Electrochemical oxidation of 1,4-dialkoxybenzenes in organic media. Reversible one-electron process in a methylene chloride/trifluoroacetic acid mixture and electropolymerization reaction in dry acetonitrile

1,4-Dialkoxybenzenes (from dimethoxy to didecyloxy) can be oxidized reversibly into stable radical cations in a CH2Cl2 + CF3COOH mixture containing 0.2 M NBu4BF4. The effects of the alkoxy substituents on the shift of the redox potentials (from the respective Taft substituent constants) as well as the variations of diffusion coefficients D and the charge transfer rate constants k(s) are examined. In particular, the D values are found to decrease from about 1.0 X 10(-5) to 0.4 X 10(-5) cm(2) s(-1) with the lengthening of ether chains, while the k(s) values are around 2 X 10(-2) cms(-1). In another investigation medium, such as dry CH3CN, the oxidation of 1,4-dialkoxybenzenes leads to the deposition onto the electrode surface of the corresponding p-doped polyphenylenes, except for the 1,4-didecyloxybenzene which does not electropolymerize. The doping level decreases with the lengthening of ether chains and polymers globally exhibit poor film properties. The apparent instability of their electrochemical response and probably the low conductivity can be attributed to a powdery structure, a poor attachment onto the electrode surface and a lack of cohesion of the deposit.