Alkyl-substituted thiophenes

The him morphology of electrochemically prepared polythiophene has been shown in numerous studies to be almost identical to that commonly observed for polypyrrole (described in Chapter 2). A nodular surface is observed for both unsubstituted and 3-alkyl substituted thiophenes.92 As with PPy, the electrochemical preparation of PTh at higher current densities produced rougher surface morphologies. The similarity in morphologies suggest a similar growth mechanism for electrochemically polymerized PPy and PTh. 

Fig. 25.17 Schematic of three types of regioisomers for the 3-alkyl-substituted thiophenes, (a) Head-to-tail (b) head-to-head (c) tail-to-tail.

Table 5. Summary of the literature reporting on the cubic susceptibility of polythiophene (PT), substituted polythiophenes and thiophene oligomers (nT). 3 alkyl-substituted thiophenes are labelled by P3C T where n indicates the alkyl chain length and PSCiaOMeT indicates an alkyl-methoxy derivative.

Thiophene 1,1-dioxide (61) is too unstable to isolate and dimerizes with loss of S02 to give 3a, 7a-dihydrobenzothiophene 1,1-dioxide (172) in 34%113. However, alkyl-substituted thiophene 1,1-dioxides can serve as dienes in the Diels-Alder reaction, since the aromatic properties of the thiophene nucleus are lost completely and the n-electrons of the sulfur atom are used for forming the bond with oxygen. The sulfones 173-178 are found to react with two moles of maleic anhydride at elevated temperature to give bicyclic anhydrides114. Thus, at high reaction temperature, S02 is split off to give cyclohexadiene... 

The rest of the sulfur in oil sand asphaltenes, which is not in the form of thiane and thiolane, is present as thiophenes. Indeed, the asphaltene pyrolysis oil contains homologous series of a and a,a n-alkyl substituted thiophenes, 2-, 4- and 2,4-... 

Among the most abundant components of the cysteine + ribose + lecithin reaction mixture were the 2-pentylpyridine and two long-chain alkyl substituted thiophenes 2-pentyl- and 2-hexylthiophenes [31,32], together with a related compound with molecular formula and... 

In this section, the structural studies of polythiophenes are reviewed the influence of structure on properties is considered in subsequent sections. The description of polythiophene structures include the alkyl-substituted thiophenes, such as poly(3-alkylthiophenes) (P3ATh s). 

As with polyanilines, polythiophenes can either be prepared directly by electropolymerization, or by casting from solutions (for alkyl-substituted thiophenes). Most interest has focused on the latter because of their improved mechanical properties compared with those of electrochemically prepared films. The factors influencing the mechanical properties of PTh s are reviewed in this section. 

Crude oil contains a number of organosulfur compounds (RS), including thiols, sulfides, thiophenes and alkyl-substituted thiophenes 18],... 

The XPS Cls and S2p core-level lineshapes for the thiophene oligomers, thiophene polymers and alkyl-substituted thiophene polymers are well documented in the literature [117,123—126]. Table 3.4 summarizes the Cl s, S2p BE values, their linewidths (full width at half maximum, FWHM) and the S/C atomic ratios for several thiophene oligomers and polymers [117]. In general, the a and p carbons in thiophene polymers with a BE separation of only 0.34 eV cannot be resolved from the main Cls component. Two other minor Cls components located at above 2.6 eV and 5.2 eV above the main peak have been assigned to the shake-up structures. The S2p core-level spectrum is curve-fitted with two components with BEs at about 163.8 and 165.5 eV, corresponding to the respective spin-orbit split doublets (S2p3/2 and S2pi/2). An n transition shake-up structure is also discernible at... 

In most cases, stability and oxidation potential issues mean that polythiophenes are prepared, studied, and used in non-aqueous media, and the influence of the solvent can be significant. The extent of solvent swelling is determined by polymer-solvent interactions, which can be manipulated both by choice of solvent and introduction of substituents, here in the 3- and 4-positions. Electropolymerization of alkyl-substituted thiophenes provides good examples of the effects one can observe. EQCM (with PBD) measurements of potentiodynamic deposition from acetonitrile benzonitrile (4 1) of poly(didodecylterthiophene) (with the substituents in the 3, 4 - or 3,3"-positions) showed that film deposition occurred primarily during the reductive half-cycle, in the form of short chain oligomers of the... 

Episulfide, thiophene, thiol-suhstituted thiophene and alkyl- and vinyl-substituted thiophenes were formed as the major products, which indicate that formation of cyclics is favonred during thermal degradation of polyethylene sulfide. The formation of thiophenes and alkyl-substituted thiophenes are due to the elimination of hydrogen sulfide cyclisation followed by a dehydrogenation reaction. The loss of hydrogen from 2,5-dihydrothiophene to form thiophene has been reported in the literature. Some of the product formations are illustrated in Equation 6.6. 

Various monoalkoxy substituted polythiophenes are the earliest examples reported in this category. These are followed by the 3-alkoxy-4-alkyl substituted thiophenes, introduced by Hoechst AG group, that result in conductivity enhancement as compared to 3-alkoxythiophenes." ... 

A decrease of the E° value in polythiophene-type polymers can be obtained only if the spacer is an electron-rich system. The electron-releasing effect of the alkyl group in the cyclopentadithiophene 21 (CPDT), coupled with the advantage, relative to alkyl-substituted thiophenes as monomers, of treating a planar system, and the jr-excessive nature of the pyrrole ring in 22 have been exploited [223,224]. In fact, the E° values of the polymers derived from both 21 and 22 are definitely lower than that of polythiophene, with the consequence that they should be much more reluctant to react with reducing agents and nucleophiles. 

When larger aromatic surfaces are involved, CH-n or van der Waals interactions controlled by alkyl chains may not be enough to prevent face-on deposition. In the two previous examples, archetypal aromatic structures such as porphyrins were shown to interact face-on with both metallic and hydrophobic surfaces. To obtain surface-tuned assemblies like those observed in alkyl substituted thiophenes with large aromatic structures, a much stronger interaction must be developed between the periphery of the porphyrin and the surface. The self-assembly of a cyano-functionalized porphyrin into extended ID structures on a KBr(OOl) surface, under ultra high vacuum, is one such example (Figure 38). ... 

Various thiolans have been prepared by ionic hydrogenation of thiophens with EtjSiH in the presence of acids. Addition of singlet oxygen to alkyl-substituted thiophens, followed by reduction with di-imine, gave the bicyclic compounds (42) these are examples of the hardly known thia-ozonides. 9-Thia-noradamantane (43) has been synthesized from cyclo-octa-2,7-dienone by a reaction sequence involving a transannular C—H carbene insertion. A bicyclic product (44) has been obtained by the reaction of (—)-carvone with... 

In other work on the concentrated H2S04-Zn system [129, 130], mono- and disubstituted thiophenes gave 2,5-dihydrothiophenes and thiophanes (2,3,4,5-tetrahydrothiophenes) with a significant preference for the former. The optimized conditions (thiophene 89% sulfuric acid zinc = 1 30 110 in dichloromethane at 20°C) converted alkyl-substituted thiophenes into a mixture of 2,5-dihydrothiophenes and tetrahydrothiophenes with the former predominating. For 2-formyl- and 2-acetylthiophenes, under slightly modified conditions, 2,5-dihydro-... 

Scheme 115 Birch reduction of alkyl-substituted thiophenes [134]...

---