Solvatochromism polythiophenes

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),... 

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 rationale for preparing this hybrid copolymer was to combine the desirable properties of polyaniline with those of polythiophene. For example, polythiophene has demonstrated thermo- and electrochromism, solvatochromism, luminescence, and photoconductivity while polyaniline has demonstrated reversible protonic dupability, excellent redox re-cyclability, and chemical stability. 

Poly thiophene [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 solvatochromism [82] and thermochromism [83]. Presently, a large variety of substituted polythiophenes with various band gaps exists (for example see Ref. [81]). 

Another interesting water-soluble polythiophene is 23 (L-POWT), bearing a chiral amino acid side chain, which was prepared by Inganas and coworkers69 by oxidative polymerization with FeCl3 (Figure 6.6). The chiroptical and solvatochromic properties of this optically active polymer are described later. 

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... 

Sandstedt, C.A., Rieke, R.D., and Eckhardt, C.J., Solid-state and solvatochromic spectra of a highly regular polythiophene, Chem. Mater. 7, 1057-1059, 1995. 

While in solution only a broad band centered at 385 nm is observed, thin films of these materials are distinctly yellow with a sharp absorption at Amax of 435 nm. The absorption behavior is mostly independent of the alkyl substituent. To understand the nature of the transition from solution to the solid state, the solvatochromic behavior of PPEs was examined. Solvatochromic behavior in conjugated polymers has long been known and has been documented by Rughooputh and Wudl"° for polythiophenes. In the case of the dialkyl-PPEs 3 addition of a nonsolvent (methanol) to a chloroform solution of 3 leads to the development of a new and very sharp band centered at 435 nm, which corresponds to the absorption observed in the solid state. In addition, the transition observed in chloroform at 385 nm is shifted to 400 nm and vibronic structure develops. 

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]. 

Typical examples are solvatochromism and thermochromism. The former chromism is observed upon dissolution of the polymers in solvent. This effect is caused by the localization of the electronic wave function as a result of the disorder introduced by the coiled (random) conformation [48]. These chromisms depend largely upon solvent quality. Choice of suitable side-groups on the polythiophene backbone brings about interesting chromism in the solid form as well. Copolymerization supplies further variations in the arrangement of the side-groups on the backbone and, hence, produees specific effects. 

Soluble polythiophenes, 317 Soluble precursor route, 295 Solvatochromism, 329, 593 Solvent and solution pH, 431 Spectroelectrochemistry, 834 Spectroscopic methods, 535 Spin density, 837 Spiro oligothiophenes, 635 Structural analysis, 3 Structural nature, 2 Structure of polytoluidines, 575 Substituted polyanilines, 532, 619 Substituted pyrroles, 442 Sulfonates, 833... 

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. 

Many experimental results have suggested that the conformational modification of the polythiophene backbone can be induced through order-disorder transitions of the side-chains [59]. It was then postulated that various external stimuli could pertuib the side-chain organization and consequently induce some chromic effects. These side-chain transitions can be induced by heating (thermochromism), varying the solvent quality (solvatochromism), ion complexation (ionochromism), photo-induced isomerization (photochromism) and affinity binding (affinity or biochromism) giving rise to a novel class of field responsive materials. 

Ingan, O. Thermochromism and Solvatochromism in Substituted Polythiophenes In Handbook of Conductive Molecules and Polymers Nalwa, H. S., Ed. John Wiley and Sons New York, 1997 Vol. 3, Chapter 15. 

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]. 

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