A series of 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 fulfide-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 chromatograaphy using the polystyrene standard,are nearly double the actual ones,indicating that they keep highly rigi 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 #pi#-#pi#~* 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.
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