Electronic Processes of Polythiophenes

Figure 4,2 CVs at 50 mV s in PC-0.2 M TEABF4 of (a) poly(TOT), (b) poly(TTOTT) and (c) poly(TTTOTTT). The Coulombic efficiencies of the p- and n-doping process from the Wth CV are (a) 98, 96%, (b) 98, 100% and (c) 96, 99 %, respectively. Adapted with permission from Figure 4 of C. Barbarella, L Favaretto, G. Sotgiu, M. Zambianchi, C. Arbizzani, /, Bongini and M. Mastragostino, Controlling the electronic properties of polythiophene through the insertion of nonaromatic thienyl S,S-dioxide units, Chem. Mater., 11, 2533-2541 (1999). Copyright 1999 American Chemical Society...

These are regarded as copolymers of thiophene and acetylene. Thus, they should have properties of polythiophene and polyacetylene, for example high conductivity of polyacetylene as well as stability, tunability of the electronic structure and processability of polythiophene. 

The development of polythiophenes since the early 1980s has been extensive. Processible conducting polymers are available and monomer derivathation has extended the range of electronic and electrochemical properties associated with such materials. Problem areas include the need for improved conductivity by monomer manipulation, involving more extensive research using stmcture—activity relationships, and improved synthetic methods for monomers and polymers alike, which are needed to bring the attractive properties of polythiophenes to fmition on the commercial scale. 

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 This value means that upon oxidation of poly(I) one electron is withdrawn from four repeat units in the backbone of the polymer upon scanning to +1.0 V vs. Ag+/Ag.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. 

Because of the ease of chemical modification of the thiophene rings, especially at the a- and fl-positions, these materials acquire good solution-processability and mechanical drawability [10]. This versatility allowed the wide investigation of their structure-property relationships. For example, spectroscopic studies related to thermochromism and solvatochromism [11] revealed that the electronic properties of the polythiophenes are strongly coupled to the backbone conformation. Moreover, polarized IR studies of drawn polythiophene films showed that the charged carrier motion is highly confined along the backbone [12]. 

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