Chromism, polythiophene

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. 

Functionalised conjugated polymers such as polythiophenes were studied from the point of view of the detection and transduction of chemical and physical information into an optical or electrical signal. Their ionochromism (reversible change of colour in the presence of ions), photochromism (reversible change of colour on exposure to light), affinity chromism (tendency to colour change) and electroluminescence of polythiophene complexes with crown ethers and other solutions are discussed in detail [295]. 

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. 

In addition to the polythiophenes, polydiacetylene and polysilane systems, for example, have also proven to be partieularly interesting from the point of view of ehromism [88-93]. Detailed studies of polydiacetylene derivatives [94] indicate that a single chain mechanism for the rod (ordered)-eoil(disordered) transition is the major cause of chromism. 

From all these examples, it is firmly believed that chromism in neutral substituted polythiophenes, polydiacetylenes, and polysilanes can be a powerful tool to... 

These chromisms were first recognized for a series of polydiacetylene [39] and polysilane [40] compounds. In these systems, extent of the disorder can be mea-smed by deviation from the fully-stretched aM-trans backbone conformation [41]. In the case of the poly thiophene and its derivatives, the conformational disorder is caused by the distortion around the cr-bonding interconnecting the thiophene rings [42]. The TT-delocalization along the polythiophene backbone will be maximized when the polymer chains assume the fully-stretched S-anti form. This delocalization will be hampered by the ring distortion of any amount. 

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