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 s even thiophene units from the 6-mer up to the 48-mer has been synthesized b y a combination of the random Eglinton coupling reaction of mono- and di-et hynyl-sexithiophenes and a subsequent sodium sulfide-induced cyclization re action of the resulting oligo(sexithienylene-diethynylene)s. Their structur es were well characterized by MALDI-TOF MS and H-1 NMR spectroscopy. The mo lecular weights of the oligothiophenes and oligo(sexithienylene-diethynylen e) s, measured by gel-permeation liquid chromatography using the polystyren e standard, are nearly double the actual ones, indicating that they keep hi ghly rigid rod-type shapes. According to a molecular model, the molecular l engths of the longest oligothiophene 48-mer and the longest oligo(sexithien ylene-diethynylene) reach approximately 18.6 nm and 25.0 nm, respectively. In the electronic absorption and emission spectra, the pi-pi* transitions o f the oligothiophenes demonstrate progressive red shifts with increasing ch ain length up to the 20-mer. In the cyclic voltammograms, furthermore, the first oxidation potentials tend to continue negative shifts up to the 34-me r. In accordance with these spectral changes, the doped conductivities stea dily increase and approach that of a structurally related polymer.