Role of the Anion Layer’s Polarity in Organic Conductors β″-(BEDT-TTF)₂XC₂H₄SO₃ (X = Cl and Br)

A two-dimensional (2D) organic conductor β″-(BEDT-TTF)2ClC2H4SO3 (1) crystallized in the P21/m and has a polar anion located on the mirror plane, parallel to the 2D BEDT-TTF conducting layer. A temperature-induced phase transition tilts the anion such that a component of its electric dipole becomes perpendicular to the conducting plane. This low-temperature phase β″-β′′-(BEDT-TTF)2ClC2H4SO3 (1L) has two crystallographically independent donor layers, A and B, each of which is bordered by the positive or negative side of the anion’s dipole (← B → A ← B → A ←). This exposes each donor layer to different effective electric fields and leads to layers of A and B with dissimilar oxidation states. Consequently, the transition can be called the temperature-induced non-doped-to-doped transition. The low-temperature phase (1L) is isomorphous with β″-β′′-(BEDT-TTF)2BrC2H4SO3 (2) from room temperature to at least 100 K, suggesting that 2 is also doped and it shows a very broad MI transition at 70 K. Applying only 2 kbar of static pressure sharpens the MI transition, indicating that the tilted anion straightens, and therefore, we suggest that it can be termed a pressure-induced doped-to-non-doped transition.