Synthesis and properties of conductive radical cation salts of MDT-TSF (methylenedithiotetraselenafulvalene)

Electrocrystallizaion of MDT-TSF (methylenedithiotetraselenafulvalene) in the presence of (n-Bu4N)AuI2, (n-Bu4N)PF6, or a mixture of Cu(SCN), KSCN, and 18-crown-6-ether as a supporting electrolyte gave (MDT-TSF)-X salts (X=AuI2, PF6, or Cu(NCS)2). X-ray structural analyses revealed that the salts adopt various types of crystal structures depending on the counter anion involved; the AuI2 salt takes a characteristic uniform stacking structure, the PF6 salt θ-type, and the Cu(NCS)2 salt κ-type structure. Regardless of the structure type, all the salts are highly conductive at room temperature (>102 S cm−1) and retain metallic down to low temperature. Of them, the AuI2 salt having a high conductivity of 2000 S cm−1 at room temperature showed a superconducting transition at 4.5 K.