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Photophysics of Molecular Materials From Single Molecules to Single Crystals Edited by Guglielmo Lanzani
Photophysics of Molecular Materials
From Single Molecules to Single Crystals
Edited byGuglielmo Lanzani
InnodataFile Attachment3527607390.jpg
Photophysics of
Molecular Materials
Edited by
Guglielmo Lanzani
Photophysics of Molecular Materials
From Single Molecules to Single Crystals
Edited byGuglielmo Lanzani
Editors
Guglielmo LanzaniDipartimento di FisicaPolitecnico di MilanoMilanoItalye-mail: [email protected]
CoverRight: Confocal laser scanning micrograph (CLSM)of a tetracene thin film.Top left: Jumps between excitonic coupling andF�rster type energy transfer in a single moleculardimer.Bottom left: Layout for ultrafast optoelectronicprobing experiments.
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ISBN-13: 978-3-527-40456-8ISBN-10: 3-527-40456-2
V
List of Contributors XV
1 Introduction 1Guglielmo Lanzani
2 Optical Microscopy and Spectroscopy of Single Molecules 5Christian H�bner and Thomas Basch�
2.1 Introduction 52.2 Photophysical Principles of Single-Molecule Fluorescence Detection 62.2.1 The Single Molecule as a Three-Level System 62.2.2 Dipole–Dipole Coupled Oscillators 102.2.2.1 Weak Coupling 112.2.2.2 Strong Coupling 122.3 Experimental Techniques 132.3.1 Signal-to-Noise Considerations 132.3.2 Room-Temperature Single-Molecule Spectroscopy 142.3.2.1 Epifluorescence Microscopy 142.3.2.2 Total Internal Reflection (TIR) Microscopy 182.3.2.3 Scanning Confocal Optical Microscopy 182.3.2.4 Two-Photon-, 4p- and STED Microscopy 212.3.2.5 Scanning Near-Field Optical Microscopy 222.3.3 Single-Molecule Spectroscopy at Cryogenic Temperatures 232.3.3.1 The Laser System 242.4 Applications 272.4.1 Photon Antibunching 272.4.2 Photon Bunching 292.4.3 Electronic Coupling Between Molecules 322.4.4 Single Molecules as Antennas: Orientation 41
Contents
VI
3 Optical Properties of Single Conjugated Polymer Chains(Polydiacetylenes) 49Michel Schott
3.1 Introduction 493.1.1 Motivation for the Study 493.1.2 Choice of the Experimental System 503.1.3 The Isolated Polydiacetylene Chain, Isolated in its
Monomer Crystal Matrix 503.1.4 Organization of the Chapter 553.2 A Short Survey of Some PDA Properties 553.2.1 Possible Electronic Structures of a PDA chain 563.2.1.1 The Colors of PDA 563.2.1.2 Ground-State Conformational Differences 583.2.1.3 Color Transitions 603.2.2 Spectroscopy of Bulk PDA Crystals 603.2.2.1 Reflection and Absorption 613.2.2.2 Electroreflectance 623.2.2.3 Fluorescence 633.2.2.4 Two-Photon Absorption 643.3 The Chosen DA 643.3.1 The Materials and How They Fulfill the Criteria 643.3.2 The Samples 673.4 Spectroscopy of Isolated Blue Chains 673.4.1 Visible Absorption Spectra 673.4.1.1 Room-Temperature Absorption and Determination of the
Polymer Content xp 673.4.1.2 Low-Temperature Absorption Spectra 693.4.1.3 Temperature Dependence 713.4.2 Electroabsorption 733.4.2.1 Results 733.4.2.2 Properties of the Exciton 753.4.2.3 Exciton Binding Energy 773.4.2.4 Properties of Electron and Hole 773.4.2.5 Electroabsorption at Higher Polymer Concentration 793.4.3 Fluorescence 793.4.3.1 Emission Spectra 793.4.3.2 Lifetime of the Emitting State 803.4.3.3 Risetime of the Emission: Relaxation Within the Singlet Manifold 823.4.4 Nonradiative Relaxation of the 1Bu Exciton 823.4.4.1 Introduction. Experimental Method 823.4.4.2 Spectra and PA Decay Kinetics 833.4.4.3 Photobleaching 863.4.4.4 Nature of the Gap States 883.4.5 The Lowest Triplet State 88
Contents
VII
3.4.5.1 Assignment of the 1.35-eV Photoinduced Absorption 883.4.5.2 Triplet Generation Processes 893.4.5.3 Triplet Energies and Triplet–Triplet Transition 913.4.5.4 Triplet Transport Properties 923.4.6 A High-Energy Exciton 933.4.7 Summary and Discussion 963.4.7.1 Summary of the Main Results Obtained on Isolated
Blue PDA Chains 963.4.7.2 Exciton Size and Binding Energy 983.4.7.3 Influence of Electronic Correlations 983.4.7.4 Comparison with Blue Bulk PDA Crystals 993.5 Red Chain Spectroscopy 1033.5.1 Another Emission in 3BCMU Crystals 1033.5.1.1 Low-Temperature Emission Spectrum 1033.5.1.2 Excitation Spectra 1043.5.2 Absorption Spectroscopy 1053.5.2.1 Absorption at 15K 1053.5.2.2 Electroabsorption 1073.5.3 Emission and Absorption Temperature Dependence 1083.5.4 Red Chain Exciton Relaxation 1093.5.4.1 Quantum Yield 1093.5.4.2 Fluorescence Decay Time 1113.5.4.3 Radiative Lifetimes 1123.5.4.4 Nonradiative Lifetimes 1133.6 Study of a Single Isolated Red Chain: a Polymeric Quantum Wire 1163.6.1 Feasibility of Studying a Single Isolated Red Chain. Experimental
Method 1163.6.2 One Exciton per Chain. Lineshape Analysis 1173.6.2.1 The Vibronic Lines. A One-Dimensional Exciton Band 1173.6.2.2 The Zero-Phonon Line. Exciton Coherence Time and Scattering
Process 1213.6.2.3 Lorentzian Component of the Vibronic Linewidth.
Optical Phonon Coherence 1233.6.3 Spatial Extension of the Emission 1243.6.3.1 The Method and a Typical Image 1243.6.3.2 Different Spatial Distributions. The Effect of Disorder 1253.6.3.3 Origin of the Spatial Distribution 1273.6.4 Several Excitons on a Chain. Effect of Excitation Power 1273.6.4.1 Absorption Cross-Section for a Single Chain 1273.6.4.2 Nonresonant Excitation 1283.6.4.3 Resonant Excitation. Exciton–Photon Interaction 1293.6.5 Summary 1303.6.5.1 Summary of the Results on Red PDA Isolated Chains 1303.6.5.2 What We Would Like to Know About Red Chains but Do Not Yet 1313.7 Answered and Open Questions 131
Contents
3.7.1 The Nature and Properties of Excited States 1323.7.2 Exciton–Phonon Interactions 1343.7.3 Electronic Correlations 1353.7.4 Influence of Disorder 136
Appendix3.A The DA Solid-State Polymerization Reaction 1373.A–1: General Description 1373.A–2: Structural Requirements 1383.A–3: Energetics and Elementary Steps 1413.B Structural Properties of 3B and 4B Monomers 1423.B–1: Phase Transitions 1433.B–2: Crystal Structures 1433.B–3: Unit Cell Parameters Along the Chain Direction 1443.C Origin of the Weak Absorption Lines in Blue Chains 145
4 Morphology-Correlated Photophysics in Organic Semiconductor ThinFilms by Confocal Laser Microscopy and Spectroscopy 153Maria Antonietta Loi, Enrico Da Como and Michele Muccini
4.1 Introduction 1534.2 Principles of Confocal Laser Scanning Microscopy 1544.3 Photoluminescence Imaging and Time-Resolved Local
Spectroscopy 1584.3.1 The Setup 1584.3.2 Morphology Correlated Spectroscopy 1594.3.3 Optical Sectioning 1604.3.4 Comparison Between Topographic and Photoluminescence
Imaging 1614.4 Supramolecular Organization in Organic Semiconductor
Ultra-Thin Films 1644.5 Imaging and Spectroscopy of Organic Bulk Heterojunctions and
Correlation with Optoelectronic Device Properties 1714.6 Conclusions 178
5 Spectroscopy of Long-Lived Photoexcitations in p-ConjugatedSystems 183Markus Wohlgenannt, Eitan Ehrenfreund and Z. Valy Vardeny
5.1 Introduction 1835.1.1 Basic Properties of p-Conjugated Polymers 1835.1.2 Optical Transitions of Photoexcitations in Conducting Polymers 1885.1.3 Optical Transitions of Solitons in Polymers with
Degenerate Ground State 1895.1.4 Optical Transitions of Charged Excitations in NDGS Polymers 1905.1.4.1 The Polaron Excitation 1905.1.4.2 The Bipolaron Excitation 190
ContentsVIII
5.1.4.3 The p-Dimer and the Delocalized Polaron Excitations 1925.1.5 Optical Transitions of Neutral Excitations in NDGS Polymers 1935.1.5.1 Singlet Excitons 1945.1.5.2 Triplet Excitons 1945.1.5.3 Polaron Pairs 1955.1.6 Infrared Active Vibrational Modes 1955.2 Experimental Methods 1985.2.1 Photomodulation Spectroscopy of Long-Lived Photoexcitations 1985.2.2 Optically Detected Magnetic Resonance Techniques 2005.2.2.1 The Electron Spin 2015.2.2.2 Electron Spin Resonance 2025.2.2.3 Basic Principles of �-Wave Resonant Transitions 2025.2.2.4 ESR Signal Strength and Population Statistics of Spin-Up and Spin-
Down Levels 2035.2.3 Magnetic Resonance Spectroscopy of Long-Lived Photoexcited States in
p-Conjugated Polymers 2045.2.3.1 The ODMR Setup 2055.3 Recombination, Relaxation and Generation Processes 2075.3.1 Mono- and Bimolecular Recombination Mechanisms 2075.3.2 Recombination Kinetics 2085.3.2.1 Steady-State Case 2085.3.2.2 Frequency Response 2105.3.2.3 Generalized Coordinates 2105.3.2.4 Dispersive Kinetics: Frequency Domain 2135.3.2.5 Lifetime Distribution for Dispersive Processes 2155.3.2.6 Inhomogeneous Distribution of Recombination Lifetimes:
General Case 2165.3.3 Polaron Recombination and Quantum Efficiency of OLEDs 2185.3.3.1 Polaron Recombination in OLEDs 2185.3.3.2 Spin-Dependent Exciton Formation Cross-Sections 2205.4 Photoinduced Absorption: Spectroscopy and Dynamics 2205.4.1 Red Polythiophenes: Regio-Regular, Regio-Random 2205.4.1.1 Photomodulation Studies of RRa-P3HT 2225.4.1.2 Photomodulation Studies in RR-P3HT 2245.4.1.3 The Polaron Relaxation Energy 2255.4.1.4 The Spectral Anti-Resonances 2285.4.2 Recombination Kinetics 2305.4.2.1 Poly(p-phenylenebipyridinevinylene) 2305.4.2.2 Poly(phenylenevinylene) 2355.4.3 Photophysics of a Blue-Emitting Polyfluorene 2375.4.3.1 Electronic Structure of PFO Phases 2395.4.3.2 Photoexcitation Dynamics in PFO 2405.4.4 Measuring the Conjugation Length Using Photoinduced Absorption
Spectroscopy 241
Contents IX
5.5 ODMR Spectroscopy: Measurement of Spin-Dependent PolaronRecombination Rates 243
5.5.1 Spin-Dependent Exciton Formation Probed by PADMRSpectroscopy 243
5.5.2 Spin-Dependent Exciton Formation Probed by PLDMRSpectroscopy 246
5.5.3 Quantitative Modeling of Spin-Dependent RecombinationSpectroscopy 248
5.5.4 Material Dependence of Spin-Dependent Exciton Formation Rates 2495.5 The Relation Between Spin-Dependent Exciton Formation Rates
and the Singlet Exciton Yield in OLEDs 2515.6 Conclusion 252
6 Charge Transport in Disordered Organic Semiconductors 261V. I. Arkhipov, I. I. Fishchuk, A. Kadashchuk and H. B�ssler
6.1 Introduction 2616.2 Charge Generation 2626.3 Charge Carrier Hopping in Noncrystalline Organic Materials 2656.3.1 Outline of Conceptual Approaches 2656.3.1.1 The Continuous Time Random Walk (CTRW) Formalism 2656.3.1.2 The Gill Equation 2666.3.1.3 The Hopping Approach 2676.3.1.4 Monte Carlo Simulation 2676.3.1.5 The Effective Medium Approach 2706.3.1.6 Effect of Site Correlation 2706.3.1.7 Polaron Transport 2726.3.2 Stochastic Hopping Theory 2736.3.2.1 Carrier Equilibration via Downward Hopping 2756.3.2.2 Thermally Activated Variable-Range Hopping: Effective Transport
Energy 2776.3.2.3 Dispersive Hopping Transport 2816.3.2.4 Equilibrium Hopping Transport 2836.3.2.5 The Effect of Backward Carrier Jumps 2856.3.2.6 Hopping Conductivity in Doped Organic Materials 2866.3.2.7 Coulomb Effects on Hopping in a Doped Organic Material 2896.3.3 Effective-Medium Approximation Theory of Hopping Charge-Carrier
Transport 2956.3.3.1 The EMATheory Formulations 2976.3.3.2 Miller–Abrahams Formalism 2986.3.3.3 Temperature Dependence of the Drift Mobility 2996.3.3.4 Electric Field Dependence of the Drift Mobility 3036.3.3.5 Hopping Transport in Organic Solids with Superimposed Disorder and
Polaron Effects 306
ContentsX
6.3.3.6 Low-Field Hopping Transport in Energetically and PositionallyDisordered Organic Solids 308
6.3.3.7 Charge Carrier Transport in Disordered Organic Materials in thePresence Of Traps 314
6.4 Experimental Techniques 3186.4.1 Charge Carrier Generation 3186.4.1.1 Generation Versus Transport Limited Photocurrents 3186.4.1.2 Delayed Charge Carrier Generation 3206.4.1.3 Optically Detected Charge Carrier Generation 3206.4.2 Experimental techniques to measure charge transport 3216.4.2.1 The Time-of-Flight Technique 3216.4.2.2 Space Charge-Limited Current Flow 3236.4.2.3 Determination of the Charge Carrier Mobility Based Upon Carrier
Extraction by Linearly Increasing Voltage (CELIV) 3256.4.2.4 Charge Carrier Motion in a Field-Effect Transistor (FET) 3266.4.2.5 The Microwave Technique 3276.4.2.6 Charge Carrier Motion Probed by Terahertz Pulse Pulses 3276.5 Experimental Results 3286.5.1 Analysis of Charge Transport in a Random Organic Solid with Energetic
Disorder 3286.5.2 The Effect of Positional Disorder 3396.5.3 Trapping Effects 3426.5.4 Polaron Effects 3476.5.5 Chemical and Morphological Aspects of Charge Transport 3506.5.6 On-Chain Transport Probed by Microwave Conductivity 3566.6 Conclusions 358
7 Probing Organic Semiconductors with Terahertz Pulses 367Frank A. Hegmann, Oksana Ostroverkhova and David G. Cooke
7.1 Introduction 3677.2 What is a Terahertz Pulse? 3717.3 Generating and Detecting Terahertz Pulses 3737.4 Terahertz Time-Domain Spectroscopy (THz-TDS) 3777.5 Conductivity Models 3817.6 THz-TDS Measurements of Conducting Polymers and Carbon
Nanotubes 3917.7 Time-Resolved Terahertz Spectroscopy (TRTS) 3937.8 TRTS Measurements of Transient Photoconductivity in Organic
Semiconductors 4047.9 Conclusion 419
Contents XI
8 Strong Exciton Polaritons in Anisotropic Crystals: MacroscopicPolarization and Exciton Properties 429Gerhard Weiser
8.1 Introduction 4298.2 Interaction of Light and Matter: Excitons and Polaritons 4328.2.1 Quantum Mechanics of Electrons Interacting with Light 4328.2.1.1 Excited Electrons and Harmonic Oscillators 4328.2.1.2 From Excitons to Polaritons 4348.2.2 Dielectric Theory 4398.2.2.1 Maxwell’s Equations, Fields and Energy Flux 4398.2.2.2 The Dielectric Tensor and Optical Properties 4428.2.2.3 Local Oscillators and Macroscopic Polarization 4448.3 Fundamental Properties of Polaritons: Dispersion X(k) 4468.3.1 Polaritons in Isotropic Solids 4468.3.1.1 Localized Electronic States 4468.3.1.2 Delocalized Electrons: Spatial Dispersion of Polaritons 4508.3.2 Polaritons in Anisotropic Solids: Directional Dispersion 4538.3.2.1 Polaritons in Uniaxial Crystals 4538.3.2.2 Biaxial Crystals and Axial Dispersion 4578.4 Experimental Results 4588.4.1 Non-Classical Absorption of Light 4588.4.2 Polaritons in Uniaxial Crystals 4608.4.2.1 Graphite 4608.4.2.2 Dye Single-Crystal CTIP 4628.4.2.3 TCNQ. Tetracyanoquinodimethane 4698.4.3 Polaritons in Biaxial Crystals 4718.4.3.1 Dye Crystal TTI. Bis(N-ethylthiazolin-2-yl)trimethine Cyanine
Iodide 4718.4.3.2 Ionic Crystals of the Dye BDH [1,7-
Bis(dimethylamino)heptamethinium] 4768.4.4 Polaritons in Thin Films of Nanocrystalline Domains 4828.4.4.1 Single Crystals of a-Sexithiophene (T6) 4838.4.4.2 Nanocrystalline Films 4878.5 Conclusion 492
9 Sub-5 fs Spectroscopy of Polydiacetylene 497Takayoshi Kobayashi, Mitsuhiro Ikuta and Yoshiharu Yuasa
9.1 Introduction 4979.1.1 Ultrafast Spectroscopy 4979.1.2 Characteristic Spectroscopic Properties of Polymers 4989.1.3 Polydiacetylenes 4989.2 Experimental 5019.2.1 Sample 5019.2.2 Laser System 501
ContentsXII
9.2.3 Data Analysis 5029.3 Results and Discussion 5029.3.1 Peak Tracking Analysis 5029.3.2 Bleaching and Induced Absorption Spectra 5099.3.3 Vibrational Thermalization in the Ground and Excited States 5119.3.4 Singular Value Decomposition for the Analysis of Mode Dependence
of Vibronic Coupling in the Excited and Ground States 5139.4 Conclusion 521
10 Ultrafast Optoelectronic Probing of Excited States in Low-DimensionalCarbon – Based p-Conjugated Materials 525T. Virgili, J. Cabanillas-Gonzales, L. L�er and G. Lanzani
10.1 Introduction 52510.2 Physics Background: Excited States in Low-Dimensional Conjugated
Carbon Materials 52610.3 Electromodulation of Steady-State Fluorescence in Organic Solids 53010.3.1 Introduction 53010.3.2 Electric Field-Assisted Photoluminescence Up-Conversion 53310.4 Electric Field-Assisted Pump–Probe 53710.4.1 Interpretation of the Pump–Probe Experiment 53710.4.2 Interpretation of the Electric Field-Assisted Pump–Probe
Experiment 54010.4.3 Review of Experimental Results 54110.4.3.1 Methyl-Substituted Ladder-Type Poly(p-Phenylene) (m-LPPP) 54110.4.3.2 Polyfluorene 54510.4.3.3 Fluorene Trimers (3F8) 54910.4.3.4 Oligo(phenylenevinylene)s 55110.5 Photocurrent Cross-Correlation: Real-Time Tracing of Mobile Charge
Carrier Formation 55510.5.1 Experimental Setup 55610.5.2 Photocurrent Cross-Correlation: the Signal 55610.5.3 Precursor Populations and the Precursor-Specific Free Carrier
Yield 55710.5.4 Stimulated Emission Dumping 55810.5.5 Formation of Mobile Charge Carriers by Re-Excitation (Pushing) of
Singlet Excitons 56310.5.6 Mobile Charge Carrier Generation by Re-Excitation of Charged States
(Detrapping) 568
Index 573
Contents XIII
XV
List of Contributors
Vladimir I. ArkhipovIMECMCP/PMEKapeldreef 753001 LeuvenBelgium
Thomas Basch�Institut f�r Physikalische ChemieJohannes-Gutenberg-Universit�tJakob-Welder-Weg 1155099 MainzGermany
Heinz B�sslerInstitute of Physical, Nuclear andMacromolecular ChemistryPhilipps-Universit�t MarburgHans-Meerwein-Strasse35032 MarburgGermany
Juan Cabanillas-GonzalezDipartimento di FisicaPolitecnico di MilanoPiazza Leonardo da Vinci 3220133 MilanoItaly
David G. CookeDepartment of PhysicsUniversity of AlbertaEdmontonAlberta T6G 2J1Canada
Enrico Da ComoIstituto per lo Studio dei MaterialiNanostrutturati (ISMN)Consiglio Nazionale delle Ricerche (CNR)via P. Gobetti 10140129 BolognaItaly
Eitan EhrenfreundPhysics DepartmentTechnion-Israel Institute of TechnologyHaifa 32000Israel
Ivan I. FishchukDepartment of Theoretical PhysicsInstitute of Nuclear ResearchNational Academy of Sciences ofUkraineProspekt Nauki 4703680 KievUkraine
XVI List of Contributors
Frank A. HegmannDepartment of PhysicsUniversity of AlbertaEdmontonAlberta T6G 2J1Canada
Christian H�bnerFachbereich PhysikMartin-Luther-Universit�t Halle-WittenbergHoher Weg 806120 Halle (Saale)Germany
Mitsuhiro IkutaDepartment of PhysicsGraduate School of ScienceUniversity of Tokyo7-3-1 HongoBunkyo-kuTokyo 113-0033Japan
Andrey KadashchukInstitute of PhysicsDepartment of PhotoactivityNational Academy of Sciences ofUkraineProspekt Nauki 4603028KievUkraineandIMECMCP/PMEKapeldreef 753001 LeuvenBelgium
Takayoshi KobayashiDepartment of PhysicsGraduate School of ScienceUniversity of Tokyo7-3-1 HongoBunkyo-kuTokyo 113-0033Japan
Guglielmo LanzaniDipartimento di FisicaPolitecnico di MilanoPiazza Leonardo da Vinci 3220133 MilanoItaly
Maria Antonietta LoiIstituto per lo Studio dei MaterialiNanostrutturati (ISMN)Consiglio Nazionale delle Ricerche (CNR)via P. Gobetti 10140129 BolognaItaly
Larry L�erIFN-CNRPolitecnico di MilanoPiazza Leonardo da Vinci 3220133 MilanoItaly
Michele MucciniIstituto per lo Studio dei MaterialiNanostrutturati (ISMN)Consiglio Nazionale delle Ricerche (CNR)via P. Gobetti 10140129 BolognaItaly
XVIIList of Contributors
Oksana OstroverkhovaDepartment of PhysicsOregon State University301 Weniger HallCorvallisOregon 97331-6507USA
Michel SchottInstitut des NanoSciences de ParisUniversit� Pierre et Marie Curieet Universit� Denis DiderotCampus Boucicaut140 rue de Lourmel75015 ParisFrance
Z. Valy VardenyPhysics DepartmentUniversity of UtahSalt Lake CityUtah 84112USA
Tersilla VirgiliIFN-CNRPolitecnico di MilanoPiazza Leonardo da Vinci 3220133 MilanoItaly
Gerhard WeiserDepartment of Physics andCenter of Material SciencesPhilipps-Universit�t MarburgRenthof 535037 MarburgGermany
Markus WohlgenanntDepartment of Physics and AstronomyThe University of IowaIowa CityIowa 52242-1479USA
Yoshiharu YuasaDepartment of PhysicsGraduate School of ScienceUniversity of Tokyo7-3-1 HongoBunkyo-kuTokyo 113-0033Japan
1
1IntroductionGuglielmo Lanzani
The field of organic semiconductors is very old. Melvin Calvin, introducing a firstcomprehensive text on the subject, Organic Semiconductors, by Felix Gutman andLawrence E. Lyons, published by John Wiley & Sons in 1966, says: “It was just overthirty years ago that I became aware of the idea that electronic conduction might beobserved in organic materials and might play a role in their biological function”. Thisplaces the birth of the field somewhere between 1930 and 1940, when quantummechanics was still young, inorganic semiconductors were in their early stagesand physics was having a fantastic evolution. The book by Gutman and Lyons col-lects the results obtained from World War II until 1966. In spite of the size andcompleteness of their text, these authors already acknowledged at that time that amuch larger, encyclopedic effort would have been required to cover the field fully.Since then much work has been done, making the “encyclopedia” even furtherout of reach. Important discoveries occurred in more recent years, especially con-jugated polymers leading to the Nobel Prize in Chemistry in 2000. The continu-ous discovery of new classes of materials, new applications and new tools forinvestigation has kept the field in a state of flux, in spite of its long history. So in2005 many of the issues reported in 1966 are still valid, such as the demand for alarge interdisciplinary approach, the effort of physicists to develop a theory forweakly bounded systems and that of chemists for understanding property–struc-ture relationships. Amid the spectacular development in science and technologyof organic semiconductors, allowed by an exponential increase in the number ofactive researchers in the field, in both academies and industrial laboratories,many questions remain open.When I decided to undertake the challenge of editing a book on molecular
materials, I had one point fixed in my mind: to make something different fromthe cutting-and-pasting of published papers. I felt that a monograph was neededthat puts new and exciting experimental techniques on a common footing when-ever possible, showing their foundations, limitations and interconnections. Thiswill help to intensify and specify communication among experts in differentexperimental fields.I asked all the authors of this book to write a broad, exhaustive tutorial on their
subject, with original contents, explanation and views. Something that could actu-
ally help the newcomers, instruct the students, support the researchers, notbecome obsolete too soon and yet have up-to-date contents. It sounds like a mam-moth task and indeed it was. Of course, selection was needed, to keep the contentssufficiently focused while preserving these general aims. For instance, the bookcontains reviews mainly on experimental results, interpretation is based on rela-tively simple models, except for a few cases, and theory is not included. There arealready many excellent books on quantum chemistry. A painful screening had tobe done, to select a few topics out of a huge amount of high-quality work existingin the field. One unquestionable criterion guiding this process was, again, avoid-ing overlap with other reviews. Yet the bibliography received special attention, tocompensate for deficiencies and provide as broad as possible review for consulta-tion. I hope most of the existing literature is properly quoted in the references andI apologize in advance for missing any contributions.In any dynamic science there are many areas of controversy. Experiments, how-
ever, “never deceive”, as Leonardo da Vinci said. Interpretation is often that of theauthors, yet I hope the reader will have the opportunity to elaborate her or hisown point of view.Radiation–matter interaction is at the foundation of material science, since it is
an integral constituent of the principal material characterization tools. Photophys-ics is the keystone of the subject. The wealth of processes that it includes may beuseful for the interpretation of results and also for the design of new device con-cepts. This is particularly true for organic semiconductors, which have the proper-ties to be highly reactive to light stimuli. Indeed, natural chromophores, light-har-vesting systems or emitters are all based on p-conjugated carbon molecules.Mimicking nature has led to the amazing development of plastic electronics.The book starts with molecular photophysics (Chapter 2). This is one important
piece of the story of organic optoelectronics, for such materials often behave asmolecular solids. In addition, single-molecule devices are at the heart of molecularelectronics, refreshing old molecular concepts for future technology. While basictopic can be a century old, the experimental results reported here are updated tostate-of-the-art techniques for single-molecule spectroscopy. This is an attractiveway to collect information on molecular dynamics, which reveals surprises andopens up new perspectives towards nanotechnology. The innovative way in whichmolecular dynamics is investigated, probing single events of isolated species andnot averages over large ensembles, provides a new point of view for looking at mo-lecular photophysics. Next are presented studies on single polymer chains, a nas-cent field (Chapter 3). Here the system investigated has a large size, challengingthe concept of localized states suitable to describe molecules and introducing theconcepts and tools of the solid state, yet in low dimensions. Such a borderlinearea is very fertile for new ideas about how to describe phenomena which areneither typical of covalent solids nor of isolated molecules. Quantum confine-ment, from three- to one-dimensional space, has dramatic effects on the natureand dynamics of excited states, as is well known from inorganic nanostructureinvestigations. In spite of a high electronic density, screening is much less effec-tive than in higher dimensions and correlation takes over. The resulting tight
1 Introduction2
bounded exciton states resemble more a molecular then a wave-like crystal excita-tion. Soft lattice and strong electron–phonon coupling, typical of organic semicon-ductors, gives an extra twist to the subject.Once the building blocks, molecules and polymers, are known, one can move
on to the solid state, where they interact. An interesting mixing of notions getsinvolved here, depending on the intermolecular coupling regime. In the weak-coupling regime, localized, molecular states are still a valid description of theelementary excitations. However, in solids new phenomena may occur: energy(excited states) can migrate, incoherently, giving rise to energy transfer, or disso-ciate, forming charge-transfer states. In the medium coupling regime, intermolec-ular “resonance” interaction may lead to delocalization of the wavefunction, thusgenerating completely new excitations with respect to the starting component,described as Frenkel excitons, which cohabit with localized states. In the strongcoupling regime, typical of covalent bonding, weakly bound electron–hole pairscan be formed, named Wannier–Mott excitons, or sometimes delocalized chargecarriers can appear. Morphology plays a crucial role in modulating the degree ofintermolecular interaction. Starting from the molecular structure, it is still a chal-lenge to predict how this happens and to what extent. A number of empiricalrules, sometime true recipes, were developed over time. Yet it is well known thateven the same molecular species can gives rise to a variety of aggregation states,depending on a number of parameters not always under control. We then intro-duce, in Chapter 4, a specifically designed technique for addressing the relation-ship between photophysics and morphology, based on the local probing of theoptical properties through confocal microscopy.Elementary excitation dynamics, including generation, relaxation and deactiva-
tion, are the next step. First we address long-lived excitations in Chapter 5, whichusually appear only in the solid state, where intermolecular processes are respon-sible for either their generation or slower recombination. Typically long-lived exci-tations are triplet states and charged states. On this time scale, typically millisec-onds, a wealth of characterization techniques are available, including the mag-netic degree of freedom, which is of critical importance in some assignments.The scenario one can obtain is fairly exhaustive. The phenomena considered hereare those occurring in most optoelectronic devices, which work in quasi-steady-state conditions. Charged excitations, rarely encountered in isolated systems,become important. They play a key role in many applications, so charge transportis the next topic to be considered (Chapter 6), and the discussion is focused ontransport in disordered media, suitable for most carbon-based p-conjugated mate-rials. Free carriers, however, are rarely encountered, if they exist at all in soft con-densed matter. The place to look for them is the far-infrared region, where“Drude-like” contributions to the radiation–matter interaction may arise. Usingelectromagnetic pulses in the THz frequency range can do this. It is a difficultexperiment, yet appealing and new to this field. The basics and a review of resultsare reported in Chapter 7. The case of highly ordered systems, as in crystallinespecimens, is addressed in Chapter 8. Strong intermolecular interactions lead towavefunction delocalization, generating new, collective excitations, which involves
31 Introduction
1 Introduction
all the molecules in the crystal and carry properties peculiar to the crystal and notthe constituents. Excitons and polaritons have to be considered. Their peculiarproperties are discussed comprehensively and some exemplifying cases arereported. In the last two chapters (9 and 10), ultrafast spectroscopy is introduced.Early time dynamics embody fundamental properties of the materials. Thebranching ratio of the nascent population into a number of subspecies, whichdetermines the final performance of the material, occurs within 100 fs. We con-sider standard pump–probe experiments with extreme time resolution and finallyelectric field-assisted pump–probe experiments, which are carried out on devicestructures. The latter provide a useful and rather unusual tool for investigatingelementary excitation dynamics, which offers a straightforward way of compari-son with the better known inorganic semiconductor counterpart.Advance in science is a collective process, which nowadays involves millions of
people. Even in our specific subject the number of active researchers is very largeand steady increasing. The essential step that keeps the whole machine runningis information exchange within the community. I hope that the publication of thisbook will contribute to this process.
4
5
2Optical Microscopy and Spectroscopy of Single MoleculesChristian H�bner and Thomas Basch�
2.1Introduction
Since its first demonstration [1], single-molecule spectroscopy (SMS) has seen rap-id development, which is evidenced by the ever-increasing number of publicationsand groups working in the field of SMS. The research topics covered range fromfundamental quantum optical experiments to applications in molecular biologyand material and nano-science. Along with this research diversity, technical prog-ress has culminated in the commercial availability of standardized versions ofoptical microscopes with single-molecule sensitivity. Considering these develop-ments, it seems natural that SMS is one important topic to be covered in a mod-ern book on Photophysics of Molecular Materials. Actually, basic photophysical pa-rameters of organic dye molecules as time constants of triplet and singlet decay orenergy transfer efficiencies (in molecular aggregates) can easily be accessed bySMS. Although some of the corresponding experiments are “just” the single mol-ecule version of well-known experiments with large ensembles of molecules, thereare many experiments, which work exclusively at the single-molecule level.The main intention of this chapter is to introduce different experimental tech-
niques of SMS and some underlying elementary photophysical principles. Manyaspects considered here have already been treated in the literature and two SMStextbooks [2, 3] and a series of review articles [4–14] are available for in-depth read-ing on specific topics. In addition, we will attempt to highlight the benefits ofSMS considering selected applications, which have some relation to other mate-rial covered in this book.Regarding applications, we will completely omit SMS in life sciences, because
the huge variety of biological issues would simply exceed the limits of this chapter.Most aspects of single-molecule fluorescence experiments discussed here, how-ever, hold also in the field of life sciences. The interested reader who has an appli-cation of SMS to a biological problem in mind will therefore profit from theunderstanding of its basic principles. Another experimental realization of single-molecule detection that will not be dealt with here is single-molecule detection insolution, which is also mainly employed in bio-oriented research. In this context,
fluorescence correlation spectroscopy (FCS) is sometimes regarded as a single-molecule technique. FCS, however, is not a strict single-molecule method,because it is by definition averaging over a large number of single-molecularevents.Whereas SMS in the early years was mainly performed at cryogenic tempera-
tures, the field of room-temperature SMS is now growing rapidly. SMS underambient conditions is also referred to as single-molecule detection (SMD). Inorder not to confuse the reader with two acronyms, we use SMS for both low-tem-perature and room-temperature experiments.What makes single-molecule fluorescence so appealing to scientists in the fields
of quantum optics and physical, chemical, material and life sciences? One of themagic words in this context is heterogeneity. If all molecules of an ensemble wereto behave identically in space and time, the study of the properties of single mole-cules would not give any extra information as compared with an ensemble experi-ment. If there is any heterogeneity, however, be it of temporal or spatial nature,the observation of isolated entities may provide a wealth of information, whichotherwise is hidden in the ensemble average. The great interest of life scientists insingle-molecule experiments is due to the notorious heterogeneity of biologicalsystems, which renders them ideal targets for SMS.On the other hand, nanostructured materials constructed in a bottom-up
approach need to be investigated on the nanoscale. Nanoscopic probes are desiredhere and fluorescence properties such as excited-state lifetime or emission wave-length of single molecules, which are determined by their surroundings, canreport on heterogeneities on molecular scales. Ultimately, a single molecule maybe a device by itself, a switch, a motor or a light source.Besides this application-driven interest, single-molecule fluorescence is fasci-
nating from a fundamental point of view. The temporal behavior of photon emis-sion is of particular interest from this perspective. Furthermore, single moleculesin a classical picture represent nanometer-sized antennas, the properties of whichare worth investigating.This chapter is organized as follows. In Section 2.2 some photophysical princi-
ples of single molecule fluorescence detection are presented. Section 2.3 dealswith the experimental techniques. Selected applications of SMS are covered inSection 2.4.
2.2Photophysical Principles of Single-Molecule Fluorescence Detection
2.2.1The Single Molecule as a Three-Level System
Most fluorescent organic molecules – referred to as fluorophores – can be approxi-mately treated as three-level systems, with the electronic ground state and the firstelectronically excited state, both being singlet states and an excited triplet state
2 Optical Microscopy and Spectroscopy of Single Molecules6
(see Fig. 2.1). From the electronic ground state (S0) the molecule can be broughtinto the first electronically excited state (S1) by interaction with the laser field. TheRabi frequency XR ” p~dd �~EE=h determines the interaction strength between theelectric field of the light wave and the molecule and thus the pump rate betweenthe electronic ground and first excited state in the electric dipole approximation.Here,~dd is the electronic transition dipole, ~EE is the amplitude of the electric field ofthe interacting laser light and h is Planck’s constant. The depopulation of S1occurs with rate constant k21, which is the sum of the radiative rate constant andthe rate constants of the radiationless transitions given by internal conversion toS0 and intersystem crossing (ISC) to T1. Because for the present we consider aresonant interaction between the purely electronic S0–S1 transition and the laserfield, the molecule can be pumped back to the electronic ground state by stimulat-ed photon emission. Of particular importance for the photodynamics of a singlemolecule is ISC form S1 to T1, which occurs with low probability with rate con-stant k23. From T1 the molecule eventually relaxes to the ground state. As was thecase for singlet relaxation, the rate constant k31 of triplet relaxation is given by thesum of radiationless and radiative transitions (phosphorescence). Typically, mole-cules suitable for SMS do not show phosphorescence, because the radiative ratefor the spin-forbidden T1 fi S0 transition is very small. Because single-moleculeoptics at present is mainly based on fluorescence detection, good single-moleculefluorophores are characterized by a high fluorescence quantum yield. Accordingly,in such molecules the radiative rate constant for singlet decay is larger than therate constants for the radiationless transitions.The quantum-mechanical treatment of the three-level system interacting with
the exciting laser can be accomplished in the framework of the density matrixformalism using optical Bloch equations. The density matrix equations in therotating wave approximation then read [12,15]:
_rr11 ¼ k21r22 þ k31r33 þ iXRðr21 � r12Þ_rr22 ¼ �k21r22 � k23r22 � iXRðr21 � r12Þ_rr12 ¼ ½iðx� x0Þ � T�12 �r12 þ iXRðr22 � r11Þ_rr33 ¼ k23r22 � k31r33
(2.1)
72.2 Photophysical Principles of Single-Molecule Fluorescence Detection
Fig. 2.1 Reduced Jablonski diagram of thethree-level system describing a fluorescentmolecule. S0 and S1 are the electronic groundand first electronically excited singlet state ofthe fluorophore, respectively, and T1 is the
first electronically excited triplet state.k21 is the spontaneous decay rate from S1and k23 and k31 are the intersystem andreverse intersystem crossing rates,respectively.
2 Optical Microscopy and Spectroscopy of Single Molecules
with x and x0 the laser and the molecule’s resonance frequency and T2 thedephasing time.The steady-state and time-dependent solutions of the set of differential equa-
tions in Eqs. (2.1) are appropriate to describe single-molecule behavior at low tem-perature, when the laser is in resonance with the purely electronic S0–S1 transitionand phase relaxation processes are slowed. The corresponding results are welldocumented in the literature [12, 15]. At room temperature, owing to the rapidloss of phase coherence, the off-diagonal elements of the density matrix can beneglected for many applications. The temporal evolution of the system is thendescribed by occupation probabilities or populations [ni(t)] of electronic statesonly. In a typical room-temperature single-molecule experiment, molecules areexcited into a vibrational level of S1. In large polyatomic molecules vibrationalrelaxation occurs on the picosecond to sub-picosecond time-scale, which is 3–4orders of magnitude faster than the time constants of other relevant transitions.Therefore, under typical experimental conditions stimulated emission must notbe considered and the three-level system can be described by the following set ofsimple rate equations:
_nn1_nn2_nn3
0@
1A ¼ �k12 k21 k31k12 �ðk21 þ k23Þ 0
0 k23 �k31
0@
1A n1n2
n3
0@
1A (2.2)
where k12 ¼ rI is excitation rate from the singlet ground state S0 to a vibrationallevel of S1 and depends on the wavelength-dependent absorption cross-section rand the laser intensity I. The solution of this system of differential equations forthe stationary case yields the emission rate for a given set of rate constants. Herewe will give only the maximum emission rate R¥ for an infinite excitation inten-sity, which depends on the excited state relaxation rate k21 and on the ISC rates k23and k31, respectively:
R¥ ¼k21 þ k231þ k23
k31
Uf ð2:3Þ
with Uf the quantum yield of fluorescence. When stimulated emission is consid-ered, as would be the case for the density matrix Eqs. (2.1), the term in thedenominator in Eq. (2.3) reads 2þ k23=k31. This can be intuitively understood,because owing to the pumping from S1 to S0 the population of the first excitedstate cannot exceed the population of the electronic ground state.The maximum emission rate is clearly limited by the ratio k23/k31, a fact that is
frequently referred to as the triplet bottleneck. Whereas ensemble fluorescencespectroscopy usually is done far from saturation of the fluorescence transition,the triplet bottleneck limits the emission rate for the high excitation rates achievedin SMS. The stationary emission intensity Iem as a function of the excitation inten-sity Iexc is as follows:
8
2.2 Photophysical Principles of Single-Molecule Fluorescence Detection
Iem�Iexc
Iexc þ Isat(2.4)
where Isat is the saturation intensity according to
Isat ¼R¥r
1þ k23k31
� �(2.5)
For low excitation intensities, the emitted fluorescence light increases linearlywith the excitation intensity and saturates for high excitation powers. This hasconsequences for the “ideal” excitation intensity, i.e. the excitation intensity withthe best signal-to-noise ratio (see Section 2.3.1).The solution of the system of differential Eqs. (2.2) under the initial conditions
n1(t = 0) = 1; n2(t = 0) = n3(t = 0) = 0 provides the time evolution of the occupationprobability of the first electronically excited singlet state n2(s), which is related tothe fluorescence intensity autocorrelation function or second-order autocorrela-tion function:
gð2ÞðsÞ ¼ nðtÞnðtþ sh inðtÞh i2 ¼
n2ðsÞn2ðsfi¥Þ
(2.6)
where n(t) is the number of photons detected at time t and the brackets denoteaveraging over t. In the short time limit (t» 1=k21 > 1=k21) from Eq. (2.3) it follows
that [16]
gð2ÞðsÞ ¼ 1þ keff23
k31e�ðk
eff23 þk31Þs (2.8)
where keff23 is the effective intersystem crossing rate, given by
keff23 ¼k12
k21 þ k12� k23 (2.9)
As a result, the probability of detecting a second photon after time s is higherthan the probability of detecting a second photon after sfi¥. This time regime istherefore called the bunching regime. The (normalized) autocorrelation curve islarger than unity in the bunching regime, from where it decays to unity in theinfinite time limit (see Fig. 2.2). The antibunching and the bunching regimes are
9
2 Optical Microscopy and Spectroscopy of Single Molecules
temporally well separated, because the excited-state relaxation rate is typicallymuch higher than the triplet population and relaxation rates.The respective solution of the density matrix Eqs. (2.1) taking into account
coherent interactions between the laser field and the molecule leads to oscillationsof gð2ÞðsÞ in the short time regime, which are called Rabi oscillations (see Fig. 2.2).Those oscillations are rapidly damped out at room temperature owing to rapidloss of phase coherence.The temporal properties of photon emission of single molecules thus differ sig-
nificantly from other light sources such as lasers or thermal emitters. Both photonantibunching and photon bunching can be intuitively understood: There is afinite time for an excitation–emission cycle separating the photons in time, andthe molecule shows periods where emission occurs due to singlet cycling, separat-ed by dark periods when it is shelved in the triplet state.
2.2.2Dipole–Dipole Coupled Oscillators
Multichromophoric aggregates recently became attractive targets for single-mole-cule fluorescence experiments driven by the interest in the study of energy trans-fer mechanisms in those systems (cf. Section 2.4.3). We will therefore present abrief overview of electronic coupling in molecular aggregates, in which one ormore excitations are present. For the sake of clarity and simplicity we will limitour discussion to Coulombic interactions taking into account only the leadingdipole–dipole term in the point multipole expansion. Consequently, electronicwavefunction overlap and electron exchange between the molecules are not con-sidered. Although this often is a good approximation for strong, dipole allowed
10
Fig. 2.2 Simulated intensity autocorrelation function for a sin-gle pentacene molecule at low temperature showing photonantibunching at short times and photon bunching at longertimes. Additionally, Rabi oscillations are visible in the shorttime region. Adapted from Ref. [17].
