Unusual Zig-Zag Effect in the Electrochemical Oxidation of Phenyl End-Capped α-Oligothiophenes

A series of phenyl end-capped α-oligothiophenes containing four to seven thiophene subunits (4T–7T) was synthesized utilizing palladium-catalyzed cross-coupling reactions. UV/Vis spectroscopic analysis revealed one broad absorption band that shifts bathochromically with increasing number units. Structured emission spectra are observed Stokes shift ν~4000 cm−1 and quantum yields up 53%. End-capping the oligothiophene molecules by units does not only extend effective conjugation but also prevents from α–α-homocoupling upon electrochemical oxidation. Accordingly, reversible redox waves in cyclic voltammetry one-electron processes for two longer congeners. Analyses first oxidation framework multiredox systems provide insight into stabilization or destabilization polaronic bipolaronic states. An unusual zig-zag trend (and a lesser second) process could be explained sterical encumbrance solubilizing hexyl chains 5T 7T which counteract formation fully planar quinoidal backbone.