Polyalkylthiophenes

We consider the case of polyhexyl and polyoctyl thiophenes (PHTh 

Samples were studied both in the solid and solution phases solid samples were prepared as KBr pellets (PHTh) or as films from melt (Ph08) or cast from solution (PHTh and POTh). PHTh and POTh were dissolved in chloroform which is a good solvent for these substances and does not show solvatochromic effects. Raman spectra were recorded either with an Ar laser at Xexc =514 nm ( with powers 30-200 mW) or a with a NdYAG laser at 1.06 (with power 100 mW). 

The main object of such analysis is to use the vibrational IR and Raman spectra of the pristine polymers to understand the molecular structure of the systems and their evolution when the material is dissoved in chloroform or when the solid material is heated. It has been shown that the class of polyalkylthiophenes show reversible thermochromism [155] whose origin at the molecular level needed an explanation. 

The problem we have to focus at is the understanding of the molecular flexibility of these systems conformational flexibility may occur topologically in two regions of the molecule, namely the heteroatomatic backbone can rotate around the inter-ring C(a)-C(a ) bonds and the long alkyl chain may take up either an all trans-structure or collapse into a liquid like conformationally distorted worm-like chain characterized by a statitical distribution [156] of gauche (G and G ) and trans (T) conformations. The fact that these two flexibilities were independent or correlated needed investigation. 

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Conjugated polymers doped with C60 become p-type semiconductors [305,306] some LB films of two polyalkylthiophenes mixed with arachidic acid and doped with C60 have been prepared [307]. The films of polyalkylthiophene + arachidic acid -l- C60 (spread from mixtures of 1.0 0.33 0.1 ratio) on ITO glass had a well-defined layer structure, as confirmed by x-ray diffraction. The bilayer distance obtained from the Bragg equation was 5.6 nm, the same as for arachidic acid LB films. Since the films were spread on subphases containing... 

G. Zerbi, B. Chierichetti, and O. Inganas, Thermochromism in polyalkylthiophenes molecular aspects from vibrational spectroscopy, J. Chem. Phys., 94 4646-4658, 1991. 

A convenient direct route has recently been described for obtaining regioregular polyalkylthiophenes using a tandem iridium-catalyzed borylation to produce the monomer, and a palladium-mediated coupling to produce the polymer [68]. The treatment of substituted thiophenes with B2pin2 in the presence of [lrCl2(COD)]2/ 4,4 -di-tert-butyl-2,2 -bipyridine (DTBPY) provided the expected monomer in 97% yield (Scheme 7.35). 

The polymers 563 to 565 appear highly fluorescent. The quantum yields of the solution polymers decrease slightly with increasing alkyl chain length, which is in contrast to other reported observation for polyalkylthiophenes. 

One of the reasons that polyalkylthiophenes have been investigated is the limited solubilities of unsubstituted thiophenes. The synthetic methods used for the preparation of both types of compounds are very similar. For example, both the parent thiophene and alkylthiophenes can be polymerized by reaction with FeCh in chloroform. Other synthetic approaches involve the reaction of dihalogen precursors in the presence of magnesium, zinc or nickel. Some examples of these approaches are shown in Figure 4.22. In all of these methods, the monomers are linked through the C2 and C5 carbon atoms rather than via 2,4 linkages. 

Figure 4.22 Examples of various chemical polymerization methods for preparing polyalkylthiophenes...

As with all supramolecular structures, one of the most important issues is whether a direct relationship between the structure of a material and its function or properties can be established. In the following, some examples of polymer systems which show such a correlation will be discussed. The materials addressed will include block copolymers, polyalkylthiophenes and a multilayer system based on the self-assembly of polyelectrolytes. Detailed studies on the electrochemical properties of redox-active polymers, based on poly(vinyl pyridine) modified with pendent osmium polypyridyl moieties, have shown that electrochemical, neutron reflectivity and electrochemical quartz crystal microbalance measurements can yield detailed information about the structural aspects of thin layers of these materials. 

Obviously, the data pertaining to ordered small molecules stand above that curve, as does the mobility measured by the Bell Labs [111] and Cambridge [112] groups on a regioregular polyalkylthiophene. In a subsequent paper [16], the Philips group extended its model to account for structure ordering. This model will be presented in Section 14.6.1.2. 

Electrochemically prepared alkylated polythiophenes have been investigated by Gamier and coworkers.90 91 When comparing polythiophene and monosubstituted polyalkylthiophenes, these workers found an increase in crystallinity of the substituted thiophenes in comparison to the unsubstituted poly thiophene. The degree of crystallinity was low (5%), but the crystal structure was assigned to a hexagonal cell... 

Polythiophenes containing electron withdrawing alkyl carboxylates (35k-351) have also been synthesized.[212] OTFTs fabricated with the regioregular polythiophene (35k) gave mobilities of up to 0.07 cm V s compared to 0.0063 cm V s for the regiorandom polythiophene (351).[212] Another electron-with-drawing moiety, tetrafluorobenzene, has been substituted on the polyalkylthiophene backbone (35m). Spin-cast films of 35m from 0.5 wt % chloroform solution showed a mobility of up to 2 x 10 cm V s. [213]... 

Kiriy, N. et al.. One-dimensional aggregation of regioregular polyalkylthiophenes. Nano. Lett. 3, 707-712, 2003. 

Aasmundtveit, K.E. et al.. Structural studies of polyalkylthiophenes with alternating side chain positioning, Synth. Met. 113, 7-18, 2000. 

Camaioni, N. et al., Polyalkylthiophene-based photodetector devices Effect of side-chain length on the device performance. Mater. Sci. Eng. C-Biomimetic Supramolec. Syst. 15, 261-263, 2001. 

Chen, T.A. and Rieke, R.D., Polyalkylthiophenes with the smallest bandgap and the highest intrinsic conductivity, Synth. Met. 60, 175-177, 1993. 

In polymer science literature, chains with pure HT-HT coupling are sometimes called isoregic. One can also imagine the synthesis of purely HH-TT coupled polyalkylthiophenes, i.e. syndioregic polymers. Such polymers have been obtained from 4,4 -dialkyl-2,2 -bithiophenes by Zagorska and Krische [13] and from 3,3 -dialkyl-2,2 -bithiophenes by Souto-Maior et al. [14]. Both 4,4 -dialkyl- 2,2 bithiopheiies ... 

Indeed both anodic and FeCls oxidation of symmetrically disubstituted 2,2 -bithiophenes result in highly regioregular polyalkylthiophenes with HH-TT coupling pattern. 

Polyalkylthiophenes for their great solubility and processability. These properties arc generally very poor for other conducting polymeis. 

Polyalkylthiophenes exhibit interesting physico-chemical properties for the fabrication of conducting polymer/insulating polymer composites. Their solubility in the reduced state in solvents such as chloroform and THF as well as the existence of a melting point around 180° make them easily processable. 

Polyaniline exhibits properties which supplement those of polypyrrole and polyalkylthiophenes. It represents an adequate candidate for radar absorption purposes. The most significant properties of this conducting polymer... 

In LB-films of poly alkylthiophenes (chainlength 7, 10, and 11) mixed with arachidic acid, a phase separation exists, which is more pronounced for small amounts of thiophene. No preferential orientation of the polyalkylthiophene domains could be observed by polarized UV-Vis spectra [94]. Also poly(3-hexade-... 

The introduction of defects like /i-links alters the electronic structure substantially, in particular concerning the 7t-orbitals because of the limited delocalization. The 7t-bands in the MNDO simulated UPS spectrum of OL-ifia-i, e.g, is very similar to adding the spectra of two trimers and a monomer [123,124,133,134]. This is also underlined by studies of the thermochromic effects in polyalkylthiophene which can be explained by conformational changes which lead to segments with shorter conjugation length [91,129]. 

E. Lacaze, K. Uvdal, P. Bodo, J. Garbarz, W.R. Salaneck, and M. Schott, Scanning turmeUng microscopy of single polyalkylthiophene molecules adsorbed on graphite. J. Polym. Sci., Part B Polym. Phys., 31, 111 (1993). 

Figure 4.14 Fluorescence confocal image of (a) P3DDT + PEO fibers (b) SEM image of a sample of fibers (c) fluorescence confocal image of P3DDT fibers (PEO was removed by washing with acetonitrile) (d) SEM image from the same sample of fibers. (Reprinted with permission from Synthetic Metals, Electrospun polyalkylthiophene/polyethyleneoxide fibers Optical characterization by A. Bianco, C. Bertarelli, S. Frisk et al., 157, 276-281. Copyright (2007) Elsevier Ltd)...

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