Optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers

The optical and redox properties of a series of 3,4-ethylenedioxythiophene oligomers (EDOTn, n=1-4) and their β,β'-unsubstituted analogues (Tn, n=1-4) are described. Both series are end capped with phenyl groups to prevent irreversible a-coupling reactions during oxidative doping. Absorption and fluorescence spectra of both series reveal a significantly higher degree of intrachain conformational order in the EDOTn oligomers. Oxidation potentials (E_PA1 and E_PA2) determined by cyclic voltammetry reveal that those of EDOTn are significantly lower than the corresponding Tn oligomers as a consequence of the electron-donating 3,4-ethylenedioxy substitution. Linear fits of E_PA1 and E_PA2 versus the reciprocal number of double bonds reveal significantly steeper slopes for the EDOTn than for the Tn oligomers. This could indicate a more effective conjugation for the EDOTn series, confirmed by the fact that coalescence of E_PA1, and E_PA2 is reached already at relatively short chain lengths (≈ 5 EDOT units) in contrast to the Tn series (> 10 thiophene units). The stepwise chemical oxidation of the EDOTn and Tn oligomers in solution was carried out to obtain radical cations and dications. The energies of the optical transitions of the radical cations and dications as determined by UV/Vis/NIR spectroscopy were similar for the two series. These spectroscopic observations are consistent with quantum-chemical calculations performed on the singly charged molecules. Cooling solutions containing T2^·+, T3^·+, EDOT2^·+, and EDOT3^·+ revealed the reversible formation of dimers, albeit with a somewhat different tendency, expressed in the values for the dimerization enthalpy.