Novel conductive radical cation salts based on methylenediselenotetraselenafulvalene (MDSe-TSF): A sign of superconductivity in κ-(MDSe-TSF)₂Br below 4 K

Seven conductive radical cation salts based on MDSe-TSF (methylenediselenotetraselenafulvalene) have been synthesized by electrocrystallization in the presence of Cl^-, Br^-, I₃^-, I₂Br^-, PF₆^-, ClO₄^-, and Cu(NCS)₂^- counter anions. The crystal appearances of these salts fairly depend on the anions employed. X-ray crystallographic analyses have revealed that the PF₆ and ClO₄ salts in the shape of brown thin plates adopt the θ-type structures characterized by the herringbone arrangement of donor stacks, whereas the Cl and Br salts in the shape of black thick plates favor the κ-type structures with the orthogonal arrangement of donor dimers. Regardless of different crystal appearances or crystal packing patterns, all these salts show high conductivity (> 10² S cm^-1) at room temperature and retain metallic properties down to 4.2 K. Of them, the Br salt shows a weak but distinct diamagnetic shielding signal below 4 K in the dc magnetization measurement under zero-field-cooled (ZFC) condition, suggesting a sign of superconductivity. The band calculations of both PF₆ and Br salts demonstrate closed Fermi surfaces indicative of two-dimensional molecular conductors.