Cyclic voltammetry and differential cyclic voltabsorptometry of soluble oligothiophenes: Evidence for a four-fold charged pi-dimer in duodecithiophene

Cyclic voltammetry (CV) and differential cyclic voltabsorptometry (DCVA) measurements have been carried out on solutions of alkyl-substituted oligothiophenes (nT, where n is the number of thiophene rings) with n = 1, 2, 3, 4, 6, 8, and 12. For 4 less than or equal to n less than or equal to 8, the CV curves present two oxidation waves, which are attributed to the successive formation of the radical cation and the dication. The DCVA curve of 6T shows similar behavior. In the case of 12T, the CV curve also presents two irreversible oxidation peaks, whereas only one peak is obtained with DCVA. This last behavior is consistent with previously reported measurements on chemically oxidized 12T. When plotted as a function of the reversed number of rings, the first and second oxidation peak potentials align along two straight lines that tend to merge at n --> infinity. The single oxidation potential of 12T, estimated from DCVA, aligns with the first oxidation potential of shorter oligothiophenes. By combining these results with previously published optical and ESR measurements on chemically oxidized 6T and 12T, we suggest a model where oxidized 12T molecules condense into four-fold charged pi-dimers.