Effect of Polymerization on Hierarchical Self-Assembly into Nanosheets

The oligomers consisting of phenyl-capped bithiophene and tetra­(ethylene glycol)­s linked by azide–alkyne Huisgen cycloaddition were synthesized. The relationship between the degree of polymerization and self-assembling ability was investigated in <i>o</i>-dichlorobenzene and dimethyl sulfoxide. From the absorption spectrum, it was confirmed that the critical degree of polymerization (CDP) for thiophene unit aggregation was 4. The morphology of the aggregated product was observed by atomic force microscopy. The oligomers <b>4mer</b> and <b>5mer</b> could not self-assemble into well-defined structures due to the weak driving force for the self-assembly. In the cases of <b>6mer</b> and <b>7mer</b>, aggregates with nonwell-defined and nanosheet structures coexisted. In the cases of <b>8mer</b> and <b>9mer</b>, the nanosheet was the main product. The critical point between <b>7mer</b> and <b>8mer</b> could be confirmed by different aggregation behaviors in the cooling process of the solution (nonsigmoidal and sigmoidal). In the cases of <b>8mer</b> and <b>9mer</b>, polymer folding prior to intermolecular self-assembly, which was supported by sigmoidal aggregation behavior, leads to the nanosheet formation. On the contrary, shorter oligomers than <b>8mer</b> experience intermolecular aggregation prior to intramolecular polymer folding, which was supported by the nonsigmoidal aggregation behavior. This is the first report to prove the existence of CDP for folded polymer nanosheet formation which requires hierarchical self-assembly, i.e., polymer folding followed by intermolecular self-assembly