Steric effects on the reversible dimerization of short-chain oligothiophene cation radicals

The reversible dimerization of cation radicals of a series of end-capped 2,5ʹʹdihexylterthiophenes with the median thiophene ring either unsubstituted (1) or monoand di-substituted by methyl and hexyl groups (2¯5) has been investigated. Concentration-dependent cyclic voltammetry, UV-Vis-NIR spectroelectrochemistry and temperature-dependent ESR spectroscopy lead to consistent results showing that, whereas the cation radical of compound 1 already dimerizes at room temperature, substitution of the median thiophene ring leads to a dramatic decrease in the propensity of the corresponding cation radical to dimerize. These results provide conclusive evidence for a steric control of the reversible dimerization of short-chain oligothiophene cation radicals.