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 b,b'-unsubstituted analogs (Tn, n = 1-4) are described. Both series are end capped with Ph 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. Oxidn. potentials (EPA1 and EPA2) detd. 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 EPA1 and EPA2 vs. the reciprocal no. 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 EPA1 and EPA2 is reached already at relatively short chain lengths (~5 EDOT units) in contrast to the Tn series (> 10 thiophene units). The stepwise chem. oxidn. of the EDOTn and Tn oligomers in soln. was carried out to obtain radical cations and dications. The energies of the optical transitions of the radical cations and dications as detd. by UV/Vis/NIR spectroscopy were similar for the two series. These spectroscopic observations are consistent with quantum-chem. calcns. performed on the singly charged mols. Cooling solns. contg. T2.bul.+, T3.bul.+, EDOT2.bul.+, and EDOT3.bul.+ revealed the reversible formation of dimers, albeit with a some-what different tendency, expressed in the values for the dimerization enthalp