Polythiophene oxidation-reduction

An important additional feature is that all of these polymer structures are amenable to facile oxidation/reduction processes that can be initiated at moderate potentials. For polypyrroles and polythiophenes two oxidation states can be reversibly switched, as shown in Eqn. 2 (Z = NH or S). The doped oxidised forms exhibit good electrical conductivity (0= 1-100 S cm ), while the reduced forms have very low conductivity (ct 10 S cm ). This ability to conduct electrons is important in that information can be readily relayed within an Intelligent... 

Some examples of ICPs are shown in Figure 11.1. The typical oxidation-reduction process between neutral (semiconducting) and oxidized (metal-hke conducting) state is shown for polythiophene in Figure 11.2. 

FIGURE 10.8. Postulated oxidation-reduction processes for (I) polypyrrole, (II) polythiophene, and (III) polyaniline. (Adapted from Ref. 156 with permission from the Royal Society of Chemistry)... 

Figure 1.13 Oxidation-reduction reactions of polythiophene derivatives showing self-doping during the oxidation reactions. (Reprinted from Synthetic Metals, 20, A. O. Patil, Y. Ikenoue, N. Basescu, N. Colaneri, F. Wudl, A. J. Heeger, 151. Copyright (1987), with permission from Elsevier.)...

The property of polythiophenes to change color upon the reversible oxidation/reduction process and the resulting electrochromic applications (e.g. smart windows) are the subject of the review Chapter 20 by Greg Sotzing and co-workers. 

Substituted polythiophenes have been synthesized and studied those polythiophenes with long alkyl side chains are soluble and suitable for the preparation of free-standing films or coatings on surfaces of a variety of materials. Optical, magnetic and liquid crystalline properties have also been studied as well as the changes of wettability resulting from the redox behavior of polythiophenes during reversible oxidation-reduction reactions. 

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. 

Pseudocapacitance is used to describe electrical storage devices that have capacitor-like characteristics but that are based on redox (reduction and oxidation) reactions. Examples of pseudocapacitance are the overlapping redox reactions observed with metal oxides (e.g., RuO,) and the p- and n-dopings of polymer electrodes that occur at different voltages (e.g. polythiophene). Devices based on these charge storage mechanisms are included in electrochemical capacitors because of their energy and power profiles. 

Conducting Polymers Electronically conducting polymers (such as polypyrrole, polythiophene, and polyaniline) have attracted considerable attention due to their ability to switch reversibly between the positively charged conductive state and a neutral, essentially insulating, form and to incorporate and expel anionic species (from and to the surrounding solution), upon oxidation or reduction ... 

Related Polymer Systems and Synthetic Methods. Figure 12A shows a hypothetical synthesis of poly (p-phenylene methide) (PPM) from polybenzyl by redox-induced elimination. In principle, it should be possible to accomplish this experimentally under similar chemical and electrochemical redox conditions as those used here for the related polythiophenes. The electronic properties of PPM have recently been theoretically calculated by Boudreaux et al (16), including bandgap (1.17 eV) bandwidth (0.44 eV) ionization potential (4.2 eV) electron affinity (3.03 eV) oxidation potential (-0.20 vs SCE) reduction potential (-1.37 eV vs SCE). PPM has recently been synthesized and doped to a semiconductor (24). 

The ratio of the integrated currents for the first reduction wave of V2+ and the oxidation wave of the polythiophene from 0.4 V to 1.0 V vs. Ag+/Ag is about 4. This value means that upon oxidation of poly(I) one electron is withdrawn from four repeat units in the backbone of the polymer upon scanning to +1.0 V vs. Ag+/Ag. At this potential, the polythiophene achieves its maximum conductivity (vide infra). The level of oxidation to achieve maximum conductivity is consistent with the result reported by Gamier and co-workers (31-33) that the doping level of oxidized polythiophene is about 25%, but the Garnier work did not establish that the 25% doping level corresponds to maximum conductivity. Scheme III illustrates the electrochemical processes of poly(I) showing reversible oxidation of the polythiophene backbone and reversible reduction of the pendant V2+ centers. 

Scheme III. Electrochemical processes of poly(I) showing reversible oxidation of the polythiophene backbone and reversible reduction of the pendant viologen centers.

The poly(I)-based transistor is the first illustration of a microelectrochemical transistor based on a combination of a conducting and a conventional redox polymer as the active material. The transistor "turns on" at VG corresponding to oxidation of the polythiophene backbone. The resistivity of poly(I) declines by a factor of 105 upon changing VG from 0.4 V to 0.8 V vs. Ag+/Ag. When Vg is moved close to the one-electron reduction potential of V2+/+, the conventional redox conductivity gives a small degree of "turn on". A sharp Iq-Vq characteristic results, with an Ip(peak) at Vq = E° (V2+/+). Though the microelectrochemical devices based on conventional redox conduction have both slow switching speed and a... 

---