Solvatochromism

Paul Suppan and Nagwa Ghoneim University ofFribourg, Switzerland

THE ROYAL SOCIETY OF CHEMISTRY

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Contents

Chapter 1 Introduction 1

1.1 Fundamentals of Electronic Absorption and Emission Spectra 1 1.1.1 Energy States of Molecules 4 1.1.2 Orbital Description of Electronic States 5 1.1.3 Potential Energy Diagrams of Electronic Transitions 6 1.1.4 Transitions Between Electronic States 7

1.1.4.1 Radiative Transitions 7 1.1.4.2 Non-radiative Transitions 7

1.1.5 The Transition Moment 8 1.1.6 Einstein Coefficients, Oscillator Strength, Extinction

Coefficient and Absorbance 9 1.1.7 Luminescence 10 1.1.8 Quenching Action of Polar Solvents 12 1.1.9 Quenching Action ofProtic Solvents 13 1.1.10 Solvent Relaxation in Luminescence Spectroscopy 13 1.1.11 Band Shapes: Spectral Broadening 14

1.1.11.1 Rotational and Vibrational Sub-levels 15 1.1.11.2 Inhomogeneous Broadening 15 1.1.11.3 The Stokes Shift 16

1.1.12 Energy Transfer 18 1.1.13 Acid-Base Equilibria 19

1.2 References 20

Chapter 2 Theory of Solute-Solvent Interactions 21

2.1 Introduction and General Remarks 21 2.2 Solvation (Theoretical and Empirical Models) 22

2.2.1 Empirical Scales of Solvent Polarity 24 2.2.1.1 The £T(30) Scale 24 2.2.1.2 The Z Polarity Scale 26 2.2.1.3 Multi-parameter Polarity Scales 28

2.2.2 'Donor Number' and 'Acceptor Number' Scales in the Spectroscopy of Metal Complexes 29

2.3 Non-specific Interactions 30

vm Contents

2.3.1 The Multipole Expansion 30 2.3.1.1 The Dipole Moment 30 2.3.1.2 The Quadrupole Moment 32 2.3.1.3 The Polarizability 33 2.3.1.4 Molecular Volume, Molecular Radius 34 2.3.1.5 Dielectric Constant, Refractive Index, Clausius-

Mosotti and Debye Equations 35 2.3.1.6 The Onsager Reaction Field Model 35

2.3.2 The Five Interaction Terms 38 2.3.3 Dipole-Dipole Interactions 39 2.3.4 Solute Dipole-Solvent Induced Dipole (Polarizability)

Interactions 41 2.3.5 Dispersion Interactions 41 2.3.6 Non-polar (but Polarizable) Solute-Polar Solvent

Interactions (Solvent Stark Effect) 43 2.3.7 The Transition Dipole Term 44 2.3.8 Effect of Solvent on Molecular Electronic Oscillator

Strengths 45 2.4 Specific Interactions 46

2.4.1 Hydrogen Bonding Effects 47 2.4.2 Physical Model of H-bonding Interaction 52 2.4.3 Red Shift H-bonding Anomalies 52

2.5 Higher Multipole Interactions 53 2.5.1 Solute Quadrupole-Solvent Dipole 53 2.5.2 Solute Dipole-Solvent Quadrupole 55

2.6 Solvation of Excimers and Exciplexes 55 2.7 Dielectric Saturation 56 2.8 Solvent Polarity Functions 57

2.8.1 Anomalous Solvents 58 2.9 Timescales of Solvation 60

2.9.1 Solvent Relaxation 60 2.9.1.1 Viscosity and Temperature Dependence of

Solvent Relaxation 63 2.9.2 Diffusion ofLiquids 64

2.10 'Outer-sphere Reorganization Energy' in Electron Transfer Processes 64

2.11 Excited States of the Solvent 66 2.12 References 67

Chapter 3 Solvatochromic Shifts of Absorption Spectra 68

3.1 Chromophores 68 3.1.1 Additivity of Polarizabilities 68 3.1.2 Types of Transitions 69

3.1.2.1 n-n* Transitions of the Carbonyl Chromophore 70 3.1.2.2 n-n* Transitions ofAromatic Chromophores 70

Contents ix

3.1.2.3 l-n*/n-n* Transitions in Substituted Aromatic Chromophores 70

3.1.2.4 71—71* Transitions in Acceptor Aromatic Molecules 70 3.1.2.5 n-n*/CT Transitions in D-Ar-A Molecules 71

3.2 Non-specific Interactions 72 3.2.1 Non-polar Solutes in Non-polar Solvents 72 3.2.2 Non-polar Solutes in Polar Solvents 74 3.2.3 Dipolar Solutes in Non-polar Solvents 75 3.2.4 Dipolar Solutes in Polar Solvents 76

3.3 Specific Solvent Effects on Absorption Spectra 78 3.4 Artefacts in Absorption Spectra 80

3.4.1 Absorption by the Solvent 80 3.4.2 Optical Artefacts 80 3.4.3 Artefacts dueto the Fluorescence of the Sample 81

3.5 Hyperchromism and Hypochromism 81 3.6 Triplet-Triplet Absorption Spectra 82 3.7 'Charge Transfer' Complexes 85 3.8 Metal Complexes 88

3.8.1 Intervalence Metal Complexes 88 3.9 Dipolar Molecules with Ions and Ion Clusters 90

3.10 Diffuse Reflectance Spectroscopy 94 3.11 References 95

Chapter 4 Solvatochromic Shifts of Luminescence Spectra 96

4.1 Introduction 96 4.2 General Remarks about Luminescence Spectra 97

4.2.1 Recording and Correction of Luminescence Spectra 98 4.3 Steady-state Fluorescence Spectra 99

4.3.1 Fluorescence Spectra of Non-polar Molecules in Non-Polar Solvents 100

4.3.2 Fluorescence Spectra of Non-polar Molecules in Polar Solvents 100

4.3.3 Fluorescence Spectra of Dipolar Molecules 101 4.3.3.1 Amino Aromatics 101 4.3.3.2 Hydroxy Aromatics 103 4.3.3.3 Carbonyl Compounds 105 4.3.3.4 Nitro Compounds 105 4.3.3.5 Cyano Derivatives 105 4.3.3.6 Halogenated Aromatic Compounds 106 4.3.3.7 Phthalimides and Related Compounds 107

4.3.4 Electron Donor-Aromatic Acceptor Molecules (D-Ar-A) 107 4.3.5 Intramolecular Electron Transfer 110

4.3.5.1 The TICT States 110 4.3.6 Intramolecular Charge Transfer in Rigid Molecules 115 4.3.7 Protolytic Equilibria 118

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4.3.7.1 The Dual Luminescence of 9-Anthracenecarboxylic Acid(9-ACA) 119

4.3.8 Shapes of Luminescence Spectra:'Red Drop' 120 4.3.9 Luminescence of Metal Complexes 120

4.4 Steady-state Phosphorescence Spectra and Lifetimes 121 4.5 Time-resolved Luminescence Spectra 123

4.5.1 Experimental Techniques 124 4.5.1.1 Combination of Kinetic Traces 124 4.5.1.2 Gated Spectra 124 4.5.1.3 Streak Camera 125

4.5.2 Solvation Processes in Single Solvents 125 4.5.3 Artefacts and Problems in Luminescence Spectroscopy 128

4.5.3.1 Second-order Excitation Scatter 128 4.5.3.2 Luminescence 'Blank' and Solvent Raman

Scattering 130 4.5.3.3 Excitation Light Absorption and Luminescence

Light Reabsorption (in Conditions of Right-angle and Front-face Illumination) 131

4.6 Excimers (Excited Dimers) 132 4.7 Exciplexes (Excited Complexes) 134

4.7.1 Dipole Moments and Charge Separation: Quenching Effect of Polar Solvents 136

4.7.2 Effect of Solvent Polarity on Dipole Moment 137 4.7.3 Intramolecular Exciplexes 138 4.7.4 Triplexes: Triple Complexes in the Excited State 139

4.8 Emission Spectra from Upper Excited States 140 4.9 References 142

Chapter 5 Solvatochromic Shifts in Rigid Media and Organized Systems 144

5.1

5.2 5.3

5.4 5.5 5.6 5.7 5.8

Introduction 5.1.1 Types of Rigid Media: 'Solids' and Organized Systems Glassy Matrices Microcrystalline Media 5.3.1 Inorganic Ions in Crystals Metal Complexes in Polyelectrolytes Micellar Suspensions Molecular Films Shpolskii Spectra References

144 145 145 146 148 149 151 153 160 162

Chapter 6 Solvatochromic Shifts in Liquid and Solid Mixtures 163

6.1 Introduction 163 6.2 Dielectric Enrichment 165

Contents XI

6.2.1 Dielectric Enrichment of Quadrupolar Solutes 169 6.2.2 PreferentialSolvationoftheSolvated Electron 170 6.2.3 Kinetics of Dielectric Enrichment; Local Polarity of

Excited States 170 6.2.3.1 The Timescale of Dielectric Enrichment 172

6.2.4 Solvatochromic Shifts of Exciplex Emission Spectra in Solvent Mixtures 172

6.3 Hydrogen Bonding 172 6.4 Solvent-Solvent Interactions 175 6.5 Time-resolved Spectra 176 6.6 MixedSolids 177

6.6.1 Polarity of Mixed Polymers 179 6.7 References 181

Chapter 7 Solvatochromic Shifts in Supercritical Fluids and Molecular Beams 183

7.1 Introduction 183 7.2 Supercritical Fluids: Two-and Three-component Mixtures 184 7.3 Clusters in Molecular Beams 186

7.3.1 Formation of Molecular Beams 187 7.3.2 Solvation by Rare Gas Solvent Clusters 187

7.3.2.1 Laser-induced Fluorescence (LIF) 187 7.3.2.2 Resonant Double-photon Ionization (R2PI) 187

7.4 Spectral Properties of Clusters 188 7.4.1 Spectra and Properties of CzAr„ Clusters 188 7.4.2 Van der Waals Clusters between Non-polar Solutes and

'Solvents' 189 7.4.3 DMABN-Water Clusters 191 7.4.4 4-Aminobenzonitrile Complexes 192

7.5 References 193

Chapter 8 Solvatochromic Shifts of Molecular Ions 194 8.1 Introduction 194

8.1.1 Modes of Formation of Molecular Radical Ions 194 8.2 Ions in Single Solvents 196

8.2.1 Absorption Spectrum of the Solvated Electron 197 8.2.2 Ion Solvation: The Born Equation 198 8.2.3 Time-resolved Spectra 200

8.3 Ions in Solvent Mixtures 202 8.4 Solvation Dynamics 203

8.4.1 Solvation Dynamics of the Electron e^lv or e~q 203 8.5 Ions in Rigid Matrices 204

8.5.1 Radical Ions in Zeolites 207 8.6 References 209

XU Contents

Chapter 9 Thermochromic Shifts 210

9.1 Introduction: Temperature Dependence of the Density, Dielectric Constant and Refractive Index 210 9.1.1 Density 210 9.1.2 Dielectric Constant 211 9.1.3 Refractive Index 211

9.1.3.1 Non-polar Solutes in Non-polar Solvents 212 9.1.3.2 Dipolar Solute-Non-polar Solvent 212 9.1.3.3 Dipolar Solutes in Polar Solvents 213

9.2 Absorption Spectra 214 9.3 Thermochromic Shifts of Luminescence Spectra 216 9.4 Thermochromic Shifts of Exciplex Fluorescence Spectra 217 9.5 Thermochromic Shifts in Solvent Mixtures 218

9.5.1 Hydrogen Bonding Mixtures 219 9.6 References 220

Chapter 10 Fluorescent Probes in Biological Systems 221

10.1 Introduction 221 10.2 Protein Detection 221 10.3 Nucleic Acid Detection 221 10.4 Physiological Ion Indicators 223 10.5 Enzyme-labelled Fluorescence 223 10.6 Intrinsic Fluorescence of Normal and Cancer Cells 224 10.7 External Fluorescence Probes of Binding Sites 226 10.8 Solvatochromic pH Indicators 229 10.9 The Environment of Visual Pigments 229 10.10 Fluorescent Probes of Serum Albumins 230 10.11 References 232

Chapter 11 External Effects on Radiative and Non-radiative Transitions 233

11.1 Introduction 233 11.2 Electrochromic Shifts 233

11.2.1 Some Examples and Applications of Electrochromic Shifts 234 11.2.1.1 Intramolecular Electrochromism 236

11.3 Pressure Shifts (Piezochromism) 236 11.4 External Magnetic Fields 238 11.5 References 239

Chapter 12 Theoretical Calculations and Computer Simulations 240

12.1 Intoduction 240 12.1.1 What is a'Computer Experiment'? 241

Contents xm 12.2 Molecular Dynamics and Monte Carlo Simulations 241

12.2.1 How Realistic are these Simulations? 242 12.3 Electron Distributions of Excited Molecules 242

12.3.1 Charge Densities in n-n* States 242 12.3.2 Electron Distributions in Charge-transfer States 243

12.4 Solvent-Solute Clusters as Models of Elementary Solvation 244 12.5 Intermolecular Potentials 244

12.5.1 Simulations of Dielectric Saturation 246 12.6 References 246

Chapter 13 Applications 247

13.1 Excited State Properties 247 13.1.1 Excited State Dipole Moments 247

13.1.1.1 Direct Measurements of Absorption or Emission Spectra 248

13.1.1.2 The Solvent-dependent Stokes Shift of the Absorption and Emission Spectra (the 'Lippert' Method) 248

13.1.1.3 The Ratio Method 248 13.1.1.4 The'Absolute'Method 250

13.1.2 Polarizabilities of Excited Molecules 252 13.2 Solvent Effects in Photochemistry 252

13.2.1 Photoreduction of 4-Aminobenzophenone 253 13.3 Analytical Methods 254 13.4 Microenvironment of Polymers 255 13.5 Fluorescent Probes for Monitoring Photopolymerization 258 13.6 References 259

Appendices 260

Subject Index 270