Lecture Notes in Chemistry - Home - Springer978-94-007-20… ·  · 2017-08-25The Lecture Notes in Chemistry The series Lecture Notes in Chemistry (LNC) reports new developments

Lecture Notes in Chemistry

Volume 78

For further volumes:http://www.springer.com/series/632

Series Editors

B. Carpenter, Cardiff, UKP. Ceroni, Bologna, ItalyB. Kirchner, Leipzig, GermanyA. Koskinen, Helsinki, FinlandK. Landfester, Mainz GermanyJ. Leszczynski, Jackson, MS, USAT-Y. Luh, Taipei, TaiwanC. Mahlke, Erlangen, GermanyR. Salzer, Dresden, Germany

The Lecture Notes in Chemistry

The series Lecture Notes in Chemistry (LNC) reports new developments inchemistry and molecular science-quickly and informally, but with a high qualityand the explicit aim to summarize and communicate current knowledge forteaching and training purposes. Books published in this series are conceived asbridging material between advanced graduate textbooks and the forefront ofresearch. They will serve the following purposes:

• provide an accessible introduction to the field to postgraduate students andnonspecialist researchers from related areas,

• provide a source of advanced teaching material for specialized seminars, coursesand schools, and

• be readily accessible in print and online.

The series covers all established fields of chemistry such as analytical chemistry,organic chemistry, inorganic chemistry, physical chemistry including electro-chemistry, theoretical and computational chemistry, industrial chemistry, andcatalysis. It is also a particularly suitable forum for volumes addressing theinterfaces of chemistry with other disciplines, such as biology, medicine, physics,engineering, materials science including polymer and nanoscience, or earth andenvironmental science.Both authored and edited volumes will be considered for publication. Editedvolumes should however consist of a very limited number of contributions only.Proceedings will not be considered for LNC.The year 2010 marks the relaunch of LNC.

Paola CeroniEditor

The Exploration ofSupramolecular Systemsand Nanostructures byPhotochemical Techniques

123

Prof. Dr. Paola CeroniDipartimento di Chimica CiamicianUniversità di BolognaVia Selmi 240126 BolognaItalye-mail: [email protected]

ISSN 0342-4901ISBN 978-94-007-2041-1 e-ISBN 978-94-007-2042-8DOI 10.1007/978-94-007-2042-8Springer Dordrecht Heidelberg London New York

Library of Congress Control Number: 2011940034

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Preface

Photochemistry and photophysics, i.e. the study of the interaction between lightand matter, is an everyday experience in our life both as a natural phenomenon andas a technology developed by humans. For example, vision is based on a photo-chemical reaction and gives us information on the world around us, while pho-tovoltaic panels convert solar energy into electricity and are expected to providesolutions to mankind’s energy thirst.

Within the scientific community, photochemistry is applied to a wide range ofdisciplines, from physical chemistry, to supramolecular chemistry, chemicalbiology, materials science and nanoscience. It is thus very important to understandthe fundamental concepts at the basis of the interaction of light with molecules andto know what information can be gained by photophysical and photochemicaltechniques, as well as practical aspects for the application of these techniques.

In the last few decades, substrates of photochemical studies have changed fromsimple organic molecules and metal complexes to supramolecular systems andnanostructures. Supramolecular chemistry, according to its most popular defini-tion, is ‘‘the chemistry beyond the molecule, bearing on organized entities ofhigher complexity that result from the association of two or more chemical speciesheld together by intermolecular forces’’. More generally, supramolecular systemscan be defined as multicomponent structures assembled by weak interactions andcharacterized by the emergence of new functions compared to their componentunits. Photochemical techniques are extremely useful to study such supramolecularsystems and nanostructures since optical inputs can be used not only to ‘‘read’’ thestate of the system, but also to provide energy for the system to function.

The essentials of a quantum mechanical treatment of the interaction of elec-tromagnetic radiation with molecules is described in the first chapter, and thesecond one deals with supramolecular photochemistry, with particular emphasis onenergy and electron transfer with a description of the Marcus theory. The fol-lowing chapters are devoted to the different photochemical and photophysicaltechniques: spectrophotometry and spectrofluorimetry, actinometry, absorptionand luminescence techniques with polarized light excitation, time-resolvedabsorption and luminescence spectroscopy, down to femtosecond resolution. Each

v

chapter comprises both the theoretical basis and the practical aspects and describesan example of the application of these techniques to the study of a supramolecularsystem.

This book is aimed at providing the newcomers of the field with an overview ofthe potential offered by the photophysical and photochemical techniques applied tosupramolecular systems and nanoobjects. Indeed, it provides the basic concepts,without introducing too many technical and mathematical details, with the aid ofself-explicative figures and schemes, and discusses the methodology to correctlyperform a photochemical experiment, as well as the most critical aspects of thelaboratory application. It is of interest also to scientists already involved in thefield because it offers technical and operative details useful in the laboratory, aswell as references to current research, pioneering contributions, and review articleson specific aspects.

I would like to express my gratitude to the authors of the chapters, most of themworking together with me in the Photochemical Nanosciences Laboratory—Department of Chemistry ‘‘G. Ciamician’’ of the University of Bologna. Thisvolume is the fruit of their long practical and teaching experience in the field ofphotochemistry and photochemical techniques.

Bologna, May 2011 Paola Ceroni

vi Preface

Contents

1 Excited-State Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 The Nature of Light. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Potential Energy Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41.4 Electronic States and Electronic Configurations . . . . . . . . . . . . 7

1.4.1 Organic Molecules. . . . . . . . . . . . . . . . . . . . . . . . . . . 71.4.2 Metal Complexes. . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.5 Light Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121.6 Intramolecular Excited State Decay . . . . . . . . . . . . . . . . . . . . . 15

1.6.1 Vibrational Relaxation . . . . . . . . . . . . . . . . . . . . . . . . 151.6.2 Radiative Deactivation . . . . . . . . . . . . . . . . . . . . . . . . 151.6.3 Radiationless Deactivation . . . . . . . . . . . . . . . . . . . . . 161.6.4 Chemical Reaction. . . . . . . . . . . . . . . . . . . . . . . . . . . 181.6.5 Kinetic Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2 Photoinduced Energy and Electron Transfer Processes . . . . . . . . . 212.1 Bimolecular Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.1.1 General Considerations. . . . . . . . . . . . . . . . . . . . . . . . 212.1.2 Thermodynamic Aspects. . . . . . . . . . . . . . . . . . . . . . . 222.1.3 Kinetic Aspects of Bimolecular Processes . . . . . . . . . . 23

2.2 Supramolecular Photochemistry. . . . . . . . . . . . . . . . . . . . . . . . 252.2.1 Definition of a Supramolecular System . . . . . . . . . . . . 252.2.2 Photoinduced Energy and Electron Transfer

in Supramolecular Systems . . . . . . . . . . . . . . . . . . . . . 262.2.3 Excimers and Exciplexes . . . . . . . . . . . . . . . . . . . . . . 27

2.3 Electron Transfer Processes . . . . . . . . . . . . . . . . . . . . . . . . . . 282.3.1 Marcus Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282.3.2 Quantum Mechanical Theory . . . . . . . . . . . . . . . . . . . 31

2.4 Optical Electron Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

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2.5 Energy Transfer Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.5.1 Coulombic Mechanism. . . . . . . . . . . . . . . . . . . . . . . . 352.5.2 Exchange Mechanism. . . . . . . . . . . . . . . . . . . . . . . . . 36

2.6 The Role of the Bridge in Supramolecular Systems . . . . . . . . . . 37References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3 Spectrophotometry, Measurements in Solution . . . . . . . . . . . . . . . 393.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.2 The Absorption Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403.3 The General Absorption Characteristics of Molecules . . . . . . . . 433.4 Qualitative Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 453.5 Quantitative Treatment of the Absorption Intensity . . . . . . . . . . 473.6 Quantitative Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

3.6.1 The Method of the Standard Additions. . . . . . . . . . . . . 513.6.2 Analysis of Mixtures of Absorbing Species . . . . . . . . . 513.6.3 Spectrophotometric Titrations . . . . . . . . . . . . . . . . . . . 52

3.7 Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523.7.1 The Light Source. . . . . . . . . . . . . . . . . . . . . . . . . . . . 533.7.2 The Monochromator . . . . . . . . . . . . . . . . . . . . . . . . . 533.7.3 The Sample Holder . . . . . . . . . . . . . . . . . . . . . . . . . . 553.7.4 The Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.7.5 The Spectrophotometers . . . . . . . . . . . . . . . . . . . . . . . 57

3.8 The Sample Measurement. . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.8.1 The Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 593.8.2 The Solvent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 603.8.3 The Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.8.4 The Instrumental Precision . . . . . . . . . . . . . . . . . . . . . 61

3.9 Experimental Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4 Photochemical Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674.1 Photochemical Apparata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

4.1.1 Light Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 684.1.2 Selection of the Exciting Radiation . . . . . . . . . . . . . . . 714.1.3 Reaction Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 724.1.4 Optical Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . 734.1.5 Control of Temperature and Stirring . . . . . . . . . . . . . . 73

4.2 Photoreaction Quantum Yield . . . . . . . . . . . . . . . . . . . . . . . . . 744.3 Chemical Actinometers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.3.1 Potassium Ferrioxalate . . . . . . . . . . . . . . . . . . . . . . . . 784.3.2 Potassium Reineckate. . . . . . . . . . . . . . . . . . . . . . . . . 814.3.3 Azobenzene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 834.3.4 Aberchrome 540 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

4.4 A Photochromic Diarylethene Compound . . . . . . . . . . . . . . . . . 88

viii Contents

4.4.1 Irradiation Experiments . . . . . . . . . . . . . . . . . . . . . . . 894.4.2 Photochemical Behaviour . . . . . . . . . . . . . . . . . . . . . . 904.4.3 Compound 1 as Actinometer. . . . . . . . . . . . . . . . . . . . 92

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

5 Spectrofluorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 975.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 985.2 The Instrumentation for Measuring Luminescence . . . . . . . . . . . 99

5.2.1 Light Source S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1005.2.2 Excitation and Emission Monochromators

M1 and M2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025.2.3 Sample-Holder C. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1025.2.4 Detector R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

5.3 Emission and Excitation Spectra and Their Correction . . . . . . . . 1045.3.1 Emission Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1045.3.2 Excitation Spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . 1075.3.3 Presence of Spurious Bands in Spectra. . . . . . . . . . . . . 109

5.4 Quantitative Measurements at Fixed Wavelength. . . . . . . . . . . . 1115.4.1 Correction for the Sample Absorption

at the Excitation Wavelength . . . . . . . . . . . . . . . . . . . 1115.4.2 Correction for the Sample Absorption

at the Emission Wavelength . . . . . . . . . . . . . . . . . . . . 1155.4.3 Determination of the Stability Constant

of a [2]Pseudorotaxane . . . . . . . . . . . . . . . . . . . . . . . . 1185.5 Determination of Luminescence Quantum Yields . . . . . . . . . . . 122

5.5.1 Reference Standards for the Determinationof Fluorescence Quantum Yields . . . . . . . . . . . . . . . . . 123

5.5.2 Reference Standards for the Determinationof Phosphorescence Quantum Yields . . . . . . . . . . . . . . 125

5.6 Luminescence Measurements on Solid Samples . . . . . . . . . . . . 1265.6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1265.6.2 Front-Face and Right-Angle Geometries. . . . . . . . . . . . 1265.6.3 Sample Inhomogeneity . . . . . . . . . . . . . . . . . . . . . . . . 1275.6.4 Concentration Effects . . . . . . . . . . . . . . . . . . . . . . . . . 128

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

6 Absorption and Emission Spectroscopy with Polarized Light . . . . . 1316.1 Linear and Circular Dichroism Spectroscopy . . . . . . . . . . . . . . 131

6.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1316.1.2 Polarized Light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1326.1.3 Birefringence and Circular Dichroism . . . . . . . . . . . . . 1346.1.4 Linear Dichroism. . . . . . . . . . . . . . . . . . . . . . . . . . . . 1396.1.5 Observables in Circular Dichroism Spectroscopy. . . . . . 140

Contents ix

6.1.6 Instrumentation and Experimental Procedures . . . . . . . . 1416.1.7 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

6.2 Circularly Polarized Luminescence Spectroscopy . . . . . . . . . . . 1466.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1466.2.2 Observables in CPL spectroscopy . . . . . . . . . . . . . . . . 1476.2.3 Instrumentation and Experimental Procedures . . . . . . . . 1496.2.4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

6.3 Steady State and Time Resolved Fluorescence Anisotropy . . . . . 1516.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516.3.2 Definition of Fluorescence Anisotropy . . . . . . . . . . . . . 1526.3.3 Fluorescence Polarization . . . . . . . . . . . . . . . . . . . . . . 1536.3.4 Fluorescence Anisotropy and Electronic Transition . . . . 1536.3.5 Depolarization due to Rotation . . . . . . . . . . . . . . . . . . 1566.3.6 Depolarization due to Energy Transfer . . . . . . . . . . . . . 1576.3.7 Emission Anisotropy Spectra . . . . . . . . . . . . . . . . . . . 1576.3.8 The G Factor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1586.3.9 Calculation of the Emission Anisotropy Spectra . . . . . . 1596.3.10 Effect of Concentration and Scattering

on the Anisotropy . . . . . . . . . . . . . . . . . . . . . . . . . . . 1606.3.11 Instrumental Configurations . . . . . . . . . . . . . . . . . . . . 1606.3.12 Fluorescence Anisotropy of a Mixture

of Fluorophores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1606.3.13 Excitation Anisotropy Spectra . . . . . . . . . . . . . . . . . . . 1616.3.14 Time Resolved Fluorescence Anisotropy . . . . . . . . . . . 1616.3.15 Fluorescence Anisotropy Decay. . . . . . . . . . . . . . . . . . 1616.3.16 Fundamental Fluorescence Anisotropy

in Time-Resolved Measurements . . . . . . . . . . . . . . . . . 1626.3.17 Acquisition of the Decay . . . . . . . . . . . . . . . . . . . . . . 1636.3.18 Interpretation of the Results . . . . . . . . . . . . . . . . . . . . 163

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

7 Time-Resolved Luminescence Techniques . . . . . . . . . . . . . . . . . . . 1677.1 General Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1677.2 Experimental Methods for Lifetime Measurements . . . . . . . . . . 170

7.2.1 Single Flash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1707.2.2 Gated Sampling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1727.2.3 Single Photon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1737.2.4 Phase Shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

7.3 Analysis of Decay Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1787.4 Example of Lifetime Measurements

on a Supramolecular System . . . . . . . . . . . . . . . . . . . . . . . . . . 1807.5 Luminescence Lifetime Standards . . . . . . . . . . . . . . . . . . . . . . 183References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

x Contents

8 Transient Absorption Spectroscopy. . . . . . . . . . . . . . . . . . . . . . . . 1858.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1858.2 Transient Absorption with Nanosecond Resolution . . . . . . . . . . 186

8.2.1 Measure of Absorbance Change . . . . . . . . . . . . . . . . . 1878.2.2 The Sample Compartment . . . . . . . . . . . . . . . . . . . . . 1898.2.3 The Optical System . . . . . . . . . . . . . . . . . . . . . . . . . . 1908.2.4 The Electronic Detection System. . . . . . . . . . . . . . . . . 191

8.3 Transient Absorption Spectroscopyin Supramolecular Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 1938.3.1 Fullerene Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . 1938.3.2 Ligand–Protein Complexes . . . . . . . . . . . . . . . . . . . . . 195

8.4 Sub-Nanosecond Transient Absorption . . . . . . . . . . . . . . . . . . . 1978.4.1 Shortening the Laser Pulse . . . . . . . . . . . . . . . . . . . . . 1988.4.2 Ti: Sapphire Laser . . . . . . . . . . . . . . . . . . . . . . . . . . . 1998.4.3 Chirped Pulse Amplification . . . . . . . . . . . . . . . . . . . . 1998.4.4 Regenerative Amplification. . . . . . . . . . . . . . . . . . . . . 200

8.5 Ultrafast Transient Absorption Spectroscopy. . . . . . . . . . . . . . . 2008.5.1 Femtochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2008.5.2 Pump and Probe Experiments . . . . . . . . . . . . . . . . . . . 201

8.6 Photoinduced Electron Transferin a Multichromophoric System. . . . . . . . . . . . . . . . . . . . . . . . 202

8.7 Femtosecond Systems, Experimental Suggestions . . . . . . . . . . . 205References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206

9 Spectroelectrochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2099.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2099.2 General Considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2179.3 Absorption Measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2199.4 Emission Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

Contents xi

Contributors

Vincenzo Balzani Department of Chemistry ‘‘G. Ciamician’’, University ofBologna, Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

Paola Ceroni Department of Chemistry ‘‘G. Ciamician’’, University of Bologna,Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

Claudio Chiorboli Istituto per la Sintesi Organica e la Fotoreattività (ISOF),Consiglio Nazionale delle Ricerche (CNR), Area della Ricerca, Via Piero Gobetti101, 40129, Bologna, Italy; Sezione di Ferrara, Via Borsari 46, 44121 Ferrara,Italy, e-mail: [email protected]

Alberto Credi Department of Chemistry ‘‘G. Ciamician’’, University of Bologna,Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

Maria Teresa Gandolfi Department of Chemistry ‘‘G. Ciamician’’, University ofBologna, Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

Alberto Juris Department of Chemistry ‘‘G. Ciamician’’, University of Bologna,Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

Mauro Maestri Department of Chemistry ‘‘G. Ciamician’’, University of Bolo-gna, Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

Luca Moggi Department of Chemistry ‘‘G. Ciamician’’, University of Bologna,Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

Marco Montalti Department of Chemistry ‘‘G. Ciamician’’, University ofBologna, Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

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Sandra Monti Istituto per la Sintesi Organica e la Fotoreattività (ISOF),Consiglio Nazionale delle Ricerche (CNR), Area della Ricerca, Via Piero Gobetti101, 40129, Bologna, Italy, e-mail: [email protected]

Luca Prodi Department of Chemistry ‘‘G. Ciamician’’, University of Bologna,Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

Margherita Venturi Department of Chemistry ‘‘G. Ciamician’’, University ofBologna, Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

Nelsi Zaccheroni Department of Chemistry ‘‘G. Ciamician’’, University ofBologna, Via Selmi 2, 40126 Bologna, Italy, e-mail: [email protected]

xiv Contributors