Polythiophene films, electrochemical polymerization

Polythiophene can be synthesized by electrochemical polymerization or chemical oxidation of the monomer. A large number of substituted polythiophenes have been prepared, with the properties of the polymer depending on the nature of the substituent group. Oligomers of polythiophene such as (a-sexithienyl thiophene) can be prepared by oxidative linking of smaller thiophene units (33). These oligomers can be sublimed in vacuum to create polymer thin films for use in organic-based transistors. 

Functionalized conducting monomers can be deposited on electrode surfaces aiming for covalent attachment or entrapment of sensor components. Electrically conductive polymers (qv), eg, polypyrrole, polyaniline [25233-30-17, and polythiophene/23 2JJ-J4-j5y, can be formed at the anode by electrochemical polymerization. For integration of bioselective compounds or redox polymers into conductive polymers, functionalization of conductive polymer films, whether before or after polymerization, is essential. In Figure 7, a schematic representation of an amperomethc biosensor where the enzyme is covalendy bound to a functionalized conductive polymer, eg, P-amino (polypyrrole) or poly[A/-(4-aminophenyl)-2,2 -dithienyl]pyrrole, is shown. Entrapment of ferrocene-modified GOD within polypyrrole is shown in Figure 7. 

Thiophene, pyrrole and their derivatives, in contrast to benzene, are easily oxidized electrochemically in common solvents and this has been a favourite route for their polymerization, because it allows in situ formation of thin films on electrode surfaces. Structure control in electrochemical polymerization is limited and the method is not well suited for preparing substantial amounts of polymer, so that there has been interest in chemical routes as an alternative. Most of the methods described above for synthesis of poly(p-phenylene) have been applied to synthesise polypyrrole and polythiophene, with varying success. 

Electrochemical deposition of polythiophene films from solution of mono- or bithiophenes is one of the commonly used methods and can also be applied to prepare oligothiophene films. The films are formed in situ during the polymerization process. Due to the oxidative polymerization only doped (oxidized) materials are obtained and appropriate counter-ions have to be added. This method can only be applied to conductive substrates. 

The most elegant approach to design polypyrrole, polyaniline or polythiophene-based porphyrin, phthalocyanine or Schiff base matrices involves the electrochemical polymerization of suitably designed substituted N4-macrocyclic monomers. We and others have shown that the electro-oxidative polymerization of such species (see significant examples in Figure 8.3) leads to the formation of films having the electrochemical properties of the monomeric complex" . ... 

Shi and co-workers prepared polythiophene films with different roughnesses by electrochemical polymerization of thiophene in boron trifluoride-diethyl etherate (BFEE) [42]. The highest WCA of 116° on polythiophene film was observed. To further increase the WCA, aligned polythiophene nano-tubes were synthesized using anodized aluminium oxide (AAO) as template. The WCA increased to 134° (Fig. 7). As a comparison, the WCA of polythiophene polymerized in acetonitrile solution was measured to be less than 75°. 

Nicolas et al. also synthesized semi-fluorinated polythiophenes (Scheme 4) [52, 53]. The monomers were chemically polymerized by oxidation with FeCls, or electrochemically polymerized in acetonitrile containing BU4NPF6 as the supporting electrolyte. The electrochemically synthesized films showed rough surfaces. The poly(fluorinated thiophene) films electropolymerized from the monomer with n = 8 and m = 2 showed a WCA of 153°, while the corresponding spin-coated films exhibited a much smaller WCA, due to their smooth surfaces. Their results indicated that the length of the fluorinated chain had weak influence on the surface property of the resulting film. 

Another interesting conjugated polymer is polythiophene, which has good environment stability compared to polyacetylenes (Fig. 49.12) (Table 49.7). Polythiophene has a value of 3 x 10 at a wavelength of 1.06 /Am. The resonantly enhanced value is about 2 orders of magnitude larger than the nonresonant value. Table 49.7 shows values of a polythiophene derivatives. Polythiophenes are colored materials [186-193]. The behavior of polythiophene has been studied in several forms. Electrochemically polymerized films of polythiophene results a value of 4 X 10 esu as measured by DFWM at 602 nm [194]. A monolayer prepared by the LB technique (thickness 22 A) yields a value of 10 esu at 602 nm as measured with femtosecond DWFM technique [195]. 

Since the discovery of a highly conductive polyacetylene film in 1977 [33], various conductive materials have been developed based on the polymerization of five-membered heteroaromatics represented by polythiophene (2). Polyselenophene (3) was also obtained by chemical polymerization [34-36] or electrochemical polymerization of selenophene (Scheme 6.2) [37-39]. The bandgap energy of polyselenophene... 

Polythiophene is readily produced by inserting a working electrode, counterelectrode, and reference electrode into a nonaqueous electrolyte in which 0.1 to 1.0 M thiophene is dissolved and then increasing the cell potential to greater than 1.6 V (versus SCE). Salts such as lithium or tetrabutylammonium perchlorate, hexafluo-rophosphate, or trifluoromethylsulfonate are typical electrolytes. Acetonitrile, ben-zonitrile, dichloromethane, and tetrahydrofuran are suitable solvents. As previously discussed for polypyrrole, polythiophene has been prepared in aqueous solutions [247]. A conductive, electroactive poly thiophene film was polymerized from a phosphoric acid-water-thiophene system using mild electrochemical polymerization conditions. 

The thiophene dimer and trimer have been electrochemically polymerized to grow poly thiophene films at lower potentials [248]. The dimer—2,2 -bithiophene—polymerizes at 1.2 V (versus SCE) to produce polythiophene with conductivities up to I S cm" [249]. The trimer— -terthiophene (2,2 5, 2"-terthienyl)—electrochemically polymerizes at 1.0 V (versus SCE) to yield films with conductivities up to 10" S cm" [201]. A mixture with sulfuric acid or use of tetrabutylammonium tetrafluoroborate as the supporting electrolyte in acetonitrile with the trimer produced no polymer, only gels or powder, respectively [250]. 

Bolognesi et al. [344] electrochemically polymerized dithienobenzene [343] (Fig. 18) and dithieno[3,4- 3, 4 -if thiophene (Fig. 17b). X-ray diffraction analysis and molecular modeling calculations were reported for dithieno[3,4-f> 3, 4 -d]thiophene [344]. The monomer polymerizes at 1.04 V (versus SCE) to yield stable films with conductivities around 1 S cm". The observed band gap was 1.1 eV, which is smaller than the band gap of polythiophene. 

Polythiophene has been prepared first by electrochemical polymerization. Since a film is produced on the anode during polymerization, this method is suitable for the preparation of polymers such as polythiophene and poly(3-methylthiophene), which is not processable after polymers are formed. However, in electrochemical polymerization, the yield of polymers is low and the polymers often do not have a well-... 

A polymeric film is obtained by electrochemical polymerization, this is a very useful method for preparing polymers such as polythiophene, poIy(3-methylthiophene) and poly(3-phenylthiophene), which are insoluble and infusible. When these polymers are obtained in the form of powder we cannot process them into a film or other useful forms. Although electrochemical polymerization is an easy method for preparing polypyrrole films, the films of polythiophene are obtained under limited electrochemical conditions. 

V(bithiophene) 1.14 (26) [46]. Electrochemical polymerization of (25) and (26) was possible under conditions close to those for thiophene and (25) and (26) was possible under conditions close to those for thiophene and bithiophene, respectively. Trimethylsilyl groups permitted specific activation of the 2 and 5 positions with respect to the electrochemical oxidation and allowed the preparation of polythiophene films, starting from oligomers, which gave rise to low selectivity without silyl substituents, or monomers with electron-withdrawing substituents. Masuda et al. elec-trochemically polymerized (25), (27) and (28) [47]. Elemental analysis of the resulting films indicated that almost all silicon atoms were eliminated during polymerization. IR spectra, absorption spectra and cyclic voltammometry indicated that the films were made up from polythiophene. Roncali et al. electro-chemically oxidized (29) and obtained a polythiophene film [48]. This particular structure of (29) allows electrochemical preparation to be performed at a very low substrate concentration while the reduction of the silicon to monomer ratio to its minimum value would lead to further improvements in the quality of the polythiophene film. 

---