Polythiophenes fundamentals

Some fundamental aspects of polythiophene preparation and chemistry have been investigated. Solution processing of a polythiophene having ester groups has been performed, which after ester cleavage and decarboxylation gave new access to the parent thiophene polymer <07MAC6012>. The mechanism of the photoinduced polymerization of thiophene... 

Intercalation of electroactive polymers such as polyaniline and polypyrrole in mica-type layered silicates leads to metal-insulator nanocomposites. The conductivity of these nanocomposites in the form of films is highly anisotropic, with the in-plane conductivity 10 to 10 times higher than the conductivity in the direction perpendicular to the film. Conductive polymer/oxide bronze nanocomposites have been prepared by intercalating polythiophene in V2O5 layered phase, which is analogous to clays. °° Studies of these composites are expected not only to provide a fundamental understanding of the conduction mechanism in the polymers, but also to lead to diverse electrical and optical properties. 

In other EFM studies,158 part of the sample was electrochemically oxidized by partially immersing in electrolyte and applying a positive potential. The EFM image clearly showed a distinction between the immersed (oxidized) and nonimmersed (less oxidized) parts of the sample in terms of relative work function. Similar shifts in work function had also been observed for polythiophenes when exposed in situ to ultraviolet (UV) radiation during EFM imaging.160 An increase in the work function upon UV exposure is due to the formation of electron-hole pairs that produce photocurrent in photovoltaic devices. Thus, the EFM provides fundamental insights into the processes involved in many of the applications for CEPs. 

The book is structured into four areas, describing the principles of molecular design and synthesis of oligo- and polythiophenes (Chapters 1-6), insight into their properties from the perspectives of quantum chemical calculations (Chapters 7 and 8), the fundamental aspects of the special electronic, photonic and self-assembly properties of oligo- and polythiophenes (Chapters 9-13) and, finally, the applications of thiophene-based materials in electronic and optoelectronic devices (Chapters 14-22). 

As synthetic chemists desired to tune the optoelectronic and redox properties of conjugated polymers in a fine manner, more complicated conjugated systems were required. The two fused heterocycles-substituted polythiophenes 16 and 17 (Chapter 18) illustrate this as electron-poor imine functionality in 16 brings donor-acceptor character to the material, while the more electron-rich thio-based system 17 provides for especially easy oxidation. Polymerization of these complicated bis-2-thienyl monomers by electrochemical methods paves the way for fundamental structure-property relationships to be understood, ultimately directing the synthetic chemist towards soluble polymers (Chart 1.6). 

Finally, we will concentrate on the chemical reactivity of silyl derivatives of thiophene. The oxidative polymerization of various silyl monomers lead to polythiophene. The evaluation of this new polymerization reaction implies a precise characterization of the produced conjugated materials. Knowledge and the control of the pertinent parameters which direct the properties of the conjugated systems are essential. Also required is the development of methods which allow a precise characterization of the samples. The role of vibrational infrared and Raman spectroscopy is of fundamental importance in this field. Optical spectroscopy is one of the few tools for unravelling the structure of these materials and understanding their properties. First, new criteria based on infrared, Raman and photoluminescence spectroscopy which allow precise estimates of the conjugation properties will be reported. Then the synthesis and characterization of polythiophene samples arising from the oxidative polymerization of silyl thiophene will be presented. 

There are two major differences between conjugated and redox polymers. These differences are of a practical rather than a fundamental nature. First, the native conjugated polymers can be brought into a state in which they do not contain charged sites (neutral polypyrrole, polythiophene, etc.). In this state the polymer is rather hydrophobic oxidation/reduction to an ionized state involves a significant structural change. Second, the polymeric redox sites (polarons, etc.) are less clearly defined than redox polymer sites, in particular with respect to their electronic energy levels. 

This review shows that a vast literature already exists on both fundamental and practical aspects of the electrochemistry of conducting polymers. This trend is expected to extend in the future to encompass other new polymer candidates in addition to polypyrrole, polythiophene, and polyaniline. Concomitantly, electrochemical studies should provide the impetus for discovering new application areas (e.g.. environmental pollution abatement and information and communication technologies) for these interesting materials. 

Combinations of polythiophenes and Cgo have also been utilised by the Santa Barbara group for more fundamental studies [211, 212], and by the Linkoping group for devices [213]. These devices actually comprise two different poly-thiophenes, poly(3-(4-octylphenyl)-2,2 -bithiophene), polymer II in Fig. 13, and poly(3- 2Lmethoxy-5 -octylphenyl thiophene), POMeOPT. The latter is used because it has a sidegroup reminiscent of anisole, which is known to interact with... 

A lot of poly(3-alkyl-thiophenes) were known at that time, and some of these special polythiophenes exhibited a remarkable conductivity in the doped state. A comprehensive overview about polythiophenes and their electronic properties available at that time can be foimd in the Handbook of Oligo- and PolythiophenesP The fundamental drawback of all these polythiophenes was the instability of the highly conductive form. In particular, humid air was destructive. 

How much do the results we have obtained here tell us about the fundamental limits to the mobilities of carriers in devices fabricated with polymer that is very much better ordered than the polyacetylene that we have used here There are recent reports of very much improved mobilities for devices based on sexithiophene (the six repeat unit oligomer of polythiophene), with a value of 0.4 cm /Vsec now reported [73], and there is now considerable interest in the development of polymer FETs as large area thin film transistors, with interest in polythiophene derivatives [74] and in poly(arylenevinylenes) such as poly(2,5-thienylene vinylene) [75]. We can see from the optical characterisations of the MIS devices that the surface layer of polyacetylene formed on SiC>2 is very much more disordered than the bulk material, but we have not made FET devices with the polymer insulator layers which give better ordered structures as characterised optically. 

---