Alkyl-substituted polythiophenes

Thiophene polymers, in particular, alkyl-substituted polythiophenes (PTH), are some of the conducting polymers being most actively investigated at present. This fact is attributable to their high degree of processability, environmental stability [49, 50] and, in some cases, ability to exhibit reversible electrochrom-ism [51] and thermochromism [52]. Another important family of sulfur-... 

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. 

Besides the oligomers based on thiophene units, polymers with similar backbones have become very represented in literature since the late nineties. Due to the fact that pure thiophene polymer chains without any substitutes show very poor solubility, mainly alkyl substituted polythiophenes are used. 

Fig. 3.2. The four possible conformational triads of alkyl substituted polythiophene, where R is the alkyl functionalizing group [20]. The HT-HT conformation has been found to stack the best. These triads can be monitored using NMR spectroscopy to determine the degree of regioregularity in a sample.

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 theoretical simulations we have reviewed here are consistent with the changes in optical absorption observed experimentally when the degree of interchain order is improved in alkyl-substituted polythiophene derivatives [60,61]. The CMS spectrum of RR-P3HT exhibits a pronounced charge-induced... 

Figure 16.38 Chemical structures of various alkyl-substituted polythiophenes...

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. 

Absorption bands, 179 A.C. conductivity of polyacetylene, 29 Actuators, 137, 162, 458-459 Aged catalyst, 9 Alkoxypyrroles, 154 Alkyl-pyrroles, 154 A-alkyl-pyrroles, 154 Alkyl-substituted oligothiophenes, 328 Alkyl-substituted polyazines, 731 3-alkyl-substituted polythiophenes, 311 3-alkylthiophene, 272 Alkylthiophenes, 145, 154... 

Figure 1. Steric interactions in a head-to head, head-to-tail triad of an alkyl substituted polythiophene. Steric clashes between alkyl side chains and between side chains and the sulfur lone pairs are indicated.

Although the structure/property relationships have become more evident for alkyl substituted polythiophenes, the synthesis and characterization of new heterofunctionalized polythiophenes continues to be an active research area because there are a number of potential applications for new materials. There is still much to be learned about the structure/property relationships in these functionalized polymers. The effects of these side chains on the stability, solubility and self-assembly of these polymers as well as their effects on the electronic, optical and electrooptical properties are largely unknown. An improved understanding of the structure/property relationships will, in turn, drive the rational synthesis of new, functionalized polymers with optimized properties. 

Alkyl-substituted polythiophenes (see Fig. 26a) have increased solubihty in common solvents [156-158] and show thermo- and sol-vatochromism see the reviews by Inganas [159] and Gustafsson et al. [160]. The self-organized structures in bulk have been systematically studied by Prosa et al. [161] and Chen and Ni [162], demonstrating the formation of self-organized lamellar phases for... 

Work reported by Liu and Gregory [35] on alkyl-substituted polythiophenes synthesized with the addition of a urethane group on the /3-substituted alkyl chain shows excellent blending characteristics with hydrogen bonding polymers such as urethanes or polyamides such as nylon. These blends have significantly reduced percolation thresholds and are solution and melt spinnable. 

Fig. 38.6 Effect of substitution on the properties of (a) unsubstituted polythiophene (b) alkyl-substituted polythiophene (c) poly(3(4-octylphenyl)thiophene (d) poly(3,4-ethylenedioxythiophene).

THG measurements on alkyl-substituted polythiophenes performed in a range of energies going from 0.6 to 1.3 eV show an intense peak at 0.8 eV with a shoulder at about 1.15 eV [136]. The 1.15 eV peak cannot be accounted for in a THG spectrum... 

A very interesting and potentially useful thermochromic effect occurs in the (undoped) alkyl-substituted polythiophenes, for alkyl groups larger than methyl [35]. The thermochromic effect is a (reversible) thermally-induced change in color. In order to characterize the observed thermochromic behavior of the P3ATs, a series of studies of the temperature-dependence of the electronic structure of P3HT have been carried out using optical absorption spectroscopy, UPS and XPS. The central results, as well as a model of the thermochromism which is consistent with all of the experimental data, are reviewed below. 

---