Thiophene ring poly

Synthetic principles for bandgap control in tt-conjugated systems of poly-(thiophene) and related systems with fused thiophene rings 97CRV173. 

Polyacetylene is considered to be the prototypical low band-gap polymer, but its potential uses in device applications have been hampered by its sensitivity to both oxygen and moisture in its pristine and doped states. Poly(thienylene vinylene) 2 has been extensively studied because it shares many of the useful attributes of polyacetylene but shows considerably improved environmental stability. The low band gap of PTV and its derivatives lends itself to potential applications in both its pristine and highly conductive doped state. Furthermore, the vinylene spacers between thiophene units allow substitution on the thiophene ring without disrupting the conjugation along the polymer backbone. 

Thin polymer films have been prepared by surface catalysis in ultrahigh vacuum and electrochemical deposition from solution. These two routes of synthesis result in poly(thiophene), poly(aniline) and poly(3,5-lutidine) films that have similar infrared spectra. These polymer films are highly orientationally ordered the rings are perpendicular to the surface in poly(thiophene) and poly(3,5-lutidine) films, and the phenyl rings are parallel to the surface in poly(aniline). 

The introduction of bridging groups on the thiophene ring modifies the physical and chemical properties of the polymers obtained. The energy of the optical absorption is reduced in FEDOT, Fig. 9.2(k), and poly(ijothia-naphthalene), (PITN) (Wudl et al., 1984), so that in the conductive state thin films are transparent. PEDOT shows high electrochemical stability in the oxidised state and, when combined with poly(styrenesulphonic acid) counter ions, can be processed from aqueous solution. 

Figure 22.6 Chemical structure of poly(3-alkyl-thiophene) P3AT with n as the number of thiophene rings and m defining the length of the alkyl-group.

An alternative, less sophisticated approach to regioregular substituted polythiophenes is the use of asymmetric 3,4-disubstituted thiophene monomers as substrates, resulting in the selective activation of either the 2- or 5- positions on the thiophene ring. Using this method, poly(3-alkoxy-4-methylthiophenes) with > 95% HT couplings have been prepared via FeCl3 oxidation.48 59... 

The switching properties are determined by the electrochemical conditions employed during growth, the counterion incorporated, and the substituents attached to the thiophene ring.146 For example, poly(3-heptoxyl-4-methylthiophene) 29 switches at 0.76 V, whereas under the same conditions, poly(3-octylthiophene) switches at 1.01 V. The electrolyte used for switching also has a marked effect on the potential, rate, and reversibility of the switching process.147... 

There were however few exceptions—the most important being polythiophene and poly(p-phenylene vinylene). It was demonstrated [4,5] that in the case of polythiophene, the substitution of hydrogen in the 3 position of the thiophene ring, by an alkyl group longer than the propyl group, renders the polymer soluble in common solvents and the decrease of conductivity in the doped state, caused by the functionalization is rather minor. Since the initial work of Elsenbaumer [4,5] hundreds of papers devoted to soluble polythiophene derivatives have been published. 

The first soluble doped polymer was the polyfp-phenylene. sulphide) with AsFs- in AsFs [44]. Even if a quite unusual solvent was needed, it was the first to show the possibility of having a solution of doped polymers. Later, the discovery that Poly-n-alkylthio-phene was soluble in a common organic solvent [45], provided a new route. Besides, the films show thermochromism properties that varj as a flinction of the chemical structure of the side groups [8]. The stability and the great variety of substituents that can be added at the thiophene ring ensure that many structural studies are still being carried out [46]. 

The attachment of long flexible side chains to the thiophene rings in poly thiophenes renders them soluble in common organic solvents. But the side-chain interaction may also lead to the phenomenon of thermal undoping of the doped polymer. In order to get over this, a benzene ring has been interposed between the thiophene and the linear alkyl chain as in (151). Polymerization of this was carried out with FeCl3 as the one-electron oxidant <92MM4297>. 

---