Polymer Films on Electrodes. 28. Scanning Electrochemical Microscopy Study of Electron Transfer at Poly(alkylterthiophene) Films

The electrochemical doping and undoping processes of poly-(3,3‘‘-didodecyl-2,2‘:5‘,2‘‘-terthiophene) (poly(33‘‘DDTT)) were studied by scanning electrochemical microscopy (SECM) to characterize the electron transfer of the polymer with a redox mediator in solution when the polymer was at different oxidation levels. SECM showed that electron exchange processes of oxidized poly(33‘‘DDTT) were localized at the polymer/solution interface rather than inside the film. Poly(33‘‘DDTT) in the neutral state did not allow the permeation of redox species to the underlying metal and behaved like a completely passivating film. A modified expression for the effective electron-transfer rate constant, k<INF>eff</INF>, was obtained by grouping the terms concerned with charge transport across the polymeric film, and the values of k<INF>eff</INF> were calculated from SECM approach curves. The values of k<INF>eff</INF> for the electron transfer between poly(33‘‘DDTT) film and methyl viologen (MV^+/2+) as the redox couple ranged between 10^-5 and 10^-1 cm s^-1, depending on polymer thickness, substrate potential, and MV concentration. Under the proper experimental conditions, k<INF>eff</INF> could be directly correlated with the conductance of the polymer film, where poly(33‘‘DDTT) thin films in the conducting state showed a metallic character.