoding 253 10.4 Molecular Devices Based on Energy Transfer 255 10.4.1 Wires 255 10.4.2 Switches 257 10.4.3 Plug/Socket Systems 258 10.4.4 Light-Harvesting Antennas 259 10.5 Molecular Devices Based on Electron Transfer 260 10.5.1 Wires 260 10.5.2 Switches 263 10.5.3 Extension Cables 265 10.6 Light-Powered Molecular Machines 268 10.6.1 Basic Remarks 268 10.6.2 The Role of Light 268 10.6.3 Rotary Motors Based on cis–trans Photoisomerization 269 10.6.4 Linear Motions: Molecular Shuttles and Related Systems 271 10.6.5 Photocontrolled Valves, Boxes, and Related Systems 275 References 276 XII Contents

11 Natural and Artificial Photosynthesis 281 11.1 Energy for Spaceship Earth 281 11.2 Natural Photosynthesis 284 11.2.1 Light Harvesting: Absorption and Energy Transfer 285 11.2.2 Photoinduced Electron Transfer Leading to Charge Separation 285 11.2.2.1 Bacterial Photosynthesis 285 11.2.2.2 Green Plants Photosynthesis: Photosystem II 287 11.2.3 Efficiency of Photosynthesis 288 11.3 Artificial Photosynthesis 290 11.3.1 Artificial Antenna 293 11.3.2 Artificial Reaction Centers 296 11.3.3 Coupling Artificial Antenna and Reaction Center 299 11.3.4 Coupling One-Photon Charge Separation with Multielectron Water Splitting 301 11.4 Water Splitting by Semiconductor Photocatalysis 302 References 304

12 Experimental Techniques 309 12.1 Apparatus 309 12.1.1 Light Sources 309 12.1.2 Monochromators, Filters, and Solvents 317 12.1.3 Cells and Irradiation Equipment 319 12.1.4 Detectors 321 12.2 Steady-State Absorption and Emission Spectroscopy 323 12.2.1 Absorption Spectroscopy 323 12.2.1.1 Instrumentation 324 12.2.1.2 Qualitative and Quantitative Applications 325 12.2.1.3 Sample Measurement 325 12.2.2 Emission Spectroscopy 326 12.2.2.1 Instrumentation 326 12.2.2.2 Emission Spectra 328 12.2.2.3 Excitation Spectra 329 12.2.2.4 Presence of Spurious Bands 330 12.2.2.5 Quantitative Relationship between Luminescence Intensity and Concentration 331 12.2.2.6 Stern–Volmer Luminescence Quenching 332 12.2.2.7 Emission Quantum Yields 333 12.3 Time-Resolved Absorption and Emission Spectroscopy 335 12.3.1 Transient Absorption Spectroscopy 335 12.3.1.1 Transient Absorption with Nanosecond Resolution 335 12.3.1.2 Transient Absorption with Femtosecond Resolution 337 12.3.2 Emission Lifetime Measurements 338 12.3.2.1 Single Flash 338 12.3.2.2 Gated Sampling 339 12.3.2.3 Upconversion Techniques 339 Contents XIII

12.3.2.4 Single-Photon Counting 341 12.3.2.5 Data Analysis 342 12.3.2.6 Phase Shift 343 12.3.2.7 Luminescence Lifetime Standards 345 12.4 Absorption and Emission Measurements with Polarized Light 346 12.4.1 Linear Dichroism 346 12.4.2 Luminescence Anisotropy 347 12.5 Reaction Quantum Yields and Actinometry 349 12.5.1 Reaction Quantum Yields 349 12.5.2 Actinometry 350 12.5.2.1 Potassium Ferrioxalate 351 12.5.2.2 Potassium Reineckate 352 12.5.2.3 Azobenzene 353 12.6 Other Techniques 353 12.6.1 Photothermal Methods 353 12.6.1.1 Photoacoustic Spectroscopy 354 12.6.1.2 Photorefractive Spectroscopy 355 12.6.2 Single-Molecule Spectroscopy 357 12.6.3 Fluorescence Correlation Spectroscopy 358 12.6.4 X-ray Techniques 360 References 361

13 Light Control of Biologically Relevant Processes 365 13.1 Introduction 365 13.2 Vision 365 13.2.1 Basic Principle 365 13.2.2 Primary Photochemical Events 367 13.3 Light, Skin, and Sunscreens 367 13.4 Photochemical Damage in Living Systems 369 13.4.1 Photochemical Damage to DNA 369 13.4.2 Photochemical Damage to Proteins 369 13.5 Therapeutic Strategies Using Light 370 13.5.1 Phototherapy 370 13.5.2 Photochemotherapy of Psoriasis 370 13.5.3 Photodynamic Therapy 371 13.5.4 Photocontrolled Delivery 373 13.6 Photocatalysis in Environmental Protection 375 13.6.1 Principles 375 13.6.2 Solar Disinfection (SODIS) 375 13.6.3 Photoassisted Fenton Reaction 376 13.6.4 Heterogeneous Photocatalysis 376 13.7 DNA Photocleavage and Charge Transport 377 13.7.1 Photocleaving Agents of Nucleic Acid 377 13.7.2 Photoinduced Electron-Transfer Processes in DNA 378 13.8 Fluorescence 379 XIV Contents

13.9 Bioluminescence 379 References 380

14 Technological Applications of Photochemistry and Photophysics 385 14.1 Introduction 385 14.2 Photochromism 385 14.3 Luminescent Sensors 388 14.3.1 Principles 388 14.3.2 Amplifying Signal 389 14.3.3 Wind Tunnel Research 389 14.3.4 Thermometers 391 14.3.5 Measuring Blood Analytes 393 14.3.6 Detecting Warfare Chemical Agents 395 14.3.7 Detecting Explosives 397 14.4 Optical Brightening Agents 399 14.5 Atmospheric Photochemistry 400 14.5.1 Natural Processes Involving Oxygen 400 14.5.2 Ozone Hole 401 14.6 Solar Cells 402 14.6.1 Inorganic Photovoltaic (PV) Cells 402 14.6.2 Organic Solar Cells (OSCs) 403 14.6.3 Dye-Sensitized Solar Cells (DSSCs) 405 14.7 Electroluminescent Materials 407 14.7.1 Light-Emitting Diodes (LEDs) 407 14.7.2 Organic Light-Emitting Diodes (OLEDs) 407 14.7.3 Light-Emitting Electrochemical Cells (LECs) 409 14.8 Polymers and Light 411 14.8.1 Photopolymerization 411 14.8.2 Photodegradation 411 14.8.3 Stabilization of Commercial Polymers 412 14.8.4 Photochemical Curing 413 14.8.5 Other Light-Induced Processes 413 14.8.6 Photolithography 414 14.8.7 Stereolithography 415 14.8.8 Holography 416 14.9 Light for Chemical Synthesis 417 14.9.1 Photochlorination of Polymers 418 14.9.2 Synthesis of Caprolactam 418 14.9.3 Synthesis of Vitamins 418 14.9.4 Perfumes 419 References 420

15 Green (Photo)Chemistry 425 15.1 Definition, Origins, and Motivations 425 15.2 Photochemistry for Green Chemical Synthesis 426 Contents XV

15.3 Photocatalysis 428 15.3.1 Heterogeneous Photocatalysis 428 15.3.2 Homogeneous Photocatalysis 429 15.4 Photocatalysis in Synthesis 429 15.4.1 Alkanes 430 15.4.2 Alkenes 430 15.4.3 Alkynes 432 15.4.4 Sulfides 432 15.5 Photocatalytic Pollution Remediation 433 15.6 Use of Solar Energy in Green Synthesis 434 References 436

16 Research Frontiers 439 16.1 Introduction 439 16.2 Aggregation-Induced Emission 439 16.3 Phosphorescence from Purely Organic Materials by Crystal Design 441 16.4 Synthesis of a 2D Polymer 443 16.5 Photocontrolled Relative Unidirectional Transit of a Nonsymmetric Molecular Wire through a Molecular Ring 444 16.6 Molecular Rotary Motors Powered by Visible Light via Energy Transfer 445 16.7 Cooperation and Interference in Multifunction Compounds 447 16.8 Singlet Fission 449 16.9 One-Color Photochromic System 452 16.10 Photonic Modulation of Electron Transfer with Switchable Phase Inversion 454 16.11 Dye-Sensitized Photoelectrosynthesis Cells (DSPECs) 457 References 459

Index 463

XVII

List of Boxes

Box 2.1: Mathematical Aspects of Group Theory 27 Box 2.2: Nd3+ complexes 50 Box 3.1: Multiphotonic Processes 66 Box 3.2: Solvatochromic Dyes 76 Box 3.3: Quantum Dots 99 Box 5.1: Energy Reservoir 132 Box 6.1: Upconversion by Energy Transfer and Triplet–triplet Annihilation 147 Box 6.2: Photocatalysis 165 Box 7.1: Singlet Oxygen 176 Box 7.2: Solid-state Photochemistry 188 Box 8.1: Oxidative Addition of Pt(II) Complexes 210 Box 8.2: Identification of the Reactive Excited State in Cr(III) Complexes by Sensitization and Quenching 218 2+ Box 8.3: Spin Crossover in [Fe(bpy)3] 225 Box 11.1: Giacomo Ciamician: A Pioneer of Photochemistry 281 Box 11.2: The Science of Leaf Color Change [12] 289 Box 11.3: Evaluation of Catalyst Efficiency in Photocatalytic Processes 302 Box 11.4: Artificial Photosynthesis Versus Photovoltaics 304 Box 12.1: Lasers 312

XIX

Preface

And God said: ‘‘Let there be light’’; And there was light. And God saw that the light was good. (Genesis, 1, 3–4)

Photochemistry and photophysics are natural phenomena as old as the world. Our life depends on photosynthesis, a natural photochemical and photophysical process. We get information about the surrounding space by photochemical and photophysical processes that occur in our eyes. Currently, photochemistry and photophysics represent a modern branch of science, at the interface between light and matter and at the crossroads of several disciplines including chemistry, physics, material science, ecology, biology, and medicine. In our daily life, we are surrounded by products obtained with the aid of photochemistry and photophysics and by devices that exploit photochemical and photophysical processes to perform useful functions in a variety of places, from industries to hospitals. We are moving toward a future in which energy and information will be the dominant features of civilization. We will be forced to exploit sunlight as our ultimate energy source, converting it into useful energy forms by photochemical and photophysical processes. We will continue to miniaturize devices for information and communication technology down to the molecular level and we will use, more and more, light signals to transfer, store, and retrieve information. The current scientific literature shows that the frontiers of photochemistry and photophysics continue to expand with the development of new molecules, new materials, and new processes. There is no doubt that photochemistry and photophysics will play an increasingly important role in the development of science and technology. The number of researchers working in the area of light–matter interaction is increasing, but several of them did (and still do) not receive appropriate training. Light is often used in chemical laboratories as a silver bullet reactant to obtain products unavailable by thermal activation. In general, however, researchers lack the basis to fully understand how photochemical and photophysical processes can XX Preface

be exploited for novel, unusual, and unexpected applications in fields such as energy conversion, information technology, nanotechnology, and medicine. In the past 5 years, several textbooks and reference books on photochemistry have been published. However, most of them essentially focus on the photoreactions of organic molecules. In some textbooks, the fundamental bases of excited-state properties are confined in a few pages; in others, theoretical aspects are presented in too much detail, including boring and unnecessary mathematical treatments. Most of the available books ignore, or barely mention, the photochemical and photophysical properties of metal complexes, a class of molecules that is attracting increasing theoretical and applicative interest. No textbook emphasizes the most recent trends in photochemistry and photophysics, such as information processing by reading, writing, and erasing molecules with light signals, the capability of powering and controlling molecular machines by light, the conversion of sunlight into electrical energy by inorganic and organic solar cells, the recent developments in the field of light-emitting devices, and the first achievements along the road toward artificial photosynthesis. For all these reasons, we felt there was the need for a book capable of (i) presenting a clear picture of the concepts required to understand the excited states properties of the most important types of molecules, (ii) showing recent applications concerning photochemistry and photophysics, and (iii) opening the eyes of young researchers toward forefront developments or even futuristic visions of the light–matter interaction. We believe that this book, which originates from our long experience in teaching photochemistry and photophysics at the University of Bologna, can be a basic text for graduate and postgraduate courses because of its balanced content. We feel that it can also be useful for scientists who desire entering photochemistry and photophysics research even if they did not have a chance, during their university training, to get the fundamental bases of this field. Scientists already active in photochemical and photophysical research may find suggestions to undertake novel scientific adventures. Chapters 1–4 of this book deal with fundamental concepts concerning the nature of light, the principles that govern its interaction with matter, and the formation, electronic structure, properties, chemical reactivity, and radiative and nonradiative decay of excited states. Each concept is illustrated making reference to important classes of molecules. The notion that an excited state is a new chemical species with its own chemical and physical properties compared with the ground state is underlined, leading to the conclusion that photochemistry is a new dimension of chemistry. Chapter 5 extends the above-mentioned concepts to supramolecular (multicom- ponent) systems, where a fundamental role is played by structural organization and component interactions. Chapter 6 illustrates the fundamental concepts and the theoretical approaches concerning the two most important photochemical and photophysical processes, namely, energy transfer and photoinduced electron transfer. Chapter 7 deals with molecular organic photochemistry, illustrating the main types of reactions of the various families of organic compounds. Chapter 8 Preface XXI is dedicated to the photochemistry and photophysics of metal complexes, with particular emphasis on the outstanding luminescence properties of some classes of these compounds. Chapter 9 describes the relationships between photochemical, photophysical, and electrochemical properties of molecules and shows how these properties can be exploited for the interconversion between light and chemical energy. Chapter 10 deals with the hot topic of light-powered molecular devices and machines. The concepts of exploiting the interaction between molecules and light to read, write, and erase information are illustrated, together with their applica- tion in the field of molecular logics. A variety of molecular devices (e.g., wires, switches, extension cables, and light-harvesting antennas) based on energy trans- fer, photoinduced electron transfer or photoisomerization processes are described, and important examples of light-powered molecular machines (e.g., linear and rotary motors) are illustrated. Chapter 11 describes the photochemical and pho- tophysical processes taking place in the natural photosynthetic process and the approaches developed toward artificial photosynthesis, with particular focus on the photosensitized water splitting process. Chapter 12 offers a detailed presentation of equipment, techniques, procedures, and reference data concerning photochem- ical and photophysical experiments, including warnings to avoid mistakes and misinterpretations. Chapters 13–16 deal with topics of great current interest. Chapter 13 illustrates the relationships between light and life, starting from vision and including damages caused by exposure to UV light, benefits deriving from light-based therapeutic processes, photocatalysis for environmental protection, fluorescence for labeling biomolecules, and a brief description of bioluminescence processes. Chapter 14 deals with applications of photochemistry and photophysics, covering a variety of topics: photochromic compounds, luminescent sensors (including, e.g., their use in fields as diverse as wind tunnel, thermometers, measuring blood analytes, detecting explosives and warfare chemical agents), optical brighteners, atmospheric photochemistry, solar cells (PV, OSC, DSSC), electrochemiluminescent materials (LED, OLED, LEC), numerous applications concerning the interaction between polymers and light (e.g., photodegradation, photostabilization, photolithography, and stereolithography), and the photochemical syntheses of industrial products. Chapter 15 illustrates the use of light as an ideal reagent for green chemical synthesis; also described is the extensive use of homogeneous and heterogeneous photocatalysis for taking advantage of sunlight in laboratory processes as well as practical applications such as pollution remediation. After having presented the fundamental concepts of photochemistry and photo- physics and described the most important natural and artificial photochemical and photophysical processes, in Chapter 16 we offer the reader the opportunity to make acquaintance with forefront research through the discussion of 10 selected topics taken from the recent literature. The choice of the examples has been based not only on their intrinsic interest but especially on their educational capacity to illustrate connections among fundamental photochemical and photophysical concepts. In several chapters, additional information on some particular topics is presented in boxes interlaced with the text. An important feature of the book is the abundance XXII Preface

of illustrations that are essential for an easier understanding of the concepts discussed. Several papers reported in the recent literature (up to mid-2013) have been cited. Before closing, we would like to express our feeling concerning science, society, and Earth, the spaceship on which we live. We are concerned about the increasing consumption of natural resources [1], the climate change [2], the energy crisis [3], and the degradation of the environment [4–6]. Until now, mankind has taken from spaceship Earth enormous amounts of resources [7]. We need to reverse this trend [8]. We need to create new resources. In principle, this is possible by exploiting the only abundant, inexhaustible, and well-distributed resource on which we can rely: solar energy. Starting from seawater and the fundamental components of our atmosphere (nitrogen, oxygen, and carbon dioxide), by means of sunshine, we need to ‘‘fabricate’’ fuels, electricity, pure water, polymers, food, and other things we need [9]. Photochemistry and photophysics can help. Maybe future generations will pay back the Earth with a capital created by human intelligence. We should not forget, however, that our society is affected by a thread more dangerous than ecological unsustainability, namely, social unsustainability, which results from the continuously increasing disparities among people living on different nations as well as within each nation. Indeed, science can greatly benefit mankind, but science and technology alone will not take us where we need to go: a fair, open, responsible, friendly, united, and peaceful society. Responsible scientists, while creating, with the greatest moral care, new science and technology, should also play an important role as authoritative, informed, and concerned citizens of planet Earth [10]. They should teach their students not only to make science but also to distinguish what is worth making with science. As pointed out by Albert Einstein, ‘‘Concern for man himself and his fate must always constitute the chief objective of all technological endeavors … never forget this in the midst of your diagrams and equations.’’ We need scientists watching that science and technology are used for peace, not for war; for alleviating poverty, not for maintaining privileges; for reducing, not for increasing the gap between developed and underdeveloped countries; for protecting, not for destroying our planet that, beyond any foreseeable development of science, will remain the only place where mankind can live.

References

1. Global Footprint Network UeG7z5tvbpA (accessed 23 September (2011) Annual Report 2011, 2013). http://www.footprintnetwork.org/en/ 3. Armaroli, N. and Balzani, V. (2011) index.php/GFN/page/annual report 2011/ Energy for a Sustainable World: From (accessed 23 September 2013). the Oil Age to a Sun-Powered Future, 2. IPCC (2007) Fourth Assessment Wiley-VCH Verlag GmbH, Weinheim. Report: Climate Change (the next report will appear at the end of 2014), 4. Wilson, E.O. (2006) The Creation: An http://www.ipcc.ch/publications and data/ Appeal to Save Life on Earth, Norton, publications and data reports.shtml#. New York. Preface XXIII

5. Brown, L.R. (2011) WorldontheEdge: 8. Roberts, L., Stone, R., and Sugden, A. How to Prevent Environmental and Eco- (2009) The rise of restoration ecology. nomic Collapse, Norton, New York. Science, 325, 555. 6. Ehrlich, P.R. and Ehrlich, A.H. (2013) 9. Gray, H.B. (2009) Powering the planet Can a collapse of global civilization be with solar fuel. Nat. Chem., 1,7. avoided? Proc. R. Soc. B, 280, 20122845. 10. Balzani, V., Credi, A., and Venturi, M. 7. Krausmann, F., Erb, K.-H., Gingrich, (2008) The role of science in our time, S., Haberl, H., Bondeau, A., Gaube, V., in Molecular Devices and Machines: Con- Lauk, C., Plutzar, C., and Searchinger, cepts and Perspectives for the Nanoworld, T.D. (2013) Global human appropriation Wiley-VCH Verlag GmbH, Weinheim. of net primary production doubled in the 20th century. Proc. Natl. Acad. Sci. U.S.A., 110, 10324–9.

XXV

Acknowledgments

In 1675, Isaac Newton in a letter to Hooke wrote: ‘‘If I have seen further it is by standing on the shoulders of Giants.’’ This aphorism can be applied to any scientific paper and especially to any scientific book. Therefore, first of all, we thank the thousands of authors whose papers have allowed us to gain a deeper understanding of the topics we have tried to illustrate and discuss in this book. We also thank a number of colleagues encountered at international meetings and on other occasions for enlightening discussions that have contributed to better focus the basic role played by photochemistry and photophysics in modern science and technology. We are profoundly grateful to all the members of the photochemistry research group of the ‘‘Giacomo Ciamician’’ Department of Chemistry of the University of Bologna for daily discussions over several years of friendly research activity. We warmly thank Professor Nick Serpone, Concordia University, Montreal, for careful reading all the manuscript; he has corrected errors, improved language, and made a number of clever suggestions that have improved precision and clarity. We also thank our colleagues Luca Moggi, Alberto Credi, Giacomo Bergamini, Serena Silvi, Giorgio Orlandi, Fabrizia Negri, and the PhD student Massimo Sgarzi (University of Bologna), Nicola Armaroli, Lucia Flamigni, Ilse Manet, Sandra Monti (ISOF Research Institute, CNR, Bologna), Maria Teresa Indelli and Franco Scandola (University of Ferrara), Angelo Albini and Maurizio Fagnoni (University of Pavia), Sebastiano Campagna (University of Messina), and A. Prasanna de Silva (University of Belfast) who have read some chapters of the manuscript, corrected errors, and suggested improvements at various levels. Last but not least, we are grateful to the students who, over the years, have attended the photochemistry and photophysical courses in our University. They have, with their clever questions and punctual observations, greatly contributed to clarifying our ideas and improving our teaching.

Bologna, September 2013 Vincenzo Balzani Paola Ceroni Alberto Juris

XXVII

List of Abbreviations acac acetylacetonate ion AO atomic orbital biq 2,2′-biquinoline BO Born–Oppenheimer approximation bpy 2,2′-bipyridine bpym 2,2′-bipyrimidine bpz 2,2′-bipyrazine CB conduction band CCD charge-coupled device CT charge transfer CTTS charge-transfer-to-solvent transitions DFT density functional theory 4,4′-dm-bpy 4,4′-dimethyl-2,2′-bipyridine 4,4′-dph-bpy 4,4′-diphenyl-2,2′-bipyridine dmphen 2,9-dimethyl-1,10-phenanthroline dpp 2,9-diphenyl-1,10-phenanthroline DSSC dye-sensitized solar cells en ethylenediamine FCS fluorescence correlation spectroscopy gly glycine HOMO higher occupied molecular orbital i-biq 3,3′-biisoquinoline ic internal conversion isc intersystem crossing ITO indium tin oxide LC ligand-centered LCAO linear combinations of atomic orbitals LEC light-emitting electrochemical cell LED light-emitting diode LMCT ligand-to-metal charge-transfer LUMO lowest unoccupied molecular orbital MC metal-centered MLCT metal-to-ligand charge-transfer XXVIII List of Abbreviations

MO molecular orbital NHE normal hydrogen electrode NIR near-infrared NLC nonlinear crystal OEP octaethylporphyrin OLED organic light-emitting diode OPA optical parametric amplifier OSC organic solar cells PCET proton-coupled electron transfer PET photoinduced electron-transfer PES potential energy surface phen 1,10-phenanthroline phq− 2-phenylquinolyl PM photomultiplier ppy− 2-phenylpyridyl pq 2-(2-pyridyl)-quinoline PS photosensitizer PV photovoltaic QD quantum dot SCE saturated calomel electrode SCO spin crossover sep 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosane SMS single-molecule spectroscopy thpy− 2-(2′-thienyl)pyridyl TICT twisted intramolecular charge transfer T.M. transition moment TPP tetrakis(phenyl)porphyrin tpy 2,2′:6′,2′′-terpyridine VB valence band v.r. vibrational relaxation YAG yttrium aluminum garnet