Aggregation Phenomena of Long α‐ and α,ω‐Substituted Oligothiophenes – the Effect of Branched vs. Linear End‐Groups

Abstract

AbstractFive homologous series of α‐ and α,ω‐substituted oligothiophenes up to an undecamer with branched and linear alkyl chains, respectively, are presented. These series are compared with respect to self‐organisation in solution and in solid state. UV/Vis absorption data for the long disubstituted branched oligothiophenes indicate the formation of aggregates in solution, expressed by a bathochromic shift of the absorption maximum and the appearance of shoulders. These changes in the spectra are obtained by variation of the solvent quality and/or the temperature. Contrary, the linear disubstituted oligothiophenes exhibit the formation of H‐aggregates in solution indicated by a hypsochromic shift in the UV/Vis spectra. Results from fluorescence spectroscopy support the existence of aggregates. No aggregation phenomena were found for the monoalkylated branched oligothiophenes. In contrast, H‐aggregation is found for the monoalkylated linear oligomers. This unexpected influence of the branching of the alkyl end groups on the aggregation behaviour in solution is discussed with the aid of mixed substituted oligothiophenes. Further information on shape and size of the aggregates present in solution is found by AFM measurements revealing that the linear substituents rather provoke the formation of anisotropic structures whereas the branched alkyl chains lead to spherical nanoobjects. Investigations of the thermal behaviour in solid state show that some of the oligomers exhibit thermotropic liquid crystalline phases. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)