| Abstract
| - Electrochemical oxidation of 4,5-ethylenedioxy-4′-methyl-tetrathiafulvalene (MeEDO-TTF) afforded three types of (MeEDO-TTF)2PF6 radical cation salts under different conditions: black powder, dark green plates, and black plates. The optical absorption spectra of these modifications were very similar to each other. The black powder and dark-green plate modifications were semiconducting and metallic, respectively, and the latter showed a semiconducting behavior below 200 K. The black plate modification exhibited a semiconductor-to-semiconductor first-order phase transition at around room temperature (303 K). The donor molecules in both the high- and low-temperature phases of this modification showed similar packing patterns, based on which the tight-binding approximation afforded similar two-dimensional Fermi surfaces. The phase transition of this modification is accompanied by a subtle change in the relative orientation of the neighboring donor molecules. The vibrational spectra proved that a nearly localized charge disproportionation takes place in the high-temperature phase and the distinct charge disproportionation is developed in the low-temperature phase. These results indicate that the slight lattice distortion assists the stabilization of charge disproportionation. The comparison of the isostructural salts of metallic (MeEDO-TTF)2X (X = BF4, ClO4) and the semiconducting high-temperature phase of the (MeEDO-TTF)2PF6 demonstrated that the side-by-side intermolecular interaction is most effectively modulated according to the anion size. The unit cell parameter corresponding to this intermolecular interaction was varied only ca. 2% among these isostructural salts. This observation demonstrates the very sensitive nature of the transport property to the lattice modulation.
- A semiconductor-to-semiconductor first-order phase transition was observed at room temperature in (MeEDO-TTF)2PF6 associated with the slight modulation of the molecular orientation. On the basis of vibrational spectra, the distinct and nearly localized charge disproportions are assigned to the origins of semiconducting properties of low-temperature low conducting and high-temperature highly conducting phases, respectively.
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