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Supramolecular organization of conjugated polythiophenes studied by harmonic light scattering

Boek - Dissertatie

Crystallization is a very common process in nature and has important applications in many scientific domains. The crystal structure determines physical and chemical material properties. It is easy and well known how to characterize crystal structures. However to gain control over the crystals structure and the properties (i.e. crystal engineering), one must control the crystallization process. To do so, fundamental understanding of the underlying growth process is key.Harmonic light scattering, a recently developed nonlinear optical scattering technique that combines second- and third-harmonic light scattering, is sensitive to symmetry, concentration and size variations from molecular level up to molecular assemblies in situ, making it an ideal technique to investigate crystallization processes. In this thesis the supramolecular organization of conjugated polythiophenes has been studied. Polythiophene has been topic of many experimental studies for its interesting opto-electronic properties. First, the supramolecular organization of a linear regioregular poly(3-alkylthiophene) was explored. Supramolecular organization was induced by reducing the solvent quality upon addition of a non-solvent. The assembly process could be divided in three conformational states; solubilized isolated single chain, pre-ordered polymer-solvent cluster and a fully planarized and electronically coupled assembled structure. The intermediate state, a pre-ordered polymer-solvent cluster resembled the final helically octopolar assembled structure. Further, there were indications that the organization process may be entirely different when the solvent quality is reduced by lowering the solution temperature. In a second study, the influence of these two types of induced supramolecular organization by reducing the solvent quality i.e. for a solvent/non-solvent system and for a temperature regime was investigated. The polythiophene in this study was a star-shaped poly(3-alkythiophene). The supramolecular organization mechanism in the solvent/non-solvent system could be divided in four regions with two intermediate clustered states. First, isolated polymer chains were well dissolved. Next, the polymer chains organized in clusters but the star arms were still randomly coiled, followed by the partial planarization of the star arms. The final octopolar assembly was fully planarized resulting in proper electronic coupling between the chains in the assembly. The resemblance with the supramolecular organization of the linear poly(3-alkylthiophene) was remarkable. The mechanistic behavior might be generalized for all conjugated systems.In the temperature regime on the other hand, there was no evidence for the presence of intermediate clusters. The mechanistic behavior could be divided in two regions; isolated single chains and assembled polymer with a markedly different structure i.e. dipolar symmetry and much larger size. As a result of these findings, the influence of solvent choice on the supramolecular organization for a solvent/non-solvent system gained interest. The solvent had an important influence on the mechanistic behavior of the supramolecular organization.
Jaar van publicatie:2020