Prediction of the Electronic Structure via Molecular Stacking Mode of Radical Cation Salts Based on Asymmetric Donor Molecule MeEDO-TTF

By the electrochemical oxidation of 4,5-ethylenedioxy-4'-methyl-tetrathiafulvalene (MeEDO-TTF), six kinds of radical cation salts were prepared: three polymorphs of 2:1 ReO4- salts, 4:1 HCTMM2- salt, and 5:2 salts with COCl42- and MnCl42-, where HCTMM2- denotes hexacyanotrimethylenemethanediide dianion. Among them, two modifications of ReO4- salts showed metal-insulator transitions, while the other salts exhibited semiconducting or insulating behaviors in the measured temperature region. The crystal structures of these salts were successfully solved except for the powder polymorph of ReO4- salt. In these crystals, the donor molecules formed head-to-tail stacking with the ring-over-ring and/or ring-over-bond type overlap modes along with the intermediate one between these two modes. The magnitudes of the face-to-face intermolecular interactions were greatly enhanced compared with those observed in the previously reported (MeEDO-TTF)(2)X (X = BF4-, ClO4-, PF6-), for which the head-to-head type molecular stacking with ring-over-bond overlap mode afforded two-dimensional Fermi surfaces. By contrast, the head-to-tail type stacking of MeEDO-TTF provided the quasi-one-dimensional electronic structure in the columnar stacks or localized charge carriers in the multimers. In the radical cation salts of the asymmetric donor molecule, MeEDO-TTF, the dimensionality of the electronic structure is greatly dependent on the molecular stacking types, head-to-head or head-to-tail.