Thermochromism, polythiophene

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

Garreau, S., M. Leclerc, N. Errien, and G. Louam. 2003. Planar-to-nonplanar conformational transition in thermochromic polythiophenes A spectroscopic study. Macromolecules 36... 

N. Di Cesare, M. Belletete, G. Durocher, M. Leclerc, Towards a theoretical design of thermochromic polythiophenes, Chem. Phys. Lett, 275, 533-539 (1997). 

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 ]). 

Levesque I, Leclerc M (1996) Ionochromic and thermochromic phenomena in a regioregular polythiophene derivative bearing oligo(oxyethylene) side chains. Chem Mater 8 2843-2849... 

Molecular self-organization in solution depends critically on molecular structural features and on concentration. Molecular self-organization or aggregation in solution occurs at the critical saturation concentration when the solvency of the medium is reduced. This can be achieved by solvent evaporation, reduced temperature, addition of a nonsolvent, or a combination of all these factors. Solvato-chromism and thermochromism of conjugated polymers such as regioregular polythiophenes are two illustrative examples, respectively, of solubility and temperature effects [43-45]. It should therefore be possible to use these solution phenomena to pre-establish desirable molecular organization in the semiconductor materials before deposition. Our studies of the molecular self-assembly behavior of PQT-12, which leads to the preparation of structurally ordered semiconductor nanopartides [46], will be described. These PQT-12 nanopartides have consistently provided excellent FETcharacteristics for solution-processed OTFTs, irrespective of deposition methods. 

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-... 

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 reversible thermochromic effect observed here and in the solid state is associated with enhanced order of the alkyl side chains with decreasing temperature, facilitating coplanar conformers in the polythiophene backbones, with the attendant enhancement in the 7t-7t -transition of the thiophene ring electronic absorption spectra and improvement in electronic conduction of the doped film. 

The colour of ICPs can also be sensitive to temperature. Substituted polythiophenes, in particular, show a marked blue shift of the highest wavelength absorption band when films or solutions are heated [7-9]. These reversible colour changes have been attributed to a twisting of the polymer backbone to a less ordered non-planar conformation. Less dramatic thermochromic effects have also been reported for polyanilines [3,10,11] where circular dichroism studies of... 

This chapter is mainly devoted to organic compounds for which the observed reversible color changes (coloring and fading reactions) are due to the effect of temperature, exclusively. The thermochromic behavior of inorganics, organo-metallics, macromolecular systems (such as polythiophenes), or supramolecular systems (such as liquid crystals) and of molecular systems for which the observed thermochromism is due to external intervention (association with another species like a metal ion or a proton) or to modification of the medium by a thermal effect (thermosolvatochromism, for instance) are not reviewed in this chapter. 

Similar thermochromic and solvatochromic effects have been observed by Bidan and coworkers71 for the optically active polymer 25 prepared in a regiospecihc fashion (100% HT-HT coupling) by the McCullough method. In contrast, the analogous regiorandom polymer prepared by oxidation with FeCl3 in chloroform has only weak optical activity. However, the disubstituted chiral polythiophene 26, synthesized by... 

Thermochromism, the change of colour upon heating, is treated in detail in Chapter 3 of this Handbook. The substituted polythiophenes exhibit this interesting phenomenon, which is manifest in pronounced shifts of the optical absorption spectra, and thus of the band gap, upon temperature variations (Figure 2.17). The... 

Thermochromic behaviour with completely conjugated planar chains at low temperature (213 K) and with small conjugated chain segments, only a few rings in length, for a temperature of 463 K, is also reported for poly(3-hexylthiophene) [91]. Depending on the position of octyl-chains in poly(3-octylthiophene) co-planar and twisted polymer backbones were found [92]. Theoretical calculations predict a planar rod-like structure for polymethylthiophene [93]. The STM observations of side-chain substituted polythiophene were reported above [71]. 

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),... 

This correlation lends itself to a most direct rationalization. As we know that polythiophene without substituents does not show any thermochromism, it is plausible that the coiling of the side chains drives the torsion of the main chain. It is thus via the steric interactions—the steric hindrance—of side chains that the main chain is forced out of planarity. The effect of... 

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., 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). 

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