Conjugated Conducting Polymers

This book reviews the current understanding of electronic, optical and magnetic properties of conjugated polymers in both the semiconducting and metallic states. It introduces in particular novel phenomena and concepts in these quasi­ one-dimensional materials that differ from the well-established concepts valid for crystalline semiconductors. After a brief introductory chapter, the second chapter presents basic theore­ tical concepts and treats in detail the various models for n-conjugated polymers and the computational methods required to derive observable quantities. Specific spatially localized structures, often referred to as solitons, polarons and bipolarons, result naturally from the interaction between n-electrons and lattice displacements. For a semi-quantitative understanding of the various measure­ ments, electron-electron interactions have to be incorporated in the models; this in turn makes the calculations rather complicated. The third chapter is devoted to the electrical properties of these materials. The high metallic conductivity achieved by doping gave rise to the expression conducting polymers, which is often used for such materials even when they are in their semiconducting or insulating state. Although conductivity is one of the most important features, the reader will learn how difficult it is to draw definite conclusions about the nature of the charge carriers and the microscopic transport mechanism solely from electrical measurements. Optical properties are discussed in the fourth chapter.

1. Introduction.- References.- 2. An Overview of the Theory of ?-Conjugated Polymers.- 2.1 Synopsis.- 2.2 Theoretical Concepts, Models and Methods.- 2.2.1 The Born-Oppenheimer Approximation.- 2.2.2 Ab Initio Calculations.- 2.2.3 Model Hamiltonians.- 2.3 The Hückel and SSH Models: Independent-Electron Theories.- 2.3.1 From Polyethylene to Polyacetylene.- 2.3.2 Bond Alternation.- 2.3.3 The Strength of the Electron-Phonon Coupling.- 2.3.4 Stability of the Dimerized State and the Phonon Spectrum.- 2.3.5 Spatially Localized Nonlinear Excitations: Solitons, Polarons and Bipolarons.- 2.3.6 Predictions of the Model.- 2.4 Hubbard Model: A Paradigm for Correlated Electron Theories.- 2.4.1 Ground State and Excitation Spectrum.- 2.4.2 Correlation Functions.- 2.4.3 Relevance for Conjugated Polymers.- 2.5 The One-Dimensional Peierls-Hubbard Model.- 2.5.1 The Model Hamiltonian and its Parameters.- 2.5.2 Methods.- 2.6 The Combined Effects of Electron-Phonon and Electron-Electron Interactions: Theory and Experiment.- 2.6.1 Ground State.- 2.6.2 Electronic Excitations and Excited States.- 2.6.3 Vibrational Excitation: Raman and Infrared Spectroscopy.- 2.7 Beyond Simple Models: Discussion and Conclusions.- 2.7.1 Effects of Disorder.- 2.7.2 Interchain Coupling and Three-Dimensional Effects.- 2.7.3 Lattice Quantum Fluctuations.- 2.7.4 Doping Effects and the Semiconductor-Metal Transition.- 2.7.5 Transport.- 2.7.6 Concluding Remarks.- References.- 3. Charge Transport in Polymers.- 3.1 Models for the Insulating and Semiconducting States.- 3.1.1 The Electronic Ground State.- 3.1.2 The Nature of the Charge Carriers.- 3.1.3 Disorder Along the Chains.- 3.1.4 Low and Intermediate Doping.- 3.2 Models for Transport Processes.- 3.2.1 Conduction in Extended States.- 3.2.2 Conduction in Localized States.- 3.2.3 Transport in One Dimension.- 3.2.4 Transport by Quasi-Particles.- 3.3 Experiments in the Insulating and Semiconducting State.- 3.3.1 Polyacetylene.- 3.3.2 Other Polymers.- 3.4 The Semiconductor-Metal Transition and the Metallic State.- 3.4.1 Models for the Highly Doped State.- 3.4.2 Experiments in the Highly Doped State.- 3.5 Summary.- References.- 4. Optical Properties of Conducting Polymers.- 4.1 Elementary Considerations.- 4.2 Dielectric Response Function and Band Structure.- 4.3 Band Gap and Band Structures of Undoped Conjugated Polymers.- 4.3.1 Results of Band Structure Calculations.- 4.3.2 Experimental Results.- 4.4 Photon-Phonon Interaction.- 4.4.1 General Remarks.- 4.4.2 Calculations of Vibrational Spectra of Polymers.- 4.4.3 Experimental Results.- 4.5 The Study of Elementary Excitations in Conjugated Polymers.- 4.5.1 General Considerations.- 4.5.2 The Electronic States of the Quasi-Particles.- 4.5.3 The Vibrational State of the Quasi-Particles.- 4.5.4 Experimental Results.- 4.6 Highly Conducting Conjugated Polymers.- 4.6.1 General Considerations.- 4.6.2 The Highly Conducting Phase of Trans-Polyacetylene.- 4.6.3 Polyacetylene: Experimental Results.- 4.6.4 Highly Conducting Polymers with Nondegenerate Ground State.- 4.6.5 Concluding Remarks.- References.- 5. Magnetic Properties of Conjugated Polymers.- 5.1 General Aspects of Magnetic Properties and Resonance Techniques.- 5.1.1 Susceptibility.- 5.1.2 Lineshapes, Linewidths and Lineshifts.- 5.1.3 Spin Relaxation (T1,T2,T1p).- 5.1.4 Double Resonance Techniques.- 5.1.5 High-Resolution NMR.- 5.2 Structure and Lattice Dynamics of Conjugated Polymers in the Non-Conducting Phase.- 5.2.1 Lattice Structure Determination from Dipole-Dipole Interactions.- 5.2.2 Bond Length Determination from Dipole-Dipole Interactions.- 5.2.3 Chemical Shift Tensor.- 5.3 Spin Dynamics of Conjugated Defects in the Non-Conducting Phase.- 5.3.1 ESR and ENDOR Lineshapes.- 5.3.2 Dynamic Nuclear Polarization.- 5.3.3 Nuclear Spin Lattice Relaxation.- 5.3.4 Electron Spin Relaxation.- 5.3.5 Light-Induced ESR.- 5.4 Magnetic Properties of Conjugated Polymers in the Conducting Phase.- 5.4.1 Susceptibility.- 5.4.2 ESR Lineshapes and Linewidths.- 5.4.3 NMR Results.- 5.5 Magnetic Properties of Polydiacetylenes (PDA).- 5.5.1 Structure.- 5.5.2 Solid-State Polymerization.- 5.5.3 Quasi-Particle Excitation.- 5.6 Other Conjugated Polymers.- 5.7 Conclusions and Remarks.- References.