Synthesis and properties of a series of the longest oligothiophenes up to the 48-mer

A series of the longest class of oligothiophenes extended at intervals of seven thiophene units from the 6-mer up to the 48-mer has been synthesized by a combination of the random Eglinton coupling reaction of mono- and di-ethynyl-sexithiophenes and a subsequent sodium sulfide-induced cyclization reaction of the resulting oligo(sexithienylene-diethynylene)s. Their structures were well characterized by MALDI-TOF MS and ¹H NMR spectroscopy. The molecular weights of the oligothiophenes and oligo(sexithienylene-diethynylene)s, measured by gel-permeation liquid chromatography using the polystyrene standard, are nearly double the actual ones, indicating that they keep highly rigid rod-type shapes. According to a molecular model, the molecular lengths of the longest oligothiophene 48-mer and the longest oligo(sexithienylene-diethynylene) reach approximately 18.6 nm and 25.0 nm, respectively. In the electronic absorption and emission spectra, the π-π* transitions of the oligothiophenes demonstrate progressive red shifts with increasing chain length up to the 20-mer. In the cyclic voltammograms, furthermore, the first oxidation potentials tend to continue negative shifts up to the 34-mer. In accordance with these spectral changes, the doped conductivities steadily increase and approach that of a structurally related polymer.