Polythiophenes regioregular substitution

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... 

Fig. 122. Significant lowering of the bandgap is achieved in regioregular substituted polythiophenes.

M.R. Andersson, W. Mammo, T. Olinga, M. Svensson, M. Theander, andO. Inganas, Synthesis of regioregular phenyl substituted polythiophenes with FeCl3, Synth. Met., 101 11-12, 1999. 

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]... 

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 synthesis of substituted polythiophenes for investigations of thermochromism has brought many examples of how the regioregular side chain substitution is central to the chromic phenomena. Some of these materials show two-phase behaviour with an isobestic point in the sequence of the spectra. As the optical absorption only reflects local properties, we have to understand what mechanism it is that will always keep the material in just two states, with no intermediate states. As the torsion of the main chain is the cause of the chromic behaviour, we have to understand more specifically what is the essential physics of a cooperative phase transition which takes the chain, or parts of the polymer chain, from one phase to the other. In many ways this might look like the falling of a row of dominoes, upon one fluctuation of one of the dominoes. In our case this would be... 

Levesque, I., and M. Leclerc. 1995. lonochromic effects in regioregular ether-substituted polythiophenes. 7 Chem Soc Chem Commun 22 2293-2294. 

Hong, X.M., and D.M. Collard. 2000. Liquid crystalline regioregular semifluoroalkyl-substituted polythiophenes. Macromolecules 33 (19) 6916-6917. 

It was also observed that conjugated polymers that are also electrical conductors (see Chap. 10) exhibit optical activity that depends critically on their structural organization [78]. Thus, strong chiroptical properties can be obtained firom substituted polythiophene [79] (Chap. 10) with optically active side chains, especially when the monomers are coupled within the polymer in a regioregular head-to-tail fashion. Actually, optical activity of these materials is only found when the polymers are aggregated at low temperature, in poor solvent, or in solution cast films. This contrasts with other optically active polymers, like polypeptides, poly(l-alkynes) and polyisocyanates that show an optically active conformation of the main chain in the absence of supramolecular association. 

G. Gotz, S. Scheib, R. Klose, J. Heinze and P. Bauerle, Synthesis and properties of a series of regioregularly amino-substituted oligo- and polythiophenes, Adv. Funct. Mater, 12, 723-728 (2002). 

A third, non-catalytic approach to regioregular polythiophenes relies on the discovery by Andersson and co-workers that the oxidation of some monomers can be controlled giving high coupling selectivities. For example, phenyl-substituted thiophene monomers can be regiospecifically polymerized by a slow addition of FeCH that maintains a low concentration of radical throughout the reaction. Yields approaching 80 %... 

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