Vincenzo Balzani,

Paola Ceroni, and

Alberto Juris

Photochemistry and Photophysics

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Vincenzo Balzani,Paola Ceroni, andAlberto Juris

Photochemistry and Photophysics

Concepts, Research, Applications

The Authors

Prof. Vincenzo BalzaniUniversity of Bologna‘‘G. Ciamician’’ Department of ChemistryVia Selmi 240126 BolognaItaly

Prof. Paola CeroniUniversity of Bologna‘‘G. Ciamician’’ Department of ChemistryVia Selmi 240126 BolognaItaly

Prof. Alberto JurisUniversity of Bologna‘‘G. Ciamician’’ Department of ChemistryVia Selmi 240126 BolognaItaly

All books published by Wiley-VCH arecarefully produced. Nevertheless, authors,editors, and publisher do not warrant theinformation contained in these books,including this book, to be free of errors.Readers are advised to keep in mind thatstatements, data, illustrations, proceduraldetails or other items may inadvertently beinaccurate.

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All rights reserved (including those oftranslation into other languages). No partof this book may be reproduced in anyform – by photoprinting, microfilm, or anyother means – nor transmitted or translatedinto a machine language without writtenpermission from the publishers. Registerednames, trademarks, etc. used in this book,even when not specifically marked as such,are not to be considered unprotected by law.

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V

To Carla, Carlo, and Teresa

VII

Contents

List of Boxes XVIIPreface XIXAcknowledgments XXVList of Abbreviations XXVII

1 Introduction 11.1 Photochemistry and Photophysics in Science and Technology 11.2 Historical Notes 21.3 A New Dimension of Chemistry and Physics 31.4 The Nature of Light 51.5 Absorption of Light 71.6 Quantum Yield, Efficiencies, and Excited-State Reactivity 8

References 10

2 Elementary Molecular Orbital Theory 112.1 Introduction 112.2 The Hydrogen Atom 112.3 Polyelectronic Atoms 132.4 From Atoms to Molecules 172.5 Electronic Structure of Homonuclear Diatomic Molecules 212.6 Electronic Structure of Heteronuclear Diatomic Molecules 252.7 Simple Polyatomic Molecules and Elements of Group Theory 262.7.1 Elements of Group Theory 262.7.2 Water 292.7.3 Ammonia 312.8 Typical Organic Molecules 332.8.1 Methane 332.8.2 Ethene 352.8.3 Benzene 372.8.4 Formaldehyde 392.9 Transition Metal Complexes 412.9.1 General Concepts 41

VIII Contents

2.9.2 Typical Metal Complexes 48References 52

3 Light Absorption and Excited-State Deactivation 553.1 Light Absorption 553.1.1 Selection Rules 573.1.2 Symmetry Selection Rules 583.1.3 Spin Selection Rules 593.1.4 The Franck–Condon Principle 603.1.5 Visualization of Photochemical Reactions on Potential Energy

Surfaces 623.2 Jablonski Diagram 643.3 Excited-State Deactivation 683.3.1 Vibrational Relaxation 683.3.2 Radiationless Deactivation 683.3.3 Radiative Deactivation 713.3.4 Radiative Lifetime 723.4 Chemical Reactions 733.5 Kinetic Aspects 743.6 Solvent and Temperature Effects 753.6.1 Solvatochromic Shift 753.6.2 Crossing of States 773.6.3 Temperature Effects on Excited-State Lifetime 793.6.4 Thermally Activated Delayed Fluorescence 803.7 Selected Molecules 813.7.1 Oxygen 813.7.2 Naphthalene 833.7.3 Benzophenone 853.7.4 Zinc(II) Tetraphenyl Porphyrin 873.7.5 [Cr(en)3]3+ 903.7.6 [Co(NH3)6]3+ 923.7.7 [Ru(bpy)3]2+ 943.8 Semiconductors 96

References 100

4 Excited States: Physical and Chemical Properties 1034.1 Excited State as a New Molecule 1034.2 Lifetime 1034.3 Energy 1044.4 Geometry 1054.4.1 Small Molecules 1064.4.2 Ethene 1074.4.3 Ethyne 1084.4.4 Benzene 1094.4.5 Formaldehyde 109

Contents IX

4.4.6 Square Planar Metal Complexes 1114.5 Dipole Moments 1124.6 Electron Transfer 1144.7 Proton Transfer 1174.8 Excimers and Exciplexes 120

References 122

5 From Molecules to Supramolecular Systems 1255.1 Supramolecular (Multicomponent) Systems and Large

Molecules 1255.2 Electronic Interaction in Mixed-Valence Compounds 1275.3 Electronic Interaction in Donor–Acceptor Complexes 1295.4 Electronic Stimulation and Electronic Interaction in the Excited

State 1315.5 Formation of Excimers and Exciplexes in Supramolecular

Systems 134References 136

6 Quenching and Sensitization Processes in Molecular andSupramolecular Species 139

6.1 Introduction 1396.2 Bimolecular Quenching 1406.2.1 Stern–Volmer Equation 1406.2.2 Kinetic Details 1436.2.3 Static versus Dynamic Quenching 1446.2.4 Sensitized Processes 1456.2.5 Spin Considerations 1466.3 Quenching and Sensitization Processes in Supramolecular

Systems 1466.4 Electron-Transfer Kinetics 1506.4.1 Marcus Theory 1506.4.2 Quantum Mechanical Theory 1536.4.2.1 The Electronic Factor 1546.4.2.2 The Nuclear Factor 1566.4.2.3 Optical Electron Transfer 1566.5 Energy Transfer 1576.5.1 Coulombic Mechanism 1596.5.2 Exchange Mechanism 1616.6 Role of the Bridge 1636.7 Catalyzed Deactivation 164

References 166

7 Molecular Organic Photochemistry 1697.1 Introduction 1697.2 Alkenes and Related Compounds 169

X Contents

7.2.1 Basic Concepts 1697.2.2 Photoisomerization of Double Bonds 1707.2.3 Electrocyclic Processes 1727.2.4 Sigmatropic Rearrangements 1737.2.5 Di-π-Methane Reaction 1747.2.6 Photocycloaddition Reactions 1747.2.7 Photoinduced Nucleophile, Proton, and Electron Addition 1757.3 Aromatic Compounds 1767.3.1 Introduction 1767.3.2 Photosubstitution 1797.3.3 Photorearrangement 1807.3.4 Phototransposition 1817.3.5 Photocycloadditions 1817.4 Carbonyl Compounds 1827.4.1 Introduction 1827.4.2 Photochemical Primary Processes 1837.5 Photochemistry of Other Organic Compounds 1857.5.1 Nitrogen Compounds 1857.5.1.1 Overview 1857.5.1.2 Photoisomerization of Azocompounds 1867.5.2 Saturated Oxygen and Sulfur Compounds 1867.5.3 Halogen Compounds 187

References 189

8 Photochemistry and Photophysics of Metal Complexes 1918.1 Metal Complexes 1918.2 Photophysical Properties 1918.3 Photochemical Reactivity 1928.4 Relationships between Electrochemistry and Photochemistry 1948.4.1 Cobalt (III) Complexes 1958.4.2 Copper (I) Complexes 1968.4.3 Ru(II) Polypyridine Complexes 1968.4.4 Excited-State Redox Potentials 1998.5 Luminescent Metal Complexes 2018.5.1 Polypyridine Metal Complexes 2018.5.2 Cyclometallated Complexes 2038.5.2.1 Ruthenium Complexes 2048.5.2.2 Rhodium Complexes 2048.5.2.3 Iridium Complexes 2058.5.2.4 Platinum Complexes 2078.5.2.5 Orbital Nature of the Emitting Excited State 2128.5.3 Porphyrin Complexes 2138.5.4 Chromium (III) Complexes 2168.5.5 Lanthanoid Complexes 2198.6 Photochemical Processes 223

Contents XI

8.6.1 Types of Photoreactions 2238.6.1.1 Photodissociation and Related Reactions 2238.6.1.2 Photooxidation–Reduction Reactions 2248.6.1.3 Intramolecular Rearrangements 225

References 226

9 Interconversion of Light and Chemical Energy by BimolecularRedox Processes 231

9.1 Light as a Reactant 2319.2 Light as a Product 2329.3 Conversion of Light into Chemical Energy 2339.4 Chemiluminescence 2359.5 Electrochemiluminescence 2359.6 Light Absorption Sensitizers 2379.7 Light Emission Sensitizers 240

References 242

10 Light-Powered Molecular Devices and Machines 24510.1 Molecules, Self-Organization, and Covalent Synthetic

Design 24510.2 Light Inputs and Outputs: Reading, Writing, and Erasing 24610.3 Molecular Devices for Information Processing 24710.3.1 Photochromic Systems as Molecular Memories 24710.3.2 Molecular Logics 24910.3.2.1 Luminescent Sensors as Simple Logic Gates 25010.3.2.2 AND Logic Gate 25110.3.2.3 XOR Logic Gate with an Intrinsic Threshold Mechanism 25110.3.2.4 Encoding and Decoding 25310.4 Molecular Devices Based on Energy Transfer 25510.4.1 Wires 25510.4.2 Switches 25710.4.3 Plug/Socket Systems 25810.4.4 Light-Harvesting Antennas 25910.5 Molecular Devices Based on Electron Transfer 26010.5.1 Wires 26010.5.2 Switches 26310.5.3 Extension Cables 26510.6 Light-Powered Molecular Machines 26810.6.1 Basic Remarks 26810.6.2 The Role of Light 26810.6.3 Rotary Motors Based on cis–trans Photoisomerization 26910.6.4 Linear Motions: Molecular Shuttles and Related Systems 27110.6.5 Photocontrolled Valves, Boxes, and Related Systems 275

References 276

XII Contents

11 Natural and Artificial Photosynthesis 28111.1 Energy for Spaceship Earth 28111.2 Natural Photosynthesis 28411.2.1 Light Harvesting: Absorption and Energy Transfer 28511.2.2 Photoinduced Electron Transfer Leading to Charge Separation 28511.2.2.1 Bacterial Photosynthesis 28511.2.2.2 Green Plants Photosynthesis: Photosystem II 28711.2.3 Efficiency of Photosynthesis 28811.3 Artificial Photosynthesis 29011.3.1 Artificial Antenna 29311.3.2 Artificial Reaction Centers 29611.3.3 Coupling Artificial Antenna and Reaction Center 29911.3.4 Coupling One-Photon Charge Separation with Multielectron Water

Splitting 30111.4 Water Splitting by Semiconductor Photocatalysis 302

References 304

12 Experimental Techniques 30912.1 Apparatus 30912.1.1 Light Sources 30912.1.2 Monochromators, Filters, and Solvents 31712.1.3 Cells and Irradiation Equipment 31912.1.4 Detectors 32112.2 Steady-State Absorption and Emission Spectroscopy 32312.2.1 Absorption Spectroscopy 32312.2.1.1 Instrumentation 32412.2.1.2 Qualitative and Quantitative Applications 32512.2.1.3 Sample Measurement 32512.2.2 Emission Spectroscopy 32612.2.2.1 Instrumentation 32612.2.2.2 Emission Spectra 32812.2.2.3 Excitation Spectra 32912.2.2.4 Presence of Spurious Bands 33012.2.2.5 Quantitative Relationship between Luminescence Intensity and

Concentration 33112.2.2.6 Stern–Volmer Luminescence Quenching 33212.2.2.7 Emission Quantum Yields 33312.3 Time-Resolved Absorption and Emission Spectroscopy 33512.3.1 Transient Absorption Spectroscopy 33512.3.1.1 Transient Absorption with Nanosecond Resolution 33512.3.1.2 Transient Absorption with Femtosecond Resolution 33712.3.2 Emission Lifetime Measurements 33812.3.2.1 Single Flash 33812.3.2.2 Gated Sampling 33912.3.2.3 Upconversion Techniques 339

Contents XIII

12.3.2.4 Single-Photon Counting 34112.3.2.5 Data Analysis 34212.3.2.6 Phase Shift 34312.3.2.7 Luminescence Lifetime Standards 34512.4 Absorption and Emission Measurements with Polarized Light 34612.4.1 Linear Dichroism 34612.4.2 Luminescence Anisotropy 34712.5 Reaction Quantum Yields and Actinometry 34912.5.1 Reaction Quantum Yields 34912.5.2 Actinometry 35012.5.2.1 Potassium Ferrioxalate 35112.5.2.2 Potassium Reineckate 35212.5.2.3 Azobenzene 35312.6 Other Techniques 35312.6.1 Photothermal Methods 35312.6.1.1 Photoacoustic Spectroscopy 35412.6.1.2 Photorefractive Spectroscopy 35512.6.2 Single-Molecule Spectroscopy 35712.6.3 Fluorescence Correlation Spectroscopy 35812.6.4 X-ray Techniques 360

References 361

13 Light Control of Biologically Relevant Processes 36513.1 Introduction 36513.2 Vision 36513.2.1 Basic Principle 36513.2.2 Primary Photochemical Events 36713.3 Light, Skin, and Sunscreens 36713.4 Photochemical Damage in Living Systems 36913.4.1 Photochemical Damage to DNA 36913.4.2 Photochemical Damage to Proteins 36913.5 Therapeutic Strategies Using Light 37013.5.1 Phototherapy 37013.5.2 Photochemotherapy of Psoriasis 37013.5.3 Photodynamic Therapy 37113.5.4 Photocontrolled Delivery 37313.6 Photocatalysis in Environmental Protection 37513.6.1 Principles 37513.6.2 Solar Disinfection (SODIS) 37513.6.3 Photoassisted Fenton Reaction 37613.6.4 Heterogeneous Photocatalysis 37613.7 DNA Photocleavage and Charge Transport 37713.7.1 Photocleaving Agents of Nucleic Acid 37713.7.2 Photoinduced Electron-Transfer Processes in DNA 37813.8 Fluorescence 379

XIV Contents

13.9 Bioluminescence 379References 380

14 Technological Applications of Photochemistry and Photophysics 38514.1 Introduction 38514.2 Photochromism 38514.3 Luminescent Sensors 38814.3.1 Principles 38814.3.2 Amplifying Signal 38914.3.3 Wind Tunnel Research 38914.3.4 Thermometers 39114.3.5 Measuring Blood Analytes 39314.3.6 Detecting Warfare Chemical Agents 39514.3.7 Detecting Explosives 39714.4 Optical Brightening Agents 39914.5 Atmospheric Photochemistry 40014.5.1 Natural Processes Involving Oxygen 40014.5.2 Ozone Hole 40114.6 Solar Cells 40214.6.1 Inorganic Photovoltaic (PV) Cells 40214.6.2 Organic Solar Cells (OSCs) 40314.6.3 Dye-Sensitized Solar Cells (DSSCs) 40514.7 Electroluminescent Materials 40714.7.1 Light-Emitting Diodes (LEDs) 40714.7.2 Organic Light-Emitting Diodes (OLEDs) 40714.7.3 Light-Emitting Electrochemical Cells (LECs) 40914.8 Polymers and Light 41114.8.1 Photopolymerization 41114.8.2 Photodegradation 41114.8.3 Stabilization of Commercial Polymers 41214.8.4 Photochemical Curing 41314.8.5 Other Light-Induced Processes 41314.8.6 Photolithography 41414.8.7 Stereolithography 41514.8.8 Holography 41614.9 Light for Chemical Synthesis 41714.9.1 Photochlorination of Polymers 41814.9.2 Synthesis of Caprolactam 41814.9.3 Synthesis of Vitamins 41814.9.4 Perfumes 419

References 420

15 Green (Photo)Chemistry 42515.1 Definition, Origins, and Motivations 42515.2 Photochemistry for Green Chemical Synthesis 426

Contents XV

15.3 Photocatalysis 42815.3.1 Heterogeneous Photocatalysis 42815.3.2 Homogeneous Photocatalysis 42915.4 Photocatalysis in Synthesis 42915.4.1 Alkanes 43015.4.2 Alkenes 43015.4.3 Alkynes 43215.4.4 Sulfides 43215.5 Photocatalytic Pollution Remediation 43315.6 Use of Solar Energy in Green Synthesis 434

References 436

16 Research Frontiers 43916.1 Introduction 43916.2 Aggregation-Induced Emission 43916.3 Phosphorescence from Purely Organic Materials by Crystal

Design 44116.4 Synthesis of a 2D Polymer 44316.5 Photocontrolled Relative Unidirectional Transit of a Nonsymmetric

Molecular Wire through a Molecular Ring 44416.6 Molecular Rotary Motors Powered by Visible Light via Energy

Transfer 44516.7 Cooperation and Interference in Multifunction Compounds 44716.8 Singlet Fission 44916.9 One-Color Photochromic System 45216.10 Photonic Modulation of Electron Transfer with Switchable Phase

Inversion 45416.11 Dye-Sensitized Photoelectrosynthesis Cells (DSPECs) 457

References 459

Index 463

XVII

List of Boxes

Box 2.1: Mathematical Aspects of Group Theory 27Box 2.2: Nd3+ complexes 50

Box 3.1: Multiphotonic Processes 66Box 3.2: Solvatochromic Dyes 76Box 3.3: Quantum Dots 99

Box 5.1: Energy Reservoir 132

Box 6.1: Upconversion by Energy Transfer and Triplet–tripletAnnihilation 147

Box 6.2: Photocatalysis 165

Box 7.1: Singlet Oxygen 176Box 7.2: Solid-state Photochemistry 188

Box 8.1: Oxidative Addition of Pt(II) Complexes 210Box 8.2: Identification of the Reactive Excited State in Cr(III) Complexes by

Sensitization and Quenching 218Box 8.3: Spin Crossover in [Fe(bpy)3]2+ 225

Box 11.1: Giacomo Ciamician: A Pioneer of Photochemistry 281Box 11.2: The Science of Leaf Color Change [12] 289Box 11.3: Evaluation of Catalyst Efficiency in Photocatalytic Processes 302Box 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 photophysicalprocess. We get information about the surrounding space by photochemical andphotophysical processes that occur in our eyes.

Currently, photochemistry and photophysics represent a modern branch ofscience, at the interface between light and matter and at the crossroads of severaldisciplines including chemistry, physics, material science, ecology, biology, andmedicine. In our daily life, we are surrounded by products obtained with the aidof photochemistry and photophysics and by devices that exploit photochemical andphotophysical processes to perform useful functions in a variety of places, fromindustries to hospitals.

We are moving toward a future in which energy and information will bethe dominant features of civilization. We will be forced to exploit sunlight as ourultimate energy source, converting it into useful energy forms by photochemical andphotophysical processes. We will continue to miniaturize devices for informationand communication technology down to the molecular level and we will use, moreand more, light signals to transfer, store, and retrieve information.

The current scientific literature shows that the frontiers of photochemistryand photophysics continue to expand with the development of new molecules,new materials, and new processes. There is no doubt that photochemistry andphotophysics will play an increasingly important role in the development of scienceand technology.

The number of researchers working in the area of light–matter interaction isincreasing, 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 obtainproducts unavailable by thermal activation. In general, however, researchers lackthe basis to fully understand how photochemical and photophysical processes can

XX Preface

be exploited for novel, unusual, and unexpected applications in fields such asenergy conversion, information technology, nanotechnology, and medicine.

In the past 5 years, several textbooks and reference books on photochemistry havebeen published. However, most of them essentially focus on the photoreactionsof organic molecules. In some textbooks, the fundamental bases of excited-stateproperties are confined in a few pages; in others, theoretical aspects are presentedin too much detail, including boring and unnecessary mathematical treatments.Most of the available books ignore, or barely mention, the photochemical andphotophysical properties of metal complexes, a class of molecules that is attractingincreasing theoretical and applicative interest. No textbook emphasizes the mostrecent trends in photochemistry and photophysics, such as information processingby reading, writing, and erasing molecules with light signals, the capability ofpowering and controlling molecular machines by light, the conversion of sunlightinto electrical energy by inorganic and organic solar cells, the recent developmentsin the field of light-emitting devices, and the first achievements along the roadtoward 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 excitedstates properties of the most important types of molecules, (ii) showing recentapplications concerning photochemistry and photophysics, and (iii) opening theeyes of young researchers toward forefront developments or even futuristic visionsof the light–matter interaction.

We believe that this book, which originates from our long experience in teachingphotochemistry and photophysics at the University of Bologna, can be a basic textfor graduate and postgraduate courses because of its balanced content. We feelthat it can also be useful for scientists who desire entering photochemistry andphotophysics research even if they did not have a chance, during their universitytraining, to get the fundamental bases of this field. Scientists already active inphotochemical and photophysical research may find suggestions to undertakenovel scientific adventures.

Chapters 1–4 of this book deal with fundamental concepts concerning the natureof light, the principles that govern its interaction with matter, and the formation,electronic structure, properties, chemical reactivity, and radiative and nonradiativedecay of excited states. Each concept is illustrated making reference to importantclasses of molecules. The notion that an excited state is a new chemical specieswith its own chemical and physical properties compared with the ground state isunderlined, leading to the conclusion that photochemistry is a new dimension ofchemistry.

Chapter 5 extends the above-mentioned concepts to supramolecular (multicom-ponent) systems, where a fundamental role is played by structural organizationand component interactions. Chapter 6 illustrates the fundamental concepts andthe theoretical approaches concerning the two most important photochemicaland photophysical processes, namely, energy transfer and photoinduced electrontransfer. Chapter 7 deals with molecular organic photochemistry, illustrating themain types of reactions of the various families of organic compounds. Chapter 8

Preface XXI

is dedicated to the photochemistry and photophysics of metal complexes, withparticular emphasis on the outstanding luminescence properties of some classes ofthese compounds. Chapter 9 describes the relationships between photochemical,photophysical, and electrochemical properties of molecules and shows how theseproperties can be exploited for the interconversion between light and chemicalenergy. Chapter 10 deals with the hot topic of light-powered molecular devices andmachines. The concepts of exploiting the interaction between molecules and lightto 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 androtary motors) are illustrated. Chapter 11 describes the photochemical and pho-tophysical processes taking place in the natural photosynthetic process and theapproaches developed toward artificial photosynthesis, with particular focus on thephotosensitized water splitting process. Chapter 12 offers a detailed presentationof equipment, techniques, procedures, and reference data concerning photochem-ical and photophysical experiments, including warnings to avoid mistakes andmisinterpretations.

Chapters 13–16 deal with topics of great current interest. Chapter 13 illustratesthe relationships between light and life, starting from vision and including damagescaused by exposure to UV light, benefits deriving from light-based therapeuticprocesses, photocatalysis for environmental protection, fluorescence for labelingbiomolecules, and a brief description of bioluminescence processes. Chapter 14deals with applications of photochemistry and photophysics, covering a varietyof topics: photochromic compounds, luminescent sensors (including, e.g., theiruse in fields as diverse as wind tunnel, thermometers, measuring blood analytes,detecting explosives and warfare chemical agents), optical brighteners, atmosphericphotochemistry, solar cells (PV, OSC, DSSC), electrochemiluminescent materials(LED, OLED, LEC), numerous applications concerning the interaction betweenpolymers 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 chemicalsynthesis; also described is the extensive use of homogeneous and heterogeneousphotocatalysis for taking advantage of sunlight in laboratory processes as well aspractical 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 andphotophysical processes, in Chapter 16 we offer the reader the opportunity to makeacquaintance with forefront research through the discussion of 10 selected topicstaken from the recent literature. The choice of the examples has been based not onlyon their intrinsic interest but especially on their educational capacity to illustrateconnections among fundamental photochemical and photophysical concepts.

In several chapters, additional information on some particular topics is presentedin 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 conceptsdiscussed. Several papers reported in the recent literature (up to mid-2013) havebeen 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 increasingconsumption 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 fromspaceship 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 exploitingthe only abundant, inexhaustible, and well-distributed resource on which we canrely: solar energy. Starting from seawater and the fundamental components ofour atmosphere (nitrogen, oxygen, and carbon dioxide), by means of sunshine, weneed to ‘‘fabricate’’ fuels, electricity, pure water, polymers, food, and other thingswe need [9]. Photochemistry and photophysics can help. Maybe future generationswill pay back the Earth with a capital created by human intelligence. We shouldnot forget, however, that our society is affected by a thread more dangerous thanecological unsustainability, namely, social unsustainability, which results from thecontinuously increasing disparities among people living on different nations aswell as within each nation. Indeed, science can greatly benefit mankind, but scienceand technology alone will not take us where we need to go: a fair, open, responsible,friendly, united, and peaceful society. Responsible scientists, while creating, withthe greatest moral care, new science and technology, should also play an importantrole as authoritative, informed, and concerned citizens of planet Earth [10]. Theyshould teach their students not only to make science but also to distinguish whatis worth making with science. As pointed out by Albert Einstein, ‘‘Concern forman himself and his fate must always constitute the chief objective of all technologicalendeavors… never forget this in the midst of your diagrams and equations.’’ We needscientists watching that science and technology are used for peace, not for war; foralleviating poverty, not for maintaining privileges; for reducing, not for increasingthe gap between developed and underdeveloped countries; for protecting, not fordestroying our planet that, beyond any foreseeable development of science, willremain the only place where mankind can live.

References

1. Global Footprint Network(2011) Annual Report 2011,http://www.footprintnetwork.org/en/index.php/GFN/page/annual report 2011/(accessed 23 September 2013).

2. IPCC (2007) Fourth AssessmentReport: Climate Change (the nextreport will appear at the end of 2014),http://www.ipcc.ch/publications and data/publications and data reports.shtml#.

UeG7z5tvbpA (accessed 23 September

2013).

3. Armaroli, N. and Balzani, V. (2011)

Energy for a Sustainable World: From

the Oil Age to a Sun-Powered Future,

Wiley-VCH Verlag GmbH, Weinheim.

4. Wilson, E.O. (2006) The Creation: An

Appeal to Save Life on Earth, Norton,

New York.

Preface XXIII

5. Brown, L.R. (2011) World on the Edge:How to Prevent Environmental and Eco-nomic Collapse, Norton, New York.

6. Ehrlich, P.R. and Ehrlich, A.H. (2013)Can a collapse of global civilization beavoided? Proc. R. Soc. B, 280, 20122845.

7. Krausmann, F., Erb, K.-H., Gingrich,S., Haberl, H., Bondeau, A., Gaube, V.,Lauk, C., Plutzar, C., and Searchinger,T.D. (2013) Global human appropriationof net primary production doubled inthe 20th century. Proc. Natl. Acad. Sci.U.S.A., 110, 10324–9.

8. Roberts, L., Stone, R., and Sugden, A.(2009) The rise of restoration ecology.Science, 325, 555.

9. Gray, H.B. (2009) Powering the planetwith solar fuel. Nat. Chem., 1, 7.

10. Balzani, V., Credi, A., and Venturi, M.(2008) The role of science in our time,in Molecular Devices and Machines: Con-cepts and Perspectives for the Nanoworld,Wiley-VCH Verlag GmbH, Weinheim.

XXV

Acknowledgments

In 1675, Isaac Newton in a letter to Hooke wrote: ‘‘If I have seen further it isby standing on the shoulders of Giants.’’ This aphorism can be applied to anyscientific paper and especially to any scientific book. Therefore, first of all, wethank the thousands of authors whose papers have allowed us to gain a deeperunderstanding of the topics we have tried to illustrate and discuss in this book. Wealso thank a number of colleagues encountered at international meetings and onother occasions for enlightening discussions that have contributed to better focusthe basic role played by photochemistry and photophysics in modern science andtechnology.

We are profoundly grateful to all the members of the photochemistry researchgroup of the ‘‘Giacomo Ciamician’’ Department of Chemistry of the Universityof Bologna for daily discussions over several years of friendly research activity.We warmly thank Professor Nick Serpone, Concordia University, Montreal, forcareful 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, SandraMonti (ISOF Research Institute, CNR, Bologna), Maria Teresa Indelli and FrancoScandola (University of Ferrara), Angelo Albini and Maurizio Fagnoni (Universityof Pavia), Sebastiano Campagna (University of Messina), and A. Prasanna de Silva(University of Belfast) who have read some chapters of the manuscript, correctederrors, and suggested improvements at various levels.

Last but not least, we are grateful to the students who, over the years, haveattended the photochemistry and photophysical courses in our University. Theyhave, with their clever questions and punctual observations, greatly contributed toclarifying our ideas and improving our teaching.

Bologna, September 2013 Vincenzo BalzaniPaola CeroniAlberto Juris

XXVII

List of Abbreviations

acac acetylacetonate ionAO atomic orbitalbiq 2,2′-biquinolineBO Born–Oppenheimer approximationbpy 2,2′-bipyridinebpym 2,2′-bipyrimidinebpz 2,2′-bipyrazineCB conduction bandCCD charge-coupled deviceCT charge transferCTTS charge-transfer-to-solvent transitionsDFT density functional theory4,4′-dm-bpy 4,4′-dimethyl-2,2′-bipyridine4,4′-dph-bpy 4,4′-diphenyl-2,2′-bipyridinedmphen 2,9-dimethyl-1,10-phenanthrolinedpp 2,9-diphenyl-1,10-phenanthrolineDSSC dye-sensitized solar cellsen ethylenediamineFCS fluorescence correlation spectroscopygly glycineHOMO higher occupied molecular orbitali-biq 3,3′-biisoquinolineic internal conversionisc intersystem crossingITO indium tin oxideLC ligand-centeredLCAO linear combinations of atomic orbitalsLEC light-emitting electrochemical cellLED light-emitting diodeLMCT ligand-to-metal charge-transferLUMO lowest unoccupied molecular orbitalMC metal-centeredMLCT metal-to-ligand charge-transfer

XXVIII List of Abbreviations

MO molecular orbitalNHE normal hydrogen electrodeNIR near-infraredNLC nonlinear crystalOEP octaethylporphyrinOLED organic light-emitting diodeOPA optical parametric amplifierOSC organic solar cellsPCET proton-coupled electron transferPET photoinduced electron-transferPES potential energy surfacephen 1,10-phenanthrolinephq− 2-phenylquinolylPM photomultiplierppy− 2-phenylpyridylpq 2-(2-pyridyl)-quinolinePS photosensitizerPV photovoltaicQD quantum dotSCE saturated calomel electrodeSCO spin crossoversep 1,3,6,8,10,13,16,19-octaazabicyclo[6.6.6]eicosaneSMS single-molecule spectroscopythpy− 2-(2′-thienyl)pyridylTICT twisted intramolecular charge transferT.M. transition momentTPP tetrakis(phenyl)porphyrintpy 2,2′:6′,2′′-terpyridineVB valence bandv.r. vibrational relaxationYAG yttrium aluminum garnet