Electrochemical and Spectroscopic Characterization of Bis(ethylenedithio)tetrathiafulvalene, Tetramethyltetraselenafulvalene and Their Perchlorate Salts.

Abstract

Electroanalytical methods (cyclic voltammetry (CV), rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) voltammetry) have been used to characterize the oxidation products of bis(ethylenedithio)-tetrathiafulvalene (BEDT-TTF or ET) and tetramethyltetraselenafulvalene (TMTSF) in two commonly used solvents, acetonitrile (CH$\sb3$CN) and 1,1,2-trichloroethane (1,1,2-TCE). Cyclic voltammetry has revealed that the electro-oxidations of ET and TMTSF each occur initially via two one-electron steps representing the formation of the monocation radical and dication species. The electrochemical behaviors of TMTSF and ET in both solvents are summarized. UV/VIS spectroscopy was used to characterize TMTSF, ET and their perchlorate salts in solution. Solution spectra of TMTSF, (TMTSF)$\sb2$ClO$\sb4$, TMTSFClO$\sb4$, ET, (ET)$\sb2$ClO$\sb4$, and ETClO$\sb4$ were determined. The solution spectra of (TMTSF)$\sb2$ClO$\sb4$ was compared to the solid state reflectance spectra of a crystal of (TMTSF)$\sb2$ClO$\sb4$. Using the available electrochemical methods--CA, RDE and RRDE, there is no electrochemical evidence for the formation of a soluble dimer species, R$\sp{+}\sb2$, by a follow up reaction. Spectroscopically, there are no discernible differences in the spectral features of the solution spectrum of the 2:1 perchlorate salt of TMTSF (or ET), and the summed solution spectrum of equal concentrations of the neutral TMTSF (or ET) and its solution spectra of the 1:1 perchlorate salt. In addition, the solution spectra of (TMTSF)$\sb2$ClO$\sb4$ in 1,1,2-TCE at varying concentrations of the salt and the solution spectra of the two 2:1 cation:anion salts at two solvent polarities did not reveal any spectral differences. Tafel experiments using crystals of (TMTSF)$\sb2$ClO$\sb4$ and (ET)$\sb2$ClO$\sb4$ as metallic substrates as well as Pt wire were performed to elucidate further the modes of electron transfer reactions. The similarities in magnitude of the exchange current densities for both crystals to the corresponding values at Pt electrode indicate that electron transfer may indeed occur at the crystal surface. However, these measurements do not prove that the electrooxidation of the neutral molecules at the crystal faces leads to crystal but only that the electron transfer is possible at comparable overvoltages. (Abstract shortened with permission of author.).