Polythiophenes, properties thermochromism

Polythiophenes are not only interesting because of their electrical properties. On oxidation (p-doping), the electrical conductivity can be enhanced from about 10 S.cm (pristine material) up to several thousand S.cm in the oxidized form. This layers of polythiophenes are also of interest due to their large number of special electrophysical properties Thermochromism, electrochromism, solvatochromism, ionochromism, color changes under pressure and effected by electricity. 

Polythiophene [78] is a promising material for certain future electronic applications, due to its relatively high stability and processability in the substituted form [79-81]. Upon substitution, with e.g. alkyl side-chains [79, 80], polythiophene exhibit properties such as solvalochromism [82] and thermochromism [83]. Presently, a large variety of substituted polythiophenes with various band gaps exists (for example see Ref. [81 ]). 

Regioregular poly(3-alkylthiophene)s have received a lot of attention, especially because of their high electrical conductivities in the doped state, and because of their unusual solvatochromic and thermochromic behavior . Hence, a lot of research has been focused on clarifying the structure of these materials, both in the solid state and in solution. Today, it is agreed that supramolecular aggregation of polythiophene chains plays an important role in their physical properties. 

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... 

Roux, C., and M. Leclerc. 1994. Structure-property relationships in thermochromic polythiophene derivatives. Macromol Symp 87 (Polymers Progress in Chemistry and Physics) l-4. 

Roux, C., K. Faid, and M. Leclerc. 1993. Thermochromic properties of polythiophenes Cooperative effects. Makromol Chem Rapid Commun 14 (8) 461—464. 

M. Toba, Y. Takeoka and M. Rikukawa. Thermochromic and solvatochromic properties of polythiophene derivatives with liquid crystal moiety. Synth. Met. 135-136, 339-340 (2003). 

Because of the ease of chemical modification of the thiophene rings, especially at the a- and fl-positions, these materials acquire good solution-processability and mechanical drawability [10]. This versatility allowed the wide investigation of their structure-property relationships. For example, spectroscopic studies related to thermochromism and solvatochromism [11] revealed that the electronic properties of the polythiophenes are strongly coupled to the backbone conformation. Moreover, polarized IR studies of drawn polythiophene films showed that the charged carrier motion is highly confined along the backbone [12]. 

Roux, C. Leclerc, M. Thermochromic properties of polythiophene derivatives formation of localized and delocalized conformational defects. Chem. Mater. 1994, 5, 620-624. 

One breakthrough in applications of conjugated polymers was the development of soluble polymers, as in the case of polythiophene [75], by substitution with, for example, alkyl side chains [76,77], The polyalkylthio-phenes exhibit properties such as solvatochromism and thermochromism [78]. A large variety of substituted polythiophenes with various band gaps exist [79]. 

C. Roux, K. Paid, and M. Leclerc, Thermochromic properties of polythiophenes cooperative effects, Makromol. Chem., Rapid Commim. 14 461 (1993). 

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