Other alkyl-substituted polythiophenes

Order-disorder, or rod-to-coil , transitions in dilute solution have been reported for polydiacetylenes (2, 5-11), polysilylenes (12-15), and alkyl-substituted polythiophenes (16). The interpretation of the experimental observations has been the subject of considerable controversy with respect to whether the observations represent a single-polymer-molecule phenomenon or a many-chain aggregation or precipitation process (3-16). Our own experimental evidence (12, 13) and that of others (5-8, 10, 16) weigh heavily in favor of the single-chain interpretation. In our theoretical interpretation, we will assume that the order-disorder transitions observed in dilute pol-ysilylene solutions represent equilibrium, single-chain phenomena. 

The phenomenon is found with many, but not all substituted polythiophenes. Studies of 3,3 -di-alkyl-substituted polythiophenes revealed no temperature-induced colour change [17]. But many other structures will give thermochromism and solvatochromism. Polythiophenes substituted both with alkoxy and alkyl side chains may show it both purely amorphous and partially crystalline polythiophenes show it short and long-chain polymers may show it both regular and irregular polymers show it (see Figure 15.1),... 

The origin of the other two modes for these polymers appears to be associated with the intrinsic modes of the neutral polymers with one of the following possibilities (1) these modes are raised very closely to the intrinsic modes (2) they are identical to the intrinsic modes. In other words, such modes would be both IR and Raman active [40,159]. Considering that partially oxidized P3HT shows the doping-associated IR characteristics to be well related to those of P3MT (see Section 2.1), these features would be common to 3-alkyl substituted polythiophenes. Introduction of the alkyl substituents does not affect the electronic structure and properties of the polythiophene backbone so much. 

Despite their low molecular weights, these unsubstituted polythiophenes are insoluble in THF and other common organic solvents, and are also infusible. Their poor processability has therefore led to extensive studies of alkyl- and alkoxy-sub-stituted polythiophenes in the hope of enhancing solubility in organic solvents and allowing melt processing. Synthetic approaches to these substituted polythiophenes are described in the following text. 

FET applications of thiophene-based materials have a two-decade history. Pioneering work was conducted by Koezuka and co-workers [33], The device utilized electrochemically synthesized polythiophene as the channel layer, its mobility being 10 cm V s . This work was followed by Garnier and co-workers [13], who used sexithiophene as the channel layer and recorded an enhanced mobility of 10 cm s . Akimichi et al. [14] made FET devices using alkyl-capped oligothiophenes and mentioned that the alkyl substitution not only improves the chemical stability of the oligothiophenes but also enhances the mobility. A similar substitution effect was also observed with other oligomers [34, 35]. 

The bandgap of PEDOT ( g = 1.6-1.7 eV)itselfis 0.5 eV lower than polythiophene, which results in an absorbance maximiun in the NIR region. Compared to other substituted polythiophenes, these materials exhibit excellent stability in the doped state which is associated with high conductivity. Doped PEDOT is almost transparent in the visible region (with a sky-blue tint) and the neutral polymer is deep blue. Because PEDOT and its alkyl derivatives are cathodically-coloring electrochromic materials, they are suitable for use with anodically-coloring conducting polymers in the construction of dual polymer ECDs (63). 

Polythiophenes with substituents other than alkyl groups at the 3 position have been prepared by the polymerization of substituted monomers. Many of these polymers have been substituted alkylthiophenes (8) where example side chains are (R =) —(86—89), —OCH (68), —NHC(0) (CH2) qCH (6 )) —0502(0112)30112 (90). Ohiral side chains have also been employed (91,92). Poly(3-alkoxythiophenes) (9) (93—95) and... 

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