Electrodeposition mechanisms and electrochemical behavior of poly(3,4-ethylenedithiathiophene)

The electrodeposition mechanisms of poly(3,4-ethylenedithiathiophene) (PEDTT), which is the sulfur analogue of the well-known poly(3,4-ethylenedioxythiophene) (PEDOT), is investigated in acetonitrile solution by means of potentiostatic methods. By analyzing the current transients within electrocrystallization theory, we observe that the electrodeposition process is a combination of two mechanisms: progressive nucleation, followed by a diffusion-controlled three-dimensional growth (PN3DD); and an instantaneous nucleation, followed by a three-dimensional growth mechanism with charge transfer as the rate-limiting factor (IN3DC). This trend is contrary to PEDOT electrodeposition mechanisms. Cyclic voltametric measurements show important differences between PEDOT and PEDTT. The most unexpected result is that, although 3,4-ethylenedithiathiophene (EDTT) has a lower oxidation potential than 3,4-ethylenedioxythiophene (EDOT), the polymer PEDTT presents a higher oxidation potential and larger band gap than PEDOT. Density functional theory (DFT) calculations reveal important structural and electronic differences between some oligomers of EDTT and EDOT. We analyze these results in terms of the electron-donating effect of the S atom, the difference in the reactivity of the radical-cations of the monomers, and the difference in the geometries of the oligomers.