Synthesis, crystal structure and physical properties of an organic semiconductor (BMDT-TTF)I 3

Abstract

Electrochemical oxidation of BMDT-TTF with tetrabutylammonium triiodide yields black rhombohedral crystals with 1:1 stoichiometry, (BMDT-TTF)I 3. Crystal structure was solved by an X-ray diffraction analysis. The BMDT-TTF cation radical and the I 3 − ion form a side-by-side coordination pair with S-I contacts shorter than the sum of van der Waals radii and two such cation-anion pairs form a dimer in which the cation radicals overlap with each other in an almost eclipsed manner. From the measurements of electrical conductivity, electron paramagnetic resonance, and polarized reflectance, it is shown that the unpaired electrons on the BMDT-TTF cation radicals are coupled to yield spin-singlet ground state stabilized by the charge-transfer interaction within the dimer and the interaction between the dimers is rather weak because of a slipped arrangement of the dimers. This results in the semiconductivity of (BMDT-TTF)I 3, although the on-site Coulomb energy of ca. 0.7 eV is significantly smaller than that of TTF salts.