Tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF) revisited: crystal structures, chiroptical properties, theoretical calculations, and a complete series of conducting radical cation salts.

The (S,S,S,S) and (R,R,R,R) enantiomers of tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF) show equatorial conformation for the four methyl groups in the solid state, according to the single-crystal X-ray analyses. Theoretical calculations at the Density Functional Theory (DFT) and time-dependent (TD) DFT levels indicate higher gas phase stability for the axial conformer than the equatorial one by 1.25 kcal ∙ mole⁻¹ and allow the assignment of the UV-vis and circular dichroism transitions. A complete series of radical cation salts of 1:1 stoichiometry with the triiodide anion I₃⁻ was obtained by electrocrystallization of both enantiopure and racemic forms of the donor. In the packing the donors are organized in dimers that further interact through S ∙ ∙ ∙ S intermolecular contacts and the triiodide anions lie parallel to pairs of oxidized donors. The conductivity of the racemate, which adopts the same, but disordered, structural type, is considerably lower, with much higher activation energy.